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Built a 250W SMPS, possible to modify?
Built a 250W SMPS, possible to modify?
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Old 2nd January 2007, 12:00 PM   #21
ifrythings is offline ifrythings  Canada
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I know this isn't quite the same thing but the horizontal circuit of a TV requires a very fast turn on but the turn off isn't as important. That why I'm a bit unsure of what is required for a fet.

I've been reading SWITCHMODE™ Power Supply Reference Manual (SMPSRM-D.pdf) by on-semi, hence why I have implemted that design into my schematic. I will put the circuit you posted earlier in my schematic so when build I don't have from toasted fets.

Is it required to have a cap in series in the secondary of the drive transfomer?

Agin, I'm still learning here so all your paciance and advice is extremly appriciated.

P.S: Hey Eva could you post or re-serve those pictures?

Thanks

Dave.
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Old 2nd January 2007, 12:10 PM   #22
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
Don't mix tv with supply, coz they have nothing in common .

I don't think the cap is needed there because trafo can't produce DC in case of in primary side of gate trafo failure.
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Old 2nd January 2007, 01:44 PM   #23
simingx is offline simingx  Singapore
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Let's not go off-topic...

I put the current sense transformer in series with the center point of the capacitors and the main transformer. On overload it limits the current (but doesn't shutdown? I haven't really tried overloading it THAT much).

However, if I short the output and THEN power it on, it (correctly) does not start... what it doesn't survive is a short AFTER power on...
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Old 2nd January 2007, 01:54 PM   #24
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
That is very strange. Some thing must be happening that shouldn't be.Did you look for any error with scope during increasing overload?
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Old 2nd January 2007, 02:18 PM   #25
simingx is offline simingx  Singapore
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Here're some shots of the gate drive waveform... directly on the primary (HV) side of the driver transformer. Note! Don't do this unless your scope has isolated inputs!

The spike looks worrying... what could be causing it? The circuit seems to be limiting the maximum duty cycle (the voltage starts to drop on overload) but it doesn't shut off.

1) No load
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Old 2nd January 2007, 02:21 PM   #26
simingx is offline simingx  Singapore
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2) Medium load (3x 12V 50W halogen bulbs)
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Old 2nd January 2007, 02:22 PM   #27
simingx is offline simingx  Singapore
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3) Overload (6x 12V 50W halogen bulbs)

Of course I could just make sure I don't short circuit the output...
Also what could be causing the little "ripple" during the off time? It's not causing any stray turn-ons of the FETs though...
I might use a proper RM core for the next project...
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Old 2nd January 2007, 02:27 PM   #28
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
With no load pic look great. But as for othe two .You will need some snubber for those big spikes and that 2v ripple to minimize it as much as possible.Did you put that gate circuit to use yet?

The ripple can come from badly winded trafo

If you have to have over current, but this won't work there are allway overrated fuses that you can use.
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Old 2nd January 2007, 02:32 PM   #29
simingx is offline simingx  Singapore
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Gate circuit? Please elaborate...

I might incorporate the snubbers from here...

The 'scope (Tektronix THS720) is going back to the office tomorrow however... it's isolated inputs are making me want to buy one

(product plug) Everyone who is even remotely serious about doing such projects should have one of these... (end product plug)
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Old 2nd January 2007, 02:42 PM   #30
luka is offline luka  Slovenia
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Built a 250W SMPS, possible to modify?
Gate circuit from post#18

You will have to see if thoose will work for you, but try gate circuit first if you have components available.

Scopes like that are great but if they wouldn't cost as much
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