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Old 11th January 2013, 11:58 PM   #91
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Quote:
Originally Posted by nigelwright7557 View Post
I now have two resonant frequencies from 600uH and 685uH.
With a 10nF Cr this gives me 60800 and 65000.
I worked out the timing resistor values and now the circuit outputs 40 volts ok.
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Old 12th January 2013, 01:24 AM   #92
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I worked out the timing resistor values and now the circuit outputs 40 volts ok.
I have Rmax as 6k and Rt as 1k2.
On mimimum load the osc is running at 400KHz
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Old 12th January 2013, 01:59 AM   #93
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I powered up the circuit on low volts dc again and the LO output was wrong again.
So I made a star ground right at pin 4 on the 27951 and now I get a square wave of the correct frequency 60100.

The circuit doesn't power up on mains unless I increase the idle frequency.
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Old 12th January 2013, 07:52 AM   #94
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Quote:
Originally Posted by nigelwright7557 View Post
How did you get 13 for primary turns ?
So if I work out 33/13 * 250uH I get 636uH which sounds about right.
See the excel sheet I sent you previously. I use that to calculate turns.

Your MATH is BAD!
The inductance (and all impedance related parameter) is proportional to the SQUARE of the turns.

Your other math is bad. The two resonant frequencies are NOT 60,8kHz and 65kHz.The lower resonant frequency should be calculated from the resonant capacitor and the total primary inductance (that is 60,8kHz), HOWEVER the upper resonant frequency should be calculated from the leakage inductance and the resonant capacitance (it gets you 173kHz). The last one is the MAIN resonant frequency.

Also the timing resistors are not set to these frequencies, Nigel, have you really understood the application note?
First you must calcuate the magnetizing current and from that the needed dead time according to FET's equivalent Coss and gate charges. This gives you the timing capacitor. Then you must calculate the minimum frequency required to maintain output stabilization (its neither the one of the previous freqs), to set the minimum freq. Then you must set the max frequency. A good rule of thumb is to set it 1,5 times the main resonant freq. It is highly limited by the transformer parasitic capacitances.

Then you must set the softstart freq and time. I reccomend you a soft start time of at least 20ms, and the soft start freq should be at least 1,7 times the main resonant freq (the IRS has a reccomended max of 500kHz).
The circuit wont power up because the lack of soft start. At power up there is a short at secondary (due to secondary el. capacitors) and the IRS protection turns on. It has almost noting to do with star ground.

Please, please, please! Use the excel spreadsheet I sent you. It does the math for you.

Look at the ferrite material datasheets, if you do you will see that there is some problem with your inductance metering.
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Old 12th January 2013, 02:29 PM   #95
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Hello people,
through this thread I have become aware of the IRS2795x. I will also try to build a resonant switching power supply.
Do you know this German site:Dimensionierung von Schaltnetzteilen
It is for dimensioning of switching power supplies. You do not need speak German to use this site.
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Old 12th January 2013, 02:34 PM   #96
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Design of switch power supplies
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Old 12th January 2013, 02:36 PM   #97
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Nigel, please use the spreadsheet (thanks lorylaci, it is very useful to get it right.
Or atleast do it right on paper first.

and thoroughly read through teh app note.

Without softstart the circuit will never get running, because it will trip on overcurrent on the first few switchcycles. It only runs at low voltage because then the peak currents will never trip the overcurrent. Otherwise you would have had to replace primary silicon repeatedly .

regards.
Rickard

edit: I think that Prof Schmidt-Walters webpage has been up on discussions before, as there was some errors in the calculations or somehting like that but I can be misstake as my memory is very short and probably incorrect. However it is useful as a conceptual design base (ie test different topologies)
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Old 12th January 2013, 02:58 PM   #98
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Thank you andrewlebon
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Old 12th January 2013, 02:59 PM   #99
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Quote:
Originally Posted by moschfet View Post
Hello people,
through this thread I have become aware of the IRS2795x. I will also try to build a resonant switching power supply.
Do you know this German site:Dimensionierung von Schaltnetzteilen
It is for dimensioning of switching power supplies. You do not need speak German to use this site.
O know this site, the professor has some lecture notes to there. However there is no lecture note about LLC converters.

If you can understand the operation of an LLC converter, can understand the ZVS switching, then building a resonant supply is very easy.

Due to the ZVS resonant switching layout is less important, you do not need to get rid of spikes etc.

Quote:
Originally Posted by rikkitikkitavi View Post
spreadsheet (thanks lorylaci, it is very useful )
You're welcome! The spreadsheet has lot of extras, whcih are not in IRS application not, and sometimes I use different claculations.

For example the AN from IR calculates with the nominal Coss of the FET. Since MOSFET Coss is highly nonliner, it is reccomended to at least calculate an approximate linear equivalent ( Cosseq=sqrt(V_datasheet/V)*Coss_datasheet), or use the equivalent values from the datasheet: Coss_er, Coss_tr.

Also the application has an error calculating the MOSFET switching off losses. MOSFET switchoff losses are not really related to Coss, however it is related the current flowing before the switch off. So far I have not been able to find a good approximation with a simple equation.

The spreadsheet also calcluates core losses, whcih is very helpsome. You can put in ferrite data on a different sheet. Also MOSFET losses is made more punctual (Rdson mosfet die temperature rise dependence also calculated). You can get the needed heatsink info etc...

I always use spreadsheets wherever I can, because as you can see there are lot of variables, so it is good to see what changes what, this can be easiyl done in the spreadsheet.

I am only sorry that it is halfway translated, maybe later I finish the translation. (and also google translator is your friend till then)
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Old 12th January 2013, 08:33 PM   #100
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Originally Posted by lorylaci View Post
See the excel sheet I sent you previously. I use that to calculate turns.

Your MATH is BAD!
The inductance (and all impedance related parameter) is proportional to the SQUARE of the turns.

Your other math is bad. The two resonant frequencies are NOT 60,8kHz and 65kHz.The lower resonant frequency should be calculated from the resonant capacitor and the total primary inductance (that is 60,8kHz), HOWEVER the upper resonant frequency should be calculated from the leakage inductance and the resonant capacitance (it gets you 173kHz). The last one is the MAIN resonant frequency.

Also the timing resistors are not set to these frequencies, Nigel, have you really understood the application note?
First you must calcuate the magnetizing current and from that the needed dead time according to FET's equivalent Coss and gate charges. This gives you the timing capacitor. Then you must calculate the minimum frequency required to maintain output stabilization (its neither the one of the previous freqs), to set the minimum freq. Then you must set the max frequency. A good rule of thumb is to set it 1,5 times the main resonant freq. It is highly limited by the transformer parasitic capacitances.

Then you must set the softstart freq and time. I reccomend you a soft start time of at least 20ms, and the soft start freq should be at least 1,7 times the main resonant freq (the IRS has a reccomended max of 500kHz).
The circuit wont power up because the lack of soft start. At power up there is a short at secondary (due to secondary el. capacitors) and the IRS protection turns on. It has almost noting to do with star ground.

Please, please, please! Use the excel spreadsheet I sent you. It does the math for you.

Look at the ferrite material datasheets, if you do you will see that there is some problem with your inductance metering.
I have looked at the spreadsheet but all it does is input values.
I cant see where Rmax and Rmin etc are calculated.

Thanks for spotting the resonant frequency mistake.

I have used the control components from the app note so soft start should be fine. This is also why I set min resonant frequency to around 60000 to match the app note transformer.
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