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Opinions please -- 200V, 70mA from 3.2V

It looks to be about 87% efficient, which is very encouraging, but I'm happy neither with the regulation nor the stability.

(Win Hill suggested two boost regulators in a row -- the best I could get with that was barely 80% efficient -- I like this better).

I am kinda proud of the snubbing.

I'm not sure if the LTC3737 is the right choice -- I started with that, then figured out that I needed stronger gate drive so I tossed in the LTC4444 and the synchronous rectification for the snubbing. But if there's a straight controller chip out there that someone wants to recommend -- speak up!

The customer has a desire to have 100mA at 200V, but they also want the thing to be "as small as possible" (I tried to tell them that to a layout guy, "as small as possible" means "kaching", but they just reiterated. OK, fine, I'm listening).

Because the transformer is off the shelf, I'm thinking of just doubling up the circuit from the LTC4444 to the output cap, possibly using external synchronization on the thing so that the two supplies are ping- ponging. I'd love to hear opinions on that, too.

Comments welcome -- I don't do a lot of switchers, so if there's anything really stupid in here that won't show up until it's on a Real Live Board I'd love to hear about it earlier rather than later.

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--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott
Loading thread data ...

On Saturday, January 23, 2016 at 7:49:39 PM UTC-5, Tim Wescott wrote:

Tim, I didn't understand the snubbing. It looked like C3 gets filled, but never emptied?

M1's drain waveform looked unhappy.

I skimmed the controller datasheet & it said it could run on

2.9V, and 3V switches the FET well enough for simulating purposes (a lower Vgs(th) FET is/may likely be needed).

Those allow eliminating the +12V supply, comparator, and gate driver. I changed the FET to one merely half as monstrous, to save on gate drive dissipation (no calculations, no optimization, the original just looked mighty hefty).

I threw a crude snubber on it and set the transformer coupling to '1' to speed the simulation, to partial effect. (Ringing makes LTSpice crawl.)

That ignores the leakage inductance--I like to get the basics working first.

Anyway, this is simplified, and makes the waveforms I'd expect. I hope you don't feel I've butchered yours too much...

Cheers, James Arthur

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SYMBOL res 1168 400 R0 SYMATTR InstName R1 SYMATTR Value 5.6m SYMBOL nmos 1136 80 R0 SYMATTR InstName M1 SYMATTR Value IPB022N04L SYMBOL ind2 1168 -128 M180 WINDOW 0 36 80 Left 2 WINDOW 3 36 40 Left 2 SYMATTR InstName L1

SYMATTR SpiceLine Rser=12.5m SYMATTR Type ind SYMBOL res 1648 368 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 1648 -32 R0 SYMATTR InstName R3 SYMATTR Value 1.24meg SYMBOL res 288 288 R0 SYMATTR InstName R4 SYMATTR Value 140k SYMBOL cap 1824 -208 R0 SYMATTR InstName C4

SYMBOL cap 384 384 R0 SYMATTR InstName C5 SYMATTR Value 10n SYMBOL res 384 464 R0 SYMATTR InstName R12 SYMATTR Value 100k SYMBOL res 2176 -240 R0 SYMATTR InstName R11 SYMATTR Value 2k SYMBOL nmos 2144 -48 R0 SYMATTR InstName M3 SYMATTR Value FDS2734 SYMBOL voltage 2096 64 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 12 3m 1u 1u 700u 1m) SYMBOL cap 192 -48 R0 SYMATTR InstName C7

SYMBOL ind2 1392 -240 M0 SYMATTR InstName L2

SYMATTR Type ind SYMATTR SpiceLine Rser=3 SYMBOL schottky 1536 -240 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D1 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 176 208 R0 SYMATTR InstName C2 SYMATTR Value 1p SYMBOL res 1024 -128 R0 SYMATTR InstName R5 SYMATTR Value 10 SYMBOL cap 1024 -192 R0 SYMATTR InstName C3 SYMATTR Value 47n SYMBOL res 1104 -224 R0 SYMATTR InstName R7 SYMATTR Value 33 TEXT 1920 424 Left 2 !.tran 5m TEXT 1216 -272 Left 2 !K L1 L2 1 TEXT 1192 -304 Left 2 ;Wurth 760871113 TEXT 1256 -224 Left 2 ;6:140

Reply to
dagmargoodboat

On Sunday, 24 January 2016 11:49:39 UTC+11, Tim Wescott wrote:

This is an adaption of the Baxandall class-D approach that I posted to the original thread, sadly using Wordpad which doesn't seem to work.

The text below was cut and pasted from the .asc file into Notepad (where it didn't have visible line feeds or carriage returns) then pasted here (wher e they reappear).

The circuit is a whole lot simpler than the competition, but you do need a special purpose transformer and an inductor which you'd almost certainly ha ve to get wound for a gapped core.

Version 4 SHEET 1 1580 736 WIRE 768 -32 -160 -32 WIRE 896 -32 768 -32 WIRE 1392 -32 1216 -32 WIRE 896 0 896 -32 WIRE 1216 0 1216 -32 WIRE 1392 0 1392 -32 WIRE -160 96 -160 -32 WIRE -112 96 -160 96 WIRE 608 96 -32 96 WIRE 1040 96 608 96 WIRE 1216 112 1216 80 WIRE 1392 112 1392 80 WIRE 1392 112 1216 112 WIRE 1040 144 1040 96 WIRE -128 192 -160 192 WIRE -32 192 -128 192 WIRE 128 192 48 192 WIRE 256 192 128 192 WIRE 368 192 336 192 WIRE 896 240 896 64 WIRE 1040 240 1040 208 WIRE 1040 240 896 240 WIRE -368 256 -464 256 WIRE -208 256 -288 256 WIRE 128 256 128 192 WIRE 128 256 -208 256 WIRE -464 336 -464 256 WIRE 768 384 768 -32 WIRE 1040 400 1040 240 WIRE 1120 400 1040 400 WIRE 1232 400 1200 400 WIRE 1376 400 1312 400 WIRE 1392 400 1376 400 WIRE 1488 400 1392 400 WIRE 608 432 608 96 WIRE -160 448 -160 192 WIRE -16 448 -160 448 WIRE 144 480 -48 480 WIRE 304 480 224 480 WIRE 368 480 368 192 WIRE 368 480 304 480 WIRE -160 496 -160 448 WIRE 368 496 368 480 WIRE 1392 496 1392 400 WIRE 1488 496 1488 400 WIRE -48 576 -48 480 WIRE -48 576 -112 576 WIRE -16 576 -16 448 WIRE 176 576 -16 576 WIRE 320 576 256 576 WIRE -464 640 -464 416 WIRE -160 640 -160 592 WIRE -160 640 -464 640 WIRE 368 640 368 592 WIRE 368 640 -160 640 WIRE 608 640 608 496 WIRE 608 640 368 640 WIRE 768 640 768 448 WIRE 768 640 608 640 WIRE 1216 640 1216 112 WIRE 1216 640 768 640 WIRE 1392 640 1392 560 WIRE 1392 640 1216 640 WIRE 1488 640 1488 576 WIRE 1488 640 1392 640 WIRE -464 672 -464 640 FLAG -208 256 Vct FLAG 1376 400 Vout+ FLAG -128 192 tank- FLAG 304 480 tank+ FLAG -464 672 0 SYMBOL ind2 -48 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 0.054m SYMATTR Type ind SYMATTR SpiceLine Rser=0.00012 SYMBOL ind2 240 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value 0.054m SYMATTR Type ind SYMATTR SpiceLine Rser=0.00012 SYMBOL nmos 320 496 R0 SYMATTR InstName M1 SYMATTR Value FDS6680A SYMBOL nmos -112 496 M0 SYMATTR InstName M2 SYMATTR Value FDS6680A SYMBOL ind -384 272 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value 0.0775m SYMATTR SpiceLine Rser=0.0001 Cpar=100p SYMBOL voltage -464 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=0.001 SYMATTR InstName V1 SYMATTR Value 3.2 SYMBOL res 272 560 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 220 SYMBOL res 240 464 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 220 SYMBOL ind2 -128 112 R270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 250m SYMATTR Type ind SYMATTR SpiceLine Rser=7.2 Cpar=100pF SYMBOL schottky 880 0 R0 SYMATTR InstName D2 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 1472 480 R0 SYMATTR InstName R4 SYMATTR Value 2.84k SYMBOL schottky 624 496 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 784 448 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 1024 144 R0 SYMATTR InstName D3 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL ind 1104 416 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L8

SYMBOL res 1328 384 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 33 SYMBOL cap 1376 496 R0 SYMATTR InstName C2 SYMATTR Value 330n SYMBOL ind2 1200 -16 R0 SYMATTR InstName L6 SYMATTR Value 0.006m SYMATTR Type ind SYMBOL res 1376 -16 R0 SYMATTR InstName R6 SYMATTR Value 36 TEXT -400 664 Left 2 !.tran 0 20m 1u 10n startup TEXT -448 8 Left 2 !K1 L1 L2 L4 L6 0.99

The big resistors at R1 and R2 are there to fool LTSpice into not making th e circuit take off at a few MHz. The N87 EPCOS ferrite that I've got in min d for the inductors helps minimise MHz ringing - it's lossy over a MHz. I' ve get LTSpice to think the same way (to some extent) by adding in a fifth linked inductor L6 - a 6uH single turn coil with a damping resistance acros s it - but I haven't got a justification for the particular damping resist ance I've chosen.

L1, L2 and L4 are 3 turns, 3 turns and 204 turns on an EPCOS ungapped RM14 N87 core - B65887E0000R087 which you can buy ex-stock from Farnell/element

14 against order number 2355135.

L3 would be 22 turns of wire on a gapped EPCOS RM14 core - B65887E9160A087

- for which the order number is 1781866. It would have been running close t o saturation in the circuit I originally posted, but it should be fine here .

The 56uH inductor at L8 is also a Farnell parts, order number 3124873.

The FDS6680A D-Mos transistors came out of LTSpice. The worst case threshol d voltage is just low enough to guarantee that the circuit will always turn on, and the maximum acceptable continuous drain current (at 12A) is fine h ere where each of M1 and M2 carries 5.3A for 50% of the time.

The circuit isn't optimised - it wouldn't be worth spending time on that un til one had built a real circuit and seen what it did in real life - but th e Spice simulation works well enough to suggest that that might be worth do ing.

The RM14 core is big and non-cheap, but the 20W application need that much winding window to keep the resistive losses in the copper windings respecta bly low.

The EPCOS ETD range might do the same job more neatly - they are mounted wi th core flat rather than vertical. The ETD 44 is in the same ball park as t he RM14 (but quite a bit cheaper), and Farnell/element 14/Newark seem to st ock cores (and formers) from ETD 29 up to 59 (via 34. 39. 44. 49 and 54).

--
Bill Sloman, Sydney
Reply to
Bill Sloman

The leakage inductance spikes at M1 M2 are reduced if you can arrange for more cross-conduction by keeping R1 R2 high and shunting them with diodes to speed up turn-on and delay turn-off. The neat thing about current fed push-pull topology is the need to have some cross-conduction

- really foxes designers used to voltage fed :)

Do you even need L8 at all? Reflected primary side L3 does the same function already?

piglet

Reply to
piglet

Could be. I'll have a poke around.

--
Bill Sloman, Sydney
Reply to
Bill Sloman

On Sunday, 24 January 2016 22:15:12 UTC+11, piglet wrote:

You are right. I didn't like the effects of the speed-up diodes - lots of r inging immediately after switch-over - but relying on L3 as the smoothing i nductor works fine, and emboldened me to increase C2 to 3.3uF - in fact a h igh voltage electrolytic would make even more sense.

This meant that I had to decrease the turns on L4 to 193 to get the 200V ou tput.

Version 4 SHEET 1 1580 736 WIRE 768 -32 -160 -32 WIRE 896 -32 768 -32 WIRE 1392 -32 1216 -32 WIRE 896 0 896 -32 WIRE 1216 0 1216 -32 WIRE 1392 0 1392 -32 WIRE -160 96 -160 -32 WIRE -112 96 -160 96 WIRE 608 96 -32 96 WIRE 1040 96 608 96 WIRE 1216 112 1216 80 WIRE 1392 112 1392 80 WIRE 1392 112 1216 112 WIRE 1040 144 1040 96 WIRE -128 192 -160 192 WIRE -32 192 -128 192 WIRE 128 192 48 192 WIRE 256 192 128 192 WIRE 368 192 336 192 WIRE 896 240 896 64 WIRE 1040 240 1040 208 WIRE 1040 240 896 240 WIRE -368 256 -464 256 WIRE -208 256 -288 256 WIRE 128 256 128 192 WIRE 128 256 -208 256 WIRE -464 336 -464 256 WIRE 768 384 768 -32 WIRE 1040 400 1040 240 WIRE 1376 400 1040 400 WIRE 1392 400 1376 400 WIRE 1488 400 1392 400 WIRE 608 432 608 96 WIRE -160 448 -160 192 WIRE -16 448 -160 448 WIRE 144 480 -48 480 WIRE 320 480 224 480 WIRE 368 480 368 192 WIRE 368 480 320 480 WIRE -160 496 -160 448 WIRE 368 496 368 480 WIRE 1392 496 1392 400 WIRE 1488 496 1488 400 WIRE -48 576 -48 480 WIRE -48 576 -112 576 WIRE -16 576 -16 448 WIRE 176 576 -16 576 WIRE 320 576 256 576 WIRE -464 640 -464 416 WIRE -160 640 -160 592 WIRE -160 640 -464 640 WIRE 368 640 368 592 WIRE 368 640 -160 640 WIRE 608 640 608 496 WIRE 608 640 368 640 WIRE 768 640 768 448 WIRE 768 640 608 640 WIRE 1216 640 1216 112 WIRE 1216 640 768 640 WIRE 1392 640 1392 560 WIRE 1392 640 1216 640 WIRE 1488 640 1488 576 WIRE 1488 640 1392 640 WIRE -464 672 -464 640 FLAG -208 256 Vct FLAG 1376 400 Vout+ FLAG -128 192 tank- FLAG 320 480 tank+ FLAG -464 672 0 SYMBOL ind2 -48 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 0.054m SYMATTR Type ind SYMATTR SpiceLine Rser=0.00012 SYMBOL ind2 240 208 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 4 56 VBottom 2 SYMATTR InstName L2 SYMATTR Value 0.054m SYMATTR Type ind SYMATTR SpiceLine Rser=0.00012 SYMBOL nmos 320 496 R0 SYMATTR InstName M1 SYMATTR Value FDS6680A SYMBOL nmos -112 496 M0 SYMATTR InstName M2 SYMATTR Value FDS6680A SYMBOL ind -384 272 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L3 SYMATTR Value 0.064m SYMATTR SpiceLine Rser=0.0001 Cpar=100p SYMBOL voltage -464 320 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 132 Left 2 SYMATTR SpiceLine Rser=0.001 SYMATTR InstName V1 SYMATTR Value 3.2 SYMBOL res 272 560 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 330 SYMBOL res 240 464 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 330 SYMBOL ind2 -128 112 R270 WINDOW 0 44 45 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 223.5m SYMATTR Type ind SYMATTR SpiceLine Rser=7.2 Cpar=100pF SYMBOL schottky 880 0 R0 SYMATTR InstName D2 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 1472 480 R0 SYMATTR InstName R4 SYMATTR Value 2.84k SYMBOL schottky 624 496 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D1 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 784 448 R180 WINDOW 0 24 64 Left 2 WINDOW 3 24 0 Left 2 SYMATTR InstName D4 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 1024 144 R0 SYMATTR InstName D3 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 1376 496 R0 SYMATTR InstName C2

SYMBOL ind2 1200 -16 R0 SYMATTR InstName L6 SYMATTR Value 0.006m SYMATTR Type ind SYMBOL res 1376 -16 R0 SYMATTR InstName R6 SYMATTR Value 36 TEXT -400 664 Left 2 !.tran 0 30m 1u 10n startup TEXT -448 8 Left 2 !K1 L1 L2 L4 L6 0.99

--
Bill Sloman, Sydney
Reply to
Bill Sloman

I assume this was a LTSpice simulation. You should be able to do much better, with the right MOSFET and inductor. Can you put up your simulation file for the 80% version?

--
 Thanks, 
    - Win
Reply to
Winfield Hill

BTW, that's an LT3757 flyback, not an LTC3737 buck.

--
 Thanks, 
    - Win
Reply to
Winfield Hill

Maybe due to the diode in the first section. I'd really suggest a Royer here. A push-pull forward would also work with good efficiency.

The LT3757A is a good choice. I use that a lot and the A version with the improved load change reaction came about the time yours truly suggested that this be improved ...

The sobering moment comes when the layouter submits the quote. As small as possible would also mean custom magnetics.

A wee problem: From the LT3757 gate transition to the LTC4444 turning M1 on there is a good 200nsec and the turn-on is too sluggish. This would need a beefier and much faster driver. The leading edge blanking period in the 3757 is only 100nsec. That's bound to cause burping, farting and poor loop behavior.

Page 14, left side:

formatting link

I usually use Zetex or now Diodes Inc BJT push-pull chips (two pnp/npn in one package) and then INTVcc to 12V for a stiff drive amplitude.

[...] 
--
Regards, Joerg 

http://www.analogconsultants.com/
Reply to
Joerg

On 24/01/2016 14:10, Winfield Hill wrote:

I had a go, see LT asc below. Put in 16.5W for 14W out or 85% which considering the big step-up ratio I think is reasonable.

Version 4 SHEET 1 1396 760 WIRE -304 16 -528 16 WIRE -192 16 -304 16 WIRE -80 16 -112 16 WIRE 64 16 0 16 WIRE 96 16 64 16 WIRE 192 16 160 16 WIRE 288 16 192 16 WIRE 400 16 288 16 WIRE 528 16 480 16 WIRE 656 16 608 16 WIRE 736 16 656 16 WIRE 880 16 800 16 WIRE 1040 16 880 16 WIRE 1168 16 1040 16 WIRE -528 64 -528 16 WIRE 880 64 880 16 WIRE 1168 96 1168 16 WIRE 192 112 192 16 WIRE -304 176 -304 16 WIRE 64 176 64 16 WIRE 656 176 656 16 WIRE 16 256 -176 256 WIRE 608 256 576 256 WIRE -176 304 -176 256 WIRE 576 320 576 256 WIRE 656 336 656 272 WIRE 1168 336 1168 176 WIRE -528 352 -528 144 WIRE -304 352 -304 240 WIRE 64 352 64 272 WIRE 192 352 192 176 WIRE 880 352 880 128 WIRE -176 448 -176 384 WIRE 384 448 -176 448 WIRE 576 448 576 400 WIRE 576 448 384 448 WIRE -176 496 -176 448 WIRE -176 608 -176 576 FLAG 288 16 vintermediate FLAG 1168 336 0 FLAG 656 336 0 FLAG 64 352 0 FLAG -528 352 0 FLAG -304 352 0 FLAG 192 352 0 FLAG 880 352 0 FLAG 1040 16 vout FLAG -176 608 0 FLAG 384 448 vgates SYMBOL nmos 16 176 R0 SYMATTR InstName M1 SYMATTR Value IPB022N04L SYMBOL nmos 608 176 R0 SYMATTR InstName M2 SYMATTR Value IPB600N25N3 SYMBOL schottky 96 0 M90 WINDOW 0 0 32 VBottom 2 WINDOW 3 32 32 VTop 2 SYMATTR InstName D1 SYMATTR Value B540C SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 736 32 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value UPSC600 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 864 64 R0 SYMATTR InstName C2 SYMATTR Value 470n SYMBOL res 1152 80 R0 SYMATTR InstName R1 SYMATTR Value 2857 SYMBOL ind -208 32 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1

SYMBOL ind 384 32 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L2

SYMBOL cap -320 176 R0 SYMATTR InstName C3

SYMBOL voltage -528 48 R0 WINDOW 123 0 0 Left 2 WINDOW 39 24 124 Left 2 SYMATTR InstName V1 SYMATTR Value 3.2 SYMATTR SpiceLine Rser=0.002 SYMBOL voltage -176 480 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value PULSE(0 8 10u 50n 50n 4.28u 5u) SYMBOL res -192 288 R0 SYMATTR InstName R2 SYMATTR Value 5 SYMBOL res 560 304 R0 SYMATTR InstName R3 SYMATTR Value 5 SYMBOL cap 176 112 R0 SYMATTR InstName C1

SYMBOL res 16 0 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1m SYMBOL res 624 0 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R5 SYMATTR Value 200m TEXT -562 632 Left 2 !.tran 5m TEXT 496 744 Left 2 ;2016 Jan 24 Two-Stage Boost - piglet inspired by Win Hill SED TEXT -152 632 Left 2 ;Use LM3478 or similar powered by Vintermediate here?

piglet

Reply to
piglet

And much smaller magnetics, since a forward converter's core doesn't have to store energy, and its filter inductor only has to handle the ripple.

Ytt, all the high-ratio converters I can think of--CRT HV, auto ignition, xenon strobe--are all flybacks.

Tim could also use a much smaller FET, which saves money and is easier to drive. The average full-load input current should be ~ (70mA * 200V output) / (3.2V input * 90%n) = 4.9A, which is not usually

1mR monster-FET territory.

A smaller, cheaper FET could save more in drive power and switching losses than it would lose in conduction losses.

Cheers, James Arthur

Reply to
dagmargoodboat

It's usually the cheapest as well. Even a Royer is large here because the transformer alone would exceed a square inch in real estate.

[...]

Either that or really low Vsat BJT if Royer. With FETs one must look for a good gate charge versus Rdson figure of merit and place that ratio in the optimum range. Using a FET with super-low Rdson is often not best because the turn-on costs too much energy and if sluggish causes losses in the power path like in the current simulation.

Regards, Joerg

formatting link

Reply to
Joerg

The Vlad memorial antiseptic converter I posted on the other thread is possibly relevant. (No, I'm not a SMPS guy--I just thought it was cool. ) It shows about 95% efficiency doing 9-400V @ 10W, with lossless inductors.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

I'm specifically trying to do it with off the shelf components.

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www.wescottdesign.com
Reply to
Tim Wescott

I had a 33mR FET in there and the gate resistance was one of the long poles in the tent. I'm sure there's a happy medium between that and 1mR, though.

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www.wescottdesign.com
Reply to
Tim Wescott

With less than unity coupling in the transformer, C3 absorbs the initial spike when the low-side FET turns off, then returns the energy to the Vin line.

Perhaps. I'll take a look at that.

But I want it to reflect reality, more or less. I'm thinking that it'll be easier to find FETs that work decently with 12V on their gates than with 3.2.

The comparator and high-side driver are there to make the push-pull pair and hence the snubbing. On reflection I think the snubbing is causing problems, but not what you see -- when the current draw goes to zero the snubber voltage goes down, then the low snubber voltage (I think) slows down the ramp-up of current to Vout.

As far as the gate driver -- I started out without synchronous rectification, with a FET that had an RdsOn of 33mR, a low threshold voltage, and a low gate charge. Conduction and switching losses were both horrendous. So I dropped in the gate driver, and a herkier FET, and happiness ensued. I'm sure I could optimize more.

I though an 87% efficient converter was a pretty good addressment of the basics.

With a reasonably accurate transformer coupling constant and a passive snubber, one ends up dumping about four or five watts into the snubber. If I _do_ just go with a diode-capacitor snubber I'll have an active buck converter that returns those four or five watts back to the Vin line -- which means, at the very least, a monolithic buck converter and an op-amp for the regulation.

I'll be trying it without the synchronous snubbing, but given my prior experience I think I'll be sticking with a gate driver and a herky FET.

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www.wescottdesign.com
Reply to
Tim Wescott

The regulator, gate driver, FETs and coil were actually just copied from the two-stage circuit, and then the coil jiggered around until it was a transformer. That did bring the diode switching losses down dramatically, but there were still significant losses in the second stage.

I'm doing the layout on this, and they do know it's adding expense.

I'll take a look. The loop is, indeed, burping and farting somewhat, but at least in simulation it's still regulating well enough.

Do you have an app note or other circuit you can point to? Or is it something simple like an emitter-follower pair? I used the LTC4444 because it's in LTSpice, but I plan on doing my homework on that part -- having a transistor pair that I could easily simulate would be nice.

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www.wescottdesign.com
Reply to
Tim Wescott

With real coupling numbers and the top FET replaced with a diode and a passive snubber (zener + series resistor to the Vin line) the snubber alone dissipates about 16W, and efficiency drops below 50%. So I think I need something better than "crude".

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

John Larkin makes the same mistake. So far you've got a complicated circuit with vile efficiency (which means that bits of it are going to get quite hot).

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Bill Sloman, Sydney
Reply to
Bill Sloman

What's wrong with 87%?

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Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com
Reply to
Tim Wescott

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