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Help with off-line smps

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uzzors2k

Mon Aug 14 2006, 11:33AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I replaced the bridge and I am glad to say the PSU works excellently. ( with only two mosfet deaths!

) I'm using a PC smps xfrmr, and I got 25volts on th 12v line, which is exactly what I got from the xfrmr while it was in the original PSU (with volt mods). I haven't stress tested it, but I think it can source enough power for now. The only limit is the transformer which came from a 250 watt supply.

Just one more newb question, should I put heatsinks on the IRFP450s? I don't dare touch them while the PSU is running. They can handle 14 amps, and the bridge rectifier is rated for 2 amps, so I think they will be fine. I'm not going to run the supply for more than 10 minutes at a time anyway.

Thanks for all the help.

ragnar

Mon Aug 14 2006, 11:48AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

A couple of clip-on heatsinks seriously wouldn't go amiss. C'mon, think how easy it would be to just grease up a mica pad and drill some metal.

If in doubt, run it for a couple of seconds. Unplug, discharge, feel temp. Then try the same for a couple of minutes and see if you're comfortable without heatsinks

uzzors2k

Mon Aug 14 2006, 12:06PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I guess some heatsinks won't hurt, I'll see what I can find.

Should I ever run the PSU without a load? Because I was going to temperature test just now, but the output hit 50 volts and my 10 ohm load resistor started smoking in seconds. If I need a load what is sufficient? A 20 watt load?

Marko

Mon Aug 14 2006, 01:25PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Good work uzzors

You can add regulation later if you come into need of a specific voltage..

Steve Ward

Mon Aug 14 2006, 03:35PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

You have discovered the need for feedback to provide voltage regulation!

Fet and diode ratings (for packages that *can* be mounted to a heatsink) are rated for being properly heatinked. Your fet, if it were running 14A would be dissipating many 10's of watts, and would melt itself without a heatsink. Some of my really low power SMPS stuff, i have gone without a heatsink, but you mention 250W, which sounds like enough to warrant at least a small heatsink for the mosfets and maybe the bridge rectifier. Most computer power supplies use a dinky heatsink, but remember they have a pretty good forced air system as well, greatly increasing the dissipation ability of the heatsinks.

uzzors2k

Sat Aug 19 2006, 07:14PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Thanks Firkragg.

I've been checking out just about every smps project on the net, so I think I can get the voltage regulation working fine. The problem is voltage regulation relies on modifying the duty cycle, something I've been having problems with.

The problem is the GDT, because input signals are perfect, but outputs are completely different. The GDT is wound 1:1:1 with 45 turns per winding. Here are the waves, 1st one is at 50% duty cycle while the other is near absolute minimum. I've marked the rise and fall of each wave since it might be confusing if you aren't actually watching it change. The phase also seems to shift about mid-duty cycle which makes things even stranger.(entire wave jumps a box(5Âµs) ahead, just check the pictures) At maximum duty cycle the waveform is perfect however.

I hope someone knows how to fix this because I've spent all week at it.

Steve Conner

Sat Aug 19 2006, 07:55PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I can see a few things that might be wrong.

1) The wiring between the TL494's output pins and the transistors that drive the GDT is wrong on your schematic.
Pin 8 should connect to the base of the top-left transistor
Pin 9 to the base of the bottom-right one
Pin 10 to top-right
Pin 11 to botom-left

2) The transistors should probably have antiparallel diodes across their collector and emitter to allow a path for the GDT magnetizing current.

3) The driver has a design flaw. It turns all four driver transistors off during deadtime, leaving the GDT primary open-circuit. You're supposed to short the primary during deadtime in order to slam all the MOSFET gates quickly to zero volts.

By the look of your screenshots, I assume you already found 1) and 2) and fixed them, but 3) explains why you get the weird ringing and reversals, as the GDT's magnetizing current tries to find a way out during deadtime. This needs a redesign of the driver circuit to fix it properly, though as a band-aid you could maybe just connect a large 22 ohm resistor across the GDT primary to damp the weirdness (and increase your gate driver power consumption hugely too)

uzzors2k

Sat Aug 19 2006, 09:15PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Thanks for the quick reply. This is the first time I'm using a tl494 so the tl494 schematic is something I got from someone else. Infact I haven't fixed anything on it, besides adding a 360 ohm resistor in series with the GDT. Without it the ouput is completely distorted.

I know enough about the tl494 now though, so I'll make my own driver. Hopefully with more success.

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