Clamping Circuit

Hi,

You could find some more information in the following application note:

http://www.powerint.com/sites/default/files/product-docs/an14.pdf

It is for TopSwich, but the principle is simiar for Tiny Switch. Unfortunately there is no application note talking about this specific topic for us yet, but I will recommand you to read this technical document:

http://www.switchingpowermagazine.com/downloads/12%20Flyback%20Snubber%20Design.pdf

Hope this is helpful 

Dear Terry;

Thank you for your support.

I have attached you our Schematic and also some Oscilloscope snap shots. Each file name is its description.

First of all please let me know why drain and source currents are different, spatially when TNY switches off the current; there is a reverse current at the drain.

Next I would like to know whether it is normal to have a peak current, as we have, in the beginning of each turn on cycle at source and drain.

There are also two other pictures from the clamping circuit signals, one of them is the voltage over one of 100ohm series resistors and the other one is voltage over four 24kohm resistors. Are they normal?

Regards 

Hi

Here is the comments regarding to your questions:

1. When you measure the drain current, I think you measure the drain current plus the current into the snubber circuit. The inverse current looks like the reverse recovery curent of the snubber diode. Would you redo the measurement, make sure the current is purely only the drain current.

2. It is normal to have a initial current during the turn on. The peak current will be higher if the power supply is working under CCM mode. This current is actually the parasitic capacitor discharge current pluse the reverse current of the secondary diode if the power supply is working under CCM mode.

3. The voltage you see across the snubber resistor is quite normal.

Hope this is helpful for you.  

Dear Terry;

Thank you for your support.

I have checked the Drain current and it is same as it and the SMPS doesn’t work under CCM(continues current mode)   mode.

Clamp diode reverse recovery current could be supplied from transformer current during the switch off time not from the switch itself.

According to above items and attached pic please let me know the following items:

Do you have any other idea about the inverse Drain current, blue circle in the right?

While I am using an ultra fast USF4007 diode in my clamp circuit, why are there inverse current in the camp at the end of clamping function in each cycle, brown circle in the left, and in the Drain current, blue circle in the right, at the end of each cycle whet the TNY turns off? Are they harmful for TNY and MosFET?

Hi,

It is expected to have the reverse current for a USF4007, this is generic characteristic of a diode.

I think the first thing is to understand why there is a difference between the source current and drain current of the MOSFET. If I understand you well, the voltage accoss the R627 is recorded get the cource current, while the drain current is recorded with a current probe? Would you please use the same method to check both the source and drain, I want to make sure the current is real first.