Does the Full Bridge Phase Shift Zero Voltage Switched Converter with Current Doubler Synchronous Rectifier Have to be Current Mode Controlled?

I think the title just about covers it.

My client requested a 24v input with 0-24v output. Max power is 600W, min power is 25W. 600W is at 12v so 50 amps of current is a lot to deal with. package is 7.5 cu-in.

I am concerned that the current signal may be buried in the noise at low voltage low power output so looking at various topologies. I might like to do a pair of 300W voltage controlled bridges interleaved, but throwing out the current doubler.

I know that this is just the type of question that you like so run with it. Brainstorm away.

Thanks Bob N9NEO

Reply to
Yzordderrex
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ALL switched-inductor topologies must be current mode.

You can use a voltage mode control with exceptions, but by the time you've hacked enough cases on to make it reliable under operating extremes and transients, you've made a shitty, overly complicated current mode control.

Better to do it right from the start.

I haven't had problems with that kind of dynamic range on any supplies I've made. Offset at the current sense amp manifests as a dead band near zero if positive, and a minimum current if negative. The one isn't so much an error as a preferred behavior (you can actually set it to zero), so just bias the current sense slightly to guarantee offset in that direction.

Don't use opamps that are prone to RF rectification, or if you do, be damn careful about filtering inputs, outputs and supply (notch ground planes if necessary to isolate loops).

And there, that's it. You've got at least 20dB useful range if the ripple fraction is very large, and probably more like 40dB (more than enough) with typical values. It may even be fine out to 60dB, but the error amp will be operating hysteretic by then so it's harder to say (effectively, the output ripple frequency drops).

My flashlight is a good illustration of this:

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When turned down to the narest sliver of setpoint, it does indeed get dimmer and dimmer, until the blinking is obvious. About 10Hz is the lowest I can get it.

(If you prefer not to have this behavior, but just a sharp cutoff instead, you can apply a little positive feedback around the error amp. Be careful that it doesn't dominate over normal loop function and turn it unstable. It's like Phil's favorite trick of making a PLL auto-sweep with positive feedback, but then when the phase detector kicks in, it more than overrides the high resistance path.)

Incidentally, this circuit is a very nice and general current mode controller; I build a Cuk variant for 18V 20A output:

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(packed like sardines into its box, I ran out of space for enough filter caps even; on the upside, it's only intended for intermittent operation, so I guess it doesn't matter much that everything gets hot really quickly. :o :^) )

Multiphase is definitely your friend here. You can't commutate impedances that low (i.e., ~0.5 ohm) without having a mess of loop inductance, even with low profile (DFN) transistors. At least, not quickly, and I'm guessing you want to go as fast as you can to keep dissipation and size down.

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

It does but I had to toggle "Show header" to read it. :-(

The usual practice is just to use the first few words in the subject line terminated with ellipsis and start the body of the text with ellipsis followed by the rest of the subject before starting the rest of the body with a new line.

This is annoying enough but at least your readers won't have to keep toggling the header just to see the full 'subject line'. :-)

As for your question... sorry, that's all I have.

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Johnny B Good
Reply to
Johnny B Good

Less than helpful.

Reply to
John S

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Thank you for the thoughtful answer Tim. I'm looking at the D2pack as ther e are quite a few sources. The specification is 90% efficient so I have a little wiggle room. I see that even on the LT spice simulator there is a l ot of ringing on the secondary side. Looks to me the leakage inductance is ringing with the Synchronous fet output capacitance. Is this a real probl em?

Did I run across you a few years back on an RC model page? I'm looking at doing a surface drone with remote video for monitoring my property.

Reply to
Yzordderrex

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