JPH0716302B2 - Switching power supply circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a switching power supply circuit, and in particular,
The present invention relates to an improvement of a snubber circuit for a switching semiconductor device.

[Outline of Invention]

In the switching power supply circuit of the present invention, a diode, a first resistor, and a capacitor are connected in series in order to remove various oscillating voltages generated when switching the input power supply. On the other hand, a snubber circuit in which a second resistor is connected in parallel is formed, and this snubber circuit is connected to the primary winding. Therefore, the spike voltage at the time of switching can be suppressed, the inrush current that flows when the transistor is conductive can be blocked, and the ringing that occurs during the off period of the transistor can be effectively absorbed.

[Conventional technology]

High-efficiency and high-stability switching power supply circuits have been put into practical use due to the development of high-speed switching elements, and they are also used as various switching power supplies for electronic devices.

FIG. 3 shows a conventional example of a switching power supply circuit. E is the input power supply (DC), L ₁ and L ₂ are the primary side of the transformer,
And a coil on the secondary side, _Tr is a switching transistor, D is a rectifying diode, and C ₀ is a smoothing capacitor.

S is a snubber circuit for suppressing a spike voltage generated when the switching transistor _Tr is turned off, and is composed of a series circuit of a resistor R and a capacitor C.

As shown in FIG. 4, the switching power supply described above has the electromagnetic energy stored in the primary coil L ₁ due to the collector current i _C flowing in the ON period of the switching transistor T _r in which the base voltage e _B is positive. but the oFF period of the switching transistor T _r to charge the capacitor C ₀ through the diode D connected to the secondary coil L _2, and supplies power to a load circuit (not shown) so ON-
It is considered to be an OFF-type converter.

In such a switching power supply circuit, unless the snubber circuit S is added, a high oscillating voltage (spike voltage) is generated at the instant when the transistor _Tr is turned off (time point t ₁ ) due to the resonance source resonance phenomenon. This spike voltage becomes a ripple output and, at the same time, adversely affects other electronic circuits as noise.

However, by adding the snubber circuit S including the resistor R and the capacitor C, it is possible to reduce the vibration period and suppress the spike voltage.

[Problems to be solved by the invention]

However, if the value of the resistance R of the snubber circuit S is reduced to increase the damping effect of the spike voltage, the inrush current i _t as shown by the dotted line at the instant when the transistor _Tr is driven on at the time point t ₂ is changed to the capacitor C. Flow through,
The power consumption increases and causes overheating of the transistor.

Further, in the snubber circuit S, when the switching power supply circuit is set to be of the mesh type as described later, the rectifying diode D is turned off at time t ₀ as shown by the collector voltage e _C in FIG. At the same time as the primary coil L ₁
And the capacitor C causes ringing, and in this case as well, the smaller the resistance R, the larger the ringing amplitude.

The amplitude of this ringing is influenced by the winding ratio L ₂ / L ₁ of the primary and secondary coils L ₁ and L _2. Especially, when the input voltage E is low, the collector voltage e _C falls below the ground potential due to ringing. It made it has, there is a possibility that reverse voltage is applied to the switching transistor T _r.

Therefore, there is a method of increasing the winding ratio L ₂ / L ₁ or designing the operation mode of the switching regulator to be a keystone type (mode in which the diode D is always conducting when the transistor is off). In order to satisfy such a condition, it is required to increase the switching cycle, and there is a problem that the optimal design of the switching regulator becomes difficult.

Further, if the ringing is large, the voltage of the collector of the switching transistor _{Tr at} the on-time point t ₂ is not constant, and
The loop gain of the feedback control system that is added to keep the rectified voltage constant causes large fluctuations.For example, when such a switching power supply circuit is used in a TV receiver, etc. The screen may fluctuate and cause a sound fluctuation.

The present invention has been made in order to reduce such a problem, and makes a slight improvement in the snubber circuit to sufficiently suppress the spike voltage at the time of switching off, and
It is intended to provide a switching power supply circuit in which the influence of ringing is reduced.

[Means for solving problems]

The snubber circuit used in the switching power supply circuit of the present invention includes a diode, a first resistor, and a capacitor connected in series, and a second resistor connected in parallel to the first resistor.

[Action]

The spike voltage generated when the switching transistor is turned off is low with the capacitor.
Effectively dumped by the resistance and diode of
Also, the rush current at the time when the switching transistor is turned on is blocked by the diode.

Further, the ringing voltage can be sufficiently suppressed by the second resistor and capacitor which are set to a relatively high value.

〔Example〕

FIG. 1 shows a main part of the switching power supply circuit of the present invention, and the same reference numerals as those in FIG. 3 indicate the same parts.

However, in the snubber circuit S indicated by the one-dot chain line, the diode and the first resistor R ₁ are connected in parallel with the second resistor R ₂ .

The second resistance R ₂ is about 10 times larger than the first resistance R ₁ .

Although not shown, the switching transistor T
A drive signal (PWM signal) whose pulse width changes depending on the magnitude of the rectified voltage, that is, the magnitude of the load current, is input to the hase electrode of _r to obtain a constant voltage characteristic as is well known. It is configured to be.

The frequency of the drive signal is set to the horizontal period of the TV, for example.

Since the switching power supply circuit of the present invention employs the snubber circuit S as described above, the following effective operation can be performed.

As shown in the collector voltage waveform in FIG. 2, when the switching transistor _Tr is turned off at time t ₁ , a high oscillation voltage (spike voltage) due to the leakage inductance and distributed capacitance of the transformer is induced as shown by the thin line. However, this spike voltage is suppressed by the capacitor C ₁ , and the oscillating voltage (current) generated immediately after that is damped by the first resistor R ₁ and the diode D ₁ which are low in value. At this time, the capacitor C ₁ is charged to the polarity shown in the figure.

Next, in the case of the present invention, since it is a mesh type switching power supply circuit, sufficient electric power charges the smoothing capacitor C ₀ during the off period of the switching transistor T _r , so that it is connected to the secondary side. Rectifying diode D
Turns off at time t ₂ . Then, at this time point t ₂ , transient vibration occurs between the capacitor C ₁ and the primary coil L ₁ and ringing starts, but the voltage charged in the capacitor C ₁ is set to a relatively large value. 1 via the second resistor R ₂
Damping is added to the ringing because it flows to the next coil L ₁ .

At time t ₃ , the ringing current flows through the diode D ₁ and the first resistor R ₁ , but at the next time t ₄ , the diode D ₁ again flows.
The reverse ringing current is blocked by and flows through the second resistor R ₂ having a high resistance value. Therefore, the damping effect of ringing is sufficiently exerted. Hereinafter, similarly, a current flows through the second resistor R ₂ which has a high value for each half-synchronization of the ringing, so that the ringing quickly converges.

At time t ₆ , the switching transistor _Tr is turned on, but the inrush current at this time is blocked by the diode D ₁ , so that the reactive power is reduced.

As described above, in the snubber circuit of the present invention, the spike voltage generated when the transistor is off is effectively prevented by the first resistor R ₁ having a low value, and when the rectifying diode D is turned off. The ringing voltage (current) generated is effectively suppressed by the second resistor R ₂ .

Therefore, the variation of the collector drop voltage e ₀ when the switching transistor T _r is inverted when the switching transistor T _r is turned on is reduced,
Fluctuation of the feedback control system for providing the constant voltage characteristic is improved.

That is, when a load is changed greatly varies in response to the pulse width output voltage of the drive signal, but on inverting the time t ₆ of the switching transistor T _r is controlled, the amplitude of the ringing voltage is suppressed The feedback system operates smoothly, and constant voltage characteristics with good followability can be obtained.

The switching power supply circuit of the present invention is a snubber circuit in which a resistor (R ₂ ) having a resistance value of 3.6 KΩ and a diode D ₁ are added to the conventional snubber circuit S having a resistance value R = 340Ω and a capacitance C ₁ = 2700PF. It was possible to reduce the heat generation temperature of the transistor by about 30 ° C (room temperature 25 ° C) under the same conditions by improving.

〔The invention's effect〕

As described above, the switching power supply circuit of the present invention is provided with the first resistor for damping the spike voltage and the second resistor for suppressing the ringing as the snubber circuit. In addition, since it is improved to effectively suppress the ringing voltage, the heat generation amount of the transistor is reduced, and
There is an effect that the feedback system for obtaining the constant voltage characteristic becomes stable and the followability to voltage fluctuation is improved.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a main part of a switching power supply circuit showing an embodiment of the present invention, FIG. 2 is a waveform diagram showing a collector voltage of a switching transistor, and FIG. 3 is a main part of a conventional switching power supply circuit. A circuit diagram and FIG. 4 are waveform diagrams for explaining the spike voltage and the ringing voltage. In the figure, E is an input power supply, _Tr is a switching transistor, L ₁ and L ₂ are primary and secondary coils (windings), D is a rectifying diode, C ₀ is a smoothing capacitor, S is a snubber circuit, R ₁ is a first resistor, R ₂ is a second resistor, C ₁ is a capacitor, and D ₁ is a diode.

Claims (1)

[Claims] 1. A switching pulse is supplied to a semiconductor switching element that switches an input power supply supplied to a primary winding of a transformer, and a DC voltage is obtained from a rectifier circuit connected to a secondary winding of the transformer. In the switching power supply circuit configured as described above, a diode, a first resistor and a capacitor are connected in series on the primary winding side, and a series circuit of the first resistor and the diode has A switching power supply circuit characterized by adding a snubber circuit in which a second resistor having a resistance value substantially higher by one digit is connected in parallel.