UCC28950: On the Current Waveform of Shim Inductor

Hello Soongil

The first thing I would say is that your waveforms look clean and quite normal. Can you take a plot of the waveform at the CS pin of the UCC28950 controller and send that to me please. I'd also like to see your full schematic and can you identify exactly where you have placed the current probe - if you don't want to post it here then please send it to me directly by email to colingillmor@ti.com

Regards
Colin

Hello Colin

First, I'm posting new waveform containing CS pin of the UCC28950 and one where i have placed the probe.

Full schematic you requested is going to be sent by e-mail for internal regulations.

Best Regards.

Hello Soongil

Thanks for the added information -  I did receive the email you sent to me. The CS signal looks correct. I will look at the current waveforms tomorrow but in the meanwhile, if you have time, you could look at the current into pin 5 of the power transformer - I've marked the point with a yellow dot below. The reason for doing this is that the current in the shim inductor is not exactly the same as that in the primary of the transformer. It's no problem if you don't have time to do this because it will be one of the first things I will check.

Regards

Colin

Good day Colin.

I couldn't see the point you have marked.
I guess it something wrong in the file!

It is very hard to look at the point that you want to look at current into pin 5 of the power transformer . So I post the waveform somewhere else.

I hope it works for you.

c1 : VDS of QD4, c2 : VDS of QD2, c3 : VDS of QD5, C4 : Drain current of QD4

load condition 28v/36A

c1 : VDS of QD4, c2 : voltage of power transformer, c3 : Vgs of QD4, C4 : Drain current of QD4

load condition 28v/4A

If you need to look at the point you want i will try again tomorrow.

Thanks you your helpful

Best Regards.

Hello Soongil

The drain current of Q4 is a good alternative.

Thanks
Colin

Hello Colin

Is there a problem with the circuit or the transformer?
We have not yet solved the heating problem of the shim inductor.
I would like advice on improving the heat of the inductor.
Will it be effective to make the turn ratio of the transformer 8: 1?

Thanks.

soongil.

Hello Soongil

What is happening here is that at heavy loads - top picture - the power stage is switching at zero volts. In the second picture - at 4A load - the system has lost ZVS because there is not enough energy to charge the stray capacitance at the switched node from 0V to Vin (or vice versa). The result is that the voltage swings part of the way and then 'falls back' in a resonant ring (orange loops below). The node finally transitions when the MOSFET is turned on. There is a large current spike associated with this - probably due to the large increase in the dv/dt rate which increases capacitive currents into the CS signal.

You can also see the 'miller plateau on the leading edge of the gate drive waveform - red circle.

All of the above is 'normal behaviour'. The question is whether you need your power stage to operate at these light loads and whether you can tolerate the increased switching losses when ZVS is lost. There are a couple of things you can do to alleviate this behaviour :

Set the UCC28950 so that it enters burst mode at light loads and set the burst mode threshold high enough to avoid loss of ZVS.

Increase the amount of energy available to drive the ZVS transition - this normally means a larger shim inductor.

Reduce the amount of stray capacitance at the switched node - different MOSFETs, smaller PCB lands at the switched nodes, different transformer construction to minimise self capacitance (this can be difficult).

the shim inductor can heat because of I^2R losses and to a lesser extent due to core losses. What is the DC resistance of the inductor. Bear in mind that this inductor carries a current at the switching frequency so skin effect and proximity losses may be partly responsible. Have  you tried winding it with Litz wire?

Regards
Colin

Hello Soongil

Please let me know how you get on.
By the Way - how hot is the shim inductor getting, what is its rating and what temperature did you expect ?

Regards
Colin

Hello Soongil

Thanks for the information. I expect that your magnetics manufacturer/designer will be able to suggest a solution. In the meanwhile, I'm going to close this thread. Please feel free to open a new thread if you have any problems that we may be able to help with.

Regards
Colin