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Old 17th June 2012, 09:33 PM   #31
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Hi, DC. what kind of efficiency should we be looking at for this converter.??
Im testing a smaller one at the moment, but for 50W out, its using 67W in.
I have a number of different EI cores, salvaged, so i dont know the specs.
thanks
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Old 18th June 2012, 04:41 AM   #32
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1) Sure D3 will work perfectly. However, what modeling doesn't warn you about is heat. With a standard diode, and the supply working at full power (400W), the diode will be dissipating ~36W! That's a lot of heat, not to mention you're losing a full volt of input voltage. Even if D3 was a Schottkey power diode, you'd still lose 0.3V and dissipate 10W. What's normally done is to put the diode in parallel with the input voltage with the cathode towards positive. That way it's out of circuit unless the battery is hooked up backwards - then it conducts blowing the fuse.
2) Yes. Paralleled FETs share current and improve performance.
4) The FF circuit is completely unnecessary. The feedback will compensate for input fluctuations & load. It just adds complexity and the potential for instability.
5) 4W of dissipation is still a lot. What savu said is correct - 1K or so is fine. For 4W of continuous dissipation you'll need a 10W resistor which is large and will require heatsinking.

For the efficiency of the supply, the model may be able to predict what it will be, but there are so many variables, I could only guess. Plus, efficiency will vary with load making it even tougher to predict. 80 to 90% is a realistic range.
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Old 18th June 2012, 02:07 PM   #33
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And which model of FET do you recommend to use? I'm thinking about IRF130 or IRF150.
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Old 18th June 2012, 02:51 PM   #34
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I believe you need to look for something that tolerates more current than those...
You will blow those quite quickly i expect...

Look for ultra low on-resistance and low gate impedance.
Example: IRF3703
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Old 19th June 2012, 01:21 AM   #35
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You can use some of the ultra-low on-resistance, super high current devices, but they're expensive. Plus, using a single device-per-phase tends to decrease reliability and surge tolerance. In a low voltage, high power SMPS, the average current running the supply is just a fraction of the instantaneous surge-current experienced each time that FET switches on. That peak current can be many times higher than the average current. So a 200A device seems like overkill, it really is just adequate.

The IRF3703 looks like a great device, but is only rated 30V VBRDSS. With a push-pull SMPS topology, when one phase is conducting, the other device experiences 2*Vin, plus some noise spikes. Since a car's electrical system can run at 14.4V, 30V can be easily be across the off-device. I recommend 50V VBRDSS devices at a minimum.

What's done in industry, and I did in mine, is to use multiple devices paralleled so that each device is only switching a portion of the load. I used 3 X IRFZ42 FET's per transformer phase.

But ultimately it's up to you. You know your budget, plus you know, or will have to figure out, your PCB, the mounting technique of the devices and rectifiers, and the chassis your project will require. These criteria can also impact your device choice as the project advances.
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Old 19th June 2012, 01:53 AM   #36
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And why not to use three IRF2805 per transformer phase? this FET also have a low on resistance.
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Old 19th June 2012, 05:44 AM   #37
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Yep. The IRF2805 looks ideal! And with three-per-phase, they should run cool even at full power continuously.

Have you identified the secondary, fast-recovery diodes yet?

Have you reconsidered running D3 in series with Vin?
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Old 19th June 2012, 08:01 AM   #38
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I agree on the IRF2805 - a good choise. I myself could even try IRF2804...

I just finished a car SMPS that could interest you.
440W 99% efficiency.
PCB is only 6x15cm (2.4x6").

This might be a bit complex from the electronics point of view - it is full bridge dual core (E30).
But the transformers are therefore crazy simple.
Just one primary and one secondary. Which makes the utilisation of them ridiculously high, which results in a very high efficiency.

Click the image to open in full size.
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Old 19th June 2012, 02:10 PM   #39
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99% efficiency?! OMG. Did you buy the transformers or you made it yourself?
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Old 19th June 2012, 02:46 PM   #40
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Take it easy, 99% is a wet dream I suppose
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