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Old 31st January 2011, 06:44 PM   #1
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Default 18 to 330Vdc with 97% efficiency.

Hi, I need to do a DC/DC power converter with at least 97% efficiency . 18Vdc to 330Vdc 150 W. The input voltage and output power almost dont varu, so it can be done with a fixed pwm.

Its possible?
Someone suggests a specific topology? a current fed topology would be better?

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Old 31st January 2011, 10:20 PM   #2
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This is not possible, or, at least, not practical. A push pull current mode topology might be a good choice. If you use very high current (low Rds on) MOSFETs for the input switches and use synchronous rectifiers, this will help with efficiency. In addition, you need to minimize the I^2*R losses in the magnetic devices. Core losses can be reduced by using relatively low frequencies.
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Old 31st January 2011, 10:42 PM   #3
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I disagree about current mode pushpull.
First, type of modulator (current, voltage) has nothing to do with efficiency. However a shunt current sensor resistor usually used with current mode limits the whole efficiency.
Second, the stray inductances in pushpull are a big challange and need lossy snubbers or avalanche operation, it may be easier to get some 2mohms Mosfets and go for fullbridge.
Synchronous rectifier for 330VDC is a joke isn't it?

Anyway, the low frequncy approach is the way to go, start around 15kHz and big transformer (etd44 minimum) with thick wiring.

Last edited by darkfenriz; 31st January 2011 at 10:44 PM.
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Old 31st January 2011, 10:52 PM   #4
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Location: Canandaigua, NY USA
IMO, the higher the conversion ratio, the harder it is to get good efficiency. Most of the high efficiency designs you see will have lower ratios and often down, not up. A huge amount will depend on the magnetics design. You might want to pick up a copy of Colonel McLyman's transformer design book. Here. I also like Billings SMPS handbook, though it's strangely organized and there may be better choices these days. There are loss mechanisms that occur at extremes of duty cycle, so some app note designs perform with much lower efficiency when the ratios depart from nominal, making choice of topology critical. People rarely ever talk about that. I'd be curious what you end up with.

I may be barking up the wrong tree, but at least I'm barking!
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Old 1st February 2011, 11:46 AM   #5
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The application is a Grid-Tie module integrated PV inverter. My first prototype was a full-bridge, it is with 94% efficiency. I'm redesigning the transformer and I believe it can reach 95.5%.

My interest now is to remove the electrolytic capacitor input. So I asked about the current-fed topologies.
I thinks the push-pull isn't the best choice, its difficulty to manage the leakage inductance. I'm trying to design a dual half-bridge, since it can be resonant and use the leakage inductance as a good factor.

Someone suggests an IC for PWM generation that consumes very low power? to replace the TL494 that I'm using.

Thanks for the comments so far.

Sorry for my english, I'm Brazilian.
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Old 3rd February 2011, 12:03 AM   #6
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I did a small change in the transformer and got 95,1% efficiency. Now i will test the dual half bridge converter (same as two inductor isolated boost converter).

Tomorrow i will post some photos so you can help me improve something.
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Old 3rd February 2011, 03:40 AM   #7
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Location: Canandaigua, NY USA
There is a very good (and long) string of posts on grid-tie inverters here if you haven't seen it.

I may be barking up the wrong tree, but at least I'm barking!
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