Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
1999-01-21
2001-09-04
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C363S097000
Reexamination Certificate
active
06285566
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a self-oscillation switching power supply apparatus.
2. Description of the Related Art
A ringing choke converter is widely used as a self-oscillation switching power supply apparatus.
FIG. 9
is a circuit diagram of a ringing choke converter (hereinafter referred to as an RCC) according to a conventional technique. As shown in
FIG. 9
, a switching transistor Q
1
is connected in series to a primary winding N
1
of a transformer T. A control circuit including a phototransistor PT serving as a photosensing element of a photocoupler is connected to a feedback winding N
B
of the transformer. A control transistor Q
2
is connected between the gate and the source of the switching transistor Q
1
.
A rectifying and smoothing circuit including a rectifying diode D
3
and a smoothing capacitor C
5
is disposed between two terminals of a secondary winding N
2
of the transformer T. The output of this rectifying and smoothing circuit is connected to a voltage detecting circuit including a resistance voltage divider consisting of resistors R
9
and R
10
, a shunt regulator SR, a light emitting diode PD of the photocoupler PC, and a resistor R
8
.
The circuit shown in
FIG. 9
operates as follows. When the circuit is started by turning on a power supply, a voltage is applied to the gate of the switching transistor Q
1
via the starting resistor R
1
and the switching transistor Q
1
turns on. As a result, an input power supply voltage is applied across the primary winding N
1
of the transformer T and a voltage with the same polarity as that of the primary winding N
1
is generated across the feedback winding N
B
. This voltage signal is applied as a positive feedback signal to the gate of the switching transistor Q
1
via a capacitor C
2
and a resistor R
2
. Furthermore, the voltage induced across the feedback winding N
B
causes a charging current to flow into a capacitor C
3
via a diode D
1
, resistors R
3
and R
5
, and the phototransistor PT of the photocoupler. If the voltage across the capacitor C
3
exceeds the forward base-emitter voltage of the control transistor Q
2
, the control transistor Q
2
turns on. As a result, the gate-source voltage of the switching transistor Q
1
becomes nearly 0 and thus the switching transistor Q
1
is forced to turn off. As a result, a voltage is generated across the secondary winding of the transformer. This causes the rectifying diode D
3
to have a voltage applied in the forward direction. As a result, the energy which has been stored in the transformer T during the on-period of Q
1
is released via the secondary winding N
2
and the capacitor C
3
is reversely charged by a flyback voltage of the feedback winding N
B
via resistors R
6
and R
7
and a diode D
2
.
If the voltage across the capacitor C
3
becomes lower than the forward base-emitter voltage of the control transistor Q
2
, the control transistor Q
2
turns off and the energy stored in the transformer T is released from the secondary winding. If the current passing through the rectifying diode D
3
becomes 0, a kick voltage is induced across the feedback winding N
B
whereby the switching transistor Q
1
again turns on. After that, the above process is repeated.
In the above operation, the output voltage across the load is detected by means of a resistance divider comprising resistors R
9
and R
10
and the detected voltage is applied as a control voltage to the shunt regulator SR. According to the detected voltage, the shunt regulator SR changes the current passing through the light emitting diode PD of the photocoupler. As a result, a corresponding change occurs in the amount of light received by the phototransistor PT serving as the photosensing element of the photocoupler, and thus the impedance of the phototransistor PT changes. This causes a corresponding change in the charging time constant associated with the capacitor C
3
. Because the charging time constant increases with the reduction in the output voltage, a reduction in the output voltage results in an increase in the period of time from an off-to-on transition of the switching transistor Q
1
to the following on-to-off transition forcedly brought about by the control transistor Q
2
, that is, an increase in the on-time of the switching transistor Q
1
, which results in an increase in the output voltage. As a result, the output voltage is controlled at a constant value.
It is known that the oscillation frequency f of the switching transistor Q
1
in the conventional self-oscillation switching power supply apparatus such as that shown in
FIG. 9
varies in approximately inverse proportion to the input or output power as shown in
FIG. 10
in which the oscillation frequency f is plotted as a function of the output power Po.
In general, the switching loss which occurs during each switching operation decreases with the reduction in the load. However, since the oscillation frequency f increases, as shown in
FIG. 10
, with the reduction in the output power Po and thus with the reduction in the load, the frequency of occurrence of switching loss per unit time increases with the increase in the oscillation frequency f. Therefore, the reduction in the switching loss which occurs when the load decreases is very small. This means that the efficiency of the power supply apparatus decreases with the reduction in the load.
The switching loss under low load condition can be reduced by designing the circuit parameters such that the oscillation frequency for the operation under the rated-load condition becomes low enough. However, in the case where the power supply apparatus is required to handle a load varying over a wide range from extremely low to high levels, it is necessary to set the oscillation frequency f under the low load condition to a relatively high value. That is, the oscillation frequency under the rated-load condition is generally determined by factors associated with components such as the magnetic flux density of the transformer and other factors such as ripples and noise. If the oscillation frequency is set to a too low value, problems such as saturation of the transformer occur.
In view of the foregoing, there is a need for a self-oscillation switching power supply apparatus capable of operating without a reduction in the efficiency due to an increase in the oscillation frequency under a low load condition even in the case where the output power to load varies over a relatively large range.
SUMMARY OF THE INVENTION
The present invention is directed to self-oscillation switching power supply apparatus that satisfied this need. The self-oscillation switching power supply apparatus is a ringing choke converter type and comprises: a transformer T including a primary winding N
1
, a secondary winding N
2
, and a feedback winding N
B
; a switching transistor Q
1
which oscillates in a self-oscillating fashion in response to a feedback signal from the feedback winding N
B
thereby turning on and off the current flowing through the primary winding; and a rectifying and smoothing circuit connected to said secondary winding. The self-oscillation switching power supply apparatus further comprises: an oscillation frequency control circuit including a control transistor Q
3
for controlling a control signal input to the switching transistor Q
1
thereby controlling the control transistor Q
3
so as to extend the off-time in the self-oscillation period of the switching transistor Q
1
; and an oscillation frequency control disabling circuit for disabling the control of the control transistor Q
3
in accordance with a remote signal.
In this self-oscillation switching power supply apparatus with the above-described circuit configuration, the oscillation frequency control circuit controls the control transistor Q
3
thereby controlling the switching transistor Q
1
so as to extend the off-time in the self-oscillation period of the switching transistor Q
1
. As a result, the switching frequency of the switching transistor Q
1
becomes lower than
Nakahira Koji
Nishida Akio
Okamura Ryuji
Tani Ryota
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
Sterrett Jeffrey
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