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Old 9th December 2008, 03:21 PM   #1
simingx is offline simingx  Singapore
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Default Questions on MC33025 current mode design

Hi all,
I'm planning on building the current-mode halfbridge describe in Brown's book, and would like to build it "as designed" to get a feel for the math involved before modifying it for my purposes.

For the first bit (where he calculates the CT secondary voltage), the equation is as such:

Vct(sec) = Vsc + 2Vfwd = 1.0V + 2(0.65) = 2.3V

The 0.65V I can determine is the voltage drop across the 2 conducting 1N4148's in the CT secondary, but I can't determine what Vsc is. Is it the "current comparator threshold" in the datasheet?
If it is, I take it that it's essentially constant as the threshold voltage shouldn't change, no matter the output requirements?

For the next bit, the slope compensation resistor:

Rsc = Vosc / (2 x Isc) = (4.5 - 2.3V) / (2 * 170uA)

The 170uA was calculated previously.
So, Vosc is the "high" level of the output voltage swing?

Thanks!
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Old 11th December 2008, 08:57 AM   #2
simingx is offline simingx  Singapore
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Oops typo, should be MC34025 instead.
No one?
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Old 12th December 2008, 06:44 PM   #3
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To get current mode half bridges to work correctly you need special balancing windings. The one shown in his book won't work correctly. Midpoint voltage is undefined, there is nothing to center the coupling capacitor voltage at Vbus/2.
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Old 13th December 2008, 12:00 AM   #4
simingx is offline simingx  Singapore
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Hi,
Thanks for the heads up on that. Converting it to a full-bridge (although overkill for low power) should solve the problem, correct? I already have a gate drive transformer suitable for a full bridge.
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Old 13th December 2008, 01:51 AM   #5
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Yes that sounds like a good idea, especially if you already have a suitable GDT. You might have to be careful if you want to use a current transformer, it could saturate instead of detecting DC current imbalances in the circuit. With a full bridge it's easy to use a resistive shunt instead though.

What is the power level you want? A two transistor forward might be a good solution too.
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Old 13th December 2008, 02:12 AM   #6
simingx is offline simingx  Singapore
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I would like to use it to power 2x UcD400's... so we're looking at about 700W. However I might instead use 2 separate supplies of about 400W each so the winding of the transformer is easier (paralleling so many wires at one time is not fun...).
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