U.S. patent application number 15/414648 was filed with the patent office on 2017-12-07 for dc-to-dc converter.
The applicant listed for this patent is SMK Corporation. Invention is credited to Toshihiro AMEI.
Application Number | 20170353106 15/414648 |
Document ID | / |
Family ID | 60328078 |
Filed Date | 2017-12-07 |
United States Patent
Application |
20170353106 |
Kind Code |
A1 |
AMEI; Toshihiro |
December 7, 2017 |
DC-TO-DC CONVERTER
Abstract
An object of the present disclosure is to provide a DC-to-DC
converter with a simple circuit configuration that continues the
supply of direct current power to a load without providing a
redundant power supply circuit even if a switching transistor
operates abnormally in the active state. A protection circuit unit
that converts an input voltage of a direct current input power
supply to a different direct current voltage obtained by stepping
down the input voltage, as in a direct current voltage conversion
circuit unit including a first switching transistor, is connected
between the direct current voltage conversion circuit unit and the
direct current input power supply and, if the first switching
transistor operates in an active state, direct current power of the
direct current voltage converted by the protection circuit unit is
supplied to a load causing the first switching transistor to
maintain operating in the active state.
Inventors: |
AMEI; Toshihiro; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SMK Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
60328078 |
Appl. No.: |
15/414648 |
Filed: |
January 25, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H02M 1/32 20130101; H02M
3/156 20130101; H02M 3/158 20130101; H02M 2001/007 20130101; H02M
3/155 20130101 |
International
Class: |
H02M 3/156 20060101
H02M003/156; H02M 1/32 20070101 H02M001/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 7, 2016 |
JP |
2016-113401 |
Claims
1. A DC-to-DC converter comprising: a direct current voltage
conversion circuit unit including a first switching transistor
connected in series to a direct current input power supply to form
a closed circuit with the direct current input power supply, a
first drive circuit configured to output a first drive signal to
control the opening and closing of the first switching transistor
in a predetermined cycle to a control terminal of the first
switching transistor, a first capacitor connected between a pair of
a high voltage-side connection line and a low voltage-side
connection line, the pair being connected to a load, a first
inductor configured to convert an output voltage between the pair
of the high voltage-side connection line and the low voltage-side
connection line to a direct current voltage different from an input
voltage of the direct current input power supply with the
interruption of a current flowing from the direct current input
power supply by the opening and closing operation of the first
switching transistor, and a first constant voltage control circuit
configured to control a closed time of the first switching
transistor with the first drive signal in accordance with the
output voltage between the pair of the high voltage-side connection
line and the low voltage-side connection line and perform constant
voltage control on the output voltage; an abnormality determination
circuit including a first comparison circuit to compare a voltage
of a connection point on a side, which is connected to the first
inductor, of the first switching transistor and a threshold voltage
freely set within the range of fluctuation of the voltage of the
connection point during the switching operation of the first
switching transistor, the abnormality determination circuit being
configured to determine that the first switching transistor is
operating in an active state upon polarity found by the comparison
of the voltage of the connection point and the threshold voltage
does not change during a detection period longer than the
predetermined cycle of the first drive signal; and a protection
circuit unit connected between the direct current input power
supply and the direct current voltage conversion circuit unit to
shift to protection operation mode at the time when the abnormality
determination circuit determines that the first switching
transistor is operating in the active state and to execute an
operation of protecting the direct current voltage conversion
circuit unit, wherein the protection circuit unit includes a second
switching transistor connected between the direct current input
power supply and the first switching transistor, a second drive
circuit configured to normally output a second drive signal to
perform control of closing the second switching transistor to a
control terminal of the second switching transistor, and output the
second drive signal to perform control of opening and closing the
second switching transistor in a predetermined cycle in the
protection operation mode, a second capacitor connected between the
pair of the high voltage-side connection line and the low
voltage-side connection line on an input side of the direct current
voltage conversion circuit unit, a second inductor configured to in
the protection operation mode convert a voltage across the second
capacitor to a direct current voltage obtained by stepping down the
input voltage of the direct current input power supply with the
interruption of the current flowing from the direct current input
power supply by the opening and closing operation of the second
switching transistor, and a second constant voltage control circuit
configured to in the protection operation mode control a closed
time of the second switching transistor with the second drive
signal in accordance with the voltage across the second capacitor
and perform constant voltage control on the voltage across the
second capacitor.
2. The DC-to-DC converter according to claim 1, wherein the second
constant voltage control circuit brings the voltage across the
second capacitor to a voltage slightly higher than the output
voltage between the pair of the high voltage-side connection line
and the low voltage-side connection line, the pair being connected
to the load, through constant voltage control so that the amount of
heat generation of the first switching transistor due to a voltage
across the first switching transistor does not exceed a maximum
heat dissipation amount of the first switching transistor.
3. The DC-to-DC converter according to claim 1, further comprising
a reporting unit configured to, upon the abnormality determination
circuit determining that the first switching transistor is
operating in the active state, report the determination result to
the outside.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The contents of the following Japanese patent application
are incorporated herein by reference,
[0002] Japanese Patent Application No. 2016-113401 filed on Jun. 7,
2016.
FIELD
[0003] The present disclosure relates to a DC-to-DC converter that
converts a direct current input voltage to a stable direct current
voltage suitable for a load by the opening and closing operation of
a switching transistor, and in more detail relates to a DC-to-DC
converter with the function of fault tolerance to a failure of a
switching transistor.
BACKGROUND
[0004] A DC-to-DC converter converts an unstable input voltage of
direct current to a stable direct current output voltage to output
it to a load. DC-to-DC converters are used as power supplies of
various electronic devices that operate on direct current voltage.
DC-to-DC converters are separated, based on the operation
principle, into an insulation type that increases and reduces the
input voltage with a transformer and a non-insulation type that
interrupts a current flowing through an inductor with a switching
transistor and converts a direct current input voltage to a direct
current output voltage of a different voltage level and polarity.
The electronic devices in which the input voltage and the output
voltage are not greatly different employ non-insulation type
DC-to-DC converters that can be configured of relatively simple
circuit elements.
[0005] A related step-down DC-to-DC converter 100 described in
Japanese Patent No. 5811237 is described below with reference to
FIG. 3. The DC-to-DC converter 100 includes a direct voltage
conversion unit 10 that steps down an input voltage Vi of direct
current and outputs it between a high voltage-side output terminal
32a and a low voltage-side output terminal 33a, which are connected
to a load RL, and an abnormality determination circuit 4 and a
protection circuit 7, which are described below. A direct current
input power supply 30 generates the input current Vi of direct
current between a high voltage-side power supply terminal 30a and a
low voltage-side power supply terminal 30b. As illustrated, a diode
D1 of which forward direction is from a low voltage side to a high
voltage side and a switching transistor Tr1, the diode D1 and the
switching transistor Tr1 being in the direct current voltage
conversion unit 10, and a switching transistor Tr2 are connected in
series between the high voltage-side power supply terminal 30a and
the low voltage-side power supply terminal 30b to form a closed
circuit.
[0006] A connection point A between the diode D1 and the switching
transistor Tr1 is connected via an inductor L1 to a high
voltage-side connection line 32, on one side of which is the high
voltage-side output terminal 32a. Moreover, the other side of the
connection point A of the diode D1 is connected to a low
voltage-side connection line 33 wired between the low voltage-side
power supply terminal 30b and the low voltage-side output terminal
33a. A capacitor C1 is connected between the high voltage-side
connection line 32 and the low voltage-side connection line 33 to
supply stable direct current power of an output current Io and an
output voltage Vo to the load RL connected between the high
voltage-side output terminal 32a and the low voltage-side output
terminal 33a.
[0007] The switching transistor Tr1 comprises an FET (field-effect
transistor). A drive signal output from a constant voltage control
circuit 40 to a gate of the switching transistor Tr1 controls the
opening and closing of the switching transistor Tr1. Assume that
the switching transistor Tr2 serving as the protection circuit 7 is
controlled to be normally closed (ON control). While the switching
transistor Tr1 is being controlled to be closed (ON control) and is
operating in a saturation state, a current flows from the direct
current input power supply 30 to the inductor L1 to charge the
capacitor C1. However, the charge voltage of the capacitor C1
becoming the output voltage Vo is a voltage lower than the input
voltage Vi due to the self-inductance of the inductor L1. Moreover,
while the switching transistor Tr1 is being controlled to be open
(OFF control) and is operating in an interruption state, electrical
energy stored in the inductor L1 becomes a charge current that is
fed back through the diode D1 to charge the capacitor C1 and
maintain the charge voltage of the capacitor C1 becoming the output
voltage Vo.
[0008] In terms of the output voltage Vo, its voltage level can be
controlled by the closing control time of the switching transistor
Tr1 during a unit time. Accordingly, the constant voltage control
circuit 40 provides the negative feedback of the duty cycle of the
drive signal to perform the control of closing the switching
transistor Tr1 from the output voltage Vo, and brings the output
voltage Vo to the operating voltage of the load RL through constant
voltage control. Hence, the constant voltage control circuit 40
includes a pair of resistive dividers R1 and R2 connected between
the high voltage-side connection line 32 and the low voltage-side
connection line 33, causes an error amplifier 41 to compare the
voltage of a connection point between the resistive dividers R1 and
R2 and a reference power supply voltage Vref that is adjusted to a
predetermined potential based on the operating voltage of the load
RL, and outputs the result to a pulse-width modulation circuit PWM.
The pulse-width modulation circuit PWM modulates the pulse width of
an oscillation signal with a constant period output from an
oscillator OSC with a comparison signal of the error amplifier 41.
The pulse-width modulation circuit PWM outputs the signal to a
drive circuit 42. The drive circuit 42 outputs, to the gate of the
switching transistor Tr1, the drive signal of which duty cycle has
been adjusted in accordance with the comparison signal of the error
amplifier 41. Consequently, for example, when the output voltage Vo
is higher than the operating voltage of the load RL, the drive
circuit 42 outputs the drive signal of which duty cycle has been
reduced to the gate of the switching transistor Tr1. Accordingly,
the ON control time within the unit time is reduced. Therefore, the
output voltage Vo reduces. Conversely, when the output voltage Vo
is lower than the operating voltage of the load RL, the drive
signal of which duty cycle has been increased is output to the gate
of the switching transistor Tr1 to extend the ON control time
within the unit time. Therefore, the output voltage Vo increases.
Accordingly, the output voltage Vo is brought to a predetermined
operating voltage that is different according to the load RL
through constant voltage control.
[0009] On the other hand, when the pulse-width modulation circuit
PWM or the like of the constant voltage control circuit 40 fails
due to some cause such as a lightening strike, and the drive
circuit 42 outputs the drive signal at a constant potential that
puts the switching transistor Tr1 in an active state to the gate
(base) of the switching transistor Tr1, the DC-to-DC converter 100
puts the switching transistor Tr1 in a normally closed state (ON
state) to operate as a series regulator (dropper circuit) that
consumes the input power by the on-resistance of the switching
transistor Tr1 and outputs the output voltage lower than the input
voltage.
[0010] However, unlike a power MOS and a power transistor, which
take measures against heat dissipation, a DC-to-DC converter that
reduces the switching loss at the switching transistor Tr1 as much
as possible and converts the input voltage to a direct current
output voltage highly efficiently uses the switching transistor Tr1
that cannot dissipate thermal energy generated by the
on-resistance, which becomes a cause of the occurrence of a serious
accident where heat is generated to cause a fire. In addition, even
if the switching transistor Tr1 operates in the active state, the
output voltage and the output current do not fluctuate largely from
the set values. Accordingly, the risk of a fire is increased while
the abnormality of the switching transistor Tr1 operating in the
active state cannot be found from the outside.
[0011] Hence, in the DC-to-DC converter 100 described in Japanese
Patent No. 5811237, the focus is concentrated on the point that
when the switching transistor Tr1 operates abnormally in the active
state, the switching transistor Tr1 does not perform the opening
and closing operation in a predetermined cycle and a voltage Vd of
the connection point A on the side, which is connected to the
inductor L1, of the switching transistor Tr1 does not change. When
the voltage Vd of the connection point A does not change during a
detection period longer than the predetermined cycle, the
abnormality determination circuit 4 connected to the connection
point A determines that the switching transistor Tr1 is operating
in the active state.
[0012] Moreover, the switching transistor Tr2 serving as the
protection circuit 7 is connected between the high voltage-side
power supply terminal 30a and the switching transistor Tr1. The
abnormality determination circuit 4 is connected to a gate of the
switching transistor Tr2. The switching transistor Tr2 is
controlled to be normally closed (ON control) by the drive signal
output from the abnormality determination circuit 4. When the
abnormality determination circuit 4 has determined that the
switching transistor Tr1 is operating in the active state, the
switching transistor Tr2 is controlled to be open (OFF control).
The supply of current to the switching transistor Tr1 from the
direct current input power supply 30 is stopped. Consequently, it
is possible to prevent abnormal heat generation in the switching
transistor Tr1.
[0013] According to the above-mentioned related DC-to-DC converter
100, when the switching transistor Tr1 operates in the active
state, the supply of current to the switching transistor Tr1 is
stopped, and fail-safe that stops the operation of the direct
current voltage conversion unit 10 works. Accordingly, a fire
accident due to heat generation in the switching transistor Tr1 can
be prevented from occurring. However, fault tolerance is not
considered. Therefore, when the abnormal operation of the switching
transistor Tr1 is detected, the supply of direct current power to
the load RL is also stopped. If the DC-to-DC converter 100 is used
in systems for, for example, the flight of an airplane, the travel
of an automobile, and the lift of an elevator, direct current power
is not supplied to these systems to become a cause of provoking a
severer accident with the risk of death.
[0014] Hence, when the function of fault tolerance is added to the
DC-to-DC converter 100, a redundant power supply circuit having the
same configuration as that of the above-mentioned direct current
voltage conversion unit 10 is normally placed in parallel between
the high voltage-side output terminal 32a connected to the direct
current input power supply 30 and the load RL, and the low
voltage-side power supply terminal 30b. When an abnormal operation
of the switching transistor Tr1 of the direct current voltage
conversion unit 10, which is caused to operate normally, is
detected, the redundant power supply circuit is activated to
operate.
[0015] However, if the redundant power supply circuit having the
same configuration is placed in parallel in this manner, the entire
circuit configuration of the DC-to-DC converter 100 becomes
complicated and is increased in size. Furthermore, the direct
current voltage conversion unit 10 where the switching transistor
Tr1 is operating abnormally continues its operation in parallel in
the face of heat generation. Accordingly, it is necessary to
provide means for stopping the operation of the failed direct
current voltage conversion unit 10. In addition, it is necessary to
provide switching means for switching operation to the redundant
power supply circuit. Especially, in terms of the switching of
operation to the redundant power supply circuit, the direct current
power of a stable output voltage cannot be supplied immediately
after the redundant power supply circuit is activated; accordingly,
it is difficult to perform the switching control of switching
operation to the redundant power supply circuit without stopping
the supply of the direct current power to the load RL.
[0016] The present disclosure has been made considering such
related problems. An objective thereof is to provide a DC-to-DC
converter with a simple circuit configuration that continues the
supply of direct current power to a load RL without providing a
redundant power supply circuit even if a switching transistor Tr1
operates abnormally in the active state.
[0017] Moreover, another objective is to provide a DC-to-DC
converter that has no risk of heat generation even if the switching
transistor Tr1 of a direct current voltage conversion unit is
caused to operate continuously in the active state.
SUMMARY
[0018] In order to achieve the above-mentioned objectives, a
DC-to-DC converter according to a first aspect includes: a direct
current voltage conversion circuit unit including a first switching
transistor connected in series to a direct current input power
supply to form a closed circuit with the direct current input power
supply, a first drive circuit configured to output a first drive
signal to control the opening and closing of the first switching
transistor in a predetermined cycle to a control terminal of the
first switching transistor, a first capacitor connected between a
pair of a high voltage-side connection line and a low voltage-side
connection line, the pair being connected to a load, a first
inductor configured to convert an output voltage between the pair
of the high voltage-side connection line and the low voltage-side
connection line to a direct current voltage different from an input
voltage of the direct current input power supply with the
interruption of a current flowing from the direct current input
power supply by the opening and closing operation of the first
switching transistor, and a first constant voltage control circuit
configured to control a closed time of the first switching
transistor with the first drive signal in accordance with the
output voltage between the pair of the high voltage-side connection
line and the low voltage-side connection line, and perform constant
voltage control on the output voltage; an abnormality determination
circuit including a first comparison circuit to compare a voltage
Vd of a connection point A on a side, which is connected to the
first inductor, of the first switching transistor and a threshold
voltage Vth freely set within the range of fluctuation of the
voltage Vd of the connection point A during the switching operation
of the first switching transistor, the abnormality determination
circuit being configured to determine that the first switching
transistor is operating in an active state upon polarity found by
the comparison of the voltage Vd and the threshold voltage Vth does
not change during a detection period longer than the predetermined
cycle of the drive signal; and a protection circuit unit connected
between the direct current input power supply and the direct
current voltage conversion circuit unit to shift to protection
operation mode at the time when the abnormality determination
circuit determines that the first switching transistor is operating
in the active state and to execute an operation of protecting the
direct current voltage conversion circuit unit. The protection
circuit unit includes a second switching transistor connected
between the direct current input power supply and the first
switching transistor, a second drive circuit configured to normally
output a second drive signal to perform control of closing the
second switching transistor to a control terminal of the second
switching transistor, and output the second drive signal to perform
control of opening and closing the second switching transistor in a
predetermined cycle in protection operation mode, a second
capacitor connected between the pair of the high voltage-side
connection line and the low voltage-side connection line on an
input side of the direct current voltage conversion circuit unit, a
second inductor configured to in protection operation mode convert
a voltage across the second capacitor to a direct current voltage
obtained by stepping down the input voltage of the direct current
input power supply with the interruption of the current flowing
from the direct current input power supply by the opening and
closing operation of the second switching transistor, and a second
constant voltage control circuit configured to in protection
operation mode control a closed time of the second switching
transistor with the second drive signal in accordance with the
voltage across the second capacitor and perform constant voltage
control on the voltage across the second capacitor.
[0019] The voltage Vd of the connection point A on the side, which
is connected to the first inductor, of the first switching
transistor fluctuates within the range of fluctuation during the
normal switching operation, in which a saturation state and an
interruption state is repeated, of the first switching transistor.
The polarity found by the first comparison circuit of the
abnormality determination circuit comparing the voltage Vd and the
threshold voltage Vth changes within the predetermined cycle when
the first switching transistor is opened and closed. When the first
switching transistor operates in the active state, an input voltage
Vi is substantially constant, and the voltage Vd of the connection
point A also maintains substantially the same potential.
Accordingly, the polarity found by the first comparison circuit
comparing the voltage Vd and the threshold voltage Vth does not
change even during the detection period longer than the
predetermined cycle when the first switching transistor is opened
and closed. Therefore, the abnormality determination circuit
determines that the first switching transistor is operating in the
active state, and the protection circuit unit shifts to protection
operation mode.
[0020] In protection operation mode, the second switching
transistor performs the opening and closing operation in a
predetermined cycle to allow the second inductor to convert the
voltage across the second capacitor to the direct current voltage
obtained by stepping down the input voltage of the direct current
input power supply. The second constant voltage control circuit
brings the voltage across the second capacitor to the stable
voltage obtained by stepping down the input voltage of the direct
current input power supply through constant voltage control.
Accordingly, the output voltage between the pair of the high
voltage-side connection line and the low voltage-side connection
line is supplied at the stable voltage obtained by stepping down
the input voltage to the load via the first switching transistor
that is operating in the active state.
[0021] A voltage on the high voltage side of the first switching
transistor that is operating in the active state is substantially
equal to the voltage across the second capacitor obtained by
stepping down the input voltage of the direct current input power
supply. Accordingly, the value of the voltage applied to the first
switching transistor is reduced since the protection circuit unit
shifts to protection operation mode. Even if the first switching
transistor operates in the active state, there is no risk of
abnormal heat generation.
[0022] In the DC-to-DC converter according to a second aspect, the
second constant voltage control circuit brings the voltage across
the second capacitor to a voltage slightly higher than the output
voltage between the pair of the high voltage-side connection line
and the low voltage-side connection line, the pair being connected
to the load, through constant voltage control so that the amount of
heat generation of the first switching transistor due to a voltage
across the first switching transistor does not exceed a maximum
heat dissipation amount of the first switching transistor.
[0023] A voltage drop Vtr across the first switching transistor
that is operating in the active state is a potential difference
between the output voltage between the pair of the high
voltage-side connection line and the low voltage-side connection
line, the pair being connected to the load, and the voltage across
the second capacitor. The amount of heat generation of the first
switching transistor that is operating in the active state is
proportional to the square of Vtr. Constant voltage control is
performed on the voltage across the second capacitor for the output
voltage between the pair of the high voltage-side connection line
and the low voltage-side connection line, the pair being connected
to the load. Accordingly, the amount of heat generation of the
first switching transistor can be controlled to be equal to or less
than the maximum heat dissipation amount of the first switching
transistor.
[0024] The DC-to-DC converter according to a third aspect further
includes a reporting unit configured to, upon the abnormality
determination circuit determining that the first switching
transistor is operating in the active state, report the
determination result to the outside.
[0025] The reporting unit reports, to the outside, the abnormal
operation of the first switching transistor that does not appear on
the outside.
[0026] The first aspect in accordance with the present disclosure
can continuously supply, to the load, the direct current power of
the output voltage obtained by stepping down the input voltage
without stopping the operation of the failed direct current voltage
conversion circuit unit without providing another redundant power
supply circuit corresponding to the direct current voltage
conversion circuit unit in parallel even if the switching
transistor of the direct current voltage conversion circuit unit
operates abnormally.
[0027] Moreover, even if the switching transistor of the direct
current voltage conversion circuit unit is caused to operate
remaining in the active state, there is no risk of abnormal heat
generation in the switching transistor.
[0028] The second aspect in accordance with the present disclosure
can securely prevent the occurrence of a fire without generating
heat abnormally even if the first switching transistor is caused to
operate remaining in the active state.
[0029] In the third aspect in accordance with the present
disclosure, the reporting unit reports, to the outside, the
abnormal operation of the first switching transistor that does not
appear on the outside. Accordingly, it is possible to take measures
for recovery such as the repair or replacement of the failed part
of the direct current voltage conversion circuit unit after the
risk of heat generation of the switching transistor is conveyed to
a user and the operation of the DC-to-DC converter is stopped
safely.
BRIEF DESCRIPTION OF DRAWINGS
[0030] FIG. 1 is a circuit diagram of a step-down DC-to-DC
converter 1 according to one embodiment of the invention of the
present application;
[0031] FIG. 2 is a block diagram of an abnormality determination
and protection control unit 11; and
[0032] FIG. 3 is a circuit diagram of a related DC-to-DC converter
100.
DESCRIPTION OF EMBODIMENTS
[0033] A DC-to-DC converter 1 according to one embodiment of the
present invention is a step-down DC-to-DC converter 1 that converts
a direct current input voltage Vi of 12 V to a direct current
output voltage Vo of 5V. The step-down DC-to-DC converter 1 is
described hereinafter with reference to FIGS. 1 and 2. FIG. 1 is a
circuit diagram of the step-down DC-to-DC converter 1 to which a
protection circuit 2 is added without changing the basic
configuration of a related DC-to-DC converter 100 illustrated in
FIG. 3 as is clear from a comparison with the DC-to-DC converter
100. Therefore, the main circuit configuration is common to that of
the above-mentioned related step-down DC-to-DC converter 100.
Accordingly, the same numerals are assigned to configurations that
act substantially in the same manner or similarly, and their
detailed descriptions are omitted.
[0034] A direct current input power supply 30 is an unstable power
supply where the direct current input voltage Vi fluctuates in
voltage by approximately 10%. A switching transistor Tr2 forming a
part of the protection circuit 2, and a switching transistor Tr1
and a diode D1 of which forward direction is from a low voltage
side to a high voltage side, the switching transistor Tr1 and the
diode D1 forming a part of a direct current voltage conversion unit
10, are connected in series between a high voltage-side power
supply terminal 30a of +12 V and a low voltage-side power supply
terminal 30b of 0 V to form a closed circuit.
[0035] The switching transistors Tr1 and Tr2 are both P-channel
FETs (field-effect transistors). The switching transistor Tr1 is
connected at its gate to a drive circuit 42 of a constant voltage
control circuit 40 including a control IC. The switching transistor
Tr1 is ON/OFF controlled by a drive signal output from the drive
circuit 42. Moreover, the switching transistor Tr2 is connected at
its gate to an output of a drive circuit 21, which is described
below, of the protection circuit 2. The switching transistor Tr2 is
normally ON-controlled by a drive signal output from the drive
circuit 21. The switching transistor Tr2 is ON/OFF controlled by
the drive signal when the operation of the switching transistor Tr1
in an active state is detected. The ON control of the switching
transistors Tr1, Tr2 indicates performing the closing control
between the drain and the source with the switching transistors
Tr1, Tr2 in a saturation state. The OFF control indicates the
opening control between the drain and the source as an interruption
state.
[0036] The drive signal output from the drive circuit 42 is, for
example, a pulse signal that repeats the ON time of 0 V and the OFF
time of +12 V in a fixed period T of 1 .mu.sec. In a normal state
where while the drive circuit 42 is outputting a drive signal of +0
V to the gate of the switching transistor Tr1, the switching
transistor Tr1 is ON-controlled and the switching transistor Tr2 is
ON-controlled, charge current that charges a capacitor C1 normally
flows from the direct current input power supply 30 to an inductor
L1. The charge voltage of the capacitor C1 becoming the output
voltage Vo during the ON control time is a voltage of +5 V lower
than the input voltage Vi of +12 V due to the self-inductance of
the inductor L1.
[0037] Moreover, when the drive circuit 42 outputs a drive signal
of +12 V to the gate of the switching transistor Tr1, the switching
transistor Tr1 is OFF-controlled. During the OFF control time, the
electrical energy stored in the inductor L1 becomes the charge
current that is fed back through the diode D1 to charge the
capacitor C1 with the same polarity as that of the charge voltage.
The output voltage Vo reduced by the power consumption of a load RL
(the charge voltage of the capacitor C1) is maintained at +5V.
[0038] The constant voltage control circuit 40 brings the output
voltage Vo to the operating voltage of the load RL through constant
voltage control. The constant voltage control circuit 40 includes a
pair of resistive dividers R1 and R2 connected between a connection
point B1 of a high voltage-side connection line 32 connected to the
load RL and a low voltage-side connection line 33, an error
amplifier 41 that compares the voltage of a connection point
between the resistive dividers R1 and R2 and a reference power
supply voltage Vref that is adjusted to a predetermined potential
based on the operating voltage of the load RL, a pulse-width
modulation circuit PWM1 that modulates the pulse width of a signal
of a triangle wave, a sawtooth wave, or the like of a fixed
frequency of 1 MHz output from an oscillator OSC1 with the output
of the error amplifier 41, and the drive circuit 42 that outputs
the modulated signal output from the pulse-width modulation circuit
PWM1 as the drive signal to the gate of the switching transistor
Tr1.
[0039] When the output voltage Vo is higher than the operating
voltage of the load RL, the pulse-width modulation circuit PWM1
controls the pulse signal to reduce the ON time within the fixed
period T of 1 .mu.sec to output the drive signal of which duty
cycle has been reduced to the gate of the switching transistor Tr1.
As a result, the ON control time of the switching transistor Tr1
within the unit time is reduced to reduce the output voltage Vo.
Conversely, when the output voltage Vo is lower than the operating
voltage of the load RL, the drive signal of which duty cycle has
been increased is output to the gate of the switching transistor
Tr1 to extend the ON control time within the unit time.
Accordingly, the output voltage Vo increases. The output voltage Vo
is brought to a predetermined operating voltage different according
to the load RL through constant voltage control.
[0040] In the step-down DC-to-DC converter 1 where the constant
voltage control circuit 40 that operates normally brings the output
voltage Vo to 5 V, which is the operating voltage of the load RL,
through constant voltage control, for example, a drive signal of
which duty cycle is 44% is output to the gate of the switching
transistor Tr1 to control the switching of the switching transistor
Tr1, and the direct current input voltage Vi of 12 V is converted
to the output voltage Vo of 5 V. The potential of a connection
point A between the switching transistor Tr1 and the inductor L1
while the switching transistor Tr1 is performing the switching
operation normally is +12 V equal to the potential of the high
voltage-side power supply terminal 30a while the switching
transistor Tr1 is being ON-controlled, and is -0.5 V that is
reduced by a voltage drop Vf across the diode D1 of approximately
0.5 V from the ground potential of the low voltage-side power
supply terminal 30b while the switching transistor Tr1 is being
OFF-controlled. Accordingly, the potential of the connection point
A fluctuates between -0.5 V and +12 V.
[0041] As illustrated in FIG. 2, an abnormality determination and
protection control unit 11 includes an abnormality determination
circuit 4 where a detection terminal SW is connected to the
connection point A between the switching transistor Tr1 and the
inductor L1 to monitor the potential of the connection point A, and
an RS flip-flop circuit 5 connected to an output of the abnormality
determination circuit 4. The abnormality determination circuit 4
and the RS flip-flop circuit 5 are connected between a constant
current circuit 6 connected to the high voltage-side power supply
terminal 30a and the low voltage-side power supply terminal 30b,
and operate using, as a power supply, the direct current input
power supply 30 where the constant current circuit 6 performs a
conversion to a stable potential.
[0042] The abnormality determination circuit 4 includes an
unillustrated comparison circuit that compares a voltage Vd of the
connection point A and a threshold voltage Vth that is freely set
within the range of fluctuation of the voltage Vd while the
switching transistor Tr1 is performing the switching operation. In
the embodiment, the voltage Vd of the connection point A fluctuates
between 0.5 V and +5 V. Accordingly, the threshold voltage Vth is
set to +0.5 V between them.
[0043] While the switching transistor Tr1 is performing the
switching operation normally, the polarity of the output of the
comparison circuit is reversed within at least the fixed period T
of 1 .mu.sec. On the other hand, if the pulse-width modulation
circuit PWM1 or the like fails due to some cause and the drive
signal output to the gate of the switching transistor Tr1 has a
constant potential, the switching transistor Tr1 operates in the
active state, and the direct current input voltage Vi is at a
substantially constant potential. Accordingly, the voltage Vd of
the connection point A also has a constant potential. The polarity
of the output of the comparison circuit is not reversed within the
fixed period T. Hence, when the polarity of the output of the
comparison circuit is not reversed even once during the detection
period set to 2 .mu.sec longer than the fixed period T of the drive
circuit 42, the abnormality determination circuit 4 determines that
it is the abnormal operation in the active state. The abnormality
determination circuit 4 normally changes the output from "L" level
to at "H" level and outputs it to a Set input of the RS flip-flop
circuit 5.
[0044] The RS flip-flop circuit 5 outputs an output signal at "L"
level until the set signal at "H" level is input from the
abnormality determination circuit 4 after a reset signal at "L"
level is input. On the other hand, if the Set signal at "H" level
that has been determined that the switching transistor Tr1 was
operating in the active state is input from the abnormality
determination circuit 4, the RS flip-flop circuit 5 outputs the
output signal at "H" level until the next time the reset signal at
"L" level is input.
[0045] An output of the RS flip-flop circuit 5 is connected to an
output terminal FLAG connected to the drive circuit 21 and an
unillustrated warning display device. While the output signal at
"L" level that has been determined that the switching transistor
Tr1 was operating normally is being output, the protection circuit
2 operates in normal operation mode, and the drive circuit 21
continuously outputs the drive signal of +0 V for ON-controlling
the switching transistor Tr2 to the gate of the switching
transistor Tr2. On the other hand, while the output signal at "H"
level that has been determined that the switching transistor Tr1
was operating in the active state is being output from the RS
flip-flop circuit 5, the protection circuit 2 operates in
protection operation mode, and the drive circuit 21 outputs the
drive signal for ON/OFF-controlling the switching transistor Tr2 to
the gate of the switching transistor Tr2 based on a modulated
signal output from a pulse-width modulation circuit PWM2 described
below. Moreover, the warning display device, into which the output
signal at "H" level has been input from the output terminal FLAG,
displays information that the switching transistor Tr1 is operating
abnormally.
[0046] As illustrated in FIG. 1, the protection circuit 2 is formed
on the direct current input power supply 30 side being the input
side of the direct current voltage conversion unit 10, and includes
circuit elements that operate as in the circuit elements of the
direct current voltage conversion unit 10. In other words, the
switching transistor Tr2 and a diode D2 of which forward direction
is from the low voltage side to the high voltage side are connected
in series for the high voltage-side power supply terminal 30a and
the low voltage-side power supply terminal 30b to form a closed
circuit between the high voltage-side power supply terminal 30a and
the low voltage-side power supply terminal 30b on the direct
current input power supply 30 side of the closed circuit including
the switching transistor Tr1 and the diode D1.
[0047] Moreover, an inductor L2 is connected between a connection
point between the switching transistor Tr2 and the diode D2 and the
input side (source) of the switching transistor Tr1. A capacitor C2
to cause the input side of the switching transistor Tr1 to have a
stable direct current voltage while the protection circuit 2 is
operating in protection operation mode is connected between a
connection point B2 of the inductor L2 and the switching transistor
Tr1, and the low voltage-side connection line 33.
[0048] While the protection circuit 2 is operating in normal
operation mode, the switching transistor Tr2 is normally
ON-controlled, and when voltage drops across the switching
transistor Tr2 and the inductor L2, through which the direct
current flows, are ignored, the input voltage Vi of direct current
is applied to the switching transistor Tr1, and the direct current
voltage conversion unit 10 steps down the input voltage Vi to
supply the direct current power of the constant voltage controlled
output voltage Vo to the load RL.
[0049] After the protection circuit 2 shifts to protection
operation mode, the switching transistor Tr2 is being
ON/OFF-controlled by the drive signal output from the drive circuit
21. While the switching transistor Tr2 is ON-controlled, a current
flows from the direct current input power supply 30 to the inductor
L2 to charge the capacitor C2. However, the charge voltage of the
capacitor C2 becoming a direct current voltage on the high voltage
side of the switching transistor Tr1 is a voltage obtained by
reducing the input voltage Vi by the self-inductance of the
inductor L2. Moreover, while the switching transistor Tr2 is being
OFF-controlled, and is operating in the interruption state, the
electrical energy stored in the inductor L2 becomes a charge
current that is fed back through the diode D2 to charge the
capacitor C2 and maintain the charge voltage of the capacitor C2
being the direct current voltage on the high voltage side of the
switching transistor Tr1.
[0050] The amount of heat generation of the switching transistor
Tr1 while the switching transistor Tr1 is operating in the active
state is expressed as (Vtr).sup.2/r where r is the on resistance
and Vtr is a differential voltage between the voltage of the source
on the connection point B2 side (the charge voltage of the
capacitor C2) and the voltage of the drain on the connection point
A side (the output voltage Vo). As described above, when the
protection circuit 2 shifts to protection operation mode, the
voltage of the source on the connection point B2 side of the
switching transistor Tr1 is reduced to a voltage sufficiently lower
than the input voltage Vi. The differential voltage Vtr from the
output voltage Vo of the drain on the connection point A side is
reduced. Accordingly, the amount of heat generation is dramatically
reduced so that the risk of abnormal heat generation
disappears.
[0051] However, in the embodiment, the protection circuit 2 further
includes a constant voltage control circuit 12 that brings the
voltage across the switching transistor Tr1 to the differential
voltage Vtr through constant voltage control so that the amount of
heat generation of the switching transistor Tr1 does not exceed a
maximum heat dissipation amount Qmax, that is,
(Vtr).sup.2/r<Qmax. The constant voltage control circuit 12 is
integrated in the abnormality determination and protection control
unit 11 as illustrated in FIG. 2.
[0052] The constant voltage control circuit 12 includes an offset
power supply 24 that outputs a direct current offset voltage
corresponding to a target differential voltage Vtrset on which
constant voltage control is performed, an error amplifier 23
connected at its inverting input terminal to a SET terminal and
connected at its non-inverting input terminal to a SENCE terminal
via the offset power supply 24, the pulse-width modulation circuit
PWM2 that modulates the pulse width of a signal of a triangular
wave, a sawtooth wave, or the like of a fixed frequency of 1 MHz
output from an oscillator OSC2 with the output of the error
amplifier 23, and the drive circuit 21 that outputs the modulated
signal output from the pulse-width modulation circuit PWM2 as the
drive signal to the gate of the switching transistor Tr2, in
protection operation mode of the protection circuit 2.
[0053] The SET terminal connected to the inverting input terminal
of the error amplifier 23 is connected to the connection point B2
between the inductor L2 and the switching transistor Tr1 on the
high voltage-side connection line 32. The SENCE terminal connected
to the non-inverting input terminal via the offset power supply 24
is connected to a connection point C of the high voltage-side
connection line 32 connected to the load RL. Consequently, the
error amplifier 23 compares a voltage to which a direct current
offset voltage Voff is added to the output voltage Vo and the
voltage on the high voltage-side of the switching transistor Tr1.
While the protection circuit 2 is operating in protection operation
mode, the output current of direct current flows through the
inductor L1. The error amplifier 23 compares the differential
voltage Vtr between the high voltage side and the low voltage side
of the switching transistor Tr1 and the target differential voltage
Vtrset specified by the direct current offset voltage Voff when a
voltage drop across the inductor L1 is ignored.
[0054] When the differential voltage Vtr is higher than the target
differential voltage Vtrset, the pulse-width modulation circuit
PWM2 controls the pulse signal to reduce the ON time within the
fixed period T. The drive signal of which duty cycle has been
reduced is output to the gate of the switching transistor Tr2. As a
result, the ON control time within the unit time of the switching
transistor Tr2 reduces; accordingly, the charge voltage of the
capacitor C2, that is, the voltage on the high voltage side of the
switching transistor Tr1 reduces. The differential voltage Vtr
reduces to approach the target differential voltage Vtrset.
Conversely, when the differential voltage Vtr is lower than the
target differential voltage Vtrset, the drive signal of which duty
cycle has been increased is output to the gate of the switching
transistor Tr2 to extend the ON control time within the unit time.
Accordingly, the charge voltage of the capacitor C2 increases. The
differential voltage Vtr increases to approach the target
differential voltage Vtrset. The above operation is repeated to
bring the voltage across the switching transistor Tr1 to the target
differential voltage Vtrset through constant voltage control.
[0055] In the embodiment, the direct current input voltage Vi is
set to 12 V, the output voltage Vo on which constant voltage
controls is performed to 5 V during normal operation. Accordingly,
the offset power supply 24 that sets the target differential
voltage Vtrset to 1 V and outputs a direct current voltage of 1 V
is used.
[0056] Consequently, even if the switching transistor Tr1 continues
operating in the active state, the voltage across the switching
transistor Tr1 with the on resistance r is brought to 1 V through
constant voltage control. The switching transistor Tr1 does not
generate heat abnormally irrespective of the voltages of the direct
current input voltage Vi and the output voltage Vo.
[0057] The operation of the switching transistor Tr1 in the active
state and the operation of the protection circuit 2 in protection
operation mode are conveyed to a user by display on the warning
display device as described above. Accordingly, a failed part can
be repaired and recovered after the operation of the DC-to-DC
converter 1 becomes possible to be stopped safely.
[0058] If while the protection circuit 2 is operating in protection
operation mode, the cause of a failure to put the switching
transistor Tr1 in the active state is somehow solved and the
switching transistor Tr1 resumes the switching operation, the
abnormality determination circuit 4 determines that it is the
normal operation and then outputs a reset signal at "L" level to
the RS flip-flop circuit 5. Accordingly, the protection circuit 2
returns to normal operation mode in which the switching transistor
Tr2 is normally ON-controlled to return the direct current voltage
conversion unit 10 to normal operation.
[0059] In the above-mentioned embodiment, a P-channel FET or an
N-channel FET is used as the switching transistors Tr1, Tr2.
However, the switching transistors Tr1, Tr2 may be an N-channel FET
or a P-channel FET in which the drain and the source are connected
the other way around, or may be a bipolar transistor.
[0060] Moreover, the constant voltage control circuit 12 that
operates when the protection circuit 2 operates in protection
operation mode performs constant voltage control on the
differential voltage Vtr of the switching transistor Tr1. However,
constant voltage control may be performed on the charge voltage of
the capacitor C2 and on the voltage on the high voltage side of the
switching transistor Tr1.
[0061] Moreover, in the above-mentioned embodiment, the example is
described in which when the abnormality determination circuit 4
determines that the switching transistor Tr1 is operating in the
active state, the warning display device displays and reports the
abnormal operation of the switching transistor Tr1. However,
another method such as the issuance of sound such as a warning
sound may make the report to the user.
[0062] Moreover, in the above-mentioned embodiment, the example is
described in which the switching transistor Tr1 operates in the
active state due to an abnormality in the drive signal. However,
the present disclosure can also be applied to a case where the
switching transistor Tr1 operates in the active state due to other
reasons such as the failure of the switching transistor Tr1 itself
and an abnormality in connection between circuit elements.
[0063] The present disclosure is suitable for a DC-to-DC converter
using a transistor as a switching element of a non-insulation type
DC-to-DC converter.
* * * * *