MOSFET dynamic ripple filter for variable supply

[Paul E. Schoen]

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

==============================================================================

Version 4
SHEET 1 880 680
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SYMATTR SpiceLine Rser=0.05
SYMBOL diode -80 336 R90
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SYMATTR Value MURS320
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SYMATTR Value MURS320
SYMBOL diode -144 128 R270
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SYMATTR Value MURS320
SYMBOL diode -144 208 R270
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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
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SYMATTR Type cap
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pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1
SYMBOL nmos 272 160 R270
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SYMATTR Value STB120NF10
SYMBOL polcap -80 -16 R90
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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
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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

 

[neon]

how an SCR is supposed to save power? once ON it stays on unless you turn it off somehow. you have a lot of copper in the front endat those currents it is very significant. Just as a suggestion remove the variable transformer and and make your regulator regulate 50k mfd man that is one hell of a cap. i can get better with regulation with a LM117 and a bypass trx then you will ever do.

 

[ehsjr]

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

==============================================================================

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
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WIRE 448 0 368 0
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SYMATTR SpiceLine Rser=0.05
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SYMATTR Value MURS320
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SYMATTR InstName D4
SYMATTR Value MURS320
SYMBOL polcap 128 176 R0
WINDOW 3 24 64 Left 0
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SYMATTR Value 50000µ
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SYMATTR Type cap
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SYMBOL polcap 224 256 R0
WINDOW 3 24 64 Left 0
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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
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SYMATTR Value STB120NF10
SYMBOL polcap -80 -16 R90
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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
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SYMATTR Value MURS120
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SYMATTR InstName D6
SYMATTR Value MURS120
SYMBOL res 384 -16 R90
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SYMATTR Value 10k
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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

How are you going to get 16 amps out
of the regulator with only 8 amps
in?

Ed

 

[Paul E. Schoen]

[snip ASCII file]

How are you going to get 16 amps out
of the regulator with only 8 amps
in?

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

 

[John]

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.

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

John

 

[Paul E. Schoen]

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

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

============================================================================

Version 4
SHEET 1 1108 680
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SYMATTR SpiceLine Rser=0.05
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SYMBOL diode -80 336 R90
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SYMATTR InstName D1
SYMATTR Value MURS320
SYMBOL diode -80 256 R90
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WINDOW 3 32 32 VTop 0
SYMATTR InstName D2
SYMATTR Value MURS320
SYMBOL diode -144 128 R270
WINDOW 0 32 32 VTop 0
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SYMATTR InstName D3
SYMATTR Value MURS320
SYMBOL diode -144 208 R270
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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
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pn="UPR2A470MPH" type="Al electrolytic" ppPkg=1
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SYMATTR Value MURS120
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SYMATTR Value 20k
SYMBOL res 672 240 R0
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SYMATTR Value 5k
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SYMATTR InstName L1
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SYMATTR Type ind
SYMBOL ind2 -304 240 R0
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SYMATTR Value 1N5818
SYMATTR Description Diode
SYMATTR Type diode
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WINDOW 3 24 64 Left 0
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SYMATTR Type cap
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SYMATTR Description Capacitor
SYMATTR Type cap
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WINDOW 3 32 56 VTop 0
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SYMATTR Value 2k
SYMBOL res 592 304 R90
WINDOW 0 0 56 VBottom 0
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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

 

[John]

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 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