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Old 30th December 2012, 08:45 PM   #31
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Quote:
Originally Posted by lorylaci View Post
These values are unrealistic. What do you measure your inductance with?
How do you short the secondaries? (you have to short all of them)

After you designed the resonant elements, you need to design the controll elements.
I have an LC meter.
Yes I shorted both secondaries at the same time.
I am using an ETD54 transformer so perhaps the extra size is giving me extra inductance ?

I put together another pcb.
Just my luck it wouldn't work at all, not even on low DC volts.

So I removed the 27951 and put one in a SOIC to dip converter and soldered that into the pcb, and it burst into life.

I can only guess I fried or static discharged the previous 27951.

I have 27952 in the circuit now and that works although the transformer buzzes a little.

I just need now to take a step back and ensure the transformer and series capacitor are correct.
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Old 30th December 2012, 09:03 PM   #32
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Quote:
Originally Posted by nigelwright7557 View Post
I have an LC meter.
Yes I shorted both secondaries at the same time.
I am using an ETD54 transformer so perhaps the extra size is giving me extra inductance ?

I put together another pcb.
Just my luck it wouldn't work at all, not even on low DC volts.

So I removed the 27951 and put one in a SOIC to dip converter and soldered that into the pcb, and it burst into life.

I can only guess I fried or static discharged the previous 27951.

I have 27952 in the circuit now and that works although the transformer buzzes a little.

I just need now to take a step back and ensure the transformer and series capacitor are correct.
An ETD54?????? That's for >10kW thing. I made >400W from an ETD29.
According to my approxiamtion your leakage should be about 100uH. 500uH can only be if you wind it really ugly.
500 uH is extremely large, there is something wrong with your transformer. Could you make a picture of it? I reccomend getting the error out first, since this is a hazrdous thing. Also you will not get regoulation without the proper setting of the resonant elements, and ALSO you can easily get ZCS or hard-switching without knowing it, and can fry your FEts (and controller also). ZVS switching canonly be set with the contorl elements, when you have set the resonant elements.
The transformer buzz is beacuase of the magnetizing current and the ugly winding.
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Old 30th December 2012, 09:23 PM   #33
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I was hoping to get 800 watts hence the large transformer.
It looks like there is one winding but there is the primary about 2/3 across the width and 2 secondaries in the last third.

Can I not use the transformer as is but change the resonant C to match ?

Click the image to open in full size.
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Last edited by nigelwright7557; 30th December 2012 at 09:33 PM.
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Old 30th December 2012, 09:40 PM   #34
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Quote:
Originally Posted by nigelwright7557 View Post
I was hoping to get 800 watts hence the large transformer.
It looks like there is one winding but there is the primary about 2/3 across the width and 2 secondaries in the last third.

Click the image to open in full size.
OMG, don't use that transformer. there is one hazardous errror and other errors.

- You did not use any insulation. Put the bobbin splitter at the middle of the bobbin, and use some plastic or paper as a splitter. Glue it with epoxy resin or other glue with good insualting properties.-
- After winding the core put some good insulation on the winding. Best is 3M yellow tapes, Kapton (by DuPont).

Without this insulation your transformer is LETHAL. The polyurethane insulation on the wire is only stated up to 500V. You need to get insulation over 2kV if you ground the secondary and the projected bos, otherwise at least 3kV insulation is needed.

- You used thick wire. Due to skin depth, this thick wire will only conduct at 10% or less of its area. Calculate the skin depth, use lots of thin wire wounded togethet 8strnaded wire). I usually use 10s of 0,2mm wire twisted together.

If you want to get over 800W you need to complete redesign the resonant circuit, the FETs and the controll elements.
What is the material of your core. For example an EPCOS ETD54 with N87 is stated for about 4 kW at 100kHz, and an N97 is stated for about 5kW at 100 kHz.

Your FETs can provide up to 300W.
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Old 30th December 2012, 09:48 PM   #35
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I just quickly knocked up a transformer to try it, that's why it is on flying leads so I can rebuild the transformer as necessary.
My previous transformer did have tape on it.

I will now go back and work through app note.
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Old 30th December 2012, 10:00 PM   #36
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Quote:
Originally Posted by nigelwright7557 View Post
I just quickly knocked up a transformer to try it, that's why it is on flying leads so I can rebuild the transformer as necessary.
My previous transformer did have tape on it.

I will now go back and work through app note.

Read the application note carefully, Nigel, everything is in it. All I know is from IT (okay, and from the UCC25600 apllication note and the TEA1713 application note). Try to understand the operation.

Feel free to ask questions, I will help.

Since its a converter working on line voltage, I reccomend you to take extra caution. I don't want you to have an injury or other.
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Old 31st December 2012, 12:13 AM   #37
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Rather than mess around with the transformer I have scaled Cr to give the correct resonant frequency.
I am now getting +/- 45 volts.
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Old 31st December 2012, 07:02 AM   #38
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Quote:
Originally Posted by nigelwright7557 View Post
Rather than mess around with the transformer I have scaled Cr to give the correct resonant frequency.
I am now getting +/- 45 volts.
What Cr value did you put in instead of 22nF?
In this way you cannot know if there is ZVS switching (unless you measure the gate signal with a signal at different load conditions).
Also load regulation will be bad.
Make a spreadsheet, calculate your conditions.
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Old 31st December 2012, 09:56 AM   #39
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what happens if this converter goes into overcurrent state, does the voltage.on the half bridge caps start tilting, ie one goes up to rail and.the other to zero. I can imagine that could lead to catastrophic failure and loss of magic smoke.
(annhilating primary silicon
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Old 31st December 2012, 10:12 AM   #40
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Originally Posted by rikkitikkitavi View Post
what happens if this converter goes into overcurrent state, does the voltage.on the half bridge caps start tilting, ie one goes up to rail and.the other to zero. I can imagine that could lead to catastrophic failure and loss of magic smoke.
(annhilating primary silicon
If the resonant network is properly designed, the IRS2795x overcurrnet protection will shut down the converter.
At a case of secondary short (after secodnary capacitors), the curent in the primary starts to increase. The speed of increase is slwoed down by the resonant network. Usually withon 2-3 cycles the urrent on the lower FET increases to, where its votlage drop is over 2V (or 3V for IRS27952), then the IC stops switching.

Just for curiosity: when the switching is stopped, the current in the resonant network circuilates (between the transofmer, resonant caps and FETs Coss) until the resistive (and other) losses causes it to go to about zero.

There are other overcurrent options (using diodes on slplit resonant capacitors, monitoring the resonant capacitor votlage, using the shunt) and the LLC is very easy to make short-circuit proof, due to the LLC network (it limits current ramping).

All of my supplies were tested to be short circuit nad overload proof. I also design them, taht at max contionous load, it shunts down when the FETs heat up to about 80°C.
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