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　　Detailed explanation of the design of turn-off snubber circuit for MOSFET in switching power supply

　　In the switching power supply topology with a transformer, when the switch tube is turned off, the loss caused by the overlap of voltage and current is the main part of the switching power supply loss. At the same time, due to the presence of stray inductance and stray capacitance in the circuit, the power switch tube is turned off when the power switch tube is turned off. When the circuit is disconnected, overvoltage will also appear in the circuit and oscillation will occur. If the peak voltage is too high, the switching tube will be damaged. At the same time, the existence of oscillation will also increase the output ripple. In order to reduce the turn-off loss and peak voltage, a snubber circuit needs to be connected in parallel at both ends of the switch tube to improve the performance of the circuit.

　　The main functions of the snubber circuit are: first, to reduce turn-on or turn-off losses; second, to reduce voltage or current spikes; third, to reduce dV/dt or dI/dt. Since the current of the MOSFET tube decreases very quickly, its turn-off loss is very small. Although the MOSFET still uses a turn-off snubber circuit, its function is not to reduce turn-off losses, but to reduce the transformer leakage inductance peak voltage. This article mainly discusses the turn-off snubber circuit of MOSFET tubes.

　　1RC buffer circuit design

　　When designing an RC buffer circuit, you must be familiar with the topology used in the main circuit. Figure l shows the buffer circuit of a forward converter composed of RC. In the figure, when Q turns off, the collector voltage begins to rise to 2Vdc, and the capacitor C limits the rising speed of the collector voltage, while reducing the overlap of the rising voltage and falling current, thereby reducing the loss of the switching tube Q. Before the next switch is turned off, C must discharge the full voltage 2Vdc, and the discharge path is C, Q, R.

　　Assuming that the switch tube does not have a snubber circuit, the reset winding and primary winding turns of the forward converter shown in Figure 1 are the same. In this way, when Q turns off, the energy stored in the excitation inductor and leakage inductance is released, the polarity of the voltage across the primary winding is reversed, and the collector voltage of the switch tube of the forward converter quickly rises to 2Vdc. At the same time, the excitation current flows to the reset winding through diode D, and finally reduces to zero. At this time, the voltage across Q drops to Vdc. Figure 2 shows the switch collector current and voltage waveforms. It can be seen that when the switch tube does not have a buffer circuit, when Q is turned off, the leakage inductance voltage spike at both ends is very large, and the resulting turn-off loss is also very large. In serious cases, the switch tube is likely to be burned out. Therefore, the switch must be tube plus buffer circuit.

　　When the switch tube has a buffer circuit, its collector voltage and current waveforms are shown in Figure 3 (taking the forward converter as an example).

　　In Figure 1, when Q begins to turn off, its current begins to decrease, and the transformer leakage inductance prevents this decrease in current. Part of the current will continue to pass through the switch tube that is about to be turned off, and the other part will pass through the RC buffer circuit and charge the capacitor C. The size of the resistor R is related to the charging current. Part of Ic flows into capacitor C, which can slow down the rise of collector voltage. By selecting a sufficiently large C, the overlap between the rising voltage and falling current of the collector can be reduced, thereby significantly reducing the turn-off loss of the switch tube, and at the same time suppressing the collector leakage inductance peak voltage. The stages A-C in Figure 3 are the switch-off stages, and C-D are the switch-on stages. Before the switch is turned off, the voltage across the capacitor C is zero. At the turn-off moment (moment B), C will slow down the rise of the collector voltage, but at the same time it will be charged to 2Vdc (when the leakage inductance peak voltage at this moment is ignored). The size of the capacitor C not only affects the rise rate of the collector voltage, but also determines the energy loss on the resistor R. At the moment Q turns off, the voltage on C is 2Vdc, and the energy it stores is 0.5C (2Vdc) 2 Joules. If all the energy is consumed on R, the energy consumed on R in each cycle is:

　　For limiting the collector rise voltage, the larger C should be, the better; but from the perspective of system efficiency, the larger C, the greater the loss and the lower the efficiency. Therefore, an appropriate C must be selected so that it can achieve a certain effect of slowing down the rising voltage of the collector without causing excessive system losses and low efficiency. In Figure 3, since it is necessary to ensure that there is no voltage across C at the next shutdown start time (time D), C must be discharged within a certain period of time between time B and time D. In fact, the capacitor C can also be discharged through the discharge circuit formed by the resistor R through Q and R during the period of C-D. Therefore, after selecting a large enough C, R should make C discharge to less than 5% of the charged charge within the minimum on-time ton, so that:

　　Equation (1) shows that the energy loss on R is proportional to C, so an appropriate C must be selected. In this way, how to choose C becomes the key to designing the RC buffer circuit. A more practical way to select the capacitor C is introduced below. method. In fact, when Q starts to turn off, it is assumed that half of the initial peak current Ip flows through C, and the other half still flows through the gradually turned off Q collector, while assuming that the leakage inductance in the transformer keeps the total current still Ip. Then, by selecting an appropriate capacitor C, the collector voltage of the switch tube rises to 2Vdc within time tf (where tf is the time for the collector current to drop from the initial value to zero, which can be queried from the switch tube data sheet), then have:

　　Therefore, the size of the capacitance C can be calculated from equations (1) and (3). After C is determined and the minimum on-time is known, the size of the resistance R can be obtained through equation (2).

　　2 Main circuit design of forward converter with RC buffer

　　2.1 Circuit design Figure 4 shows a forward converter main circuit with an RC buffer circuit. The main circuit parameters are: Np=Nr=43 turns. Ns=32 turns, switching frequency f=70kHz, input voltage range is DC 48~96V, output is DC 12V and DC 0.5A.

　　Switch tube Q is a MOSFET, model IRF830, and its tf is generally 30ns. Dl, D2, and D3 are fast recovery diodes with very small tf (usually tf=30ns). The output power of this design is p0=V0I0=6W. Assuming that the efficiency of the converter is 80%, the power lost by each RC buffer circuit accounts for 1% of the output power. Take Vdc=48V here. 2.2 Experimental analysis The following is an experimental analysis of this design in two situations. One is that the primary winding has buffering and the secondary has no buffering; the other is that the primary winding has no buffering and the secondary has buffering. (1) The primary winding is buffered, and the secondary winding is unbuffered. This experiment measures the drain-source voltage at both ends of the switch tube Q. The experiment is divided into the following two situations: The first situation is RS1=1.5k, CS1 is uncertain, and the input DC voltage Vdc is 48V. The experimental results are: when RS1 remains unchanged, the larger CSl is, although the leakage inductance peak voltage of the switch tube Q does not decrease significantly, its drain-source voltage becomes gentle, which shows that the RC buffer of the primary switch tube In the circuit, CSl should choose a relatively small value. The second case is CSl=33pF, RS1 is uncertain, and the input DC voltage Vdc is 48V. The result is: when CS1 remains unchanged, the larger RS1 is, the larger the leakage inductance peak voltage of switch tube Q is (the increase is relatively small). It can be seen that in the RC buffer circuit, the size of the parameter R has a great influence on reducing the leakage inductance spike. When selecting a suitable C and satisfying equation (2), R should choose a relatively small value. (2) The secondary winding is buffered, and the primary winding is unbuffered. In this experiment, the cathodes of D2 and D3 are used as the common terminal to measure the terminal voltage of the fast recovery diode. The result is that when R remains unchanged, the larger C, the greater the leakage at both ends of the diode. The smaller the sensory peak. At the same time, theoretically, if C is infinite, there will be no leakage inductance spike in the voltage across the diode. In practice, it is sufficient to make the leakage inductance peak voltage of the voltage across the diode within 30% of its terminal voltage peak value, so that the cost will not be too high at the same time. 2.3 Determination of design parameters It can be seen from experimental analysis that in the RC snubber circuit of the secondary fast recovery diode, when the capacitor C of appropriate size is selected, the resistor R should be selected to be larger when formula (2) is satisfied. Smaller is better. After actual debugging, the RC snubber circuit parameters selected for this design are: Primary: RS1=200, CSl=100pF Secondary: RS2=RS3=5l, CS2=CS3=1000pF In the RC snubber circuit of the primary switch tube of this design Although the C value is chosen slightly larger than the calculated value, the loss is not very large, so it is still acceptable. Compared with the primary, the C value in the RC snubber circuit of the secondary fast recovery diode is chosen to be much larger than the calculated value, and the system loss will inevitably increase. However, the RC buffer circuit connected in parallel at both ends of the fast recovery diode is mainly to improve the system output performance. Therefore, although choosing a relatively large C value will reduce the overall efficiency of the system, the leakage inductance peak at both ends of the diode will be reduced a lot, and The ripple of the output voltage can also meet the specified requirements.

　　3Conclusion

　　According to the formula given above, a suitable RC snubber circuit can be selected very well and conveniently. However, in engineering applications, truly appropriate parameters can be obtained through actual debugging based on the performance indicators of the system design. Sometimes, in order to achieve system performance indicators, it is necessary to sacrifice a certain amount of efficiency. In short, when designing the parameters of the RC buffer circuit, system performance and efficiency must be considered comprehensively, and the appropriate RC parameters must be finally selected.

　　At the Technology Zone Mu Exhibition, SiC, GaN, and three-level products brought by Shiqiang will bring your efficiency to the highest point. How to use a secondary output filter to prevent switching power supply noise. Welding precautions for ceramic vertical mount packages (CVMp) and What are the common methods for generating reference regulated power supplies for average small signal mathematical modeling and loop compensation design of DC-DC converters?

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