JPS6033740Y2 - Switching control type power supply circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching control type power supply circuit, and particularly to improvements in the drive circuit section.

Generally, in a constant voltage power supply circuit, a detection resistor is connected in series with the load current path, and when the load current increases beyond the rated value, an overcurrent protection circuit detects the rise in voltage that occurs in the resistor and shuts off the power supply circuit. The same applies to switching control type power supply circuits.

However, in such conventional overcurrent protection circuits, if the value of the resistor is set to a large value, the power loss in this resistor becomes a problem and the regulation of the output voltage deteriorates; If it is set to a small value, there is a drawback that changes in load current due to temperature changes etc. cannot be detected accurately, which has been a bottleneck in design.

In addition, a reference voltage source, voltage comparison circuit, etc. are required to detect the voltage generated in the low pile.
The disadvantage is that the number of circuit elements increases.

Therefore, the present invention proposes a switching control type power supply circuit that eliminates such drawbacks.

Hereinafter, a case will be described in which an embodiment of the present invention shown in FIG. 1 is used as a power supply circuit for a television receiver.

The circuit in Figure 1 can be roughly divided into a rectifier circuit section 1 that receives a commercial AC power source as an input, a switching control section 2 that controls the rectified output intermittently, a smoothing circuit section 3 that smoothes the switching output, and It is comprised of a variable pulse width oscillation circuit section 4 that is controlled according to the obtained DC voltage.

The switching circuit section 2 includes a switching transistor Tr1 connected in series between the rectifier circuit section 1 and the smoothing circuit section 2, a drive transistor Tr2 to whose base the output pulse of the oscillation circuit section 4 is applied, and this drive transistor. The main element is a drive transformer T connected so that the switching transistor Tr1 is turned on when Tr2 is turned on.

Particular attention should be paid to the input winding of the drive transformer T and the input line 1 in the switching circuit section 2.
A first impedance circuit consisting of a capacitor C□ and a resistor R□ is connected between the input winding 1 and the output line 1, and a second impedance circuit consisting of a backflow prevention diode D1 and a resistor R2 is connected between the input winding and the output line 1□. This is the point where the circuit is connected.

The capacitor C2 is used to prevent switching noise generated in the winding of the drive transformer T from entering the input line 11, and the capacitor C3 and resistor R3 are used to prevent excessive switching noise generated in the winding. This is for suppressing pulse voltage.

On the other hand, the oscillation circuit section 4 includes a transistor Tr3°Tr,
An astable multivibrator whose main elements are: and the DC output voltage V obtained on the output line 1□.

It consists of a control transistor Tr5 that operates with variable impedance in accordance with The configuration is such that the oscillation frequency is fixed by an appropriate trigger pulse TP.

In addition, a resistor R is connected to the oscillation circuit section 4 from the input line 11.
The starting current is supplied through 4, and after starting, the DC voltage +V obtained from the flyback transformer of the television receiver, etc.
b is supplied as a driving power source through diode D2 and resistor R5, but at this time, the above DC voltage is supplied to diode D
3. It is clamped to a constant voltage (approximately 6.OV) by D4.

Next, the operation of such a power supply circuit will be explained.

First, when the power switch SW is turned on, the input line 11
A starting current flows from the source to the oscillation circuit section 4 through the resistor R4,
This causes the astable multivibrator in the oscillation circuit to self-oscillate.

On the other hand, since the collector of the drive transistor Tr2 is connected to the input line 11 via the input winding of the drive transformer T, the resistor R1, and the capacitor C1, the drive transistor Tr2 is connected to the input line 11 through the input winding of the drive transformer T, the resistor R1, and the capacitor C1. When turned on, the collector current flows from the input line 1□ side through the input winding of the drive transformer T.

As a result, the switching transistor Tr□ is also turned on, and therefore, the output voltage V is applied to the output line 12 through the smoothing circuit 3.

appears.

Therefore, the current flowing from the output line 1□ through the diode D□ and the resistance loss A2 flows to the drive transistor Tr2 together with the starting current passing through the capacitor C1 and the resistor R1.

While repeating this operation several times, the output voltage V.

As the voltage gradually rises, the horizontal output circuit of the television receiver, which receives power from output line 1□, begins to operate.
As a result, the DC voltage vb gradually increases and the trigger pulse TP appears.

Therefore, after a certain period of time has passed since the power switch SW is turned on, the voltage vb is transferred to the diode D3. A constant DC voltage obtained by clamping with D4 is supplied to the oscillation circuit section 4,
Moreover, this circuit section is triggered at regular intervals.

Further, in this state, since the capacitor C0 for the previous start-up is charged, the collector current of the drive transistor Tr2 is supplied only from the output line 1° side.

In such a steady state, the output line 1° has a constant output voltage V determined by the on/off time ratio of the switching transistor Tr.

is obtained, but the control operation to maintain this voltage at a constant value is performed as follows.

That is, now, for example, the output voltage V. increases, the collector-emitter impedance of the control transistor Tr5 increases, so that this Tr5. R6, R7 and C
The time constant determined by 4 becomes larger.

For this reason, the turn-on timing of Tr3 of the astable multivibrator is delayed, but on the other hand, the turn-off timing is fixed by the trigger pulse ■, so this T
The on period of r3, that is, the pulse width T of the oscillation output pulse OP becomes narrower.

Therefore, the on-period of the drive transistor Tr2 and therefore the on-period of the switching transistor TR1 are shortened, resulting in an output type IEV.

In other words, it is controlled in a direction that reduces the

Next, when an overload occurs, when an overcurrent flows due to a failure of the load circuit connected to the output line 1□, the output voltage V.

decreases. This is because the maximum power that can be taken out to the output line 12 is limited by the maximum length of the on period of the switching transistor Tr1, and in such a case, the maximum power will be outside the constant voltage control region.

When the output voltage decreases in this way, the diode D1
Since the collector current supplied to the drive transistor Tr2 through the resistor R2 decreases, the current induced in the output winding of the drive transformer T also decreases.

As a result, the switching transistor Tr1 is no longer driven sufficiently, and its collector-emitter impedance increases, resulting in an output voltage V.

decreases. This decrease in output voltage further reduces the collector current of the drive transistor Tr2, so by repeating this operation, the switching transistor Tr2
1 turns off instantly, cutting off power supply to the load circuit.

FIG. 2 shows the load current versus output voltage characteristics in this case.

As explained above, in a switching controlled power supply circuit in which a switching transistor is transformer-coupled to a drive transistor, the present invention supplies a starting current to the drive transistor from the power supply input side at startup, and in a steady state, the power supply circuit Since the operating current is supplied from the output line side, the overcurrent protection function can be achieved without the need for a special protection circuit, which simplifies the circuit configuration and reduces power consumption within the power supply circuit. Loss can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing one embodiment of the power supply circuit of the present invention;
The figure shows the load current versus output voltage characteristics. Trl...Switching transistor, Tr2
...Drive transistor, T...Drive transformer, C□, R1...Capacitor and resistor forming the first impedance circuit, Dl, R2
...Diode and resistor that constitute the second impedance circuit.

Claims (1)

[Scope of utility model registration request] A switching transistor that controls DC input intermittently,
This transistor generates variable pulse width! A drive transistor that is driven in response to the output of the circuit is transformer-coupled,
A first impedance circuit is connected between the input winding of the transformer connected to the drive transistor and the DC input side for supplying a starting current to the drive transistor, which is conductive only at the time of starting. A switching control characterized in that a second impedance circuit for supplying a steady operating current of the drive transistor, which is conductive in a steady state, is connected between the output side of a smoothing circuit that smoothes the output and the input winding. Type power circuit.