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Old 6th October 2005, 12:13 PM   #1
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Please help.

Can you use a half bridge or full bridge converter for +12V to +/- 63V PSU?

I have been working on a 1000W PSU for 2 amplifier modules using a Push-Pull converter. I just cannot seem to get pass 750W out of the converter without the output voltage falling to +/- 54V.
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Old 6th October 2005, 04:03 PM   #2
subwo1 is offline subwo1  United States
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Archimedes, there is no inherent power limitation with either of those two topologies, but practicality may be a factor. Maybe if you open a new thread after you believe the search function cannot lead you to an answer, someone may be able to give a more detailed response.
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Old 7th October 2005, 06:26 AM   #3
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Sorry Subwo1 I should have stated the case more clearly.

The topology that is giving me headaches is a CT Push pull converter not a half or full bridge. I have been doing some reading and many people claim that the CT Push Pull is troublesome because of symmetry issues.

I would like to know though if I could use a Full Bridge for a converter that is powered by a car battery. My problem is that I have 2 x 276Wrms amplifier modules that require + and - 63 V at 3.76A per side (1 module) x 2 modules. If I test the converter on 1 module only the output voltages hold up very well. The moment I introduce the second amp the output voltage falls to about + and - 54V and thus I am only able to get out around 200Wrms/ch into a 4R load.

I am using 6 Mosfets per side and full wave rectifier for the output consisting of Fast recovery TO220 diodes.

Any help or directions is most appreciated.
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Old 7th October 2005, 03:18 PM   #4
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I think this is pretty off-topic here.

Please, could any moderator move Archimedes' posts into a new "12V push pull SMPS" thread?

Thanks.
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Old 9th October 2005, 03:25 PM   #5
subwo1 is offline subwo1  United States
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Archimedes, here is an idea. On the push-pull circuit, it is possible that your transformer may be starting to saturate. More transformer core volume may help. If you have a scope, look for a sharp rise in MOSFET current at the end of the duty cycle.

It may help to raise the frequency some if you are getting saturation. Then, you will probably have to reduce the number of turns on both the primary and secondary. ?
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Old 9th October 2005, 03:49 PM   #6
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Hello Archimedes,
It is very difficult to diagnose a problem without any real data on what is going on.
There have been some good recommendations so far. It is possible that your core is the wrong size for the frequency and power level you want to use.
It is also possible that your windings and source have too high resistance.
What do you have for capacitors on the center tap of the primary side?
What frequency are you running at?
Do you have any pictures or schematics to show us?
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Old 9th October 2005, 04:25 PM   #7
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Thanks to the moderator for moving this.

The trafo is a Siemens EPCOS toroid R58,3 40,8 17,6 (Ae=152.4 sq.mm, Ve=23230 cu.mm) with N87 material. It has Pri:2+2 (8 x 1.61mm wire per primary)
and Sec:14+14 (40 x 0.28 wires twisted per secondary) running at 43kHz. There are 5 (not 6 as previously mentioned) IRFZ44's per switching side. These are driven by a complimentary follower pair made from BD139 and BD140 per side. The chip is an SG3525. I have 4 x 4700uF caps near the centre tap. There is a 22uH choke on the input from the battery + to the caps +.

When I tested the psu I noticed that it had pulled almost 110A at 10.6V battery input measured with a clamp.
The voltages on the secondary of the converter was then + and - 54V at +/- 3A per side. Looking at the waveform on a scope the pk.pk value measured at any drain had fallen to 21V and the waveform was completely square (Full on).

Is the transformer to small ? Just curious Has anyone ever built a Full bridge converter powered by a car battery. Is it really impractical to do so ?

Phew... this looks like a thesis :-)
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Old 9th October 2005, 04:27 PM   #8
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If you post the following information, it may be helpful in troubleshooting the supply.

Core data (link to datasheet showing core material and dimensions would be very helpful).

Number of primary turns (for example, 4+4 or 5+5).

Number of secondary turns.

The amount of copper on the primary and secondary (number of conductors and size of conductors for each turn).

The operating frequency.

The part numbers of the FETs and the rectifier diodes.

A schematic.
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Old 9th October 2005, 04:36 PM   #9
subwo1 is offline subwo1  United States
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Your input power being so much greater than output appears to provide assurance that once you locate the bottleneck, whether core saturation or something, the supply should work as you wish.
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Old 9th October 2005, 06:38 PM   #10
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4 x BYW29-200 used as full wave rectifier. There are 2 x 470uF/100v 105 deg low esr caps across the outputs of the diodes to the input of the 47uH chokes. This is then smoothed by 1 x 2200uF/100v cap per side at the output of the chokes.

Voltage feedback is provided by ILD55 and BC547.
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