Boost Converter Efficiency Improvements

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Sure, but it seems silly to add an opamp active diode to fool a chip that's already too complex.

TI has a DSP-core digital power controller with picosecond PWM resolution. I'll ask Rob what the part number is.

You could also give a cheap uP a little help around the edges, like a gate driver that shuts off when peak current is sensed, something like that. One advantage of designing everything yourself is that you can understand it; lots of complex analog chips are mysterious about what they actually do. Like an opamp on the block diagram with three inputs... what's that supposed to do?

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Nah, foldback makes a lot of sense in many situations. For example, I just designed a unit that also has to supply a couple amps to miscellaneous external 12V gear. Small displays and such, and if the regulator were capable of dumping 2A into a shorted out electrolytic, changes are there would be a serious *KAPHOOF* situation. If I wanted no foldback for some reason all I need to do that is place three parts. Two resistors and one diode. About 50 millibucks worth.

I'll

That is nice. As long at it has the horsewpower for a reasonably fast loop (as in tens of kHz) and won't require the San Onofre power plant to be fired up again in order to operate.

See? There the digital situation already requires extra hardware, including extra ICs. There is no way to properly drive a big fat switcher from a 5V logic device of any kind. Because you need +10V (or better 12-15V) to turn the power FETs on and -5V or so to turn the off with gusto. Else the efficiency will suffer. All this logic-level driving is no good when efficiency is paramount.

Pretty soon you'll find out that leading-edge blanking just isn't in the cards with a uC, its internal comparator has the acceleration of an Isetta, some high priority interrupt keeps trampling on the loop performance, and it can't prevent a loud bang when the regulator goes into CCM and the inductor core saturates within a microsecond or so.

That almost drove people nuts when we had transistors on the schematic of an IC design that had numerous collectors.

Good grief, why use complicated LTC chips and microprocessors and stuff?

Scope the fet drain; it's really cool.

Version 4 SHEET 1 1920 968 WIRE 240 144 112 144 WIRE 320 144 240 144 WIRE 464 144 400 144 WIRE 528 144 464 144 WIRE 656 144 608 144 WIRE 816 144 768 144 WIRE 960 144 896 144 WIRE 1040 144 960 144 WIRE 1184 144 1104 144 WIRE 1232 144 1184 144 WIRE 1360 144 1312 144 WIRE 1424 144 1360 144 WIRE 112 176 112 144 WIRE 768 208 768 144 WIRE 240 240 240 144 WIRE 320 240 240 240 WIRE 464 240 464 144 WIRE 464 240 384 240 WIRE 1184 240 1184 144 WIRE 1232 240 1184 240 WIRE 1360 240 1360 144 WIRE 1360 240 1296 240 WIRE 1360 288 1360 240 WIRE 112 320 112 256 WIRE 656 320 656 144 WIRE 720 320 656 320 WIRE 768 320 720 320 WIRE 768 368 768 320 WIRE 960 368 960 144 WIRE 304 384 240 384 WIRE 400 384 368 384 WIRE 1360 400 1360 352 WIRE 48 416 -32 416 WIRE 96 416 48 416 WIRE 240 416 240 384 WIRE 240 416 176 416 WIRE 656 432 656 320 WIRE -32 448 -32 416 WIRE 240 448 240 416 WIRE 304 448 240 448 WIRE 400 448 400 384 WIRE 400 448 368 448 WIRE 768 480 768 448 WIRE 960 480 960 448 WIRE 240 512 240 448 WIRE 304 512 240 512 WIRE 400 512 400 448 WIRE 400 512 368 512 WIRE 432 512 400 512 WIRE 560 512 512 512 WIRE 608 512 560 512 WIRE -32 528 -32 512 WIRE 48 576 48 416 WIRE 80 576 48 576 WIRE 128 576 80 576 WIRE 240 576 240 512 WIRE 240 576 192 576 WIRE 304 576 240 576 WIRE 400 576 400 512 WIRE 400 576 368 576 WIRE 656 576 656 528 WIRE 768 592 768 544 WIRE 960 592 960 544 WIRE 1184 592 1136 592 WIRE 1360 592 1264 592 WIRE 1408 592 1360 592 WIRE 1424 592 1408 592 WIRE 48 608 48 576 WIRE 1136 640 1136 592 WIRE 1360 656 1360 592 WIRE 224 688 96 688 WIRE 320 688 224 688 WIRE 448 688 384 688 WIRE 480 688 448 688 WIRE 224 720 224 688 WIRE 48 752 48 704 WIRE 1136 800 1136 720 WIRE 1360 800 1360 720 WIRE 224 832 224 800 FLAG 1424 144 VP FLAG 656 576 0 FLAG 112 320 0 FLAG 720 320 DRAIN FLAG 560 512 GATE FLAG 768 208 0 FLAG 960 592 0 FLAG 1136 800 0 FLAG 1408 592 EFF FLAG 768 592 0 FLAG 1360 800 0 FLAG -32 528 0 FLAG 80 576 rc FLAG 48 752 0 FLAG 224 832 0 FLAG 448 688 VP FLAG 1360 400 0 SYMBOL ind2 512 160 R270 WINDOW 0 -41 56 VTop 2 WINDOW 3 -53 53 VBottom 2 SYMATTR InstName L1 SYMATTR Value 10µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.06 SYMBOL schottky 1040 160 R270 WINDOW 0 -41 30 VTop 2 WINDOW 3 -49 27 VBottom 2 SYMATTR InstName D1 SYMATTR Value 10MQ060N SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage 112 160 R0 WINDOW 0 48 55 Left 2 WINDOW 3 54 83 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 9 SYMBOL nmos 608 432 R0 WINDOW 0 -48 -65 Left 2 WINDOW 3 -87 -29 Left 2 SYMATTR InstName M1 SYMATTR Value Si7218DN SYMBOL ind2 800 160 R270 WINDOW 0 -32 56 VTop 2 WINDOW 3 -41 57 VBottom 2 SYMATTR InstName L2 SYMATTR Value 90µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.06 SYMBOL res 528 496 R90 WINDOW 0 77 56 VBottom 2 WINDOW 3 84 57 VTop 2 SYMATTR InstName R3 SYMATTR Value 25 SYMBOL res 1216 160 R270 WINDOW 0 47 31 VTop 2 WINDOW 3 74 86 VBottom 2 SYMATTR InstName R6 SYMATTR Value 1m SYMBOL cap 1296 224 R90 WINDOW 0 68 59 VBottom 2 WINDOW 3 42 -8 VTop 2 SYMATTR InstName C3 SYMATTR Value 100m SYMBOL res 304 160 R270 WINDOW 0 48 28 VTop 2 WINDOW 3 76 86 VBottom 2 SYMATTR InstName R8 SYMATTR Value 1m SYMBOL cap 384 224 R90 WINDOW 0 73 60 VBottom 2 WINDOW 3 47 -14 VTop 2 SYMATTR InstName C6 SYMATTR Value 100m SYMBOL res 944 352 R0 WINDOW 0 -50 38 Left 2 WINDOW 3 -57 75 Left 2 SYMATTR InstName R9 SYMATTR Value 500 SYMBOL cap 944 480 R0 WINDOW 0 -46 35 Left 2 WINDOW 3 -50 68 Left 2 SYMATTR InstName C7 SYMATTR Value 30p SYMBOL bv 1136 624 R0 WINDOW 3 -558 118 Left 2 WINDOW 0 -89 56 Left 2 SYMATTR Value V= LIMIT (0, 100 * I(R6) * V(VP) / ( I(R8) * 9 ), 100 ) SYMATTR InstName B1 SYMBOL res 1280 576 R90 WINDOW 0 70 60 VBottom 2 WINDOW 3 78 61 VTop 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL res 752 352 R0 WINDOW 0 57 39 Left 2 WINDOW 3 51 80 Left 2 SYMATTR InstName R2 SYMATTR Value 200 SYMBOL cap 752 480 R0 WINDOW 0 60 34 Left 2 WINDOW 3 56 72 Left 2 SYMATTR InstName C1 SYMATTR Value 50p SYMBOL cap 1344 656 R0 WINDOW 0 -37 63 Left 2 WINDOW 3 -43 94 Left 2 SYMATTR InstName C2 SYMATTR Value 50n SYMBOL Digital\\schmtinv 128 512 R0 SYMATTR InstName A1 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL cap -48 448 R0 WINDOW 0 42 14 Left 2 WINDOW 3 42 51 Left 2 SYMATTR InstName C4 SYMATTR Value 1n SYMBOL res 80 432 R270 WINDOW 0 -36 57 VTop 2 WINDOW 3 -43 57 VBottom 2 SYMATTR InstName R4 SYMATTR Value 1K SYMBOL Digital\\schmtinv 304 448 R0 SYMATTR InstName A2 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 384 R0 SYMATTR InstName A3 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 512 R0 SYMATTR InstName A4 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 320 R0 SYMATTR InstName A5 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL nmos 96 608 M0 WINDOW 0 100 28 Left 2 WINDOW 3 79 72 Left 2 SYMATTR InstName M2 SYMATTR Value 2N7002 SYMBOL res 208 704 R0 WINDOW 0 -55 50 Left 2 WINDOW 3 -63 91 Left 2 SYMATTR InstName R5 SYMATTR Value 10K SYMBOL zener 320 704 R270 WINDOW 0 -47 34 VTop 2 WINDOW 3 -54 32 VBottom 2 SYMATTR InstName D2 SYMATTR Value ZNR48 SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 1344 288 R0 WINDOW 0 56 10 Left 2 WINDOW 3 56 55 Left 2 SYMATTR InstName C5 SYMATTR Value 5µ TEXT 1032 488 Left 2 !.tran 0 5m 0 10n uic TEXT 1064 392 Left 2 ;DUMB BOOST TEXT 1048 440 Left 2 ;JL June 28 2013 TEXT 648 104 Left 2 !K L1 L2 0.95 TEXT 144 624 Left 2 ;74HC14

That's how I designed the boost converter on my first mass-produced design in the early 90's, less the transformer (used only an inductor). But I used a BJT instead of the 7002 because that saved another penny. No kidding, and we also had the inductor made in Asia because then it cost less than a catalog part from a distributor.

I like your use of a B source, to plot efficiency. Must remember that one.

I rehashed it a bit, to include "proper" 74HC14s (actually a 74HC .lib file that purports to come from LT, but I can't remember where I got it from), and a real-life Zetex 39V zener.

Little if any difference.

I'd use a CD40106 instead. Even such logic-level FETs aren't all that happy at 5V or 6V if in a converter job.

On Sat, 29 Jun 2013 09:05:35 -0700, Fred Abse wrote:

I had to put in the 0,100 limits to avoid startup goofies, like teravolt spikes. So far, LT Spice doesn't seem to mind dividing by zero.

I used the generic LT Spice schmitt, tweaked a little. In real life, 5 paralleled HC14 or AC14 gates should drive a small mosfet pretty well.

I like the way it makes single shots after the cap is charged up. That's probably better, by accident, than most burst-mode chips would work.

One bug is the high continuous-mode startup current, so I added a soft-start. The ss cap will have to be discharged, of course.

I'd like to find a snubber circuit that recycles the energy, but that's probably too complex.

Version 4 SHEET 1 1920 968 WIRE 240 144 96 144 WIRE 320 144 240 144 WIRE 464 144 400 144 WIRE 528 144 464 144 WIRE 656 144 608 144 WIRE 816 144 768 144 WIRE 960 144 896 144 WIRE 1040 144 960 144 WIRE 1152 144 1104 144 WIRE 1200 144 1152 144 WIRE 1328 144 1280 144 WIRE 1344 144 1328 144 WIRE 1360 144 1344 144 WIRE 96 176 96 144 WIRE 768 208 768 144 WIRE 240 240 240 144 WIRE 320 240 240 240 WIRE 464 240 464 144 WIRE 464 240 384 240 WIRE 1152 240 1152 144 WIRE 1200 240 1152 240 WIRE 1328 240 1328 144 WIRE 1328 240 1264 240 WIRE 1328 288 1328 240 WIRE 656 304 656 144 WIRE 720 304 656 304 WIRE 768 304 720 304 WIRE 96 320 96 256 WIRE 656 336 656 304 WIRE 768 336 768 304 WIRE 960 336 960 144 WIRE 304 384 272 384 WIRE 400 384 368 384 WIRE 1328 400 1328 352 WIRE 656 432 656 416 WIRE 96 448 -48 448 WIRE 144 448 96 448 WIRE 272 448 272 384 WIRE 272 448 224 448 WIRE 304 448 272 448 WIRE 400 448 400 384 WIRE 400 448 368 448 WIRE 768 480 768 416 WIRE 960 480 960 416 WIRE -48 496 -48 448 WIRE 272 512 272 448 WIRE 304 512 272 512 WIRE 400 512 400 448 WIRE 400 512 368 512 WIRE 432 512 400 512 WIRE 560 512 512 512 WIRE 608 512 560 512 WIRE 96 576 96 448 WIRE 160 576 96 576 WIRE 272 576 272 512 WIRE 272 576 224 576 WIRE 304 576 272 576 WIRE 400 576 400 512 WIRE 400 576 368 576 WIRE 656 592 656 528 WIRE 768 592 768 544 WIRE 960 592 960 544 WIRE -48 608 -48 560 WIRE 1216 624 1152 624 WIRE 1344 624 1296 624 WIRE 1360 624 1344 624 WIRE 96 672 96 576 WIRE 160 672 96 672 WIRE 272 672 160 672 WIRE 464 672 400 672 WIRE 560 672 464 672 WIRE 688 672 624 672 WIRE 720 672 688 672 WIRE 1152 672 1152 624 WIRE 1360 688 1360 624 WIRE 272 704 272 672 WIRE 464 704 464 672 WIRE -48 720 -48 688 WIRE 96 720 96 672 WIRE 400 784 400 672 WIRE 400 784 320 784 WIRE -48 832 -48 784 WIRE 96 832 96 784 WIRE 272 832 272 800 WIRE 464 832 464 784 WIRE 1152 832 1152 752 WIRE 1360 832 1360 752 FLAG 1344 144 VP FLAG 656 592 0 FLAG 96 320 0 FLAG 720 304 DRAIN FLAG 560 512 GATE FLAG 768 208 0 FLAG 960 592 0 FLAG 1152 832 0 FLAG 1344 624 EFF FLAG 768 592 0 FLAG 1360 832 0 FLAG 96 832 0 FLAG 160 672 rc FLAG 272 832 0 FLAG 464 832 0 FLAG 688 672 VP FLAG 1328 400 0 FLAG -48 832 0 SYMBOL ind2 512 160 R270 WINDOW 0 -41 56 VTop 2 WINDOW 3 -53 53 VBottom 2 SYMATTR InstName L1 SYMATTR Value 3µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.06 SYMBOL schottky 1040 160 R270 WINDOW 0 -41 30 VTop 2 WINDOW 3 -49 27 VBottom 2 SYMATTR InstName D1 SYMATTR Value 10MQ060N SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage 96 160 R0 WINDOW 0 48 55 Left 2 WINDOW 3 54 83 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 9 SYMBOL nmos 608 432 R0 WINDOW 0 -55 -55 Left 2 WINDOW 3 -101 -22 Left 2 SYMATTR InstName M1 SYMATTR Value Si7218DN SYMBOL ind2 800 160 R270 WINDOW 0 -32 56 VTop 2 WINDOW 3 -41 57 VBottom 2 SYMATTR InstName L2 SYMATTR Value 27µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.06 SYMBOL res 528 496 R90 WINDOW 0 77 56 VBottom 2 WINDOW 3 84 57 VTop 2 SYMATTR InstName R3 SYMATTR Value 10 SYMBOL res 1184 160 R270 WINDOW 0 -38 31 VTop 2 WINDOW 3 -11 86 VBottom 2 SYMATTR InstName R6 SYMATTR Value 1m SYMBOL cap 1264 224 R90 WINDOW 0 68 59 VBottom 2 WINDOW 3 42 -8 VTop 2 SYMATTR InstName C3 SYMATTR Value 100m SYMBOL res 304 160 R270 WINDOW 0 -37 24 VTop 2 WINDOW 3 -11 88 VBottom 2 SYMATTR InstName R8 SYMATTR Value 1m SYMBOL cap 384 224 R90 WINDOW 0 73 60 VBottom 2 WINDOW 3 47 -14 VTop 2 SYMATTR InstName C6 SYMATTR Value 100m SYMBOL res 944 320 R0 WINDOW 0 -54 67 Left 2 WINDOW 3 -58 98 Left 2 SYMATTR InstName R9 SYMATTR Value 500 SYMBOL cap 944 480 R0 WINDOW 0 -46 35 Left 2 WINDOW 3 -50 68 Left 2 SYMATTR InstName C7 SYMATTR Value 30p SYMBOL bv 1152 656 R0 WINDOW 3 -557 141 Left 2 WINDOW 0 -83 97 Left 2 SYMATTR Value V= LIMIT (0, 100 * I(R6) * V(VP) / ( I(R8) * 9 ), 100 ) SYMATTR InstName B1 SYMBOL res 1312 608 R90 WINDOW 0 70 60 VBottom 2 WINDOW 3 78 61 VTop 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL res 752 320 R0 WINDOW 0 53 70 Left 2 WINDOW 3 51 100 Left 2 SYMATTR InstName R2 SYMATTR Value 200 SYMBOL cap 752 480 R0 WINDOW 0 60 34 Left 2 WINDOW 3 56 72 Left 2 SYMATTR InstName C1 SYMATTR Value 200p SYMBOL cap 1344 688 R0 WINDOW 0 -37 63 Left 2 WINDOW 3 -43 94 Left 2 SYMATTR InstName C2 SYMATTR Value 50n SYMBOL Digital\\schmtinv 160 512 R0 SYMATTR InstName A1 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=0.5 SYMATTR SpiceLine Td=5n SYMBOL cap 80 720 R0 WINDOW 0 44 60 Left 2 WINDOW 3 37 92 Left 2 SYMATTR InstName C4 SYMATTR Value 500p SYMBOL res 128 464 R270 WINDOW 0 88 58 VTop 2 WINDOW 3 74 57 VBottom 2 SYMATTR InstName R4 SYMATTR Value 2K SYMBOL Digital\\schmtinv 304 448 R0 SYMATTR InstName A2 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 384 R0 SYMATTR InstName A3 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 512 R0 SYMATTR InstName A4 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 320 R0 SYMATTR InstName A5 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL nmos 320 704 M0 WINDOW 0 77 0 Left 2 WINDOW 3 63 35 Left 2 SYMATTR InstName M2 SYMATTR Value 2N7002 SYMBOL res 448 688 R0 WINDOW 0 47 64 Left 2 WINDOW 3 40 94 Left 2 SYMATTR InstName R5 SYMATTR Value 10K SYMBOL zener 560 688 R270 WINDOW 0 50 33 VTop 2 WINDOW 3 -13 35 VBottom 2 SYMATTR InstName D2 SYMATTR Value 1N5369B SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 1312 288 R0 WINDOW 0 -61 48 Left 2 WINDOW 3 -59 81 Left 2 SYMATTR InstName C5 SYMATTR Value 5µ SYMBOL res 640 320 R0 WINDOW 0 47 70 Left 2 WINDOW 3 41 102 Left 2 SYMATTR InstName R7 SYMATTR Value 1m SYMBOL cap -64 720 R0 WINDOW 0 61 23 Left 2 WINDOW 3 50 60 Left 2 SYMATTR InstName C8 SYMATTR Value 330n SYMBOL res -64 592 R0 WINDOW 0 59 52 Left 2 WINDOW 3 67 87 Left 2 SYMATTR InstName R10 SYMATTR Value 1K SYMBOL schottky -64 496 R0 WINDOW 0 55 23 Left 2 WINDOW 3 25 77 Left 2 SYMATTR InstName D3 SYMATTR Value SMS7621 SYMATTR Description Diode SYMATTR Type diode TEXT 1064 432 Left 2 !.tran 0 5m 0 10n uic TEXT 1040 488 Left 2 ;PRETTY DUMB BOOST TEXT 1072 544 Left 2 ;JL June 29 2013 TEXT 648 104 Left 2 !K L1 L2 0.95 TEXT 192 624 Left 2 ;74HC14 TEXT -40 384 Left 2 ;SOFT TEXT -48 416 Left 2 ;START TEXT 160 504 Left 2 ;OSC TEXT 696 168 Left 2 ;1:3

You could also get a cheap inverting gate driver with Schmitt input, runs that as oscillator. If it is one without UVLO you could feed it directly off of your 9V input.

We stock LM5112, a pretty cool part. That would work. It might turn out that there is some fet with nice specs but prefers more gate drive. Might need another resistor to futz the duty cycle.

But it costs over a dollar! An AC14 is 10 cents!

It should cost less than 50c. Where do you buy them? Other chips:

And, if you calculate the energy savings over the next 200 years you'll have it all back. When it has to be ral chep I use a CD40106 and a PNP/NPN follower. Some seciton of the 40106 can be rented out.

Simulation slows to a crawl as the converter starts up. Not so much a slow, as a delayed, start. Surge current is substantially the same, just delayed.

This is what I'm seeing. Without the soft-start, the input current peaks at about 1.6 amps.

This run took a couple of minutes on my 5-year old HP. The soft-start doesn't affect the sim speed much; I get around 13 PPM of real time during chargeup.

That's not what I see. I see about 10 microseconds chargeup, followed by a slowdown to a few ps/s as the converter starts, then molasses.

With, or without the soft start, current draw as the switch starts is a tad under 5 amps peak.

Without the slow start, a run takes about a minute.

Fred Abse wrote:

Didn't slow down much here. But yesterday the fan speed cranked itself to the hilt. I took the PC apart and found an incredible amount of dog hair and dust in the cooling fins for the processor. One of our Labradors has a habit of curling himself up in the small space under my PC workplace.

Here is a version with a better soft-start:

Version 4 SHEET 1 1920 1076 WIRE 208 48 96 48 WIRE 96 144 96 48 WIRE 240 144 96 144 WIRE 320 144 240 144 WIRE 464 144 400 144 WIRE 528 144 464 144 WIRE 656 144 608 144 WIRE 816 144 768 144 WIRE 960 144 896 144 WIRE 1040 144 960 144 WIRE 1152 144 1104 144 WIRE 1200 144 1152 144 WIRE 1328 144 1280 144 WIRE 1344 144 1328 144 WIRE 1360 144 1344 144 WIRE 96 176 96 144 WIRE 768 208 768 144 WIRE 240 240 240 144 WIRE 320 240 240 240 WIRE 464 240 464 144 WIRE 464 240 384 240 WIRE 1152 240 1152 144 WIRE 1200 240 1152 240 WIRE 1328 240 1328 144 WIRE 1328 240 1264 240 WIRE 1328 288 1328 240 WIRE 656 304 656 144 WIRE 720 304 656 304 WIRE 768 304 720 304 WIRE 96 320 96 256 WIRE 656 336 656 304 WIRE 768 336 768 304 WIRE 960 336 960 144 WIRE 304 384 272 384 WIRE 400 384 368 384 WIRE 1328 400 1328 352 WIRE 656 432 656 416 WIRE 144 448 96 448 WIRE 272 448 272 384 WIRE 272 448 224 448 WIRE 304 448 272 448 WIRE 400 448 400 384 WIRE 400 448 368 448 WIRE 768 480 768 416 WIRE 960 480 960 416 WIRE 272 512 272 448 WIRE 304 512 272 512 WIRE 400 512 400 448 WIRE 400 512 368 512 WIRE 432 512 400 512 WIRE 560 512 512 512 WIRE 608 512 560 512 WIRE 96 576 96 448 WIRE 160 576 96 576 WIRE 272 576 272 512 WIRE 272 576 224 576 WIRE 304 576 272 576 WIRE 400 576 400 512 WIRE 400 576 368 576 WIRE 656 592 656 528 WIRE 768 592 768 544 WIRE 960 592 960 544 WIRE 1216 624 1152 624 WIRE 1344 624 1296 624 WIRE 1360 624 1344 624 WIRE 96 672 96 576 WIRE 160 672 96 672 WIRE 272 672 160 672 WIRE 464 672 400 672 WIRE 560 672 464 672 WIRE 688 672 624 672 WIRE 720 672 688 672 WIRE 1152 672 1152 624 WIRE 1360 688 1360 624 WIRE 272 704 272 672 WIRE 464 704 464 672 WIRE 96 720 96 672 WIRE 400 784 400 672 WIRE 400 784 320 784 WIRE 96 832 96 784 WIRE 272 832 272 800 WIRE 1152 832 1152 752 WIRE 1360 832 1360 752 WIRE 240 912 144 912 WIRE 368 912 320 912 WIRE 464 912 464 784 WIRE 464 912 432 912 WIRE 464 928 464 912 WIRE 464 1056 464 1008 FLAG 1344 144 VP FLAG 656 592 0 FLAG 96 320 0 FLAG 720 304 DRAIN FLAG 560 512 GATE FLAG 768 208 0 FLAG 960 592 0 FLAG 1152 832 0 FLAG 1344 624 EFF FLAG 768 592 0 FLAG 1360 832 0 FLAG 96 832 0 FLAG 160 672 rc FLAG 272 832 0 FLAG 464 1056 0 FLAG 688 672 VP FLAG 1328 400 0 FLAG 208 48 9V FLAG 144 912 9V SYMBOL ind2 512 160 R270 WINDOW 0 -41 56 VTop 2 WINDOW 3 -53 53 VBottom 2 SYMATTR InstName L1 SYMATTR Value 3µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.06 SYMBOL schottky 1040 160 R270 WINDOW 0 -41 30 VTop 2 WINDOW 3 -49 27 VBottom 2 SYMATTR InstName D1 SYMATTR Value 10MQ060N SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage 96 160 R0 WINDOW 0 48 55 Left 2 WINDOW 3 54 83 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V2 SYMATTR Value 9 SYMBOL nmos 608 432 R0 WINDOW 0 -55 -55 Left 2 WINDOW 3 -101 -22 Left 2 SYMATTR InstName M1 SYMATTR Value Si7218DN SYMBOL ind2 800 160 R270 WINDOW 0 -32 56 VTop 2 WINDOW 3 -41 57 VBottom 2 SYMATTR InstName L2 SYMATTR Value 27µ SYMATTR Type ind SYMATTR SpiceLine Rser=0.06 SYMBOL res 528 496 R90 WINDOW 0 77 56 VBottom 2 WINDOW 3 84 57 VTop 2 SYMATTR InstName R3 SYMATTR Value 10 SYMBOL res 1184 160 R270 WINDOW 0 -38 31 VTop 2 WINDOW 3 -11 86 VBottom 2 SYMATTR InstName R6 SYMATTR Value 1m SYMBOL cap 1264 224 R90 WINDOW 0 68 59 VBottom 2 WINDOW 3 42 -8 VTop 2 SYMATTR InstName C3 SYMATTR Value 100m SYMBOL res 304 160 R270 WINDOW 0 -37 24 VTop 2 WINDOW 3 -11 88 VBottom 2 SYMATTR InstName R8 SYMATTR Value 1m SYMBOL cap 384 224 R90 WINDOW 0 73 60 VBottom 2 WINDOW 3 47 -14 VTop 2 SYMATTR InstName C6 SYMATTR Value 100m SYMBOL res 944 320 R0 WINDOW 0 -54 67 Left 2 WINDOW 3 -58 98 Left 2 SYMATTR InstName R9 SYMATTR Value 500 SYMBOL cap 944 480 R0 WINDOW 0 -46 35 Left 2 WINDOW 3 -50 68 Left 2 SYMATTR InstName C7 SYMATTR Value 30p SYMBOL bv 1152 656 R0 WINDOW 3 -557 141 Left 2 WINDOW 0 -83 97 Left 2 SYMATTR Value V= LIMIT (0, 100 * I(R6) * V(VP) / ( I(R8) * 9 ), 100 ) SYMATTR InstName B1 SYMBOL res 1312 608 R90 WINDOW 0 70 60 VBottom 2 WINDOW 3 78 61 VTop 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL res 752 320 R0 WINDOW 0 53 70 Left 2 WINDOW 3 51 100 Left 2 SYMATTR InstName R2 SYMATTR Value 200 SYMBOL cap 752 480 R0 WINDOW 0 60 34 Left 2 WINDOW 3 56 72 Left 2 SYMATTR InstName C1 SYMATTR Value 200p SYMBOL cap 1344 688 R0 WINDOW 0 -37 63 Left 2 WINDOW 3 -43 94 Left 2 SYMATTR InstName C2 SYMATTR Value 50n SYMBOL Digital\\schmtinv 160 512 R0 SYMATTR InstName A1 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=0.5 SYMATTR SpiceLine Td=5n SYMBOL cap 80 720 R0 WINDOW 0 44 60 Left 2 WINDOW 3 37 92 Left 2 SYMATTR InstName C4 SYMATTR Value 500p SYMBOL res 128 464 R270 WINDOW 0 88 58 VTop 2 WINDOW 3 74 57 VBottom 2 SYMATTR InstName R4 SYMATTR Value 2K SYMBOL Digital\\schmtinv 304 448 R0 SYMATTR InstName A2 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 384 R0 SYMATTR InstName A3 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 512 R0 SYMATTR InstName A4 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL Digital\\schmtinv 304 320 R0 SYMATTR InstName A5 SYMATTR Value2 Vhigh=5 Vt=2.5 Vh=1 SYMATTR SpiceLine Td=5n SYMBOL nmos 320 704 M0 WINDOW 0 77 0 Left 2 WINDOW 3 63 35 Left 2 SYMATTR InstName M2 SYMATTR Value 2N7002 SYMBOL res 448 688 R0 WINDOW 0 47 64 Left 2 WINDOW 3 40 94 Left 2 SYMATTR InstName R5 SYMATTR Value 10K SYMBOL zener 560 688 R270 WINDOW 0 50 33 VTop 2 WINDOW 3 -13 35 VBottom 2 SYMATTR InstName D2 SYMATTR Value 1N5369B SYMATTR Description Diode SYMATTR Type diode SYMBOL cap 1312 288 R0 WINDOW 0 -61 48 Left 2 WINDOW 3 -59 81 Left 2 SYMATTR InstName C5 SYMATTR Value 5µ SYMBOL res 640 320 R0 WINDOW 0 47 70 Left 2 WINDOW 3 41 102 Left 2 SYMATTR InstName R7 SYMATTR Value 1m SYMBOL res 448 912 R0 WINDOW 0 47 64 Left 2 WINDOW 3 40 94 Left 2 SYMATTR InstName R11 SYMATTR Value 10K SYMBOL cap 368 928 R270 WINDOW 0 32 32 VTop 2 WINDOW 3 0 32 VBottom 2 SYMATTR InstName C9 SYMATTR Value 0.33µ SYMBOL res 224 928 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 0 56 VBottom 2 SYMATTR InstName R10 SYMATTR Value 10k TEXT 1064 432 Left 2 !.tran 0 15m 0 10n uic TEXT 1040 488 Left 2 ;PRETTY DUMB BOOST TEXT 1072 544 Left 2 ;JL June 29 2013 TEXT 648 104 Left 2 !K L1 L2 0.95 TEXT 192 624 Left 2 ;74HC14 TEXT 296 1016 Left 2 ;SOFT TEXT 296 1040 Left 2 ;START TEXT 160 504 Left 2 ;OSC TEXT 696 168 Left 2 ;1:3

Sorry, mistaken, 490 microseconds chargeup.

-- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)

Cute, but the oscillator runs a bit at powerup. The zener forward conducts and clamps the 2N7002 gate voltage to VP+0.6. Not lethal, since VP jumps up and fixes things pretty quickly. With my 4000 uF load, it might be a problem.

With 4000 uF, my soft-start would have to be pretty slow. Maybe we'll do it digitally after all.

I'm in a situation where the customer is in a frenzy to get this thing done. We quoted 5 weeks ARO for a first article, so they waited a few weeks and then cut a PO stating 4 weeks. Two weeks later, we have no spec hard enough to design around, and no answers to basic questions, like how does this thing mount? How big can it be? What's the orientation of the connector? What's the required output, into what load?

So I play with the power inverter now and then, and rest up for the panic.

That's better. Initial current draw at switch start down to ~1.2 amps.

Runs faster, too, similar to the original circuit.

I was playing with something similar, hold the switch off by pushing an artificial feedback voltage, then let it ramp down to meet the real feedback coming up. Feedforward, overtaken by feedback.

Bingo! It was the Skyworks schottky model that was causing the problem. It's a 2V mixer diode (BV=3), not suitable. What model were you using? I was using the "official" model per Skyworks datasheet.

Using a Diodes inc. 1N5711, I now get what you got.

Your SMS 7621 model must differ from mine.

Must admit I prefer Joerg's soft start, though;-)

I just pulled it out of the LT Spice list. I'm not sure where it came from. The devices aren't sorted alphabetically, so I tend to pick the first thing that might work.

The SMS7621 is actually good to maybe 6 volts in real life. 0.22 pF. We use a lot of them.

It does have the startup burst bug.