MOS tube reverse peak and RCD absorption circuit in switching power supply design

For a switching power supply engineer, it is common to make choices between one or more pairs of mutually opposing conditions. The topic we are discussing today is a pair of mutually opposing conditions. (That is, to limit the maximum reverse peak of the main MOS tube, and to minimize the power consumption of the RCD absorption circuit)

Let us make a few assumptions before discussing:

① The operating frequency range of the switching power supply: 20~200KHZ;

② The forward conduction time of the diode in the RCD is very short (usually tens of nanoseconds);

③ Before adjusting the RCD circuit, the parameters of the main transformer, MOS tube and output circuit have been completely determined.

With the above assumptions, we can start with the calculation:

1. First, segment the VD of the MOS tube:

ⅰ, input DC voltage VDC;

ii, secondary reflects primary VOR;

ⅲ, Main MOS tube VD margin VDS;

iv. RCD absorbs effective voltage VRCD1.

2. Calculate several parts of the above main MOS tube VD:

ⅰ, input DC voltage VDC.

When calculating VDC, the highest input voltage is used as the standard. For example, if the voltage is wide, AC265V should be selected, which is DC375V.

VDC＝VAC*√2

ii, Secondary reflects the primary VOR.

VOR is calculated based on the maximum secondary output voltage and the maximum voltage drop of the rectifier diode. For example, the output voltage is 5.0V±5% (calculated based on Vo = 5.25V), and the diode VF is 0.525V (this value is the VF value at rated current found in the 1N5822 data).

VOR＝(VF Vo)*Np/Ns

ⅲ, margin VDS of main MOS tube VD.

VDS is the minimum value of 10% of the VD of the MOS tube. For example, if the VD of KA05H0165R is 650, DC65V should be selected.

VDC＝VD*10%

ⅳ, RCD absorbs VRCD.

The maximum value of VRCD is obtained by subtracting items ⅰ and ⅲ from the VD of the MOS tube. The actual VRCD should be 90% of the maximum value (this is mainly due to the dispersion of the components of the switching power supply, the influence of temperature drift and time drift, etc.).

VRCD＝(VD-VDC -VDS)*90%

Note: ① VRCD is calculated as a theoretical value, and then adjusted through experiments to make the actual value consistent with the theoretical value.

② VRCD must be greater than 1.3 times of VOR. (If it is less than 1.3 times, the VD value of the main MOS tube is too low)

③ The VD of the MOS tube should be less than 2 times of VDC. (If it is greater than 2 times, the VD value of the main MOS tube is too large)

④ If the measured value of VRCD is less than 1.2 times of VOR, the RCD absorption circuit will affect the power supply efficiency.

⑤ VRCD is composed of VRCD1 and VOR

ⅴ, RC time constant τ is determined.

τ is determined by the operating frequency of the switching power supply, and is generally selected as 10 to 20 switching power supply cycles.

3. Experimental adjustment of VRCD value

First, assume an RC parameter, R=100K/RJ15, C="10nF/1KV". When the power is on, the principle of low voltage first, then high voltage, and then from light load to heavy load should be followed. During the test, the voltage value on the RC component should be closely watched, and VRCD must be less than the calculated value. If it is found that the calculated value is reached, the power should be turned off immediately, and the above test should be repeated after the R value is reduced. (The voltage value on the RC component is observed with an oscilloscope, and the ground of the oscilloscope is connected to one RC point of the "+" pole of the input electrolytic capacitor, and the test point is connected to another RC point). A suitable RC value should be that the test value of VRCD is equal to the theoretical calculated value under the highest input voltage and the heaviest power load.

IV. Phenomena worth noting during the experiment

The lower the input grid voltage, the higher the VRCD, and the heavier the load, the higher the VRCD. If the test value of VRCD at the lowest input voltage and heavy load is greater than the VRCD value calculated theoretically above, does this contradict the content of (III)? Not at all. The theoretical value is the calculation result at the highest input voltage, and now it is a low input voltage. Heavy load refers to the maximum load that the switching power supply can reach. The limit power of the switching power supply is mainly measured through experiments.

5. Power selection of R value in RCD absorption circuit

The power selection of R is calculated based on the maximum value of the measured VRCD. The actual selected power should be greater than twice the calculated power.

Postscript: If the R value in the RCD absorption circuit is too small, the efficiency of the switching power supply will be reduced. However, if the R value is too large, the MOS tube is in danger of being broken down.