Flyback power supply problem help

Aguilera

Flyback power supply problem help [Copy link]

 

The following flyback power supply

Teachers, the following is a commonly used UC3842 controlled flyback power supply circuit

Two questions

1.What are the functions of the 820p capacitor, VD3 and 2.5k resistor in the red box?

2. How to remove the power frequency ripple of flyback switching power supply

maychang

『1. What are the functions of the 820p capacitor, VD3, and 2.5k resistor in the red box? 』

The functions of these three components are basically the same as R12C9VD2, which is used to absorb the peak voltage generated by the leakage inductance of the transformer.

maychang

『2. How to remove the power frequency ripple of flyback switching power supply』

Does “power frequency ripple” refer to the power frequency ripple across capacitor C1, or the power frequency ripple across capacitor C10?

Aguilera

maychang posted on 2022-7-20 12:11 『1. What is the role of the 820p capacitor and VD3 in the red box, and the resistor 2.5k? 』 The role of these three components is roughly the same as R12C9VD2, which is used to absorb...

Thank you teacher for your answer

Since these three components are used to absorb the peak voltage generated by the transformer leakage inductance

Why is the capacitor 820pF?

The resistor is 2.5k, can't I use other diodes?

Is there any formula here?

Please give me some advice, teacher

Aguilera

maychang published on 2022-7-20 12:12 『2. How to remove the power frequency ripple of the flyback switching power supply』 "Power frequency ripple" refers to the power frequency ripple at both ends of capacitor C1, or the capacitor C ...

Power frequency ripple across C10

maychang

Aguilera posted on 2022-7-22 07:29 Thank you for your answer. Since these three components are used to absorb the peak voltage generated by the leakage inductance of the transformer, why is the capacitor selected as 820pF?

"Can't I use other diodes?"

VD1, VD2, and VD3 all work at PWM frequencies, at least tens of kHz, so ordinary industrial frequency diodes such as 1N4007 cannot be used, and fast recovery diodes must be used.

However, the reverse withstand voltages that these three diodes need to withstand are different. VD1 needs to withstand the lowest reverse voltage, and only needs to withstand the reverse voltage of winding N1 (negative on top and positive on bottom) plus the voltage across capacitor C1. The reverse voltages that VD2 and VD3 need to withstand are much higher, because the voltage after AC mains rectification is as high as 300V or more.

maychang

Aguilera posted on 2022-7-22 07:29 Thank you for your answer. Since these three components are used to absorb the peak voltage generated by the leakage inductance of the transformer, why is the capacitor selected as 820pF?

"Then why is the capacitor 820pF and the resistor 2.5k?"

The two RCD circuits have different time constants. The time constant of the group in the red box is slightly smaller, and the time constant of the group above is relatively large. Therefore, the action in the red box is slightly faster, but the group above can last slightly longer (can absorb more energy).

The calculation requires knowing the leakage inductance value of the transformer, but the leakage inductance is not very certain and is closely related to the transformer winding method. Different winding methods can cause the leakage inductance to differ by several times or even more.

maychang

Aguilera posted on 2022-7-22 07:30 Power frequency ripple at both ends of C10

If the switching power supply feedback circuit is designed correctly, the power frequency ripple across C10 is very small and usually does not need to be considered.

The switching power supply with regulated output should keep the output stable when the input voltage (voltage across C1) changes. For example, when the voltage across C1 changes from 200V to 350V, the voltage across C10 can maintain 5V. The power frequency ripple frequency of the voltage across C1 is 100Hz, that is, the period is 10ms. If the feedback circuit responds quickly enough and can restore the original output value within 1ms or even less time after the sudden change of input voltage causes the output to change, then the voltage across C10 will not be affected by the change of the ripple voltage across C1, or at least the impact will be very small.

maychang

Aguilera posted on 2022-7-22 07:30 Power frequency ripple at both ends of C10

Using an analog oscilloscope to observe the drain voltage of the power switch tube, you can see an unclear rectangular wave (a digital oscilloscope requires repeated triggering). The reason for the unclearness is that the PWM duty cycle is constantly changing. The changing PWM duty cycle indicates that the duty cycle is constantly adjusting with the power frequency ripple at both ends of C1 to keep the output voltage stable.

maychang

Aguilera posted on 2022-7-22 07:30 Power frequency ripple at both ends of C10

For the circuit in the first post, the output voltage is sampled from the voltage across C2, because the voltage across the N2 winding is proportional to the voltage across the N3 winding at any time. The capacity of C2 is relatively large, and the voltage on it cannot change quickly, and cannot reflect the change of the voltage across C10. In this case, the suppression of the power frequency ripple across C10 mainly depends on the current feedback loop of UC3843, that is, the feedback formed by the voltage across the resistor R10. This matter is a long story, and I am afraid it cannot be explained clearly in one or two posts.

maychang

Aguilera posted on 2022-7-22 07:30 Power frequency ripple at both ends of C10

The output voltage stability of this circuit, which obtains the output voltage from the power supply winding N2 through rectification and filtering, is much worse than that of the circuit that obtains the output voltage from both ends of C10, isolates it through an optocoupler, and then feeds it back to pin 2 of UC3842.

lkh747566933

I am a newbie here to learn. I have never designed a switching power supply circuit and I know nothing about it.

fjdeepblue

The above person is right. It is the absorption circuit, which has the same function as R12, C9 and VD2. Generally, only one absorption circuit is needed for low power.

qwqwqw2088

Power supply ripple, no matter what kind of ripple it is, can only be suppressed, not removed

The power frequency ripple belongs to the low-frequency input ripple.
Its size is related to the size of the filter capacitor in the output circuit.