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Old 3rd February 2012, 08:19 AM   #1
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Default SMPS Efficiency - bad...

I'm struggling with the bad efficiency I have managed to build in to a DC-DC full bridge SMPS. Vin=12V.

I am pretty sure I can not make any better transformer (ETD34, n1=5, n2=26). f=100kHz (actually adjustable between 15kHz and 200kHz...).

So I need to concentrate to the switches and diodes - or what?
Switches at the moment 4xIRFP2907.
Any recomendations to replace these?
Rectifier diodes at the output 4xSR5100.
Recomendations?
Panasonic Low-ESR capacitors everywhere, so can't go any better with those...?

Or why is my efficiency only about 70%????
Power is good, efficiency is not!
I need at least 80 to 85%...

It is a SG3525 + 2xIR2110 circuit...

Thanks for all suggentions!
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Old 3rd February 2012, 12:24 PM   #2
Romek is offline Romek  Poland
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Hi

For small voltages full bridge is bad choice, I think this is the problem.
Better for 12V is push pull.

Regards Roman
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Old 3rd February 2012, 02:59 PM   #3
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Absolutely.
This is a low power prototype.
Best choise would probably have been Flyback, if I needed only a couple off hundred watts...
But I want to get this right before I go Hi-power...
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Old 3rd February 2012, 09:18 PM   #4
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Another vote for push pull, this definitely topology of choice for 30V and below.
The voltage swing for push pull is doubled while Rdson is halved, which results in 1/4 of conduction losses.
First check where the power is being lost, hint: temperature.
Also if you want to stick to full bridge, the Mosfet choice is poor, try like IRFB3004 or similar.
There is also no data on the secondary power inductor if you have one (yes, there should be some)
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Old 4th February 2012, 08:29 AM   #5
PChi is offline PChi  United Kingdom
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Hello,
I don't have any suggestions on what to concentrate on.
What power level are you measuring the efficiency at? It's surprising where it all can go such as in the control circuit, switching losses in the Power MOSFETs and the Diodes with all that capacitance to charge and discharge at 100 kHz.
Without more information it's difficult to guess what the problem is.
If there isn't a capacitor in series with the transformer primary and an imbalance in switching times the resultant DC component passes through the primary. Seen by using a current probe or a low value resistor in series with an Oscilloscope.
I'm surprised about the variable frequency because usually the transformer is designed to work at a fixed frequency with the minimum number of turns to give an acceptable magnetizing current.
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Old 5th February 2012, 07:16 AM   #6
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Default Yet another vote for....

....Push-Pull
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Old 5th February 2012, 09:46 AM   #7
luka is offline luka  Slovenia
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yet another push-pull vote, this is the best way for 12v setup
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Old 6th February 2012, 01:39 AM   #8
dtproff is offline dtproff  United States
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Let's all try and give him a hand on what he already has.

So... You told us Vin and we can assume Vo from the turns ratio. What is the output power you were trying to push when you measured the efficiency? How did you measure the efficiency?

You might also talk a little on the ETD34 construction, what wire gauges you used and how you wound the windings (for example 3 parallel 18 AWG wires for the primary and then 2 parallel 20AWG for the secondary with tape between... those types of things).

Some hints on where you think the power loss is and a schematic would be helpful.

Tony
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Old 6th February 2012, 02:41 PM   #9
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Very good points PChi and dtproff! Thanks!
I will concentrate on those first.
I started to make a metering amplifier to make crazy accurate measurements of this.
Will come back with the results soon...

Thanks again! Good points!
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Old 9th February 2012, 07:45 AM   #10
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OK, some news...

Max. output power: 50.2V x 6.12A = 307W
Input power: 12.5V x 28.1A = 351W
Efficiency: 87%!
What do you say? :-)

I pretty much did nothing else than changed the input wires from the battery to the PS.
And measured the current from the negative power line directly using it as the 'shunt'.

I am so ashamed now...
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