U.S. patent application number 09/748293 was filed with the patent office on 2001-08-30 for switching power supply apparatus.
Invention is credited to Nagahara, Kiyokazu.
Application Number | 20010017779 09/748293 |
Document ID | / |
Family ID | 18505333 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017779 |
Kind Code |
A1 |
Nagahara, Kiyokazu |
August 30, 2001 |
Switching power supply apparatus
Abstract
A power supply apparatus comprises a variable frequency
oscillating circuit, a driver, soft start circuit, switching
elements for receiving a switching signal, a resonating capacitor
connected at a connection point of the switching elements via a
primary coil of a transformer, rectifying circuit provided at a
secondary coil of the transformer, an amplifier for comparing an
output voltage, Vb obtained at the rectifying circuit and a
reference voltage, Vref, a photo-coupler for controlling an
impedance of an oscillating element of a variable frequency
oscillating circuit based on the comparison output and a charge
voltage control circuit for controlling an oscillation frequency
when the variable oscillation circuit is initially driven. A
frequency control signal when power is ON is made nonlinear
relevant to a time. As a result, a change in oscillation frequency
immediately after power is ON is made gentle, and a rapid change in
current that flows in a primary coil is eliminated. Therefore, no
over-current flows in the switching elements, damage to switching
elements can be reduced more significantly than conventionally, and
these switching elements can be reliably protected.
Inventors: |
Nagahara, Kiyokazu; (Tokyo,
JP) |
Correspondence
Address: |
JAY H. MAIOLI
COOPER & DUNHAM LLP
1185 Avenue of the Americas
New York
NY
10036
US
|
Family ID: |
18505333 |
Appl. No.: |
09/748293 |
Filed: |
December 26, 2000 |
Current U.S.
Class: |
363/16 |
Current CPC
Class: |
H02M 3/335 20130101;
H02M 1/36 20130101 |
Class at
Publication: |
363/16 |
International
Class: |
H02M 003/335 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 28, 1999 |
JP |
P11-375323 |
Claims
The invention claimed is:
1. A switching power supply apparatus comprising: switching signal
generating means having a variable frequency oscillating circuit; a
pair of switching elements for receiving the switching signal from
the switching signal generating means; a resonating capacitor
connected to a connection point of a pair of these switching
elements via a primary coil of a transformer; a rectifying circuit
provided at a secondary side of said transformer; comparison means
for comparing an output voltage obtained at the rectifying circuit
with a reference voltage; impedance control means for controlling
an impedance of an oscillation element of said variable frequency
oscillating circuit based on the comparison output from said
comparison means; and frequency control means for controlling an
oscillation frequency when said variable frequency oscillating
circuit is initially driven, wherein a frequency control signal of
said frequency control means is formed to provide nonlinear
characteristics relevant to a time.
2. The switching power supply apparatus as claimed in claim 1,
wherein said frequency control means comprises a soft start circuit
during startup, a charging capacitor and charge voltage control
means connected to both ends of this charging capacitor, wherein
non-linear charge characteristics allows a frequency control signal
output from said soft start circuit to be made nonlinear
characteristics.
3. The switching power supply apparatus as claimed in claim 2,
wherein said charge voltage control means owns charge
characteristics in which at least one transition point relevant to
a startup time of said variable frequency oscillating circuit, and
wherein charge characteristics after an elapse of said transition
point is gentler in gradient than charge characteristics before
said transition point.
4. The switching power supply apparatus as claimed in claim 2,
wherein said charge voltage control means comprises first and
second resistors connected in series between a power supply and a
ground, and a third resistor connected in parallel to the first
resistor via a switching transistor, and Wherein said charging
capacitor is connected at a neutral point between said first and
second resistors.
5. The switching power supply apparatus as claimed in claim 1,
wherein said charge voltage control means comprises a soft start
circuit during startup and said charging capacitor connected to the
soft start circuit; wherein a constant current source is connected
with said charging capacitor and a switching transistor operated by
a first reference voltage is connected into a charging path to said
charging capacitor; and wherein turning ON/OFF the switching
transistor allows charge characteristics for said charging
capacitor to be made nonlinear characteristics.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to a switching power supply
apparatus. More particularly, it relates to a switching power
supply apparatus in which a rectifying current that flows in a
primary side of a transformer connected to a load side is formed to
provide nonlinear characteristics when a power supply is started
up, thereby preventing an excessive current from flowing in a
switching transistor or the like provided at the primary side of
the transformer.
[0002] As a switching power supply apparatus, there is known an
apparatus based on a current resonance system. FIG. 1 shows a
conventional example of switching power apparatus of this current
resonance system, the apparatus having a SEPP (Single Ended Push
Pull) arrangement.
[0003] A switching power supply apparatus 10 shown in FIG. 1
comprises switching signal generating means 12 including a variable
frequency oscillating circuit 14 and a drive circuit 16. The
oscillating circuit's oscillation signal is supplied to the drive
circuit, thereby generating a pair of switching signals, for
example, having a reverse phase relationship therebetween. In the
case where the switching generating means 12 is composed of IC
circuits, an oscillating element (capacitor 18 and resistor 20)
that determines an oscillation frequency is externally provided at
any of external terminals 12a and 12b of this IC circuit.
[0004] A pair of switching signals Sp, Sp bar are supplied to a
pair of switching elements 22 and 24 having SEPP arrangement. A MOS
type electric field effect transistor or the like may be utilized
as the switching elements 22 and 24. An resonating capacitor 28 is
connected to both a ground and a connection neutral point `p`
between a pair of these switching elements 22 and 24 via a primary
coil 26a of an insulation transformer 26.
[0005] Respective diodes 30a and 30b rectify a secondary current
that flows in a pair of secondary coils 26b and 26c of the
insulation transformer 26 as full-wave rectifier. The full-wave
rectified current allows a smoothing capacitor 32 to be charged.
Therefore, voltage `vb` obtained at both ends 34 of the smoothing
capacitor 32 is supplied to a load (not shown) as an output
voltage.
[0006] The output voltage is supplied to an amplifier 36, as
voltage comparison means, wherein the voltage is compared with a
reference voltage, Vref. Its comparison output is supplied to a
photo-coupler 38 that configures inductance control means 37
provided in order to insulate the primary and secondary sides of
the transformer 26. The photocoupler 38 comprises a photodiode 40
and a phototransistor 42 that functions as a variable inductance
element. A current based on the comparison output flows in this
phototransistor 42.
[0007] The phototransistor 42 is connected to the external terminal
12b through a stationary resistor 44. Therefore, when the
phototransistor 42 is ON, this resistor 44 and serial impedance
caused by the phototransistor 42 are connected in parallel to a
resistor 20, which is an oscillation element.
[0008] In this arrangement, it is known that a relationship between
a resonation frequency `f` and a resonation impedance Z of the
resonating circuit on the primary side of the transformer 26 formed
of its primary coil 26a and the capacitor 28 is based on upper side
operation as indicated by a curve `Lo` in FIG. 2.
[0009] In this resonating circuit, when switching frequencies of
the switching signals Sp and Sp bar supplied to a pair of switching
elements 22 and 24 are increased, the resonance impedance Z
increases. The resonance impedance Z is lowered as the switching
impedance Z is lowered. Such change in the resonance impedance Z
causes a resonance current i1 that flows in the primary coil 26a to
be changed. Thus, controlling this resonance current i1 allows an
output voltage Vb induced at the secondary side of the transformer
26 to be controlled.
[0010] When the output voltage Vb obtained at an output terminal 34
is illustratively higher than the reference voltage Vref, the
phototransistor 42 has its impedance according to the comparison
output. Thus, the composite resistance of the external terminal 12b
becomes smaller than a case of a simplex of the resistor 20,
whereby an oscillation frequency `fsw` increases.
[0011] When the oscillation frequency `fsw` is increased, the
resonance impedance Z determined depending on the primary coil 26a
and the capacitor 28 increases. Thus, a current that flows in this
primary coil 26a is limited, and its value decreases. With this
decrease in current, the currents induced at the secondary coils
26b and 26c are reduced as well. As a result, a charge voltage with
the capacitor 32 decreases. Namely, the output voltage Vb is
controlled in the direction of the reference voltage Vref.
[0012] Conversely, when the output voltage Vb is lower than the
reference voltage Vref, the impedance of the phototransistor 42
increases, and the composite resistance value at the external
terminal 12b increases. Then, the variable frequency oscillating
circuit 14 is controlled so that its oscillation frequency `fsw`
may be lowered. As a result, the switching frequency is lowered
relevant to the switching elements 22 and 24, and the primary
resonance impedance Z of the transformer 26 is lowered accordingly.
This causes the resonance current to increase. When the resonance
current increases, the secondary current increases as well. Thus,
the charge voltage Vb with the capacitor 32 rises, and a closed
loop control is performed so as to be close to the reference
voltage Vref.
[0013] In the meantime, in this switching power supply apparatus
10, a large amount of resonance current flows from a time when a
power supply is turned ON to a time when the capacitor 32 rises to
a voltage in its constant state. Thus, this current may damage the
switching elements 22 and 24.
[0014] In order to reduce such damage, there has been
conventionally provided a soft start circuit 50, which functions as
frequency control means 60, for limiting a resonance current during
startup. This soft start circuit 50 is provided in the switching
signal generating means 12. An external charging capacitor 52 is
connected to an external terminal 12c arranged at this soft start
circuit 50, so that charging for this capacitor 52 is started in
synchronism with turning ON the power. Then, a change in charge
voltage Va at this time causes a charge current of the oscillating
capacitor 18, which is an oscillating element, connected to the
external terminal 12a to be changed.
[0015] When a charge current with the oscillating capacitor 18
changes with an elapse of time, the oscillation frequency `fsw`
changes accordingly. This fact will be described with reference to
FIGS. 3A to 3E.
[0016] FIG. 3A shows a change in charge voltage Va when and after
the power is turned ON, wherein the charge characteristics are
linear as indicated by line La. The variable frequency oscillating
circuit 14 is changed in the oscillation frequency `fsw` by the
charge voltage Va of the capacitor 52 associated with the soft
start circuit 50 connected to the oscillating circuit 14. The
oscillation frequency fsw changes almost linearly as indicated by
characteristic line Lb in FIG. 3B. When a charge voltage Va is zero
volt, oscillation occurs at a high frequency, and the oscillation
frequency `fsw` is lowered as the charge voltage Va increases.
[0017] On the other hand, the primary resonance impedance Z is
characterized by characteristic curve Lo such that the resonance
impedance Z increases as a frequency increases from the resonance
frequency `fo` as shown in FIG. 2. A relationship between the
impedance Z and a time is illustrated as shown in FIG. 3C. Namely,
there are nonlinear characteristics that the resonance impedance Z
is initially high, and then, lowers rapidly; the impedance gently
changes as the charge voltage Va is close to a full charge.
[0018] As a result, there is provided nonlinear characteristics
such that, although not so much primary current i1 flows in this
primary resonance circuit system from a time when the power is
turned ON to a predetermined time, as indicated by curve Lc in FIG.
3D, the current i1 increases rapidly after a certain period of time
has elapsed. Accordingly, there is established a charge mode in
which, although the output voltage (charge voltage) Vb of the
capacitor 32 connected to the output terminals 34 is initially
charged gently as indicated by the curve Ld in FIG. 3E, rapid
charging is then performed. Immediately before a time `tb` when a
soft start mode terminates, the result is gentle charging; and at
the time and after the time `tb` this mode transits to a voltage
control mode caused by a closed loop. In this control mode, voltage
control is performed such that the reference voltage Vref is
obtained as indicated by the line Le in FIG. 3E.
[0019] Thus, a rapid current i1 flows in the primary resonance
system until a time has come immediately before the soft start mode
terminates because of an effect due to a change in the primary
impedance Z. This rapid current i1 causes a pair of switching
elements 22 and 24 an excessive stress, and thus, the switching
elements 22 and 24 or the like may be damaged.
[0020] Although a voltage change applied to a load while the power
is ON depends on charge characteristics of the soft start circuit,
a voltage applying state that is the most suitable to the load can
be achieved if the voltage change state matching such load can be
freely set. However, conventional art as described above has been
such a disadvantage that flexible response cannot be made, since
the charge characteristics of the soft start circuit is merely
linear.
[0021] Accordingly, it is an object of the present invention to
provide a switching power supply apparatus in which the charge
characteristics of a capacitor connected to the soft start circuit
are made gentle when the power is turned ON, whereby damage to at
least the switching elements can be reduced.
SUMMARY OF THE INVENTION
[0022] In accordance with the invention, the object is accomplished
in switching power supply apparatus comprising a frequency control
device, preferably such as a soft start circuit and a charging
capacitor, for controlling an oscillation frequency when switching
signal generating device, preferably such as variable frequency
oscillating circuit is initially driven. A frequency control signal
of the frequency control devices is formed to provide nonlinear
characteristics relevant to a time. In carrying out the present
invention in one preferred mode, since the charge characteristics
of the soft start circuit are made nonlinear, charging for a
capacitor connected to the soft start circuit is rapidly performed
when the power is ON, and then, the charging is performed
gradually.
[0023] By doing this, the primary impedance Z that originally
changes nonlinearly changes almost linearly. This resonance
impedance Z determines the primary current i1, and thus, the
excessive current of this primary current i1 is inhibited.
Therefore, no excessive current flows in switching elements, and
damage to these switching elements can be reduced.
[0024] In addition, an output voltage to be applied to a load,
particularly a voltage change when power is ON depends on the
charge characteristics of a soft start circuit. The charge
characteristics are provided as charge characteristics suitable to
the load, whereby more stable circuit operation can be
achieved.
[0025] According to the present invention, the switching power
supply apparatus involves a pair of switching elements for
receiving the switching signal, a resonating capacitor connected to
a connection point of a pair of these switching elements via a
primary coil of a transformer, a rectifying circuit provided at a
secondary side of the transformer, comparison devices, preferably
such as an amplifier, for comparing an output voltage obtained at
this rectifying circuit with a reference voltage, and impedance
control devices, preferably such as a photo-coupler, for
controlling an impedance of an oscillation element of the variable
frequency oscillating circuit based on this comparison output.
[0026] The switching power supply apparatus according to the
present invention is very preferable on applying it to a switching
converter having a SEPP configuration or the like.
[0027] The concluding portion of this specification particularly
points out and distinctly claims the subject matter of the present
invention. However those skilled in the art will best understand
both the organization and method of operation of the invention,
together with further advantages and objects thereof, by reading
the remaining portions of the specification in view of the
accompanying drawing(s) wherein like reference characters refer to
like elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a connection diagram showing a conventional
switching power supply apparatus;
[0029] FIG. 2 is a characteristic view showing a relationship
between an oscillation frequency and impedance that show primary
resonance impedance characteristics;
[0030] FIGS. 3A to 3E are waveform charts provided for a
description of operation of the switching power supply
apparatus;
[0031] FIG. 4 is a connection diagram showing essential portions of
a switching power supply apparatus embodying the present
invention;
[0032] FIG. 5 is a connection diagram showing an example of a soft
start circuit used in a switching power supply apparatus embodying
the present invention;
[0033] FIG. 6 is a characteristic view showing charge
characteristic of a charge voltage control circuit; and
[0034] FIG. 7 is a connection diagram showing essential portion of
another example of a soft start circuit used in a switching power
supply apparatus embodying the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0035] Now, embodiments of the switching power supply apparatus
according to the present invention will be described in detail with
reference to the accompanying drawings.
[0036] According to the present invention, the charge
characteristics of the soft start circuit provided as switching
signal generating means are changed from linear to nonlinear,
whereby a change in characteristics of the primary resonance
impedance of an insulation transformer is made gentle, and thus,
damage to the switching element connected to the primary side of
the insulation transformer can be reduced.
[0037] FIG. 4 shows a switching power supply apparatus 10 embodying
the present invention when a soft start circuit 50 is used. An
arrangement of the soft start circuit 50 will now be described here
according to FIG. 5.
[0038] In the soft start circuit 50, a current path 74 of a first
current mirror circuit 72 for charging a capacitor 52 at a constant
current is connected to the capacitor 52. The first current mirror
circuit 72 comprises a constant current portion 77 composed of a
pair of transistors 75 and 76, a third transistor 78 connected to a
base of the transistor 75 and a diode (composed of transistor) 79
for preventing back flow. A current identical to the current that
flows in the constant current portion 77 flows in the capacitor 52
via the diode 79 with the capacitor 52 being charged at a constant
current. Thus, the charge voltage Va with the capacitor 52
indicates linear charge characteristics.
[0039] A second current mirror circuit 82 is connected to the
current path 74 via a pair of transistors 80 and 81 that are
Darlington-connected, which may determine the value of a constant
current that flows in a second constant current portion 85. The
second current mirror circuit 82 is also arranged similar to the
first constant current portion 72. This circuit comprises a
constant current portion 85 composed of a pair of transistors 83
and 84, a third transistor 86 connected to a base of the transistor
83 and a diode (composed of transistor) 87 for preventing back
flow. A current identical to the current that flows in the constant
current portion 85 flows in the capacitor 18 via the diode 87.
Thus, the charge characteristics of the capacitor 18 are controlled
according to those of the charge voltage Va. As a result, the
oscillation frequency `fsw` of the variable frequency oscillating
circuit 14 is controlled as desired, and a soft start mode is
achieved.
[0040] According to the present invention, there is also provided
frequency control means for the variable frequency oscillating
circuit 14. In the illustrative embodiment, charge characteristics
of the charge capacitor 52 provided at the soft start circuit 50 is
controlled, thereby obtaining a control signal for controlling the
oscillation frequency.
[0041] An oscillation frequency control circuit comprises the soft
start circuit 50, the charging capacitor 52 particularly connected
to the soft start circuit 50 and a charge voltage control circuit
90 connected to the charging capacitor 52 in the illustrative
embodiment of FIG. 4. The circuit 90 has nonlinear charge voltage
characteristics relevant to the capacitor 52.
[0042] This circuit 90 has a pair of resistors 91 and 92 connected
in series as shown in FIG. 4, and its connection neutral point `d`
is connected to an external terminal 12c. Namely, the resistor 92
is connected in parallel to the capacitor 52. A switching
transistor 94 is further connected between this connection neutral
point `d` and a power supply Vcc through resistor 93. The partial
pressure voltage caused by a pair of resistors 95 and 96 is applied
to the transistor 94 as its base voltage.
[0043] According to the thus configured charge voltage control
circuit 90, the capacitor 52 is charged from a time when the power
is ON, and thus, a soft start mode starts. When the power is ON,
the transistor 94 is turned ON. At this time, the capacitor 52 is
charged by the charge current from the current path 74 and the
charge current determined depending on the values of the resistors
91, 92, and 93 (indicated by straight line Pal in FIG. 6). When a
certain degree of power is charged, an emitter voltage of the
transistor 94 rises, whereby the transistor 94 is cut off.
Therefore, subsequently, the capacitor 52 is charged by the charge
current from the current path 74 and the charge current determined
depending on the values of the resistors 91 and 92 (indicated by
straight line Pa2 in FIG. 6).
[0044] As a result, a shown in FIG. 6, the charge characteristics
Pa relevant to the capacitor 52 differ before and after a
transition point `y` at a point `ta` at which the transistor 94 is
cut off as shown in FIG. 6 while the transition point is defined as
a reference. Namely, a straight line `Pa1` is obtained until the
transistor 94 has been cut off, and then, the straight line `Pa2`
with its gentler gradient than the line `Pa1` is obtained after the
transistor has been cut off. Therefore, comparatively rapid
charging is performed up to the point `ta` when the transistor 94
is cut off (provided if a small amount of current is produced). In
contrast, after the transistors 94 have been cut off, gentle
charging is performed. Namely, there is provided nonlinear charge
characteristics having one transition point.
[0045] A relationship between the primary current `i1` and the
output voltage Vb when the nonlinear characteristics are obtained
will be described with reference to FIGS. 3A to 3E.
[0046] The curve Pa shown in FIG. 3A indicates charge
characteristics relevant to the capacitor 52. In the variable
frequency oscillating circuit 14, its oscillation frequency `fsw`
varies depending on the charge voltage Va of the capacitor 52
associated with the soft start circuit 50 connected to the
oscillating circuit 14. There are provided characteristics in which
oscillation frequency `fsw` also changes almost nonlinearly. When
the charge voltage Va is zero volt, oscillation occurs at a high
frequency as indicated by straight line `Pb` shown in FIG. 3B and
the oscillation frequency `fsw` is lowered as the charge voltage Va
increases. The frequency change rate, however, differs before and
after the transition point `y`. Since the frequency change rate
after the transition point `y` is smaller than that before the
transition point, the oscillation frequency `fsw` changes gently at
a timing when a soft start mode terminates.
[0047] The primary resonance impedance Z of the insulation
transformer 26 changes according to this change in oscillation
frequency `fsw`, as shown in FIG. 3C. In this resonance impedance
Z, there are provided nonlinear characteristics such that the
impedance change rate is originally small where the oscillation
frequency (switching signal) `fsw` is high as indicated by the
curve `Lo` shown in FIG. 2, and the impedance change rate is great
where the oscillation frequency `fsw` is comparatively low.
However, since the change in oscillation frequency indicates
nonlinear characteristics as shown in FIG. 3B, the impedance Z
conversely indicates almost linear change as indicated by the curve
Po.
[0048] As a result, the primary current `i1` also changes almost
linearly as indicated by the curve Pc shown in FIG. 3D. Namely,
although a change rate of a current that flows is different from
another, there are provided almost linear current characteristics
before and after the transition point `y`. This prevents a current
from rapidly flowing in the primary coil 26a.
[0049] Due to the current characteristics, it is found that the
charge voltage Va relevant to the capacitor 52 be charged linearly
even before and after the transition point `y` as indicated by the
curve Pd in FIG. 3E.
[0050] The charge characteristics relevant to the capacitor 52 is
thus made nonlinear, and the frequency of the variable frequency
oscillating circuit 14 is controlled so as not to cause the
frequency change to be partially rapid, whereby the current that
flows in the primary coil 26a of the insulation transformer 26 can
be limited linearly. In this manner, a current that flows in a pair
of switching elements 22 and 24 is made gentle, and the damage to
the switching elements 22 and 24 can be significantly reduced.
[0051] In addition, the aforementioned output voltage Vb can be
changed according to the charge characteristics of the capacitor
52. When design is made in consideration of the position of the
transition point `y` or a gradient of the charge characteristics
before and after the transition point `y`, there can be achieved a
voltage change state that is the most suitable to a load to be
connected to the output terminal 34 when the power is turned ON. As
a result, there can be achieved characteristics on an output
voltage rise suitable to the load, and more stable circuit
operation can be obtained.
[0052] The soft start circuit 100can be also arranged as another
example shown in FIG. 7. In this example, charge voltage control
means relevant to the capacitor 52 is not arranged as an external
circuit, but the means is arranged as an IC circuit directly
incorporated in switch signal generating means 12. Therefore, in
this case, the charge voltage control circuit 90 shown in FIG. 1 is
not required.
[0053] In the soft start circuit 50 shown in FIG. 7, a current path
101 relevant to a DC power source 104 is connected to the capacitor
52. To this current path 101, a switching transistor 102 is
connected in series via a resistor 103 and a diode (composed of
transistor) 105 for preventing back flow. Further, A first current
mirror circuit 106 supplies a constant current to a neutral point
`s` of connection between the resistor 103 and the diode 105.
[0054] The first current mirror 106 comprises a MOS transistor 107
as a constant current source and a MOS transistor 108 connected to
a gate of the transistor 107. A transistor 109 for determining the
value of the constant current is connected to the MOS transistor
107 via a resistor 110. At the transistor 109, the minimum partial
pressure voltage obtained at the neutral point `r3` of connection
of a partial pressure circuit 111 made of a plurality of resistors
Ra to Rd is supplied to a base of the transistor 109. The partial
pressure circuit 111 applies an intermediate partial pressure
voltage obtained at the connection neutral point `r2` to the
switching transistor 102 connected to the current path 101.
[0055] A pair of transistors 120 and 121 that are
Darlington-connected amplifies a current flowing in the current
path 101. The amplified current is used as a current that flows in
a constant current source 125 of a second current mirror circuit
122. Therefore, this current path is connected to the constant
current portion 125 that configures the second current mirror
circuit 122 via a switching transistor 123 and a resistor 124. At
the transistor 123, the maximum partial pressure voltage obtained
at the neutral point `r1` of connection of the partial pressure
circuit 111 is supplied to a base of the transistor 123.
[0056] In this embodiment, there is provided an arrangement in
which the capacitor 18 that determines the aforementioned
oscillation frequency is charged with the current flowing in the
other transistor 126 of the second current mirror 122.
[0057] With this circuit configuration, the circuit configuration
from the switching transistor 102 connected to the current path 101
to the first current mirror circuit 106 functions as charge voltage
control means. Therefore, when the power is ON, the capacitor 52 is
charged with the current made by composing the constant current
that flows in the transistor 108 and a current that flows in the
transistor 102. Due to the charging, the terminal voltage Va of the
capacitor 52 rises, and a potential of the connection neutral point
`s` rises, then the potential becomes higher than a base potential
of the transistor 102. Thus, this transistor 102 is cut off. As the
result, the capacitor 52 is charged with only a constant current
from the first current mirror 106.
[0058] Therefore, the voltage change rate of the charge voltage
before the transistor 102 is cut off differs from that after it is
cut off. Namely, the voltage change rate after the transistor 102
is cut off is smaller than that before cut off, and the charge
characteristics similar to those shown in FIG. 6 are obtained.
[0059] Since a change in the charge current identical to this
charge characteristics is also transmitted to the second current
mirror circuit 122, the charge characteristics relevant to the
capacitor 18 that determines an oscillation frequency is also
provided as nonlinear characteristics having a transition point `y`
as shown in FIG. 6. Therefore, nonlinear characteristics similar to
the cases of FIGS. 3A to 3E can be achieved.
[0060] The nonlinear characteristics relevant to the soft start
circuit 50 can be provided in a way other than the aforementioned
method. In addition, in the aforementioned embodiments, a voltage
change rate as nonlinear characteristics is expressed by a single
transition point. However, the nonlinear characteristics can be
achieved by a pure curve, and nonlinear characteristics having a
plurality of transition points can be provided.
[0061] In the illustrative embodiments, although the present
invention is applied to a switching power supply apparatus having
an SEPP configuration, it can be applied to a push-pull type
switching power supply apparatus or a half-bridge configured
switching power supply apparatus.
* * * * *