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Old 28th September 2005, 04:37 PM   #1
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Default Current ringing in SMPS

Let me start by saying, I've found all kinds of terrific information on this board- it is a great resource for SMPS design!

I am trying to work out the final kinks in my half-bridge 2000W audio amplifier power supply. Everything is working pretty good as far as regulation and ripple. I am using 2 punch-through Advanced Power IGBT's (thanks to Eva for the IGBT suggestions ) The converter is running at 100kHz effective (200kHz clock) with a MC34025 controller (current mode). I am using a gate transformer wound on an EE19 core with 11TS primary and 11TS secondaries.

My main transformer is on an ETD49 core. Primary turns is 18TS wound between the 2 secondary halves of 10TS. (Split secondary construction) I'm using 0.1mm copper strip for adaqate current capacity and lower leakage (I think ) No PFC on this supply, 320 DC peak on the bulk DC caps. +/-68 VDC rails on the output. Output has coupled inductor with 320uH per output and 940uF capacitance per rail. I'm usng some great ST diodes (STTH8R06D) on the secondary in full bridge config.

My problem is my IGBT's are heating up quite rapidly because I had to slow thier on time switching speed rate a bit (w/ 47ohm gate R's). This is due to the fact the with very fast rise, the current spike and ringing as seen through the primary of the transformer was so great it was causing all kinds of RF noise on the amplifier and who knows what the EMI would look like on the spectrum analyser. As I increase the gate resistance, the ringing is almost gone and so is the noise on the amp. This, of course, creates unaccaptable heat on the IGBT's.

I've tried different transformer arrangements and can't seem to improve this at all. I think my layout is pretty tight with wide tracks in the power loop. (I'm not sure I have a way to post the PCB layout or the schem.) I keep thinking I'm doing something wrong with the way the transformer is wound and I'm getting excessive leakage inductance. I can't live with the heat, so do I try some sort of current snubber arrangement, and if so, where does it go? I currently don't have any additional inductors or caps in the primary path- should there be?

Thanks for any and all help- it's been frustrating me for some time
Matt.
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Old 28th September 2005, 11:18 PM   #2
Eva is offline Eva  Spain
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Are you using copper foil high voltage transformer windings?
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Old 28th September 2005, 11:33 PM   #3
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Eva,

Yes- with mylar sandwiched in between. It was my understanding that this would lead to lowest leakage inductace. I tried wire wound transformers, but saw no difference in performance that I could tell. Perhaps I should go back to the transformer design stage

I also see the same ringing as a voltage ring on the output diodes. Snubbing them only helps a little.
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Old 28th September 2005, 11:51 PM   #4
Eva is offline Eva  Spain
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Are you aware that 18 turns of copper foil with the width allowed by an ETD49 coil former are going to produce a huge winding capacitance?

Also, leakage inductance is not such a bad thing, it has some great properties like smoothing reverse recovery from secondary winding diodes and allowing the switches to saturate before the current has reached its maximum value, thus reducing dissipation.
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Old 29th September 2005, 09:12 PM   #5
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I tried winding another xfmr with triple insulated wire. S=SH=P=SH=S (SH means 'shield'), one layer each for each half of the secondary and the primary. I also added a shield layer in between each half of the secondary and the primary. I see no difference The shields help a bit, but not as much as I hoped.

Back to the drawing board...

I may try winding it more conventionally as split-primary P=SH=S=S=SH=P and see if I have better results.
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Old 1st October 2005, 02:09 AM   #6
Eva is offline Eva  Spain
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Could you post some pictures of the prototype? The faster you switch and the higher the winding capacitances, the more critical the layout is.
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Old 5th October 2005, 04:53 PM   #7
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I have not been able to find a way to get a picture of it yet, and my layout/schem is done on a Mac, of all things, so I need to find a way to transfer to JPEG. I do have a way to post waveforms- I will try to post the current waveform with gate drive and Vce waveforem for one IGBT.

I'm going to stick with wire windings since there is no improvement with foil. I suppose foil would be useful for high current, low voltage secondary windings. I had just noticed that a QSC amp had a transformer with all foil windings and thought I'd give it a try. Wire is WAY easier to work with anyways and that makes things much easier.
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Old 5th October 2005, 08:31 PM   #8
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Thanks to a nice scope borrowed from work, I'm able to show these waveforms. This is with barely any load. (Amplifier at idle.)

The light blue is the current through the primary of the transformer.
The dark blue is the Vce of one of the IGBT's
The green is the gate voltage of the same IGBT.
Attached Files
File Type: pdf smps1.pdf (31.3 KB, 207 views)
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Old 5th October 2005, 08:33 PM   #9
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Here's another with the horizontal scale reduced- same waveforms. Also, the amplifier now has a small sine input.
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File Type: pdf smps2.pdf (37.4 KB, 123 views)
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Old 8th October 2005, 10:45 AM   #10
sbrads is offline sbrads  United Kingdom
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I would guess that you need a smoother gate drive. Still fast rising for efficiency, but rounded off a bit to cut down on transformer ringing. High power SMPS FET stuff I see at work tends to use low source impedance transformer drive feeding the gates through a 10R in parallel with a small fast forward diode, with a capacitor from gate to source or the earthy end of the drive winding depending if there's a source current sense. The capacitor is usually somewhere in the 1nF to 10nF region and is tuned to as low a value as necessary to stop ringing. I haven't used IGBT's so whether this is any use, dunno.
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