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Old 3rd February 2013, 07:24 PM   #191
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I don't suppose the using of longish leads and crocodile clips could be causing problems? I can't imagine that this would be a great setup for higher frequencies and high currents. Maybe this could be causing stability issues or problems with the voltage regulation/feedback. Of course you've shown that the values expected for the transformer aren't quite right and it's most likely this, but have you tried it with shorter leads?
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Old 3rd February 2013, 07:56 PM   #192
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I don't suppose the using of longish leads and crocodile clips could be causing problems? I can't imagine that this would be a great setup for higher frequencies and high currents. Maybe this could be causing stability issues or problems with the voltage regulation/feedback. Of course you've shown that the values expected for the transformer aren't quite right and it's most likely this, but have you tried it with shorter leads?
The crocodile clips are just used to testing leakage/main inductance not when the circuit is live.

I had an ETD49 transformer so I put the same windings on that and get the same results as the etd54. I am only getting about 53uH leakage when I need at least a 100uH.
Gapping the transformer makes no difference to leakage inductance.
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Old 3rd February 2013, 08:02 PM   #193
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OK I found a problem.

My 47nf Cr turned out I had put in a 22nf !!!
This made my calculations out by a factor of 2.
Leakage inductance is now 137uH and main inductance is 862uH.

I found a 47nf in my box and tried it but it is still dropping out of regulation.
I adjusted the control components to match correct inductances.
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Old 4th February 2013, 12:35 AM   #194
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I had a play around with the Rmin resistor to see if I could get the circuit to stop dropping out even on low currents.

Rmin was originally 8k.
I found if I dropped this to 3k9 I could get a few amps out of the circuit.
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Old 4th February 2013, 12:57 AM   #195
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The crocodile clips are just used to testing leakage/main inductance not when the circuit is live.
Ah, that makes more sense.

I am enjoying this thread greatly though, learning to build a simple, but regulated SMPS is always something that I've wanted to do, but have never attempted. This type of supply appears to do and be just that and this thread has turned out to be a great resource towards anyone who is interested.

The bit that has always put me off is the part where you make your own transformer and for good reason too it seems, they are critical towards the success of the design and lori's input in this regard has, at least to me, been extremely helpful in helping to bring some clarity to the problem.
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Old 4th February 2013, 01:08 AM   #196
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Ah, that makes more sense.

The bit that has always put me off is the part where you make your own transformer and for good reason too it seems, they are critical towards the success of the design and lori's input in this regard has, at least to me, been extremely helpful in helping to bring some clarity to the problem.
I have made about 6 transformers now.
The first I had the secondaries wound on top of the primary which is wrong if you want decent leakage inductance. A split bobbin is the way to go.

I was completely thrown by a cheap LC meter I have giving nonsense readings. It told me I had 10uH leakage when I had 100uH.
I found the best way to measure it was to apply through a resistor a sweeped waveform looking for the low point. I then measured the frequency on the scope and could work out the inductance from that.
L=1/ 4 PI PI F F C

I was advised to wind the 2 secondaries bifilar to keep the inductances the same.

The maths is quite hard going in the app notes.
lorylaci gave me a spreadsheet to make it much easier.

For some reason I have had terrible troubles with the 27952 SMD device. I have blown up a few now. I am not sure if the yare being cooked or it is a static problem.
I ma finding only 1 in 3 work which is a nuisance.
I bought a smd heat gun which I found just overcooked the 27952's but it has come in handy for removing them from the pcb.
I try to solder the legs as quickly as I can to reduce thermal shock.
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Old 4th February 2013, 07:37 PM   #197
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I have got a bit further tonight.

I found reducing Rmin stopped the transformer buzzing.
But I am still not getting anywhere near 10 amps, I get 3 and thats with the output voltage sagging.
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Old 5th February 2013, 05:13 PM   #198
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Originally Posted by nigelwright7557 View Post
I have got a bit further tonight.

I found reducing Rmin stopped the transformer buzzing.
But I am still not getting anywhere near 10 amps, I get 3 and thats with the output voltage sagging.
How much is the pirmary votlage when you load the secondary?
If your primary sinks to much, then regalutaion will fail.

Also you must keep in mind, that reducing the Rmin will increase your minimum frequency, and so regaulation will also fail.

The buzzing of the transformer is simply caused by the forces between the wires (F=IxB). If you wind the wires tighther, it will buzz less.
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Old 5th February 2013, 08:40 PM   #199
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How much is the pirmary votlage when you load the secondary?
If your primary sinks to much, then regalutaion will fail.

Also you must keep in mind, that reducing the Rmin will increase your minimum frequency, and so regaulation will also fail.

The buzzing of the transformer is simply caused by the forces between the wires (F=IxB). If you wind the wires tighther, it will buzz less.
I am getting 320 volts on the primary side.
I added another capacitor but it made no difference. I have 440uf total.

If Rmin gets too large then the transformer starts to buzz and then the over current on the 27952 kicks in. I guess it is going into ZCS mode, taking too much current and tripping out.

I have juggled Rmin the best I can and I get 5amps max now.

I shorted out my thermistor and common mode choke in case they were dropping volts but it made no difference.
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Old 5th February 2013, 10:58 PM   #200
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I checked the spreadsheet against all my components and I found my transformer had more turns than suggested by the spreadsheet.

I adjusted the transformer and now I am getting 6 amps per secondary.

The transformer no longer buzzes.

I adjusted the control components for the new leakage inductance.

It powers up mostly now but still sometimes the 27952 resets on power up.

I just need to decrease Rmin a little to stop the resetting.
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Last edited by nigelwright7557; 5th February 2013 at 11:10 PM.
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