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Old 13th October 2012, 02:06 AM   #1
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Default SMPS power up problems.

I have taken a L6565 smps design and altered it to be driven by a PIC.
The PIC fires the mosfet driving the transformer and also switches off the mosfet if an over current event occurrs.

I am finding the overcurrent event happens at the very first mosfet pulse at power up. So the PIC turns off the mosfet and flashes the error LED.

The circuit should work fine.
I have tried ignoring the over current event but this just blows the fuse.

The only part that might not be right is the transformer.
I have wound on 50 turns on the primary and 13 on the secondary.
There wasnt enough room on the transformer to wind on 50 turns horizontally so I put on 25 turns then went back along the previous 25 turns with the final 25 turns. I thought this would be OK.
The secondary is simply 13 turns wound horizontally.

Any clues as to why the over current event occurrs would be helpful.
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Old 13th October 2012, 03:42 AM   #2
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Did you gap the transformer? If not, it will saturate right away.
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Old 13th October 2012, 11:06 AM   #3
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Quote:
Originally Posted by wrenchone View Post
Did you gap the transformer? If not, it will saturate right away.
No I didnt.
Do you mean the ferrite cores need a gap ?
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Old 13th October 2012, 09:25 PM   #4
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I tried it with a gap tonight and stupidly just parted the cores slightly. Of course Iswitched it on and the two halves were attracted and blew the fuse and the mosfet. I have no mosfets spare so will have to order some more.

How do you normally cut down the cores ? Do you put something of 1mm between them or physically cut them down with a hack saw ?
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Old 13th October 2012, 10:19 PM   #5
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You need to do some background reading on how flyback SMPS work. I would stick with the basic L6565 circuit and get it working first before trying to do anything fancy with a PIC.Search the ST web site - they probably have a design spreadsheet somewhere that will allow you to design a proper transformer.
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Old 14th October 2012, 02:00 AM   #6
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I took the transformer from a datasheet example.

I found out tonight I have phased the transformer wrong.
The output is taking current as the transformer primary charges up.
It is supposed to take current after the mosfet switches off.
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Old 14th October 2012, 02:26 AM   #7
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And it still needs a gap, too -not just any gap. Do you have any spec goals for this supply? Post them, (AC input voltage range, Vout, Iout, proposed operating frequency),along with the core you're using, and maybe I can help some.

You should also realize that the L6565 is not your usual fixed-frequency PWM controller. Since it's quasi-resonant, it operates in variable frequency borderline continuous mode, kinda like a self-oscillating RCC flyback with some extra smarts. Your choice of primary inductance will affect the operating frequency.
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Old 14th October 2012, 01:29 PM   #8
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Quote:
Originally Posted by wrenchone View Post
And it still needs a gap, too -not just any gap. Do you have any spec goals for this supply? Post them, (AC input voltage range, Vout, Iout, proposed operating frequency),along with the core you're using, and maybe I can help some.
.

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.
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Old 14th October 2012, 04:57 PM   #9
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I have ordered a etd34 bobbin with 1mm gapped ferrites.
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Old 17th October 2012, 12:16 AM   #10
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I built up a new pcb today with the et34 gapped transformer.
The pic still kept getting an overcurrent event.
So I backed off the duty cycle and the PIC now doesnt get the over current event.
Thats as far as I got tonight, will spend some more time on it tomorrow.
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