JPH08340675A - Intermittent oscillation prohibiting circuit for switching power supply circuit - Google Patents

Abstract

PURPOSE: To facilitate the supply of a stable output voltage over the range from no-load to full-load by a method wherein a dummy load is connected to the output of a switching power supply circuit when the light load state of the power supply circuit is detected by a light load detecting means. CONSTITUTION: An intermittent oscillation prohibiting circuit has a dummy load 4, a light load detecting means 2 and a dummy load connecting means 3. If the load of an RCC switching power supply circuit 1 is in a light load state, the ON-time of a transistor T1 is much shortened and a pulse whose width is very small is outputted. Then the light load state is detected by the light load detecting means 2 and the dummy load 4 is connected to the output of the RCC switching power supply circuit 1 by a dummy load connecting means 4. As a result, the ON-time of the transistor T1 is widened again by the dummy load 4 and the load of the RCC switching power supply 1 is quickly shifted to a heavy load side but the ON-time of the transistor T1 is quickly widened, so that the influence upon an output circuit is very little.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermittent oscillation preventing circuit for a ringing choke converter type switching power supply circuit.

[0002]

2. Description of the Related Art FIG. 5 shows an example of a conventional switching power supply circuit of ringing choke converter type (hereinafter, ringing choke converter is abbreviated as RCC). First, the operation of the RCC switching power supply circuit will be described. In FIG. 5, the RCC switching power supply circuit includes a transformer PT having a primary coil NP, a secondary coil NS, and a tertiary coil NB, and a first switching transistor T1 (hereinafter,
And a DC power supply circuit.
The primary coil NP, the collector / emitter circuit of the transistor T1 and the overcurrent detection resistor RE are connected in series with Vin, and electromagnetic energy is stored in the inductance of the primary coil NP of the transformer PT when the transistor T1 is conductive. The output of the RCC switching power supply circuit is the electromagnetic energy emitted when the transistor T1 is cut off, and the reverse voltage induced in the secondary coil NS when the transistor T1 is cut off is rectified and smoothed by the diode D3 and the capacitor C3. The output voltage is controlled by controlling the conduction / cutoff time of.

The switching operation and output voltage control of the transistor T1 are performed by a circuit on the side of the tertiary coil NB. That is, the voltage induced in the tertiary coil NB is a diode
Via the bias resistor RB in series with the parallel circuit of D1 and the capacitor C1, the base / emitter circuit loop of the transistor T1 is connected to the polarity of the positive feedback circuit to ensure the switching operation of the transistor T1. On the other hand, through the diode D2 in series with the parallel circuit of the resistor RC and the capacitor C2, the transistor T1 charges the capacitor C2 with the induced voltage at the time of interruption, and secures the voltage proportional to the output voltage of the RCC switching power supply circuit, Connected to the base circuit of the transistor T1 via the diode DZ,
It controls the output voltage of the switching power supply circuit. That is, RC
When the output voltage of the C switching power supply circuit increases and the charging voltage of the capacitor C2 increases in proportion to this output voltage, the base current ib of the transistor T1 is bypassed via the Zener diode DZ, and the cutoff state of the transistor T1 is maintained. .

The base-emitter circuit of the transistor T2 is connected in parallel with the overcurrent detection resistor RE, and the collector circuit of the transistor T2 is connected to the base circuit of the transistor T1. The operation of this circuit is that when the overcurrent flows in the emitter circuit of the transistor T1 and the voltage drop in the overcurrent detection resistor RE exceeds the base-emitter forward voltage of the transistor T2, the base of the transistor T2 flows and the collector of the transistor T2 .Conductor between emitters, transistor
Bypassing the base current ib of T1 and controlling the overcurrent of the transistor T1.

The operation of the RCC switching power supply circuit will be described below with reference to FIGS. The RCC switching power supply circuit ensures that the transistor T1 conducts when the load on the secondary side fluctuates.
The control described below is performed so that the output voltage on the secondary side is kept constant by dividing the time of interruption (hereinafter, conduction / interruption is expressed as ON and OFF). First, the switching operation will be described.

When the base starting current ig flows through the transistor T1 through the starting resistor RG, the transistor T1 enters the conducting direction, the collector current ic flows, and the voltage Vin of the DC power supply circuit is applied to the primary coil NP of the transformer PT. It As a result, a voltage VB, which is a voltage Vin divided in the winding ratio of the primary coil NP and the tertiary coil NB, is induced in the tertiary coil NB, and the induced current VB causes a base current ib to flow in the transistor T1 to ensure the transistor T1. To turn it on. As a result, the collector current ic of the transistor T1 flowing in the primary coil NP of the transformer PT
Increase in a triangular wave shape that increases with time as shown in FIG. 6, and the electromagnetic energy (described later)
(See formula (1)).

Next, the collector current ic increases and icm ≥ h
When the condition of fe · ib is entered, the collector current icm cannot be increased any more and the voltage applied to the primary coil NP decreases. Therefore, the voltage VB induced in the tertiary coil NB decreases and the capacitance
The base current ib decreases sharply in the differentiation circuit via C1, which causes the collector current icm to decrease more rapidly, the base current ib disappears, and the transistor T1 is turned off.
F

When the transistor T1 enters the OFF state, the collector current icm flowing in the primary coil NP immediately before the transistor T1 enters the OFF state is converted into the secondary induced current is in the secondary coil NS side in inverse proportion to the winding ratio. And is reduced as the electromagnetic energy stored in the transformer PT is released. This secondary induced current is charges the capacitance C3 via the diode D3 and supplies the DC voltage V0 to the load circuit RL. Transistor
T1 enters the OFF state, and the secondary induced current is is 0
Then, there is a small amount of residual energy in the secondary coil NS winding of the transformer PT, and this residual energy causes ib
When a flows in the opposite direction, the tertiary coil NB winding is activated, the transistor T1 is turned on again, and by repeating this operation, the voltage Vin of the DC power supply circuit is insulated and becomes an output.

Next, the control of the output voltage will be described. The output voltage is controlled by the operation of the Zener diode DZ and the capacitor C2. The electromagnetic energy stored in the transformer PT is
It becomes formula (1).

[0010]

[Equation 1]

In equation (1), in order to stabilize the output power V0 · I0 of the RCC switching power supply circuit, the repetition frequency f
Alternatively, the ON time ton of the transistor T1 may be controlled. This control is the output voltage of the RCC switching power supply circuit.
Capacity C2 that is almost proportional to the value obtained by multiplying V0 by the secondary / tertiary winding ratio
The charging voltage Vc of the Zener diode DZ
When approaching the value of Vz, part of the base current ib flowing in the transistor T1 is shunted to the circuit of the Zener diode DZ. As a result, the base current ibb of the transistor T1 decreases and the collector current ic that turns off the transistor T1.
m (= hfe ・ ibb) also decreases. That is, here, the output voltage V0 of the RCC switching power supply circuit can be controlled by controlling the ON time ton of the transistor T1.

Next, the load RL of the RCC switching power supply circuit
However, if it is reduced at time t1, for example, the ON time of the transistor T1 is shortened as shown in FIG. In addition, the time
When the load RL becomes lighter at t2, the transistor T1 remains in the OFF state for a long time, and the intermittent oscillation operation that intermittently turns on the transistor T1 for a very short time continues occasionally (the time scale from time t2 to t4 in Fig. 7 is shortened. To illustrate). In this state, if the load RL of the RCC switching power supply circuit fluctuates rapidly in the heavy direction at time t4, the output voltage V0 will suddenly drop, and the CPU, memory, etc. connected to the load circuit of the RCC switching power supply circuit will decrease. This may cause problems such as erasing the data in the integrated circuit IC or stopping the operation.

[0013]

However, in the RCC switching power supply circuit as described above, when the load of the output circuit fluctuates rapidly in the heavy direction when the first switching transistor is in the cutoff state for a long time, the output voltage is increased. V0
Drops sharply. Therefore, in the conventional technology, the load range is selected so that the load does not become lighter than a certain value, or a dummy load is always connected to the output circuit to reduce the allowable load capacity or the switching load power supply circuit is reduced by the dummy load. It was done to increase the capacity.

The present invention has been made in view of the above points, and an object thereof is to solve the above-mentioned problems and to stabilize the RCC switching power supply circuit from the no load to the full load without increasing the capacity. To provide an intermittent oscillation prevention circuit for a switching power supply circuit capable of supplying various output voltages.

[0015]

In order to achieve the above object, the present invention comprises a transformer having a primary coil, a secondary coil, and a tertiary coil, and a first switching transistor. The primary coil and the collector-emitter circuit of the first switching transistor are connected in series to the, and the voltage induced in the tertiary coil is connected to the base-emitter circuit of the first switching transistor in the polarity of the positive feedback circuit. In the intermittent oscillation prevention circuit of the switching power supply circuit of the ringing choke converter type that controls the conduction / cutoff of the first switching transistor by rectifying and smoothing the voltage induced in the secondary coil at the time of cutoff and using it as the output of the switching power supply circuit, a light load The detection means and the dummy load connection operated by this light load detection means And means, and when the light load detecting means detects the light load state of the switching power supply circuit output, the dummy load connection means shall connect a dummy load to the output of the switching power supply circuit.

The light load detecting means detects the light load state from the pulse signal induced in the tertiary coil when the conduction time of the first switching transistor is shorter than a predetermined time. The light load detection means detects the light load state from the pulse signal induced in the tertiary coil when the conduction ratio of the first switching transistor is smaller than a predetermined value.

The dummy load connecting means amplifies the dummy load, the photocoupler for insulating the signal detected by the light load detecting means, the signal from the photocoupler, and connects the dummy load to the output circuit of the switching power supply circuit. And a second switching transistor.

[0018]

With the above structure, the conduction time of the first switching transistor is shorter than a predetermined time from the pulse voltage induced in the tertiary coil, or the conduction ratio of the first switching transistor is smaller than a predetermined value. Detects a light load state, and only when the switching power supply circuit is in a light load state, conduct the second switching transistor and connect a dummy load to the output circuit of the switching power supply circuit to secure the conduction time width of the first switching transistor. However, the switching power supply circuit is prevented from performing the intermittent oscillation operation.

[0019]

1 is a conceptual diagram of an intermittent oscillation prevention circuit of a switching power supply circuit according to the present invention, FIG. 2 is an operation explanatory diagram according to a load state of a switching power supply circuit having an intermittent oscillation prevention circuit of the present invention, and FIG. FIG. 4 is a circuit diagram of the switching power supply circuit according to the embodiment, and FIG. 4 is a circuit diagram of the switching power supply circuit according to the second embodiment. The same functional members corresponding to FIGS.

In FIG. 1, the RCC switching power supply circuit 1 surrounded by the one-dot chain line shows the same circuit as the switching power supply circuit described in the prior art of FIG. 5, and therefore the duplicated description will be omitted. The intermittent oscillation prevention circuit according to the present invention comprises a dummy load 4, a light load detecting means 2 and a dummy load connecting means 3 operated by the light load detecting means 2.

In such a structure, when the light load detecting means 2 detects the light load state of the output of the switching power supply circuit 1, the dummy load connecting means 3 connects the dummy load 4 to the output of the switching power supply circuit 1. The operating state according to the load state of the switching power supply circuit of the present invention will be described with reference to FIG. In FIG. 2, the horizontal axis is the time axis, the upper part of the vertical axis shows the ON-OFF operation of the transistor T1, and the lower part shows the output voltage of the RCC switching power supply circuit. When the load RL of the RCC switching power supply circuit 1 is in the steady load range, the ON time of the transistor T1 is controlled by changing the pulse width according to the load state. When the light load state is entered at time t1, the ON time of the transistor T1 has a very short pulse width, and at time t3, the light load detection means 2 detects the light load state of the RCC switching power supply circuit 1 and the dummy load connection means 3 The dummy load 4 is connected to the output of the RCC switching power supply circuit 1.
As a result, the ON time of the transistor T1 is expanded again by the dummy load 4, and at time t4, the load RL of the RCC switching power supply circuit suddenly changes in the heavy direction, and in the prior art, the output voltage V0 suddenly increases as shown by the dotted line. However, in the present invention, the ON time of the transistor T1 is immediately extended, and the influence on the output circuit is small.

FIG. 3 shows, as a first embodiment according to the present invention, an RCC switching power supply circuit in which the light load detecting means 2 detects a light load state when the conduction ratio of the first switching transistor T1 is smaller than a predetermined value. Show. In FIG. 3, the light load detecting means is composed of diodes D4 and D5 and resistors R1 and R2.
Pulse signal waveform shaping circuit 2A consisting of a transistor and a transistor T3
And a filter circuit composed of a resistor R3 and a capacitor C4, and a comparator 2C having a reference voltage Vref and a hysteresis characteristic.
And Further, the dummy load connecting means 3 is
Connect the dummy load 4 to the dummy load 4, the photocoupler PC that insulates the signal detected by the light load detection means 2, the circuit of the transistor T4 that amplifies the signal from this photocoupler PC, and the output circuit of the RCC switching power supply circuit 1. And a second switching transistor T5 that operates.

In such a configuration, the pulse voltage signal VB induced in the tertiary coil NB is waveform shaped by the waveform shaping circuit 2A, and the ON-OFF signal of the transistor T1 is obtained. This ON-O
The FF signal is filtered by the filter circuit of the resistor R3 and the capacitor C4 and becomes the conduction ratio of the transistor T1. When this conduction ratio becomes equal to or lower than the preset reference voltage Vref, the comparator 2C
Is activated and outputs 0V. The 0V output of the comparator 2C sends a forward current to the light emitting diode of the photocoupler PC to cause it to emit light, transmit a signal to the light receiving element by optical coupling, amplify it with the transistor T4, activate the second switching transistor T5, and operate the dummy load. 4 is connected to the output circuit of RCC switching power supply circuit 1.

The load of the RCC switching power supply circuit 1 increases, the conduction ratio of the transistor T1 increases, and the comparator 2C
When the hysteresis characteristic of is exceeded, the output of the comparator 2C rises to the H level, the forward current does not flow to the light emitting diode of the photocoupler PC, and the second switching transistor
T5 turns off and the dummy load 4 is cut off from the output circuit of the RCC switching power supply circuit 1.

As a second embodiment according to the present invention, FIG. 4 shows an RCC switching power supply circuit which detects a light load state when the light load detecting means 2 has a conduction time of the first switching transistor T1 shorter than a predetermined value. Show. 3 is different from FIG. 3 in that the light load detecting means detects the conduction ratio of the first switching transistor T1 in the circuit of FIG. 3, but detects the conduction time of the first switching transistor T1 in the circuit of FIG. Is. Hereinafter, the light load detection means different from FIG. 3 will be mainly described.

In FIG. 4, the light load detection means is a pulse signal waveform shaping circuit 2A consisting of the above-mentioned diodes D4, D5, resistors R1, R2 and transistor T3, a timer 21, and an intermittent oscillator (OSC) with reset function. 22 and flip-flop 23
And In such a configuration, the tertiary coil
The pulse voltage signal VB induced in the NB is waveform-shaped by the waveform shaping circuit 2A and input to the timer 21. With timer 21,
When the conduction time of the input first switching transistor T1 is shorter than a predetermined time, the circuit configuration is such that the reset output (RESET pulse) does not occur due to the timer setting time. Therefore, when the conduction time of the transistor T1 is shorter than a predetermined time, the output bar Q of the flip-flop 23 is triggered by the output pulse of the intermittent oscillator with reset function (OSC) 22 to maintain the L level, and the photo coupler
Forward current is applied to the light emitting diode of the PC,
Activate T6 and connect the dummy load 4 to the output circuit of the RCC switching power supply circuit 1.

When the conduction time of the first switching transistor T1 input to the timer 21 is longer than a predetermined time, a reset output (RESET pulse) is output,
On the other hand, the intermittent oscillator with reset function (OSC) 22 is reset, and the intermittent oscillator (OSC) 22 outputs and flip-flop 2
The pulse output that triggers 3 is delayed, while the reset output (RESET pulse) resets the flip-flop 23 and maintains the output bar Q of the flip-flop 23 at the H level. Therefore, the forward current does not flow to the light emitting diode of the photocoupler PC, and the second switching transistor T5 is turned off.
However, the dummy load 4 can be cut off from the output circuit of the RCC switching power supply circuit 1.

Further, in the RCC switching power supply circuit shown in FIG. 4, when the power is turned on, the load is generally in a light load state. Therefore, a start lock signal is forcibly applied to the reset output (RESET pulse). And RC dummy load 4
C It is configured to connect to the output circuit of the switching power supply circuit. Further, in the illustrated examples of FIGS. 1, 3, and 4, the light load detection unit 2 directly connects the input circuit of the waveform shaping circuit 2A to the tertiary coil NB, but the extraction of the pulse signal induced in the tertiary coil NB is not performed. Alternatively, the diodes D5 and D6 of the waveform shaping circuit 2A may be omitted by extracting from the connection point of the diode D2 and the resistor RB.

In one embodiment of the present invention, in an RCC switching power supply circuit having an output capacity of about 10 W, intermittent oscillation that frequently occurs when the input voltage is high and the load of the output circuit is light,
By consuming about 0.5W of power consumption as a dummy load, we were able to sufficiently prevent intermittent oscillation even at an input voltage of AC264V.

[0030]

As described above, according to the configuration of the present invention, the RCC switching power supply circuit, which is omitted in the illustrated example, is provided with a rectifying and smoothing circuit in the input circuit.
AC and DC input power supply voltage can be wide range, input power is about 10 watts, especially as a power supply circuit for electronic devices and electronic equipment, AC power supply AC85V ~ AC2
It is possible to obtain a DC power supply which is stable over a wide range of 64 V from no load to the entire load range and which is insulated from the primary power supply circuit.

As a result, as a power supply for an electronic device or an electronic device which receives power from a commercial AC power supply, AC100V system, AC
For 200V, 220V AC, and 240V AC power supplies,
Since the same RCC switching power supply circuit can be shared, the flexibility of electronic devices and equipment for various power supply systems can be increased.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of an intermittent oscillation prevention circuit of a switching power supply circuit according to the present invention.

FIG. 2 is an operation explanatory diagram according to a load state of a switching power supply circuit including an intermittent oscillation prevention circuit of the present invention.

FIG. 3 is a circuit diagram of a switching power supply circuit according to a first embodiment.

FIG. 4 is a circuit diagram of a switching power supply circuit according to a second embodiment.

FIG. 5 is an explanatory diagram illustrating a switching power supply circuit according to a conventional technique.

FIG. 6 is an operation waveform diagram of each part of the switching power supply circuit.

FIG. 7 is an operation explanatory diagram according to a load state of the switching power supply circuit.

[Explanation of symbols]

 1 RCC Switching power supply circuit 2 Light load detection means 3 Dummy load connection means 4 Dummy load PT Transformer NP, NS, NB Coil T1 to T5 Transistor C1 to C4 Capacitance D1 to D5 Diode DZ Zener diode PC Photocoupler R1 to R6, RB, RC, RE, RG Resistance Vin Voltage of DC power supply circuit ig Base start current ib Base current ic, icm Collector current is Secondary induced current V0 Output voltage I0 Output current

Claims (4)

[Claims] 1. A transformer having a primary coil, a secondary coil, and a tertiary coil, and a first switching transistor, wherein the primary coil and the collector / emitter circuit of the first switching transistor are connected in series to a DC power supply circuit. Then
The voltage induced in the tertiary coil is connected to the base / emitter circuit of the first switching transistor in the polarity of the positive feedback circuit, and the voltage induced in the secondary coil when the first switching transistor is cut off is rectified / smoothed to switch the power supply circuit. As output, conduction of the first switching transistor
An intermittent oscillation prevention circuit for a switching power supply circuit of a ringing choke converter type for controlling interruption is provided with a light load detection means and a dummy load connection means operated by the light load detection means, and the light load detection means is a switching power supply circuit. An intermittent oscillation prevention circuit for a switching power supply circuit, wherein the dummy load connecting means connects a dummy load to the output of the switching power supply circuit when a light load state of the output is detected. 2. The intermittent oscillation prevention circuit of the switching power supply circuit according to claim 1, wherein the light load detection means is shorter than a predetermined conduction time of the first switching transistor from the pulse signal induced in the tertiary coil. An intermittent oscillation prevention circuit for a switching power supply circuit, characterized in that it detects a light load state. 3. The intermittent oscillation prevention circuit of the switching power supply circuit according to claim 1, wherein the light load detection means has a conduction ratio of the first switching transistor smaller than a predetermined value from the pulse signal induced in the tertiary coil. An intermittent oscillation prevention circuit for a switching power supply circuit, characterized in that it detects a light load state. 4. An intermittent oscillation prevention circuit for a switching power supply circuit according to claim 1, wherein the dummy load connection means insulates the dummy load from the signal detected by the light load detection means. And a second switching transistor for amplifying a signal from the photocoupler and connecting a dummy load to the output circuit of the switching power supply circuit.