JPS62262659A - Switching power circuit - Google Patents

Abstract

PURPOSE:To improve the follow-up adaptability to the voltage fluctuation as well as to reduce the calorific value of a transistor, by providing the first and second resistance in a snubber circuit and by suppressing the spike voltage and ringing voltage by means of these resistance. CONSTITUTION:A switching power circuit comprises a switching transistor Tr, a snubber circuit S and a transformer with the primary and secondare coils L1 and L2. As the snubber circuit a diode D1, the first resistance R1 and a capacitor C1 are connected in series, while the second resistance R2 is connected in parallel to the serial circuit of the first resistance R1 and a diode D1. The spike voltage, set up when the switching transistor Tr is turned OFF, is effectively damped by the first resistance R1 and the diode d1. The ringing voltage is sufficiently suppressed by the second resistance R2.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a switching power supply circuit, and in particular,
This invention relates to improvements in snubber circuits for switching semiconductor devices.

[Summary of the 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 that occur when switching the input power supply. In contrast, a second resistor is connected in parallel to form a snubber circuit, and this snubber circuit is connected to the primary winding. Therefore, spike voltage during switching can be suppressed, rush current flowing when the transistor is turned on can be blocked, and ringing that occurs during the off period of the transistor can be effectively absorbed.

[Conventional technology]

Due to the development of high-speed switching elements, switching power supply circuits with high efficiency and high stability have been put into practical use, and are also used as so-called switching regulators in various power supplies for electronic devices.

Figure 3 shows a conventional example of a switching power supply circuit, where E is a human power supply (DC), Ll and L2 are the primary and secondary coils of the transformer, T is a switching transistor, and D is a rectifier. The diode Co is a smoothing capacitor.

S is a snubber for suppressing the spike voltage that occurs when the switching transistor T is turned off.
r) circuit, which consists of a series circuit of a resistor R and a capacitor C.

In the switching power supply described above, as shown in Fig. 4, the electromagnetic energy accumulated in the primary coil L1 is generated by the collector current fR,jC flowing during the on period of the switching transistor Tr whose base voltage eQ is positive. During the off-period of the switching transistor T, the capacitor Co is charged via the diode D connected to the secondary coil L2, and the converter is of the so-called 0N-OFF operation type and supplies power to a load circuit (not shown). There is.

In such a switching power supply circuit, if the snubber circuit S is not added, a high oscillating voltage (spike voltage) will be generated due to the resonance image due to the leakage inductance at the moment when the transistor T is turned off (time 1+).
This spike voltage becomes a ripple output and at the same time adversely affects other electronic circuits as noise.

However, by adding a snubber circuit S consisting of a resistor R and a capacitor C, the vibration period can be reduced and the spike voltage can be suppressed.

[Problem that the invention seeks to solve]

However, if the value of the resistance R of the snubber circuit S is reduced to increase the damping effect of the spike voltage, then at time t2
At the moment when the transistor Tr is driven on, an inrush current it as shown by the dotted line flows through the capacitor C, increasing power consumption and causing overheating of the transistor.

In addition, when the snubber circuit S is set to be a mesh type as described later in the switching power supply circuit, at the same time as the diode D diverted to 9 is turned off at time t as shown in the collector voltage ec in Fig. 4. Ringing will occur due to the primary coil Ll and the capacitor C, and in this case as well, the smaller the resistance R, the larger the amplitude of the ringing.

The amplitude of this linking is
In particular, when the input voltage E is low, the collector voltage e r
, may fall below the ground potential, and there is a risk that a reverse voltage will be applied to the switching transistor T'.

Therefore, should we increase the winding ratio L2/Ll?

Alternatively, there is a way to design the switching regulator so that its operating mode is a keystone type (a mode in which diode D is always conductive when the transistor is off), but in order to satisfy this condition, the switching period must be increased. However, there is a problem in that it makes it difficult to optimally design a switching regulator.

In addition, if the ringing is large, the voltage at the collector at the ON point t2 of the switching transistor T will not be constant, and the loop gain of the feedback control system, which is normally added to keep the rectified voltage constant, will cause large fluctuations. ,
For example, when such a switching power supply circuit is used in a TV receiver or the like, high audio output may cause sound fluctuations that cause the screen to shake.

This invention was made to reduce such problems, and by making slight changes to the snubber circuit, the spike voltage at the time of switching off is sufficiently suppressed, and
The present invention provides a switching power supply circuit in which the influence of ringing is reduced.

[Means for solving problems]

In the switching power supply circuit of the present invention, a diode, a first resistor, and a capacitor are connected in series as a snubber circuit, and a snubber circuit is connected to the first resistor.

A second resistor connected in parallel is used.

[Effect]

The spike quasi-pressure that occurs when the switching transistor is turned off is caused by the capacitor and the first, which is said to have a low value.
is effectively dumped by the resistor and diode,
Further, the inrush current when the switching transistor is turned on is blocked by the diode.

Furthermore, 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 the main parts of a switching power supply circuit according to the present invention, and the same reference numerals as in FIG. 3 indicate the same parts.

However, the snubber circuit S shown by the dashed line has a diode and a first resistor R1, and a second resistor R2.
are connected in parallel.

Note that the second resistance R2 is approximately 10% relative to the first resistance R1.
The value is said to be about twice as large.

Also, although not shown, a 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 base electrode of r, and as is well known, a constant voltage characteristic is obtained. It is configured so that

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

Since the switching power supply circuit of the present invention employs the snubber circuit S as described above, it is possible to perform the effective operation shown below.

As shown in the collector voltage waveform in Fig. 2, when the switching transistor Tr is turned off at time t1, a high oscillating voltage (, spike voltage I → due to the cage inductance and distributed capacitance of the transformer, as shown by the thin line) However, this spike voltage is suppressed by the capacitor C1, and the oscillating voltage (current) generated immediately after is suppressed by the first resistor R1 and the diode DI.
damped by. Further, at this time, the capacitor C1 is charged to the illustrated polarity.

Next, in the case of the present invention, since it is a mesh type switching power supply circuit, sufficient power charges the smoothing capacitor Co during the off period of the switching transistor Tr, so that the rectifier diode connected to the secondary side D turns off at time t2. Then, at this time t2, a transient vibration occurs between the capacitor C,1 and the primary coil L1, and ringing starts, but the voltage charged in the capacitor C1 is a relatively large value. damping is added to the ringing because it flows to the primary coil L1 via the resistor R2.

At time t3, a ringing current flows through the diode DI and the first resistor R1, but at the next time t4, the ringing current in the opposite direction is blocked again by the diode DI, and the second resistor R2 has a high resistance value. flows.

Therefore, the ringing damping effect is fully exhibited. Thereafter, the current flows through the second resistor R2, which is set to a high value every half cycle of ringing, so that the ringing quickly converges.

At time t6, the switching transistor T'' is turned on, but since the inrush current at this time is blocked by the diode D1, the reactive power is reduced.

As mentioned above, in the snubber circuit of the present invention, the spike voltage that occurs when the transistor is turned off is effectively prevented by the first resistor R1, which has a low value, and the spike voltage that occurs when the rectifier diode D is turned off. The ringing voltage (
', it flow) is effectively suppressed by the second resistor R2.

Therefore, fluctuations in the collector drop voltage eo when the switching transistor Tr is inverted when turned on are reduced, and fluctuations in the feedback control system for providing constant voltage characteristics are improved.

That is, when the load fluctuates greatly, the pulse width of the drive signal changes in accordance with the output voltage, and the on-inversion time t of the switching transistor Tr is also controlled, but since the amplitude of the linking voltage is suppressed, The feed 7 to ivy system operates smoothly, and constant voltage characteristics with good followability can be obtained.

Note that the switching power supply circuit of the present invention has a resistance value R=340Ω and a capacitance C,=27 of the conventional snubber circuit S.
For 00PF, by modifying the snubber circuit by adding a resistor (R2) of 3.6Ω and a diode D1, the heat generation temperature of the transistor can be reduced to approximately 30°C under the same conditions.
(room temperature 25°C).

〔Effect of the invention〕

As explained above, the switching power supply circuit of the present invention is provided with a first resistor for damping spike voltage and a second resistor for suppressing ringing as a snubber circuit, and these resistors act as a snubber circuit to damp spike voltage. Since it has been improved to effectively suppress ringing voltage, the amount of heat generated by the transistor is reduced, and the feedback system for obtaining constant voltage characteristics is stabilized.
This has the effect of improving followability to voltage fluctuations.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of the main parts of a switching power supply circuit showing an embodiment of the present invention, Fig. 2 is a waveform diagram showing the collector voltage of a switching transistor, and Fig. 3 shows the main parts of a conventional switching power supply circuit. The circuit diagram and FIG. 4 are waveform diagrams for explaining spike voltage and ringing voltage. In the figure, E is a human power source, Tr is a switching transistor, Ll, L2 are primary and secondary coils (windings), D is a rectifying diode, co is a smoothing capacitor, S is a snubber circuit, and R1 is a first coil. , R2 is a second resistor, C1 is a capacitor, and Dl is a diode.゛[)几%'7 Agent Waki Atsushi Husband';2, t+ t2t3ta ts t6'? Sui・
Set the collector voltage of the trunk trunk to 7r, ″1′ urinary Nz Fig. 2 Fig. 3 Fig. 4

Claims (1)

[Claims] It is configured to supply a switching pulse to a semiconductor switching element that switches an input power supply supplied to a primary winding of the transformer, and to obtain a DC voltage from a rectifier circuit connected to a secondary winding of the transformer. In a switching power supply circuit, a diode, a first resistor having a relatively low value, and a capacitor are connected in series on the primary winding side, and a capacitor having a relatively high value is connected to the diode and the first resistor. A switching power supply circuit characterized by adding a snubber circuit in which a second resistor is connected in parallel.