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Old 18th October 2012, 12:59 AM   #21
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Quote:
Originally Posted by whizgeek View Post
post the current waveforms used to detect oc.
I cant, I dont have an isolation transformer. I can only look at the secondary safely.

I have got a bit further.
If I decrease the pulse width to the primary I find I can then decrease the gap between the pulses to almost meeting up.

I might be fooling myself a bit coz i am using a lamp in series with my setup to limit current.

Last edited by nigelwright7557; 18th October 2012 at 01:21 AM.
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Old 20th October 2012, 11:09 PM   #22
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Almost certain you are saturating the transformer.

thats a dual scope you have there.
Put a resistor in the return leg of the switching element, use probe each side of the resistor, differential mode on the scope
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Old 20th October 2012, 11:18 PM   #23
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Only one channel works, I got the scope very cheap.

Yes the transformer is definitely saturating hence the over current problems.
I am using an etd34 with 2mm gapped core.
If I output just a short pulse to the mosfet it works fine.
The output looks OK now after I snubbered the primary.
I have tested the output upto 500mA, I am waiting for high power resistors to arrive so I can test higher currents.
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Old 23rd October 2012, 12:25 AM   #24
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Quote:
Originally Posted by nigelwright7557 View Post
I am trying to get around 5 amps at 40 volts out.
Input is 240VAC mains. I am running the PIC at 50KHz with 50% duty cycle.
I am using an ETD54 with n97 core, 50 turns primary and 13 turns secondary.
I am now running the PIC mosfet driver at 500KHz so there is a 2uS pulse to the mosfet. Anything longer causes an overcurrent trip.
I am now using an ETD34 with a 2mm gapped core.
I am managing to get 2amps at 30 volts out at the moment, but I would like to get more.
The transformer is getting quite warm which is a bit of a worry but I have read SMPS transformers often do.
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Old 23rd October 2012, 09:47 PM   #25
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I found a fault in the datasheet circuit for the L6565.
It shows 100nf with a 220k in series in the tL431 circuit.
This causes problems with the feedback signal.
If I make the 220k 0r then the feedback circuit works fine.
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Old 27th October 2012, 09:06 PM   #26
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I added another small winding to the transformer to detect zero voltage crossing on the output so I know when to apply the next pulse to primary side of the transformer.
Seems to work very well.
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Old 27th October 2012, 10:50 PM   #27
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Got a bit further tonight.

I found I was getting an over current event because I was firing another primary pulse before the previous secondary discharge had finished.

I can now get full secondary voltage that I wanted at a decent current level.
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Old 27th October 2012, 11:07 PM   #28
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Good to hear you are getting there.
Re the datasheet; your application is substantially different, so values dont really apply.

What PIC are you using ?
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Old 27th October 2012, 11:11 PM   #29
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I am trying to emulate the L6565 with a PIC.
The PIC now does output pulses, zero voltage crossing detect and primary over current detect. I also detect voltage feedback from the secondary.

I am just using a PIC16f506 running at 20MHz.
There is very little code to do.
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Old 27th October 2012, 11:13 PM   #30
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Not that I know much about SMPS design (something i'd like to change at some point), but it seems like you've managed to solve the original problems. And even though there haven't been that many contributions you've still kept us informed with the process of how you figured it all out, which is rather helpful. What voltage/current will this thing now work at/provide?
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