Ask about the flyback power supply

Aguilera

Ask about the flyback power supply [Copy link]

Please advise on the power supply issue.

As shown in the red box in the picture, under what circumstances should the backup be selected?

normal

spare

Also, what is the function of C107?

不亦心

The primary switch tube and C107 capacitor may be used to understand EMC and improve dv/dt.

I didn't see the RCD circuit, so the drawing is incomplete...

From the drawing, in normal mode, 24V is connected to a dummy load to prevent poor cross regulation and severe voltage drift;

In standby mode, the 24V dummy load is disconnected and feedback is directly involved.

Aguilera

Bu Yixin posted on 2023-5-20 23:43 The primary switch tube and C107 capacitor may be for understanding EMC and improving dv/dt. I didn’t see the RCD circuit, so the drawing is incomplete. . . From the drawing...

Thank you teacher for your answer

The drawing is complete

Why should we choose 1KV withstand voltage for this 107 capacitor to improve dv/dt?

不亦心

Aguilera posted on 2023-5-25 13:27 Thank you for your answer. The drawing is complete for this 107 capacitor. Why do we need to choose a 1KV withstand voltage to improve dv/dt?

This power is not small. Without RCD absorption, the power tube cannot handle it.

When the input voltage is 265VAC, the peak value of the rectified voltage is 375V. Add the reflected voltage and calculate it as 100V. Add the leakage inductance peak and calculate it as 80V. This is almost 600V. Is it abnormal to use 1KV?

Aguilera

Bu Yixin published on 2023-5-25 23:21 This power is not small. Without RCD absorption, the power tube can't handle it. When 265VAC input, the rectified voltage peak is 375V, plus the reflected voltage is 1...

This circuit has no RCD absorption, and is about to add RCD absorption.

Teacher, do you have any good suggestions for adding RCD absorption?

Aguilera

Bu Yixin published on 2023-5-25 23:21 This power is not small. Without RCD absorption, the power tube can't handle it. When 265VAC input, the rectified voltage peak is 375V, plus the reflected voltage is 1...

Do I need to consider using RCD to clamp the drain voltage of the switch tube? I have no idea about this.

不亦心

Aguilera posted on 2023-5-26 13:43 Do you need to consider using RCD to clamp the drain voltage of the switch tube? I have no idea about this

The power level in the blueprint should be increased.

If your transformer has a very small leakage inductance (which is not easy to do with multiple windings), and the capacitor C107 in the drawing is relatively large (the loss is considerable), perhaps the leakage inductance peak can be controlled, but the probability of being able to do so without an RCD is very small. I personally have not played with it this way.

Aguilera

Bu Yixin posted on 2023-5-27 15:49 The power level on the drawing should be added. If your transformer leakage inductance is very small (multiple windings seem to be difficult to achieve), C107 on the drawing...

How to test transformer leakage inductance?

The secondary side is short-circuited, so measure the primary side. But how to measure it specifically? I haven't been able to find a suitable method recently.

What should I use, a digital multimeter or an oscilloscope?

Aguilera

When testing, must the transformer be separated from the input circuit?

maychang

Aguilera posted on 2023-6-15 10:43 When testing, must the transformer be separated from the input circuit?

To test the leakage inductance of a transformer, short-circuit the secondary of the transformer (if there are multiple secondaries, all of them need to be short-circuited), and use a multi-bridge or inductance and capacitance tester to measure the inductance of the primary winding when the secondary is short-circuited.

If the transformer primary is connected to other components, such as common-mode inductors and capacitors, the primary winding needs to be separated from these components. Only one end of the primary winding needs to be separated.

maychang

Aguilera posted on 2023-6-15 10:43 When testing, must the transformer be separated from the input circuit?

[What to use, digital multimeter or oscilloscope? ]

Some digital multimeters have an inductance range, which can be used to measure the leakage inductance of the transformer. However, if the digital multimeter does not have an inductance range, a multibridge or inductance and capacitance tester is required.