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Old 14th January 2013, 07:48 PM   #121
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Quote:
Originally Posted by nigelwright7557 View Post
I used a single 100nf 500 volt AC capacitor.

Do you mean put two 47nf in parallel ?
No, see the atached pictures.
Nigel_your is the single capacitor. Nigel_split is the split capacitor.
The error is because of the IRS27951 longer first impulse. At startup the middle of the split capacitor has voltage of half the primary supply voltage. So this cause less current transient at startup. It can solve the problem.

This all error is about the OC prot of the IRS27951. Its oversensitive
The datasheets recommends using FET with lower Rdson (it recommend that the calculated peak current times the Rdson should about 1V). The conduction lossws will be very small with this FET, so you can cahnge to IRS27952 if you want.
Attached Images
File Type: png Nigel_your.png (68.0 KB, 114 views)
File Type: png Nigel_split.png (71.4 KB, 111 views)
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Old 14th January 2013, 08:09 PM   #122
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I tried the circuit with a 9 volt batteries and it didn't work.

However, I tried it with my lab power supply and now I get 45 volts out.

I have used a 6666R resistor dropping the 320 VDC for the 27951 so it is getting 48mA.
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Old 14th January 2013, 08:13 PM   #123
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Quote:
Originally Posted by lorylaci View Post
This all error is about the OC prot of the IRS27951. Its oversensitive
The datasheets recommends using FET with lower Rdson (it recommend that the calculated peak current times the Rdson should about 1V). The conduction lossws will be very small with this FET, so you can cahnge to IRS27952 if you want.
I am already using 27952.
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Old 14th January 2013, 08:48 PM   #124
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Quote:
Originally Posted by nigelwright7557 View Post
I tried the circuit with a 9 volt batteries and it didn't work.
The 9V is below the turn on treshold of the IRS2795x.
So even with IRS27952 the OC protection turns on?
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Old 14th January 2013, 08:54 PM   #125
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Quote:
Originally Posted by lorylaci View Post
The 9V is below the turn on treshold of the IRS2795x.
So even with IRS27952 the OC protection turns on?
I put 2 batteries in series to get 18 volts.

I also tried with my bench power supply and now it works.

So the problem must be to do with how I power the 27952 or the power up sequence.
I use a 6666R resistor dropping the 320VDC.
This should give 48mA which should be more than enough to power the 27952.
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Old 14th January 2013, 09:06 PM   #126
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I thought I had connected 15 volts to the 27952 but I had connected it through a 1k resistor. So I tried with 20/25/30 volts but through a 1k resistor and it doesn't power up.

On power up with the 15 volts connected it is also getting some current from the 6666R
dropper resistor off 320VDC.
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Old 14th January 2013, 09:57 PM   #127
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If I connect 27951 to 15 volts through a 1k resistor this gives 12 volts at the 27951.
Then if I apply mains it powers up ok. At this point the 27951 is powered via the lab supply and via a dropper resistor from 340VDC.
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Old 14th January 2013, 10:21 PM   #128
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I have ordered some lower RDSon mosfets 0r22.
I have also ordered the 47nf capacitors to give that a try.

I think the circuit is just on the edge given the power supply bodge that got it running.
So hopefully the mosfets will cure it.
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Old 15th January 2013, 06:44 PM   #129
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Quote:
Originally Posted by nigelwright7557 View Post
I have ordered some lower RDSon mosfets 0r22.
I have also ordered the 47nf capacitors to give that a try.

I think the circuit is just on the edge given the power supply bodge that got it running.
So hopefully the mosfets will cure it.
I agree, the circuit at start is on its overcurrent limit.
Using 2x47nF as I drawn on schematic will decrease the current transient at startup. Also a lower Rdson FET will have less voltage drop.
Also you can cahnge the 100nF to a single 47nF, then recalculate values for a higher frequency etc... This will decrease power output,and also peak current.

I few pages earler I sent you a Hungarian link. In that I used STP12NM50FP (Rdson<0,35Ohm). The resonant limited peak current is 4,98A. With IRS27951's 2V protection it starts with 50ms softstart time and 500kHz soft start frequency.
However I use some trick. The charge of primary capacitors is slown down by an NTC. I use a relatively low startup restior (100kOhm 2W) from 320V, and the supply capactior of IRS27951 is only 1uF. This way, the IRS27951 starts up when the capacaitors are halfway charged (about 200V), so at startup the current trasients are also less.
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Old 15th January 2013, 09:10 PM   #130
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I put in a thermistor on my pcb (there was a place for it but I had shorted it out) and the board powers up sometimes.

I get the other lower Rdson mosfets tomorrow so will try them.
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