I ran my signal generator into the LC network and I get a huge peak roughly around 23KHz. I tried to find the upper resonant frequency but if it is there it is small compared to the lower resonant frequency.
I designed a USB LC meter a couple of years back and I built one up to check my readings compared to my LC hand meter. The USB meter says it is 625uH and 10uH. I don't believe its 10uH I believe the 125uH open circuit reading on my handheld meter. Even if it was only 10uH it isn't the main resonant frequency used by the system.
So it looks like the resonant circuit is ok.
I tried 1000pf as the dead-time capacitor and there was no joy there, I obviously altered the Rmin and Rmax to suit.
The softstart should dictate that high frequency when starting up.
Your Rmin values is bad, as i pointed out previously (Fmin is not the lower resonant frequency), I get different values fromt he spreasheet also.
No, no, no Nigel. You are messing things up again.
With no load, on a bode plot you will see the lower resonant frequency.
Now: short the outputs of the transofrmer. Then you will see the upper (MAIN) resonant frequency on the bode plot. Try this experiment!
Also it will check your LC meter.
Do your transformer wound on split bobbin? (Or do your priamry and secondary wound on the oppsite side of the bobbin?).
Dead time is (normally) not related to the max current. Dead-time should be set to get ZVS with your FETs Coss. If you do not understand what ZVS means, do not worry. I can tell you.
For some reason my pcb has stopped working, there is no HO and LO outputs.
So I tried another pcb and cant get any 27952's to run. I tried 3 and none worked.
Something is seriously going wrong with my soldering. I am using a heat gun but I think it is blowing up the 27952's.
I think I have spent too much time chasing something that is not going to work for me.
I am going to see if I can find a through hole IC that I can work with instead.
So I tried another pcb and cant get any 27952's to run. I tried 3 and none worked.
Something is seriously going wrong with my soldering. I am using a heat gun but I think it is blowing up the 27952's.
I think I have spent too much time chasing something that is not going to work for me.
I am going to see if I can find a through hole IC that I can work with instead.
SOIC8 not thermally enhanced aren't hard to solder and I'd be surprised if you were cooking them, unless you were continuing to apply heat for a long time after the solder had started to melt. If your eyes are bad just use a nice magnifier. I've got good eyes but msop parts are hard for anyone to see. I solder them somewhat blind and then use a magnifier with solder wick/fine tip/solder and the continuity sensor to check things are soldered properly.
The easiest method to solder a soic8 part would be to try soldering each pin manually with an iron and not caring if you solder two legs together. Then use solder wick to remove any excess solder.
The magnifier is essential in allowing me to see what needs to be seen with those smaller parts.
The easiest method to solder a soic8 part would be to try soldering each pin manually with an iron and not caring if you solder two legs together. Then use solder wick to remove any excess solder.
The magnifier is essential in allowing me to see what needs to be seen with those smaller parts.
Nigel, search youtube for soldering video tips. 90 seconds is "90 seconds of death" for the ICs.
I use simple temperature controlled soldering iron, and I usually use excess solder. If it is too much, I just use a coopper strand (image below) to remove excess.
An externally hosted image should be here but it was not working when we last tested it.
It is pretty easy.
It is so sorry to hear you give up. If you are interested, I can send you a presoldered controller board by post
(pictured a few post earlier)From the Hungarian forum so far two guys built LLC converters after my article. One of them was quite successful, now he is on a Class-D project. None of them complained about SMD soldering.
As you could figure out, LLC converter is my favourite magical thing. From my first design (weel, actually the third prototype of that design), tahat is on that article I was able to pump out 400W cont and 500W peak from 12x8x4 cm.
I have already done the class d thing. It took a couple of revisions of circuit and pcb to get there but I have a design that does 250 watts to 750 watts.
I bought some more 27952's.
I soldered in the first and checked my circuit for shorts between pins.
It had a short between pins 7 + 8 so I took the chip back out and there was no short on the pcb but pins 7/8 were shorted on the 27952.
So I soldered another one in and this one powers up on low volts, with a nice square wave on the output.
I tried to power it up off mains but I just get a brief pulse on the output then it stops and discharges.
Any suggestions where to go to next ?
I currently have:
Resonant frequency = 95k
Max switching frequency = 190k
soft start frequency = 400k
Rmin = 10k
Rmax = 2k2
Cr = 1000pf
Running off 30 volts the switching frequency is 55k
I soldered in the first and checked my circuit for shorts between pins.
It had a short between pins 7 + 8 so I took the chip back out and there was no short on the pcb but pins 7/8 were shorted on the 27952.
So I soldered another one in and this one powers up on low volts, with a nice square wave on the output.
I tried to power it up off mains but I just get a brief pulse on the output then it stops and discharges.
Any suggestions where to go to next ?
I currently have:
Resonant frequency = 95k
Max switching frequency = 190k
soft start frequency = 400k
Rmin = 10k
Rmax = 2k2
Cr = 1000pf
Running off 30 volts the switching frequency is 55k
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Nigel, You were talking about using a sigal generator to drive the LC network.
Could you do two measurements:
- Normal measurement, you did prevously (you have got a peak at 23kHz) (you don't need to repeat this, if you havent change anything in the LC network)
- A measurement with secondaries shorted. Short the secondaries, then run the signal generator. Check where is the peak!
I am waiting for your measurement!
With 47nF and my transformer secondary shorted I get a resonant frequency of 200KHz
With 47nF and secondaries open I get 28KHz.
THAT IS INTERESTING!
From 47nF and 200kHz (thomsone-equation) we get: 13,5uH
Your transformer according to these measurements has a leakage inductance of 13,5uH
From 47nF and 28kHz we get: 688uH
Your transfromer has a total primary inductance of 688uH.
I said previously that your inductance measurements can be bad. From this measurement we got a totally differnt leakage inductnce. Since this values is mslaler, your maximal currents are much larger (and this correlates with that your circuit don't start).
Now which measruement should be believe? 120uH or 13,5uH. Could you make a photo of the actual transformer? From the photo I can make an approximation, of which value couldbe true.
(of course the best is to do an other measurement, with an other inductance meter)
I couldn't get enough leakage inductance so I had to wind the secondary on top of the primary.
My handheld meter said 125uH and my USB inductance meter said 10uH, so it looks like my USB one is closer by miles after all. I didn't believe it at first.
I will rewind the transformer and see what the USB meter says.
My handheld meter said 125uH and my USB inductance meter said 10uH, so it looks like my USB one is closer by miles after all. I didn't believe it at first.
I will rewind the transformer and see what the USB meter says.
An externally hosted image should be here but it was not working when we last tested it.
What is the type of your handheld meter?
It seems that your USB inductance meter is more acurate measureing the leakage inductance. Of course using a signal generator is the best method.
Grats, Nigel.
Now it is time to fine-tune your circuit.
So you have 120uH leakage, 700uH total primary inductance and your resonant capacitor is 47nF.
What frequencies have set?
Could you make some scopepictures about the following:
- low side FET gate voltage (with no load - max frequency, and with about 50-100W load)
- half bridge output (drainof low side FGET) (with no load - max frequency, and with about 50-100W load)
With this pictures I can tell if the dead-time is good (enoguh, too much, too less).
My low frequency is 27,777Hz which gives 120uH
My Fr1 is 66,666Hz which gives 700uH
I used a large dead-time so I could be sure that wasn't the problem. I have 1000pf as the timing capacitor. 10k as Rmin and 2k2 as Rmax.
My inductance and capacitor are a bit off from the spread-sheet so there is still a bit of work to do yet.
My LC meter was £27 so isn't the best one.
The LC USB meter I made myself using a PIC micro and oscillator circuits based around comparators.
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I found 390pf caused the transformer to vibrate a little.
With 1000pf it is dead quiet.
I obviously recalculated Rmin and Rmax to suit from the spread sheet.
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