Push-Pull driver <<1ohm Rdson?

Hello Folks,

Looking for a staunch chip to drive transformers in the 30-150kHz range. The most powerful (and available) one I found is the MIC4451 but it's

1ohm Rdson. Way too much.

There are bigger ones but they usually have a charge pump and the efficiency is pretty poor when operated at less than 3A:

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What I need is n/p channel push-pull output (not two n-channels), no charge pump because the clock sometimes stops, and ideally 100-200mohm Rdson. External FETs aren't so hot, too much cross conduction. I was hoping there'd be n/p synchronous buck converters. But nope, all with charge pumps.

Any ideas?

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Regards, Joerg

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Joerg
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FS---FDD8424H---Dual N & P Channel half-bridge 40V@20A 54mOhm.pdf

Reply to
Jon Slaughter

If you had no luck on a driver and wanted to use a charge pump for the widest choice of output transistors, could you turn the bottom FET on when the clock stops?

This would keep the charge pump capacitor primed and ready to go on the first positive clock. The same thing would work for a full h-bridge. It might take a bit of glue logic to get things started again in the correct phase but it shouldn't be too difficult.

Regards,

Mike Monett

Reply to
Mike Monett

Hi Joerg

Why is a charge pump not acceptable? Are you concerned about the time it takes to build up the gate charge.

What do you mean when you say "External FETs have too much cross conduction?

Dick

Reply to
dick.milun

This is a nice part:

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It has a charge pump, but it also has an oscillator to keep it pumped even when the input parks at one level for a long time. I've used it for driving microsteppers, works fine.

John

Reply to
John Larkin

Thanks, Jon. Although they conduct already quite well between 2-3V Vgs so there'll still be considerable cross conduction. I am operating at

12V. Maybe I'll place zeners in the gate drive to burn off some drive level.
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Regards, Joerg

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Reply to
Joerg

Yes, that would be no problem.

I was thinking about glue logic with some "poor man's" one-shots in there. Certainly an option but I wanted to avoid it, keeping complexity down.

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Reply to
Joerg

Thanks, John. I had looked at that one and the Rdson is a bit highish,

600mohm max. I've also had some issues with National motor drivers with obsolescence. Thing is, a lot of my stuff remains in production well over 10 years.

I wish there was a big brother of the LMD18201 somewhere, 1/3rd of its Rdson or so. I am running this at 12V supply and cost is not a real concern for this part of the design. IOW if it was $10 a pop it would be ok.

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Regards, Joerg

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Joerg
[...]

You could also put a small inductor in the positive leg to limit the max current while both FETs are on.

Say the cross conduction current was a rectangular pulse, and you wanted to limit the current spike to 1A from your 12V supply. If the conduction overlap was 40ns, a rough calculation give an inductance of

L = E * dt / di = 12 * 40e-9 / 1 = 480nH

If you knew the actual pulse shape, a simulation in LTspice would give a more accurate value. But it would be lower than the above calculation.

A value this small would have little or no effect on the operation at 100KHz. But limiting the current spike would greatly reduce EMI to the rest of the circuit.

Best Regards,

Mike Monett

Reply to
Mike Monett

You could certainly make it out of parts... a couple of LM5112's, or a dual gate driver chip, a couple of non-overlap parts, and two fets.

But I bet one of the fiercer fet gate driver chips would just work... maybe parallel 2 or 4 sections.

Take a look at the LM5112... it's an interesting part. They cost us $1.30 in small quantities. Maybe one or three of them could drive your load.

John

Reply to
John Larkin

Yeah, if I don't find anything I'll do that this afternoon. Some logic and external FETs. I wanted to avoid it but I guess it's the usual, I might be alone in the marketplace for this stuff so the mfgs don't bother. Kind of surprising because motor-PWM sometimes needs this stuff.

Well, that's just the point, I haven't found any fiercer ones. And none where I could parallel on the same chip and get to Take a look at the LM5112... it's an interesting part. They cost us

Nah, I did look at that in detail after you mentioned it last year. The high side is too wimpy, only the BJT in there has enough oomph and that leaves too much headroom. The MIC4451 is much better. But not good enough here. Also comes in a cheaper 9A peak edition.

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Joerg

Ok, guys, don't bother with this one anymore. I just wrapped it up with discrete parts. As usual :-)

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Joerg
[...]

That's exactly what a small inductor in the top FET will prevent. Check it out. You might be surprised how effective it can be.

You really have to get off this Phssst. *BANG* habit. It makes too much noise, and everyone gets nervous. Plus it stinks up the place:)

Best Regards,

Mike Monett

Reply to
Mike Monett

I tried the inductor and wasn't too enthused. With a small damper resistor inparallel it was kind of ok but still cost efficiency. But it's done, I just lashed up the usual discrete concoction that provides proper dead time and all that. Now it's moving on to the dreaded packaging design. As much fun as eating pea soup and I don't like pea soup. Ok, with some Johnsonville Brats in there I'll eat it.

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Regards, Joerg

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Reply to
Joerg

Joerg, I'm surprised it affected the efficiency enough to make a difference. Did you simulate it or try it on the bench? And did you use the smallest inductor needed to limit the current? Did it somehow drastically increase the turnoff times?

With the FETs that were mentioned, the overlap is only 40 or 50 ns or so. At 100KHz, it occurs every 5uS. That is only 1% of the duty cycle, so it's not clear how that can cause a significant loss of efficiency. If you have the time, I'd like to learn more how you did the test.

Xnews chopped the rest of your post, and I can't figure out how to make it put it back. So I'll have to trust that most people know you:)

Best Regards,

Mike Monett

Reply to
Mike Monett

Mike Monett wrote:

I did a small simulation in LTspice. The SPICE model for the FDD8424H is available from Fairchild, but it is for PSPice. I didn't want to take the time to make a model for LTspice, so I used IRF530 and IRF7204 for complimentary MOSFETs. I adjusted the rise and fall time of the gate drive to give about 50ns cross-conduction.

I increased the series inductor between the MOSFETs to 3uH and used a 47 ohm damping resistor.

The switching losses end up heating the damping resistor, so the integral of the power dissipated gives the total loss. For this simulation, the result is 52.114mW.

If the power delivered to the load is 3 watts, using a series inductor to minimize shoot-through adds 52.114e-3 / 3 = 1.73% to the total power dissipation. This is a rather small amount for the simplicity and reliability gained.

Any circuit changes to reduce the power loss in switching will probably cost additional power, so the overall gain might be small or negative.

The conclusion is a small series inductor can be a viable option to minimize shoot-through and reduce circuit complexity.

The LTSPICE ASC file is below, followed by the PLT file. The .tran analysis string is set to 1uS to show the switching waveforms. Increase it to 1ms to calculate the power in R2.

Best Regards,

Mike Monett

Version 4 SHEET 1 948 800 WIRE 224 16 48 16 WIRE 528 16 224 16 WIRE 48 32 48 16 WIRE 224 32 224 16 WIRE 528 32 528 16 WIRE 176 48 144 48 WIRE 480 48 448 48 WIRE 48 128 48 112 WIRE 224 144 224 128 WIRE 256 144 224 144 WIRE 320 144 256 144 WIRE 528 144 528 128 WIRE 560 144 528 144 WIRE 608 144 560 144 WIRE 688 144 608 144 WIRE 320 160 320 144 WIRE 608 208 608 144 WIRE 688 208 688 144 WIRE 256 256 224 256 WIRE 320 256 320 240 WIRE 320 256 256 256 WIRE 224 272 224 256 WIRE 96 352 48 352 WIRE 144 352 144 48 WIRE 144 352 96 352 WIRE 176 352 144 352 WIRE 48 368 48 352 WIRE 560 368 528 368 WIRE 608 368 608 288 WIRE 608 368 560 368 WIRE 688 368 688 288 WIRE 688 368 608 368 WIRE 224 384 224 368 WIRE 528 384 528 368 WIRE 48 464 48 448 WIRE 144 464 144 352 WIRE 448 464 448 48 WIRE 448 464 144 464 WIRE 480 464 448 464 WIRE 528 496 528 480 FLAG 96 352 M1G FLAG 48 128 0 FLAG 256 256 M1D FLAG 224 384 0 FLAG 48 464 0 FLAG 256 144 M2S FLAG 560 368 M3D FLAG 528 496 0 FLAG 560 144 M4S SYMBOL Nmos 176 272 R0 SYMATTR InstName M1 SYMATTR Value IRF530 SYMBOL voltage 48 352 R0 WINDOW 123 24 134 Left 0 WINDOW 3 -128 159 Left 0 WINDOW 39 0 0 Left 0 SYMATTR Value PULSE(0 12 0 100n 100n 5u 10u) SYMATTR InstName V2 SYMBOL Voltage 48 16 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 12V SYMBOL Pmos 176 128 M180 SYMATTR InstName M2 SYMATTR Value IRF7204 SYMBOL res 304 144 R0 SYMATTR InstName R1 SYMATTR Value 1 SYMBOL Nmos 480 384 R0 SYMATTR InstName M3 SYMATTR Value IRF530 SYMBOL Pmos 480 128 M180 SYMATTR InstName M4 SYMATTR Value IRF7204 SYMBOL ind 672 192 R0 SYMATTR InstName L1 SYMATTR Value 3µ SYMBOL res 592 192 R0 SYMATTR InstName R2 SYMATTR Value 47 TEXT 40 -40 Left 0 ;'Complimentary PWM Switch Series Inductor R2=52.114mW TEXT 200 -8 Left 0 !.tran 0 1u 0 10n

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[Transient Analysis] { Npanes: 2 { traces: 3 {34603011,0,"I(L1)"} {34603012,0,"Id(M3)"} {524293,0,"- Is(M4)"} X: ('m',1,0,0.0001,0.001) Y[0]: ('m',0,-0.9,0.1,0.5) Y[1]: (' ',1,1e+308,0.3,-1e+308) Amps: ('m',0,0,0,-0.9,0.1,0.5) Log: 0 0 0 GridStyle: 1 }, { traces: 2 {34603010,0,"I(R1)"} {524294,1,"V(M4S,M3D)*I(R2)"} X: ('m',1,0,0.0001,0.001) Y[0]: (' ',0,-1,1,11) Y[1]: (' ',1,0,0.3,3.6) Amps: (' ',0,0,0,-1,1,11) Units: "W" (' ',0,0,1,0,0.3,3.6) Log: 0 0 0 GridStyle: 1 } }
Reply to
Mike Monett

Oops - typo. M2S and M4S are mislabeled. They should be M2D and M4D. This has no effect on the result.

Best Regards,

Mike Monett

Reply to
Mike Monett

That's just the thing. Whatever you do, in the end you just move the dissipation from one part to another.

Yep, so I just did the usual, a concoction of Schmitts, resistors and diodes. No more cross conduction :-)

Thanks, Mike, but I am done with that part of the design now. I was just hoping there was a push-pull driver with much less than 1ohm so I could drive the transformer directly. Such drivers are usually process-controlled so their internal cross conduction is minimized. Something you can't do with tolerance-prone external parts where things such as Vth stray a lot.

--
Regards, Joerg

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Joerg

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