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Old 10th May 2005, 06:31 AM   #1
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Location: ontario
Default SMPS Design Outcome and Problems!

Hello Everyone!,

I have designed and built a switching power supply for a car amplifier. PWM with deadband comes from a PSoC microcontroller. The 50-200KHz (adjustable) PWM runs to a 6A mosfet driver and finally to IRF2804 mosfets through a 4.6Ohm 2W gate resistor.

I wound the tranformer on a 2.5" OD core. I used roughly 5+5 turns of 4X18 gauge magnet wire. This was wrapped flat around the core and centertapped at the adjacent end. The secondary consists of 40 turns of 18 gauge magnet wire, centertapped where the primary was started.

I used the exact filter from here:
http://sound.westhost.com/project89.htm
on the input to snub the voltages..

I have a total of 33000uF at the input, and 10000uF at the output with 4 10uH power inductors to form the PI filter.

I used two lightbulbs in parallel (measured 12Ohms) and got 35.5V at the output. I haven't tried a lower load, but I hope it can produce more than 200W.

The problem that I am having is that I hear a ringing sound for about 1/2 second when I disconnect the 12V source. I have no clue what this is, but it doesn't sound healthy. It actually sounds a lot like the noise from an NPN transistor amplifier I blew up a couple months ago... The MOSFETs do not get too hot, either do the ultrafast rectifier diodes..

Can anyone help??

Steve
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Old 12th May 2005, 06:41 AM   #2
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Join Date: Apr 2005
Location: ontario
Well, the SMPS failed miserably...

I had mounted the TO220 packages on a rather large heatsink and I think I might have shorted something. I did use silpads and thermal adhesive on the devices..

I used a bar to apply pressure evenly to the front of the TO220s and perhaps tightened them too much.

I tested the SMPS several times and got over 400Wrms and perhaps more if I loaded it more. When I ran the supply with the Tripath board, it whined quite loadly and ceased to work. I didn't even get to see anything blow up, just the sound of MOSFETs failing and eventually dying.

I was pleased with its performance while it worked and before I did something wrong. Not bad for my first SMPS supply design..

Steve
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Old 19th May 2005, 10:15 AM   #3
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Location: nr Oxford UK
Default SMPS failures

Steveo:

Looking at the schematic on the website (by Moreno and Elliott) I see there is no cycle by cycle current limit on the schematic!

The mosfets in this circuit are in 'Kamikaze mode' - they die first so that the fuse can blow. Thus nothing to protect the output devices in the event of an overload. The only thing a fuse will help here is to stop a fire.

Also, in the Moreno and Elliott circuit, I would be concerned about the absence of a secondary filter inductor/s, because at fs, the primary will see only the ESR of the filter caps.

(Others will disagree on this last point based on the number of successful converters using this topology - we must agree to disagree then)

You don't mention how your own circuit is protected.


John
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