U.S. patent application number 11/951408 was filed with the patent office on 2011-01-13 for dc/dc converter control circuit, and power supply apparatus, light emitting apparatus and electronic device using the same.
This patent application is currently assigned to ROHM CO., LTD.. Invention is credited to Kyoichiro Araki, Yoichi Tamegai, Isao Yamamoto.
Application Number | 20110006694 11/951408 |
Document ID | / |
Family ID | 37498338 |
Filed Date | 2011-01-13 |
United States Patent
Application |
20110006694 |
Kind Code |
A9 |
Tamegai; Yoichi ; et
al. |
January 13, 2011 |
DC/DC CONVERTER CONTROL CIRCUIT, AND POWER SUPPLY APPARATUS, LIGHT
EMITTING APPARATUS AND ELECTRONIC DEVICE USING THE SAME
Abstract
A control circuit is provided for a separately excited DC/DC
converter which directly monitors output voltage to detect a
short-circuit state, and performs overcurrent protection. A
switching controller of the control circuit controls a switching
operation of a switching transistor of the separately excited DC/DC
converter. A voltage comparator compares the output voltage and a
threshold voltage, to detect the short-circuit state. After a
predetermined start-up time has elapsed after beginning start-up of
the separately excited DC/DC converter, when the voltage comparator
detects the short-circuit state, the switching controller halts the
switching operation of the switching transistor, and makes
detection of the short-circuit state by the voltage comparator
non-operative before elapse of the start-up time. After detecting
the short-circuit state and halting the switching operation of the
switching transistor for a predetermined halt time, the switching
controller begins start-up of the separately excited DC/DC
converter once again.
Inventors: |
Tamegai; Yoichi; (Kyoto,
JP) ; Yamamoto; Isao; (Kyoto, JP) ; Araki;
Kyoichiro; (Kyoto, JP) |
Correspondence
Address: |
CANTOR COLBURN LLP
20 Church Street, 22nd Floor
Hartford
CT
06103
US
|
Assignee: |
ROHM CO., LTD.
Kyoto
JP
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20080136342 A1 |
June 12, 2008 |
|
|
Family ID: |
37498338 |
Appl. No.: |
11/951408 |
Filed: |
December 6, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP2006/311065 |
Jun 2, 2006 |
|
|
|
11951408 |
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Current U.S.
Class: |
315/209R ;
323/282 |
Current CPC
Class: |
H02M 3/33507 20130101;
H02M 1/32 20130101 |
Class at
Publication: |
315/209.R ;
323/282 |
International
Class: |
H05B 41/36 20060101
H05B041/36; G05F 1/00 20060101 G05F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 6, 2005 |
JP |
2005-166046 |
Claims
1. A control circuit which controls a switching operation of a
switching transistor of a separately excited DC/DC converter, the
control circuit comprising: a switching controller which controls
the switching operation of the switching transistor; and a voltage
comparator which compares output voltage of the separately excited
DC/DC converter and a predetermined threshold voltage, to detect a
short-circuit state; wherein after a predetermined start-up time
has elapsed after beginning start-up of the separately excited
DC/DC converter, when the voltage comparator detects the
short-circuit state, the switching controller halts the switching
operation of the switching transistor, and males detection of the
short-circuit state by the comparator non-operative before elapse
of the start-up time.
2. A control circuit according to claim 1, wherein after detecting
the short-circuit state and halting the switching operation of the
switching transistor for a predetermined halt time, the switching
controller begins start-up of the separately excited DC/DC
converter once again.
3. A control circuit according to claim 1, wherein the start-up
time is set to be longer than time required for the output voltage
to become higher than the threshold voltage, after beginning
start-up of the separately excited DC/DC converter.
4. A control circuit according to claim 1, wherein, when the output
voltage is continuously below the threshold voltage for a
predetermined short-circuit detection time, according to the
voltage comparator, the switching controller halts the switching
operation of the switching transistor.
5. A control circuit according to claim 1, wherein the switching
controller comprises: a state machine which holds a control state
of the switching transistor, the state machine comprising three
modes: a start-up mode in which detection of a short-circuit state
by the comparator is made non-operative, while executing a step-up
operation by the separately excited DC/DC converter, a normal mode
in which detection of a short-circuit state by the voltage
comparator is performed, while executing the step-up operation by
the separately excited DC/DC converter, and a halt mode in which
the step-up operation by the separately excited DC/DC converter is
halted, and a transition is made to the normal mode after the
start-up time has elapsed after transiting to the start-up mode, a
transition is made to the halt mode when the voltage comparator
detects a short-circuit state when in the normal mode, and a
transition is made to the start-up mode after the predetermined
halt time has elapsed after transiting to the halt mode.
6. A control circuit according to claim 5, wherein, in the normal
mode, when the output voltage of the separately excited DC/DC
converter decreases due to driving a load, the state machine
transits to the start-up mode.
7. A control circuit according to claim 5, wherein the switching
controller further comprises: a pulse width modulator which
generates a pulse signal; a driver circuit which drives the
switching transistor based on the pulse signal; and a hysteresis
comparator which compares the output voltage and a threshold
voltage set in a vicinity of a target value of the output voltage;
and wherein the pulse width modulator gradually changes a duty
ratio of the pulse signal in the start-up mode, and fixes the duty
ratio of the pulse signal at a predetermined value in the normal
mode and the halt mode; and the driver circuit drives the switching
transistor based on the pulse signal, in the start-up mode and the
normal mode, and halts driving of the switching transistor in the
halt mode.
8. A control circuit according to claim 5, wherein the switching
controller further comprises a timer circuit, and the state machine
operates using the timer circuit for required time measurement.
9. A control circuit which controls a switching operation of a
switching transistor of a self-excited type DC/DC converter, the
control circuit comprising: a switching controller which controls
the switching operation of the switching transistor; and a voltage
comparator which compares output voltage of the self-excited DC/DC
converter and a predetermined threshold voltage, to detect a
short-circuit state; wherein after a predetermined start-up time
has elapsed after beginning start-up of the self-excited DC/DC
converter, when the voltage comparator detects the short-circuit
state, the switching controller halts the switching operation of
the switching transistor, and makes detection of the short-circuit
state by the voltage comparator non-operative before elapse of the
start-up time.
10. A control circuit according to claim 1, wherein the switching
controller and the voltage comparator are integrated on one
semiconductor substrate.
11. A power supply apparatus comprising: a separately excited DC/DC
converter comprising a switching transistor, and in which a step-up
operation is controlled by putting the switching transistor ON and
OFF; and a control circuit according to claim 1 which controls
putting the switching transistor ON and OFF.
12. A light-emitting device comprising: a power supply apparatus
according to claim 11; and a light-emitting element which is driven
by output voltage of the separately excited DC/DC converter of the
power supply apparatus.
13. A battery-driven electronic device comprising: an imaging unit;
and a light-emitting device according to claim 12, used as a flash,
when taking an image with the imaging unit; wherein the
light-emitting device steps up battery voltage to drive the
light-emitting element.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a switching power supply,
and a drive system for a DC/DC Converter.
[0003] 2. Description of the Related Art
[0004] A step-up type of switching power supply for generating
voltage higher than an input voltage is widely used in various
electronic devices. This step-up type of switching power supply is
provided with a switching element, and an inductor or a
transformer, and by putting the switching element ON or OFF in a
time-division way, a back electromotive force is generated in the
inductor or the transformer, and input voltage is stepped up and
outputted.
[0005] Among such switching power supplies, two types of
insulating-type DC/DC converters that use the transformer are
known: a self-excited type and a separately excited type. These
types are selected in accordance with a characteristic required of
the switching power supply, such as range of output voltage, or the
like. In the insulating-type of DC/DC converter, when a switching
transistor is OFF, a current flows on a primary side of the
transformer, and energy is stored in the transformer. When the
switching transistor is OFF, energy stored in the transformer on a
secondary side of the transformer is transferred to an output
capacitor, via a rectifier diode, as a charging current, and the
output voltage rises.
[0006] In this type of separately excited DC/DC converter, when an
overcurrent flows due to a short-circuit of a load, or the like,
since the transformer is saturated, or reliability of the switching
transistor is effected, there are cases in which an overcurrent
protection circuit is provided. For example, Patent Document 1
discloses technology in which the current on the primary side of
the transformer is monitored to detect an overcurrent state.
[0007] Patent Document 1: Japanese Patent Application, Laid Open
No. 2002-374671
[0008] However, in the technology described in the abovementioned
document, in which the current on the primary side of the
transformer is monitored, when a load suddenly short-circuits,
there have been cases in which current flowing on the primary side
of the transformer suddenly rises at a speed exceeding response
speed of a voltage comparator, so that overcurrent protection
response is delayed, and the overcurrent flows in the switching
transistor.
SUMMARY OF THE INVENTION
[0009] The present invention was made in view of these issues, and
a general purpose thereof is the provision of a control circuit for
a DC/DC converter that performs overcurrent protection.
[0010] An embodiment of the present invention relates to a control
circuit for controlling a switching operation of a switching
transistor of a separately excited DC/DC converter. The control
circuit for controlling the switching operation of the switching
transistor of the separately excited DC/DC converter includes a
switching controller for controlling the switching operation of the
switching transistor, and a voltage comparator for comparing an
output voltage of the separately excited DC/DC converter and a
predetermined threshold voltage, to detect a short-circuit state.
After a predetermined start-up time has elapsed after beginning
start-up of the separately excited DC/DC converter, when the
voltage comparator detects the short-circuit state, the switching
controller halts the switching operation of the switching
transistor, and makes detection of a short-circuit state by the
voltage comparator non-operative before elapse of the start-up
time.
[0011] According to this embodiment, it is possible to distinguish
between a state in which a load short-circuits and the output
voltage decreases, and a state in which, before the output voltage
at start-up time rises to a target value thereof, the output
voltage is lower than the threshold voltage, and it is possible to
halt the switching transistor and realize circuit protection only
in cases in which the load has really short-circuited.
[0012] After detecting the short-circuit state and halting the
switching operation of the switching transistor for a predetermined
halt time, the switching controller may begin the start-up of the
separately excited DC/DC converter once again. At a time of a
short-circuit of the load, in cases in which a long-term
short-circuit of the load is maintained, by halting the switching
transistor for the predetermined halt time, since there is
intermittent operation in which current flows during the start-up
time and is shut off during the halt time, it is possible to
prevent a large current flowing continuously in the switching
transistor or transformer.
[0013] The start-up time may be set to be longer than the time
required for the output voltage to become higher than the threshold
voltage, after beginning the start-up of the separately excited
DC/DC converter.
[0014] When the output voltage is continuously below the threshold
voltage for a predetermined short-circuit detection period,
according to the voltage comparator, the switching controller may
halt the switching operation of the switching transistor. In such
cases, the long-term short-circuit state can be preferably detected
and the circuit protection can be performed.
[0015] The switching controller may include a state machine for
holding a control state of the switching transistor. The state
machine may have three modes: a start-up mode in which detection of
a short-circuit state by the comparator is made non-operative,
while executing a step-up operation by the separately excited DC/DC
converter, a normal mode in which detection of a short-circuit
state by the voltage comparator is performed, and a halt mode in
which the step-up operation by the separately excited DC/DC
converter is halted. The state machine may transit to the normal
mode after the start-up time has elapsed after transiting to the
start-up mode; the state machine may transit to the halt mode when
the voltage comparator detects a short-circuit state when in the
normal mode; and the state machine may transit to the start-up mode
after the predetermined halt time has elapsed after transiting to
the halt mode.
[0016] By defining three states using the state machine, and
transiting in accordance with a drive state, it is possible to
preferably execute the abovementioned short-circuit protection.
[0017] In the normal mode, when the output voltage of the
separately excited DC/DC converter decreases due to driving the
load, the state machine may transit to the start-up mode. In such
cases, the output voltage can be increased once again by the
start-up mode, and during that time, the detection of the
short-circuit state can be made non-operative.
[0018] The switching controller may include a pulse width modulator
which generates a pulse signal, a driver circuit which drives the
switching transistor based on a pulse signal, and a hysteresis
comparator which compares the output voltage and a threshold
voltage set in a vicinity of a target value of the output voltage.
The pulse width modulator may gradually change a duty ratio of the
pulse signal in the start-up mode, and, in the normal mode and the
halt mode, may fix the duty ratio of the pulse signal at a
predetermined value; and the driver circuit may drive the switching
transistor based on the pulse signal in the start-up mode and the
normal mode, and may halt driving of the switching transistor in
the halt mode.
[0019] In such cases, it is possible to execute a soft start in the
start-up mode. In addition, in the normal mode, since the switching
transistor is driven at the fixed duty ratio, irrespective of the
output voltage, the output voltage gradually increases. After that,
when the output voltage reaches a first threshold voltage of the
hysteresis comparator, the switching transistor is halted, and the
output voltage gradually decreases. When the output voltage
decreases to a second threshold voltage of the hysteresis
comparator, driving of the switching transistor is restarted. As a
result, in the normal mode, the output voltage is stabilized
between the first threshold voltage and the second threshold
voltage. Furthermore, by putting the machine into a halt mode when
there is a short-circuit, a step-up operation can be halted and the
circuit protected.
[0020] The switching controller may further include a timer
circuit, and the state machine may operate using the timer circuit
for required time measurement.
[0021] The switching controller and the voltage comparator may be
integrated on one semiconductor substrate. The integration here may
include cases in which all component elements of the circuit are
formed on the semiconductor substrate, and cases in which main
component elements of the circuit are integrated, with some
resistors, capacitors, or the like, for adjustment of a circuit
constant, arranged outside of the semiconductor substrate.
[0022] Another embodiment of the present invention relates to a
control circuit that controls a switching operation of the
switching transistor of a self-excited DC/DC converter. This
control circuit is provided with a switching controller that
controls a switching operation of the switching transistor, and a
voltage comparator that compares output voltage of the self-excited
DC/DC converter and a predetermined threshold voltage, to detect a
short-circuit state. After a predetermined start-up time has
elapsed after beginning start-up of the self-excited DC/DC
converter, when the voltage comparator detects the short-circuit
state, the switching controller halts the switching operation of
the switching transistor, and males detection of a short-circuit
state by the voltage comparator non-operative before elapse of the
start-up time.
[0023] A further embodiment of the present invention is a power
supply apparatus. This power supply apparatus is provided with a
separately excited DC/DC converter that includes a switching
transistor, and in which a step-up operation is controlled by
putting the switching transistor ON or OFF, and a control circuit
that controls putting the switching transistor ON or OFF.
[0024] According to this embodiment, it is possible to preferably
protect the switching transistor and the transformer of the
separately excited DC/DC converter from overcurrent.
[0025] A still further embodiment of the present invention is a
light-emitting device. This light-emitting device is provided with
the abovementioned power supply apparatus, and a light-emitting
element that is driven by an output voltage of the separately
excited DC/DC converter of the power supply apparatus.
[0026] According to this embodiment, when the light-emitting
element, which is a load, is driven normally without a
short-circuit or the like occurring, it is possible to stably emit
light from the light-emitting element, and in cases of a
short-circuit, it is possible to protect the circuit from
overcurrent.
[0027] A further embodiment of the present invention is a
battery-driven electronic device. This battery-driven electronic
device is provided with an imaging unit, and the abovementioned
light-emitting device, used as a flash when talking an image with
the imaging unit, and the light-emitting device steps up the
battery voltage to drive a light-emitting element.
[0028] According to this embodiment, in cases in which the
light-emitting element, which is connected to the power supply
apparatus as a load, short-circuits, it is possible to prevent a
large current from flowing out continuously for a long time from
the battery, and it is possible to curtail heat generation in the
electronic device.
[0029] It is to be noted that any arbitrary combination or
rearrangement of the above-described structural components and so
forth is effective as and encompassed by the present
embodiments.
[0030] Moreover, this summary of the invention does not necessarily
describe all necessary features so that the invention may also be a
sub-combination of these described features.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Embodiments will now be described, by way of example only,
with reference to the accompanying drawings which are meant to be
exemplary, not limiting, and wherein like elements are numbered
alike in several Figures, in which:
[0032] FIG. 1 is a circuit diagram showing a configuration of a
light-emitting device according to an embodiment;
[0033] FIG. 2 is a block diagram showing a configuration of an
electronic device in which the light-emitting device of FIG. 1 is
installed;
[0034] FIG. 3 is a state transition diagram of a state machine;
and
[0035] FIG. 4 is a time chart showing an operation state of the
light-emitting device of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
[0036] The invention will now be described based on preferred
embodiments which do not intend to limit the scope of the present
invention but exemplify the invention. All of the features and the
combinations thereof described in the embodiment are not
necessarily essential to the invention.
[0037] FIG. 1 is a circuit diagram showing a configuration of a
light-emitting device 200 according to the embodiment. This
light-emitting device 200 is installed in an electronic device
provided with a camera, and when an image is taken by the camera,
functions as a light source used as a flash.
[0038] FIG. 2 is a block diagram showing a configuration of the
electronic device 300 in which the light-emitting device of FIG. 1
is installed. In the present embodiment, the electronic device 300
is a mobile telephone in which a camera is installed, and is
provided with a battery 310, a communication processor 312, a DSP
(Digital Signal Processor) 314, an imaging unit 316, and the
light-emitting device 200.
[0039] The battery 310 is, for example, a lithium-ion battery, and
outputs a voltage of approximately 3 to 4 volts, as a battery
voltage Vbat. The DSP 314 is a block that performs overall control
of the entire electronic device 300, and is connected to the
communication processor 312, the image unit 316, and the
light-emitting device 200. The communication processor 312 includes
an antenna, a radio frequency circuit, and the like, and is a block
that performs communication with a base station. The imaging unit
316 is an imaging device such as a CCD (Charge Coupled Device), a
CMOS sensor, or the like.
[0040] The light-emitting device 200 is provided with a separately
excited DC/DC converter 210, a light-emitting element 212, and a
trigger circuit 214. A xenon tube or the like is preferably used as
the light-emitting element 212. The separately excited DC/DC
converter 210 steps up the battery voltage Vbat supplied from the
battery 310, and supplies a drive voltage (below, referred to as
output voltage) Vout of approximately 300 volts to the
light-emitting element 212. The drive voltage Vout is stabilized at
a target voltage Vtgt of a predetermined level. The trigger circuit
214 is a circuit that controls timing of light emission of the
light-emitting device 200. The light-emitting element 212 emits
light in synchronization with image-taking by the imaging unit
316.
[0041] The explanation returns to FIG. 1. The light-emitting device
200 includes a control circuit 100, a switching transistor Tr1, a
transformer 50, a rectifier diode 52, an output capacitor C1, a
first resistor R1, a second resistor R2, a light-emitting element
212, and an IGBT (Insulated Gate Bipolar Transistor) 214a. The
control circuit 100 is an integrated circuit that is integrated on
one semiconductor substrate. The switching transistor Tr1 is
additionally integrated on this integrated circuit.
[0042] The control circuit 100, the switching transistor Tr1, the
transformer 50, the rectifier diode 52, the output capacitor C1,
the first resistor R1, and the second resistor R2, shown in FIG. 1,
correspond to the separately excited DC/DC converter 210 of FIG. 2.
Furthermore, the IGBT 214a and a light emission controller 214b of
FIG. 1 correspond to the trigger circuit 214 of FIG. 2. The IGBT
214a is provided on a current pathway of the light-emitting element
212, and light emission of the light-emitting element 212 is
controlled by putting the light-emitting element ON or OFF.
[0043] The control circuit 100 controls gate voltage of the
switching transistor Tr1 of the separately excited DC/DC converter
210, to control switching operations, that is, ON and OFF
operations. The control circuit 100 switches between three modes: a
start-up mode that executes a step-up operation by the separately
excited DC/DC converter 210, a normal mode, and a halt mode that
halts the step-up operation by the separately excited DC/DC
converter 210, and controls the switching transistor Tr1. The
control circuit 100 is provided with a function for detecting a
short-circuit state of a load, details of which will be described
below, performing short-circuit detection in the normal mode, and
making short-circuit detection non-operative in the start-up
mode.
[0044] The control circuit 100 is provided with an output terminal
102, a feedback terminal 104, and a light emission control terminal
106. The output terminal 102 is connected to a gate of the
switching transistor Tr1, and a switching signal Vsw, which is an
output signal of the control circuit 100, is outputted. The output
voltage Vout of the separately excited DC/DC converter 210, which
is divided by the first resistor R1 and the second resistor R2, is
fed back to the feedback terminal 104. The light emission control
terminal 106 is connected to a gate of the IGBT 214a.
[0045] The battery voltage Vbat is applied to one end of a primary
coil of the transformer 50, and a drain of the switching transistor
Tr1 is connected to the other end. The switching transistor Tr1 is
an N-channel MOS transistor, whose source is grounded.
[0046] One end of a secondary coil of the transformer 50 is
grounded, and an anode of the rectifier diode 52 is connected to
the other end. A cathode of the rectifier diode 52 is grounded via
the output capacitor C1. The output voltage Vout of the separately
excited DC/DC converter 210 occurs at a connection point of the
output capacitor C1 and the rectifier diode 52. This output voltage
Vout is supplied to the light-emitting element 212.
[0047] The control circuit 100 includes a switching controller 10,
a voltage comparator 30, and the light emission controller 214b.
The switching controller 10 generates the switching voltage Vsw
based on voltage fed back to the feedback terminal 104, and
controls switching operations of the switching transistor Tr1. The
voltage comparator 30 compares the output voltage Vout of the
separately excited DC/DC converter 210 and a predetermined
threshold voltage Vth, to detect a short-circuit state. The light
emission controller 214b generates a light emission control signal
SIG20, and controls a base voltage of the IGBT 214a.
[0048] When the voltage comparator 30 detects a short-circuit
state, after a predetermined start-up time Tp1 has elapsed after
beginning start-up of the separately excited DC/DC converter 210,
the switching controller 10 halts a switching operation of the
switching transistor Tr1, and makes detection of a short-circuit
state by the voltage comparator 30 non-operative before the elapse
of the start-up time Tp1. A detailed explanation is given below
concerning a configuration and operation of the switching
controller 10 and the comparator 30.
[0049] The switching controller 10 includes a hysteresis comparator
12, a state machine 14, a timer circuit 16, a driver circuit 18,
and a pulse width modulator 20.
[0050] The pulse width modulator 20 generates a pulse width
modulation signal Vpwm in which pulse varies, at a constant
frequency, and outputs to the driver circuit 18. The driver circuit
18 is configured to include an inverter and the like, and generates
the switching voltage Vsw based on the pulse width modulation
signal Vpwm, to drive the switching transistor Tr1. This driver
circuit 18 is provided with two enabling terminals 18a and 18b, and
a mode signal MODE1 outputted from the hysteresis comparator 12 and
the state machine 14, described below, and an overvoltage detection
signal Vov is inputted to each of the enabling terminals 18a and
18b.
[0051] The hysteresis comparator 12 detects an overvoltage state in
which the output voltage Vout of the separately excited DC/DC
converter 210 is higher than a predetermined threshold voltage, and
generates the overvoltage detection signal Vov. This overvoltage
detection signal Vov has a high level in the overvoltage state, and
a low level otherwise. When the overvoltage detection signal Vov
has a high level, irrespective of a pulse width modulation signal
Vpwm outputted from the pulse width modulator 20, the driver
circuit 18 halts a switching operation of the switching transistor
Tr1, and when the overvoltage detection signal Vov has a low level,
drives the switching transistor Tr1 based on the pulse width
modulation signal Vpwm.
[0052] As described above, the output voltage Vout of the
separately excited DC/DC converter 210 is divided in the feedback
terminal 104, and a feedback voltage Vout'=Vout.times.R1/(R1+R2) is
fed back. The feedback voltage Vout' inputted to the feedback
terminal 104 is inputted to a plus (non-inverting) terminal of the
hysteresis comparator 12, and a reference voltage Vref is inputted
to a minus (inverting) terminal, respectively. When its output is
at a low level, the hysteresis comparator 12 compares a first
threshold voltage Vref1 and the feedback voltage Vout', and when
its output is at a high level, compares a second threshold voltage
Vref2 and the feedback voltage Vout'. Here, a relationship
expressed as Vref1>Vref2 is established between the first
threshold voltage Vref1 and the second threshold voltage Vref2.
[0053] When the output voltage Vout rises, by a step-up operation,
and reaches the first threshold voltage given by
Vmax=Vref1.times.(R1+R2)/R1, the overvoltage detection signal Vov
outputted from the hysteresis comparator 12 has a low level, and
when the output voltage Vout decreases, by halting the step-up
operation, and reaches the second threshold voltage given by
Vmin=Vref2.times.(R1+R2)/R1, the overvoltage detection signal Vov
has a high level. The reference voltage Vref of the hysteresis
comparator 12 is set so that Vref=Vtgt.times.R1/(R1+R2) is
established, using the target voltage Vtgt of the output voltage
Vout of the separately excited DC/DC converter 210.
[0054] The pulse width modulator 20 includes a voltage comparator
22, an oscillator 24, and a soft start circuit 26. The oscillator
24 generates a cyclic frequency Vosc of a triangular waveform or a
sawtooth waveform. The soft start circuit 26 generates a soft start
voltage Vss that gradually increases in the start-up mode. The
cyclic Voltage Vosc and the soft start voltage Vss are inputted to
a plus (non-inverting) terminal of the voltage comparator 22, and a
fixed voltage Vc1 is applied to the minus (inverting) terminal. The
maximum value of the soft start voltage Vss is set to be equal to
the fixed voltage Vc1. The voltage comparator 22 compares the
cyclic voltage Vosc and the lower of the fixed voltage Vc1 and the
soft start voltage Vss. Accordingly, with regard to the pulse width
modulation signal Vpwm outputted from the pulse width modulator 20,
a duty ratio gradually becomes large in the start-up mode, and in
the normal mode and the halt mode, the duty ratio is fixed at a
predetermined value determined by the fixed voltage Vc1.
[0055] The driver circuit 18, referring to a comparison result of
the hysteresis comparator 12, in the start-up mode and the normal
mode, drives the switching transistor Tr1 based on the pulse width
modulation signal Vpwm when the output voltage Vout is lower than a
threshold voltage, and halts driving of the switching transistor
Tr1 when the output voltage Vout is higher than the threshold
voltage. Moreover, in the halt mode, driving of the switching
transistor Tr1 is halted.
[0056] The voltage comparator 30 is provided for detecting a
short-circuit state of a load, by monitoring the output voltage
Vout of the separately excited DC/DC converter 210. The voltage
comparator 30 compares the feedback voltage Vout' inputted to the
plus (non-inverting) terminal, and the predetermined threshold
voltage Vth' inputted to the minus (inverting) terminal, and when
Vout'>Vth' outputs at a high level, and when Vout'<Vth'
outputs at a low level. Below, the output of the voltage comparator
30 is referred to as a short-circuit detection signal Vsc. That is,
the voltage comparator 30 detects the short-circuit state by
comparing the output voltage Vout of the separately excited DC/DC
converter 210 with the threshold voltage Vth=Vth'.times.(R1+R2)/R1.
For example, in cases in which the target voltage Vtgt of the
output voltage Vout is 300 volts, the threshold voltage Vth is set
at approximately 30 volts.
[0057] The short-circuit detection signal Vsc outputted from the
voltage comparator 30 is inputted to the state machine 14. The
state machine 14 holds control states of the switching transistor
Tr1, that is, four states: the start-up mode, the normal mode, the
halt mode, and standby mode. FIG. 3 is a state transition diagram
of the state machine 14. When power is supplied to the electronic
device 300, the state machine 14 is in a standby mode S4. After
that, when an enabling signal EN inputted to the state machine 14
goes to a high level, there is a transition to the start-up mode
S1. The enabling signal EN is given from outside the control
circuit 100.
[0058] The state machine 14 makes detection of the short-circuit
state by the voltage comparator 30 non-operative in the start-up
mode S1, and after the start-up time Tp1 has elapsed, transits to
the normal mode S2. The start-up time Tp1 is set to be longer than
time required for the output voltage Vout to become higher than the
threshold voltage Vth, after beginning start-up of the separately
excited DC/DC converter 210.
[0059] Moreover, when the voltage comparator 30 detects the
short-circuit state, in the normal mode S2, the state machine 14
transits to the halt mode S3. When the short-circuit detection
signal Vsc outputted from the voltage comparator 30 continues at a
high level for a predetermined short-circuit detection time Tp2,
the state machine 14 may transit from the normal mode S2 to the
halt mode S3.
[0060] In addition, the state machine 14 transits to the start-up
mode S1 after the predetermined halt time Tp3 has elapsed after
transiting to the halt mode S3, and begins the start-up of the
separately excited DC/DC converter 210.
[0061] Furthermore, when a light emission control signal SIG20
outputted from the light emission controller 214b is inputted to
the state machine 14, and a light-emitting operation of the
light-emitting element 212 is completed in the normal mode S2,
there is a transition to the start-up mode S1. That is, when the
output voltage Vout of the separately excited DC/DC converter
decreases by the light-emitting element 212, which is a load, being
driven, in the normal mode S2, the state machine 14 transits to the
start-up mode S1.
[0062] The state machine 14 outputs mode signals MODE1 and MODE2
expressing present state, to the driver circuit 18 and a soft start
circuit 26, in each state. The state machine 14 performs required
time measurement, that is, the start-up time Tp1, the short-circuit
detection time Tp2, the halt time Tp3, and the like, using the
timer circuit 16.
[0063] An explanation is given of operations of the light-emitting
device 200 configured as above. FIG. 4 is a time chart showing
operation states of the light-emitting device 200 of FIG. 1.
[0064] At time T0, when power is supplied to the electronic device
300, the state machine 14 is in standby mode S4. At time T1, when
the enabling signal EN goes to a high level, there is a transition
to the start-up mode S1. When the start-up mode S1 occurs, the soft
start circuit 26 is controlled by the mode signal MODE2 outputted
from the sate machine 14, the duty ratio of the pulse width
modulation signal Vpwm gradually becomes larger, and the output
voltage Vout of the separately excited DC/DC converter 210 begins
to gradually increase.
[0065] In a period from time T1 to time T2, the output voltage Vout
is lower than the threshold voltage Vth. During this time, in the
start-up mode S1, the state machine 14 makes detection of the
short-circuit state by the voltage comparator 30 non-operative. At
time T3 after elapse of the start-up time Tp1 from time T1, the
state machine 14 transits from the start-up mode S1 to the normal
mode S2. In the normal mode S2, monitoring of a short-circuit state
by the voltage comparator 30 is operative.
[0066] At time T4, the output voltage Vout reaches the target
voltage Vtgt, operation of the driver circuit 18 by the hysteresis
comparator 12 is controlled, and stabilization is carried out so
that Vout.apprxeq.Vtgt. In FIG. 4, the output voltage Vout
indicates a constant value, but in reality varies between voltages
Vmax and Vmin determined by the hysteresis comparator 12.
[0067] At time T5 when the load short-circuits, the output voltage
Vout suddenly drops, and just after that, the output voltage Vout
at time T6 becomes lower than the threshold voltage Vth, and the
short-circuit detection signal Vsc, which is output of the voltage
comparator 30, goes to a high level. The short-circuit state of the
load continues, and at time T7 after the short-circuit detection
time Tp2 has elapsed after the short-circuit detection signal Vsc
has gone to a high level, the state machine 14 transits from the
normal mode S2 to the halt mode S3. At this time, the state machine
14 has the mode signal MODE2, which is outputted to the driver
circuit 18, at a high level, and driving of the switching
transistor Tr1 by the driver circuit 18 is halted.
[0068] At time T8 after the halt time Tp3 has elapsed after
transiting to the halt mode S3, the state machine 14 transits to
the start-up mode S1. The output voltage Vout in the start-up mode
S1 rises again by a soft start operation. At this time, the load is
assumed to be released from the short-circuit state. At time T9 the
short-circuit detection signal Vsc goes to a low level, and at time
T10, after the start-up time Tp1 has elapsed after beginning the
start-up, the state machine goes to the normal mode S2. At time T11
the output voltage Vout reaches the target voltage Vtgt.
[0069] At time T12, when a user of the electronic device 300
presses a shutter of the imaging unit 316, the light emission
control signal SIG20 outputted from the light emission controller
214b goes to a high level, the IGBT 214a is ON, and the
light-emitting element 212 emits light. At this time, charge
accumulated in the output capacitor C1 is discharged, and the
output voltage Vout decreases suddenly. In the period in which the
light emission control signal SIG20 has a high level, the state
machine 14 is in the standby mode S4. After that, at time T13 the
light emission control signal SIG20 goes to a low level, and the
state machine 14 goes to the start-up mode S1. At time T14 after
the start-up time Tp1 has elapsed from time T13, the state machine
14 transits to the normal mode S2.
[0070] According to the control circuit 100 according to the
present embodiment, by switching the start-up mode S1 and the
normal mode S2, and by switching the detection of the short-circuit
state by the voltage comparator 30 between operative and
non-operative, it is possible to distinguish between a state in
which the load short-circuits and the output voltage Vout
decreases, and a state in which, before the output voltage Vout at
start-up time rises to its target value Vtgt, the output voltage
Vout is lower than the threshold voltage Vth, and it is possible to
halt the switching transistor Tr1 and realize circuit protection
only in cases in which the load has really short-circuited.
[0071] Furthermore, after detecting a short-circuit state and
halting switching operation of the switching transistor Tr1 for a
halt time Tp3, in order for the switching controller 10 to begin
the start-up of the separately excited DC/DC converter, in cases in
which a long-term short-circuit of the load is maintained, since
there is intermittent operation in which current flows during the
start-up time Tp1 and current is shut off during the halt time Tp3,
it is possible to prevent a large current flowing continuously in
the switching transistor Tr1 or the transformer 50.
[0072] Moreover, with regard to the switching controller 10, when
the output voltage Vout is continuously below the threshold voltage
Vth for the short-circuit detection time Tp2, since the voltage
comparator 30 determines that there is a short-circuit state, it is
possible to preferably detect a long-term short-circuit state and
perform circuit protection, without judging, as short-circuits,
cases in which the output voltage Vout has decreased for a very
short time.
[0073] In addition, when the output voltage Vout of the separately
excited DC/DC converter decreases by the light-emitting element
212, which is a load, being driven, in the normal mode S2, the
state machine 14 transits to the start-up mode S1, so that after
emission of light the output voltage Vout is once again increased
in the start-up mode S1, and in this time, detection of the
short-circuit state by the voltage comparator 30 can be made
non-operative.
[0074] The abovementioned embodiment is an example, and a person
skilled in the art will understand that various modified examples
in combinations of various component elements and various processes
thereof are possible, and that such modified examples are within
the scope of the present invention.
[0075] In the embodiment, an explanation of the DC/DC converter has
been given concerning cases in which the light-emitting element 212
is driven; however, there is no limitation thereto, and various
other loads requiring high voltage can be driven. In the present
embodiment, an explanation has been given concerning cases in which
the output voltage Vout of the separately excited DC/DC converter
210 decreases due to driving the load, that is, light emission from
the light-emitting element 212; however, in cases in which the
output voltage Vout does not decrease so much due to driving the
load, a transition to the start-up mode S1 synchronous with driving
the load, shown at time T13 in FIG. 4, need not be carried out.
[0076] In the embodiment, an explanation has been given concerning
the separately excited DC/DC converter; however, a self-excited
type is also possible.
[0077] Furthermore, in the present embodiment, setting of high
level and low level logical values is one example, and it is
possible to make changes freely, by appropriately making inversions
by an inverter or the like.
[0078] The present invention has been explained based on the
embodiment; however, clearly the embodiment merely shows principles
and applications of the present invention, and many modified
examples and changes to arrangements are possible within a scope
that does not depart from the spirit of the present invention as
prescribed in the scope of the claims.
[0079] While the preferred embodiments of the present invention
have been described using specific terms, such description is for
illustrative purposes only, and it is to be understood that changes
and variations may be made without departing from the spirit or
scope of the appended claims.
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