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Old 30th December 2012, 04:31 PM   #21
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I will have to build up a new pcb with a split bobbin transformer.
This will also allow me to fit an irs27952 which has a 3 volt over voltage cut out.
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Old 30th December 2012, 05:03 PM   #22
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Quote:
Originally Posted by nigelwright7557 View Post
I will have to build up a new pcb with a split bobbin transformer.
This will also allow me to fit an irs27952 which has a 3 volt over voltage cut out.
Good luck!
But first, plan and design the values!
I will help you if you are stuck.
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Old 30th December 2012, 05:49 PM   #23
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Quote:
Originally Posted by lorylaci View Post
Good luck!
But first, plan and design the values!
I will help you if you are stuck.
The transformer windings seem to be the first decision.

I have used 36 turns primary with two 8 turn secondaries.
Is it ok to stay with this or do I need to change it ?
My resonant capacitor is currently 22nf.

What is the next step ?
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Old 30th December 2012, 05:53 PM   #24
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Quote:
Originally Posted by nigelwright7557 View Post
The transformer windings seem to be the first decision.

I have used 36 turns primary with two 8 turn secondaries.
Is it ok to stay with this or do I need to change it ?
My resonant capacitor is currently 22nf.

What is the next step ?
What is the input voltage? Is there a PFC? What is the output voltage?

Output turn ratio is the first deicision not the turns.
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Old 30th December 2012, 05:59 PM   #25
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Quote:
Originally Posted by lorylaci View Post
What is the input voltage? Is there a PFC? What is the output voltage?

Output turn ratio is the first deicision not the turns.
Input voltage is 320 VDC.
No PFC just common mode choke plus 2 100nf capacitors for a filter.

Turns ratio needs to be 4:1. 320/4 = 80 volts max.

I am looking to get +/- 40 volts DC out but it is variable from 30 to 50 volts via a pot in the feedback loop.

Last edited by nigelwright7557; 30th December 2012 at 06:02 PM.
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Old 30th December 2012, 06:03 PM   #26
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Quote:
Originally Posted by nigelwright7557 View Post
Input voltage is 320 VDC.
No PFC just common mode choke plus 1 100nf capacitors for a filter.

Turns ratio needs to be 4:1. 320/4 = 80 volts max.

I am looking to get +/- 40 volts DC out but it is variable from 30 to 50 volts via a pot in the feedback loop.
You have a 230V AC line in England. Then input voltage will be 290V - 330V
(you ahve to yount with high line conditions, and the depletion of the primary capacitors).

Irs27951 has to stabilzie output even at high input votlage so for 80V output voltage you will need at least n=4,125 turn ratio.

For 40V you need a turn ratio of 8,25.
You won't able to set it in such a range with a pot. It is not a PWM converter. (it can set in a range of +/- 10% depending of gain of the LLC circuit)
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Old 30th December 2012, 06:10 PM   #27
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Quote:
Originally Posted by lorylaci View Post
You have a 230V AC line in England. Then input voltage will be 290V - 330V
(you ahve to yount with high line conditions, and the depletion of the primary capacitors).

Irs27951 has to stabilzie output even at high input votlage so for 80V output voltage you will need at least n=4,125 turn ratio.

For 40V you need a turn ratio of 8,25.
You won't able to set it in such a range with a pot. It is not a PWM converter. (it can set in a range of +/- 10% depending of gain of the LLC circuit)
The pot is on the secondary TL431 circuit to set when the opto coupler output goes low.

If all I can get is a set voltage then 40 volts should be fine.
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Old 30th December 2012, 06:16 PM   #28
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Quote:
Originally Posted by nigelwright7557 View Post
The pot is on the secondary TL431 circuit to set when the opto coupler output goes low.

If all I can get is a set voltage then 40 volts should be fine.
I simply used a zener, and managed to get <5% voltage drop from no load to full load. With the pot you will be able to fine tune the circuit with the TL431, to get less than 2% voltage drop.
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Old 30th December 2012, 07:18 PM   #29
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I decided to go with the app note example transformer and split the bobbin as you suggested.

I got 1.5 mH primary inductance with 500uH leakage inductance.
This is way higher than the app note got.
So I gapped the core and now get 768uH primary inductance and 500uH leakage inductance.
The circuit still wont run.
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Old 30th December 2012, 08:31 PM   #30
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Quote:
Originally Posted by nigelwright7557 View Post
I decided to go with the app note example transformer and split the bobbin as you suggested.

I got 1.5 mH primary inductance with 500uH leakage inductance.
This is way higher than the app note got.
So I gapped the core and now get 768uH primary inductance and 500uH leakage inductance.
The circuit still wont run.
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.

For the split bobbin there is an approximation for the leakage inductance. Measere the winding width of there half of the bobbin. For ETD34 the full width is 20,9mm, half of the bobbin width the split is about 9mm. Calculate the inductance with this gap. AL=(gap^K2)*K1 so AL=32nH for 9mm gap, so leakage inductance will be about 42uH.
This is only an approximation, and only stands for split bobbins.
So there must be an error with your measurements.
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Last edited by lorylaci; 30th December 2012 at 08:52 PM.
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