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MOSFET dynamic ripple filter for variable supply

I have a high current DC supply consisting of a 0-140 VAC variable transformer driving the primary of a transformer with 20-0-20 VAC output at about 8 amps. I have the output switchable for FWCT or FWB with some 50,000 uF capacitors to get about 0-25 VAC or 0-50 VAC at nominal 10A or 5A, with occasional surges up to 3x. It works OK for rough work, but the regulation and ripple are very poor.

I designed a circuit which provides some output voltage load regulation, but especially very good output ripple of only about 20 mV P-P at 8 volts out into 0.5 ohms load (16 A). At 100 ohms load the output voltage is 11.6 VDC, and at 10 ohms it is 11.2 VDC, and at 2 ohms it is 10.17 VDC. Not bad for a simple supply. I also added optional current limiting, which actually provides current regulation by using a transistor monitoring the current sensing shunt, but I may replace the transistor with an SCR to latch the output off, rather than burn up lots of power in the MOSFET.

The MOSFET burns about 25 watts with the 0.5 ohm load, 11 watts with the 2 ohm load, and less then 3 watts for 10 ohms. These figures are all for 16 VAC from the transformer. These are all values obtained using a simulation in LTSpice. The ASC file follows.

I have not built it yet, but I think it is a workable design. However, it is somewhat dependent on the characteristics of the MOSFET, as discussed some time ago. I tried several models, and all worked, with some adjustment of the bias resistors. I'll let you know if and when I actually build it.

Paul

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Version 4 SHEET 1 880 680 WIRE -144 0 -256 0 WIRE 0 0 -80 0 WIRE 96 0 64 0 WIRE 144 0 96 0 WIRE 240 0 224 0 WIRE 288 0 240 0 WIRE 448 0 368 0 WIRE 96 32 96 0 WIRE -256 112 -256 0 WIRE -208 112 -256 112 WIRE -144 112 -208 112 WIRE 144 112 -80 112 WIRE 272 112 144 112 WIRE 624 112 368 112 WIRE 656 112 624 112 WIRE 656 144 656 112 WIRE 144 176 144 112 WIRE -144 192 -176 192 WIRE -80 192 -80 112 WIRE 0 208 0 0 WIRE 352 208 352 160 WIRE 496 208 352 208 WIRE 240 224 240 0 WIRE 352 224 352 208 WIRE 352 224 240 224 WIRE 240 256 240 224 WIRE 656 256 656 224 WIRE 656 256 640 256 WIRE -256 272 -256 112 WIRE -208 272 -208 112 WIRE -144 272 -208 272 WIRE -176 352 -176 192 WIRE -144 352 -176 352 WIRE -80 352 -80 272 WIRE 96 352 96 96 WIRE 96 352 -80 352 WIRE 144 352 144 240 WIRE 144 352 96 352 WIRE 240 352 240 320 WIRE 240 352 144 352 WIRE 384 352 240 352 WIRE 448 352 448 0 WIRE 448 352 384 352 WIRE 496 352 496 304 WIRE 496 352 448 352 WIRE 656 352 656 336 WIRE 656 352 496 352 WIRE 384 384 384 352 WIRE -256 416 -256 352 WIRE -176 416 -176 352 WIRE -176 416 -256 416 WIRE 0 416 0 272 WIRE 0 416 -176 416 FLAG 384 384 0 FLAG 624 112 Vout SYMBOL voltage -256 256 R0 WINDOW 3 -37 181 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 -37 204 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 16 60) SYMATTR SpiceLine Rser=0.05 SYMBOL diode -80 336 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D1 SYMATTR Value MURS320 SYMBOL diode -80 256 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D2 SYMATTR Value MURS320 SYMBOL diode -144 128 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMATTR Value MURS320 SYMBOL diode -144 208 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D4 SYMATTR Value MURS320 SYMBOL polcap 128 176 R0 WINDOW 3 24 64 Left 0 SYMATTR InstName C1 SYMATTR Value 50000µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=16 Irms=2.62 Rser=0.022 MTBF=5000 Lser=0 ppPkg=1 SYMBOL res 640 128 R0 SYMATTR InstName R1 SYMATTR Value 1 SYMBOL res 240 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 9k SYMBOL polcap 224 256 R0 WINDOW 3 24 64 Left 0 SYMATTR InstName C2 SYMATTR Value 47µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1 SYMBOL nmos 272 160 R270 WINDOW 0 -14 27 VRight 0 WINDOW 3 71 12 VRight 0 SYMATTR InstName M1 SYMATTR Value STB120NF10 SYMBOL polcap -80 -16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 47µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1 SYMBOL polcap 80 32 R0 SYMATTR InstName C4 SYMATTR Value 47µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1 SYMBOL diode 16 272 R180 WINDOW 0 24 72 Left 0 WINDOW 3 -20 -18 VRight 0 SYMATTR InstName D5 SYMATTR Value MURS120 SYMBOL diode 0 16 R270 WINDOW 0 -30 33 VTop 0 WINDOW 3 62 38 VBottom 0 SYMATTR InstName D6 SYMATTR Value MURS120 SYMBOL res 384 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 10k SYMBOL res 640 240 R0 SYMATTR InstName R4 SYMATTR Value 0.02 SYMBOL npn 560 208 M0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL res 656 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 100 TEXT -32 432 Left 0 !.tran 2s TEXT 440 384 Left 0 ;32 A current limit (optional) TEXT 712 176 Left 0 ;Load

Reply to
Paul E. Schoen
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How are you going to get 16 amps out of the regulator with only 8 amps in?

Ed

Reply to
ehsjr

[snip ASCII file]

In a FWCT configuration, each arm of the transformer contributes 16 amps at

50% duty cycle, so it effectively sees 8 amps. The actual RMS current through the transformer is about 40% higher than the DC output, or about 11 amps, because of the capacitor charging to peak. But transformers can be overloaded by 40% for intermittent duty, about 5 minutes on and 5 minutes off. At my maximum output current of about 35 amps, the transformer will see the equivalent of 25 amps, or about 3x overload. At that level it will only be on for about 10% duty cycle, perhaps 10 seconds on and 2 minutes off.

Paul

Reply to
Paul E. Schoen

Watch out for the ripple current in C1 (about 12.5A RMS in the model you posted).

John

Reply to
John

"John" wrote in message news:ep5eef$sgm$ snipped-for-privacy@aioe.org...

I changed the ESR to 0.5 ohms which is probably more realistic. Now the ripple current is more like 4 amps, but the power dissipation is about 10 watts. Of course that changes with applied voltage and load.

I made a modification to the circuit so that the MOSFET will be biased according to the amount of ripple on the output. This results in a maximum ripple of 1 volt P-P under most conditions. This also works for either FWCT or FWB configurations, where the fixed bias resistors R2 and R3 previously had to be changed. They could also be replaced with a pot, which could be set for minimum ripple and best efficiency. Probably an op-amp circuit could be added to perform this function even better, but I like the simpler approach.

I also added a center tapped transformer model so the FWCT and FWB configurations could be tested. The new ASCII file follows.

Paul

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Version 4 SHEET 1 1108 680 WIRE -144 0 -256 0 WIRE 0 0 -80 0 WIRE 96 0 64 0 WIRE 144 0 96 0 WIRE 240 0 224 0 WIRE 288 0 240 0 WIRE 448 0 368 0 WIRE 96 32 96 0 WIRE 976 48 816 48 WIRE 1008 48 976 48 WIRE -288 112 -384 112 WIRE -256 112 -256 0 WIRE -256 112 -288 112 WIRE -208 112 -256 112 WIRE -144 112 -208 112 WIRE 144 112 -80 112 WIRE 272 112 144 112 WIRE 624 112 368 112 WIRE 688 112 624 112 WIRE 736 112 688 112 WIRE 864 112 800 112 WIRE 896 112 864 112 WIRE 1008 112 1008 48 WIRE 1008 112 960 112 WIRE -384 128 -384 112 WIRE -288 144 -288 112 WIRE 688 144 688 112 WIRE 144 176 144 112 WIRE -144 192 -176 192 WIRE -80 192 -80 112 WIRE 240 192 240 0 WIRE 352 192 352 160 WIRE 352 192 240 192 WIRE 384 192 352 192 WIRE 496 192 464 192 WIRE 864 192 864 112 WIRE 1008 192 1008 112 WIRE -384 208 -384 176 WIRE 0 208 0 0 WIRE -288 240 -288 224 WIRE -224 240 -288 240 WIRE 576 240 560 240 WIRE 816 240 816 48 WIRE 816 240 656 240 WIRE -288 256 -288 240 WIRE 240 256 240 192 WIRE 688 256 688 224 WIRE -208 272 -208 112 WIRE -144 272 -208 272 WIRE 496 320 496 288 WIRE 576 320 576 240 WIRE -176 352 -176 192 WIRE -144 352 -176 352 WIRE -80 352 -80 272 WIRE 96 352 96 96 WIRE 144 352 144 240 WIRE 144 352 96 352 WIRE 240 352 240 320 WIRE 240 352 144 352 WIRE 384 352 240 352 WIRE 448 352 448 0 WIRE 448 352 384 352 WIRE 496 352 496 320 WIRE 496 352 448 352 WIRE 688 352 688 336 WIRE 688 352 496 352 WIRE 864 352 864 256 WIRE 864 352 688 352 WIRE 1008 352 1008 256 WIRE 1008 352 864 352 WIRE 384 384 384 352 WIRE -384 416 -384 208 WIRE -288 416 -288 336 WIRE -288 416 -384 416 WIRE -176 416 -176 352 WIRE -176 416 -288 416 WIRE 0 416 0 272 WIRE 0 416 -176 416 WIRE -224 448 -224 240 WIRE 96 448 96 352 WIRE 96 448 -224 448 FLAG 384 384 0 FLAG 624 112 Vout FLAG 976 48 Vripple SYMBOL voltage -384 112 R0 WINDOW 3 -146 120 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 -118 146 Left 0 SYMATTR Value SINE(0 30 60) SYMATTR SpiceLine Rser=0.05 SYMATTR InstName V1 SYMBOL diode -80 336 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D1 SYMATTR Value MURS320 SYMBOL diode -80 256 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName D2 SYMATTR Value MURS320 SYMBOL diode -144 128 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D3 SYMATTR Value MURS320 SYMBOL diode -144 208 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D4 SYMATTR Value MURS320 SYMBOL polcap 128 176 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 40000µ SYMATTR InstName C1 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=40 Irms=30 Rser=0.5 MTBF=5000 Lser=0 ppPkg=1 SYMBOL res 672 128 R0 SYMATTR InstName R1 SYMATTR Value 2 SYMBOL res 240 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL polcap 224 256 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 47µ SYMATTR InstName C2 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 ppPkg=1 SYMBOL nmos 272 160 R270 WINDOW 0 -14 27 VRight 0 WINDOW 3 71 12 VRight 0 SYMATTR InstName M1 SYMATTR Value STB120NF10 SYMBOL polcap -80 -16 R90 WINDOW 0 0 32 VBottom 0 WINDOW 3 32 32 VTop 0 SYMATTR InstName C3 SYMATTR Value 47µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1 SYMBOL polcap 80 32 R0 SYMATTR InstName C4 SYMATTR Value 47µ SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 mfg="Nichicon" pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1 SYMBOL diode 16 272 R180 WINDOW 0 24 72 Left 0 WINDOW 3 -20 -18 VRight 0 SYMATTR InstName D5 SYMATTR Value MURS120 SYMBOL diode 0 16 R270 WINDOW 0 -30 33 VTop 0 WINDOW 3 62 38 VBottom 0 SYMATTR InstName D6 SYMATTR Value MURS120 SYMBOL res 384 -16 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 20k SYMBOL res 672 240 R0 SYMATTR InstName R4 SYMATTR Value 0.02 SYMBOL npn 560 192 M0 WINDOW 3 32 57 Left 0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL res 672 224 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 5k SYMBOL ind2 -304 128 R0 SYMATTR InstName L1 SYMATTR Value 100m SYMATTR Type ind SYMBOL ind2 -304 240 R0 SYMATTR InstName L2 SYMATTR Value 100m SYMATTR Type ind SYMBOL schottky 880 256 R180 WINDOW 0 24 72 Left 0 WINDOW 3 24 0 Left 0 SYMATTR InstName D7 SYMATTR Value 1N5818 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 896 128 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName D8 SYMATTR Value 1N5818 SYMATTR Description Diode SYMATTR Type diode SYMBOL polcap 992 192 R0 WINDOW 3 24 64 Left 0 SYMATTR Value 10µ SYMATTR InstName C6 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 ppPkg=1 SYMBOL polcap 800 96 R90 WINDOW 3 32 32 VTop 0 WINDOW 0 0 32 VBottom 0 SYMATTR Value 10µ SYMATTR InstName C5 SYMATTR Description Capacitor SYMATTR Type cap SYMATTR SpiceLine V=100 Irms=210m Rser=0.37 MTBF=2000 Lser=0 ppPkg=1 SYMBOL res 480 176 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 2k SYMBOL res 592 304 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 15 -3 VTop 0 SYMATTR InstName R7 SYMATTR Value 50k TEXT 160 416 Left 0 !.tran 5s TEXT 440 384 Left 0 ;32 A current limit (optional) TEXT 728 200 Left 0 ;Load TEXT -360 440 Left 0 !K1 L1 L2 1

Reply to
Paul E. Schoen

I don't think .5 ohms is realistic. A quick scan of the Mouser catalog showed ESRs around .02 ohms for the capacitance and voltage ratings in your netlist. I adjusted your design to 47000uF, 25V, .0218 ohms ESR (Vripple specified at 9.3A RMS) and ran it for ripple currents. Inrush is about 140A peak the first half cycle if the input voltage starts at zero. Starting the input voltage off at a peak gives a peak current of 300A.

Steady state ripple current is over 10A.

Add more capacitance and put in some inrush limiting. For inrush limiting in AC motor speed controllers, they used to put in series resistors and a contactor. The coil was connected to the line (after the power switch) and the contacts shorted out the series resistors after the short time it takes for the contacts to transition. You could use a second set of contacts to connect the load so it would be less of a burden during inrush.

Just some thoughts. Good luck.

John

Reply to
John

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