IRS27951 / IRS27952

[nigelwright7557]

When it does power up I get 45 volts out but I cant alter it on my pot.

[nigelwright7557]

I tried the new mosfets and it sometimes powers up on its own.

I tried the 2 off 47nf trick but that wouldn't power up at all.

I will do what you said in regards using Cr as 47nF then alter timing resistors and see how I get on.

[lorylaci]

Originally Posted by nigelwright7557

I tried the new mosfets and it sometimes powers up on its own.

I tried the 2 off 47nf trick but that wouldn't power up at all.

I will do what you said in regards using Cr as 47nF then alter timing resistors and see how I get on.

If you can also borrow and other inductance meter (not a multimeter type). And measure again the leakage and magnetizing inductance of your transformer.
Previously I said I have doubts about those values. Some inductance meters tend to give false values with ferrite transformers (but good values with iron powder cores etc...).

[nigelwright7557]

I started again from scratch tonight from measuring inductance to redoing the spread sheet input values.

Due to my poor eyesight I had not connected one of the secondaries to the output and this made my inductance readings wrong.
The inductances are now 620uH and 60uH.

I noticed with the spreadsheet altering it didn't change Rmin which seemed stuck at 68k.
So I downloaded your spreadsheet again and input all the correct values. It now gives much smaller values of resistors. This would mean my previous values put the 27951 into ZCS mode seriously.

However I tried to power it up again and it has gone back to the previous problem of needing a lab power supply attached to start up.

[nigelwright7557]

I am using a ct_ertek of 390pf.
What is the Ct ? that is 133pf

Is it telling me to use 133pf for Ct ?

Just out of interest I put in a low value resistor for Rmin and the circuit powers up every time.
I guess you are right that I am not getting a value right somewhere.
My inductance meter does measure down to 22uH with an inductor correctly so I don't think that is the problem.

Perhaps I am missing something in the spreadsheet ? perhaps I have misunderstood it somewhere ?

[lorylaci]

Originally Posted by nigelwright7557

I am using a ct_ertek of 390pf.
What is the Ct ? that is 133pf

Is it telling me to use 133pf for Ct ?

Ct is the calculated timing capacitor.
The timing capacitor should be choose to set the proper dead-time.
Will in the FET datahseet values to the sheet "FETek", then you can choose it at cell B70.
I am sore there is some error, normal Ct values are between 330-470pF.

In Ct_ertek you should fill in the nearest (but larger) standard value.

[nigelwright7557]

Originally Posted by lorylaci

Ct is the calculated timing capacitor.
The timing capacitor should be choose to set the proper dead-time.
Will in the FET datahseet values to the sheet "FETek", then you can choose it at cell B70.
I am sore there is some error, normal Ct values are between 330-470pF.

In Ct_ertek you should fill in the nearest (but larger) standard value.

I am currently using 390pf.

[nigelwright7557]

With 600uH and 60uH and 47nF.
I get:
Fr1 = 95000
Fmin = 28575
Fmax = 190000

Does this sound in the ballpark ?

[lorylaci]

Originally Posted by nigelwright7557

With 600uH and 60uH and 47nF.
I get:
Fr1 = 95000
Fmin = 28575
Fmax = 190000

Does this sound in the ballpark ?

There is some error with Fmin.
I get 56kHz. The 28kHz is unreal.

[nigelwright7557]

[QUOTE=nigelwright7557;3329063
Just out of interest I put in a low value resistor for Rmin and the circuit powers up every time.[/QUOTE]

If I use 1k as Rmin the 27951 powers up every time.
This gives a frequency of 500,000Hz

Something is seriously wrong as it should run easily at frequencies below this.