Hot Clamp

You should look at the drain voltage and current waveforms to verify that the power supply is operating properly. If the waveforms are normal, check that the transformer's primary leakage inductance is less than 3% of the primary inductance. If not, you may have to split the primary winding such that the secondary winding is between both halves of the primary.

Z. Cochrane

Dear Z. Cochrane

Thank you very much for your attention but my circuit works properly. I checked waveforms with typical waveforms on application nots. The voltage is a puls with an overshoot and vanishing sinusoidal waves. In addition, I have winded the transformer with two primery halves and put the secondry winding between them. I think the remaining parameter is leakage inductance which I don't know what it is exactly and how to measure it. maybe my problem due to it.thus, would you please help me to understand it better and measure it to correct my circuit.
thank you again

best regards
Pooria Saiyad Khodashenas

Primary leakage inductance shows up as the energy that is not coupled from the primary to the secondary. This can be modeled as an inductor in series with the primary winding. You can measure the leakage inductance by removing the transformer from your circuit and shorting the secondary winding. Measure the inductance across the primary. The value you read is the leakage inductance.
The energy stored in the leakage inductance will be transferred to the primary clamp circuit. Power in the clamp is = 1/2 *L*(I^2)*f. where L is the leakage inductance, I is the peak drain current and f is the switching frequency.
For your application, it must be less than 3% of the primary inductance or the power dissipated in the clamp circuit will result in hot components.

Z. Cochrane

Dear Z. Cochrane

thank you very much for your answer. I will check this in my circuit and let you know about it. in addition, I thought to myself that this power consuming in clamp circuit may because of reflected output voltage and by change this parameter to a lower level this power consumption will modified.
am i rigth ? because I think the clamp circuit runs after this voltage applied to the circuit so current flows via the elements during this period. Thus, by low voltag we can get low power consumption.

Wow it is to high.
I removed the transformer from my Circuit and for the first time the secondary winding shorted and 166 uH achieved while the primery inductance was 316 uH. it seems to high ( (316 - 166)/316 = 0.47 , it means 47%). for the second time bais winding shorted and the test repeated. 130 uH achieved it means (316-130)/316 = 0.58 or 58%. yes it is too high. How could I reduce it ?

I would suggest to work with your transformer manufacturer for suggestion on how to reduce the leakage inductance.
The transformer should be wound with the primary windings placed neatly across the entire bobbin width. You indicated that you have a split primary winding with the secondary winding between the two primary layers. This should give you the lowest leakage inductance. The primary winding for each half of the winding should only be one layer of wire across the bobbin. The secondary should be neatly wound on one layer that fills the entire bobbin width. The other half of the primary should be wound next. Use the minimum layers of tape between windings to meet your safety requirements. Wind the bias winding last. This construction technique should get your leakage inductance to a reasonable level.

Z. Cochrane

Thank you very much
Transformer was wound by myself. I wound them manually so maybe this is the main problem because we are going to demonstrate some prototypes then extend it in our factory.
I will use your structure to realize my transformer and I hope to make it well.
if you have an alternative slotion please let me know.
thank you again.

I have one additional comment. For 75 W output power you can expect a dissipation of approximately 1.5 W in the clamp. Therefore, a single TVS zener will almost certainly get hot.
I'd suggest that you use either a RCD clamp with TVS in parallel of use multiple TVS zeners for thermal management.
For component values put the design through PI Expert and see what values of R and C the software suggests.

Thank you very much.
I already do what you suggested. I use RCD and 2 zeners in series parallelly to reduce the heat and It works well.
Thank you agian

Glad that its working for you. BTW one last comment - For 75 W if you are not using a split primary construction for your transformer I'd highly recommend that you do. This will further reduce your leakage inductance (which the root cause of clamp power loss)