Driving capacitive loads with an H bridge
in [Design]
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From: Tim Williams on 26 Jul 2010 14:35 "Grant" <omg(a)grrr.id.au> wrote in message news:rjmq46tuls8137jsu1fo71r8p4srcs023k(a)4ax.com... > Rarely see push-pull transformer these days, more common to see half or > full bridge drive, to utilise copper all the time instead of half the > time? > > Though centre-tap output windings are common, to save on expensive > diodes, so copper utilisation perhaps a poor argument? Typical of automotive stuff, for the same reason: conduction loss dominates over copper loss at low voltages. And for really low voltages, you use MOSFETs instead of schottkies, in a synchronous rectifier. PP at line voltages would be silly, because you have twice the peak voltage (requiring 800V+ transistors for 240V input) and a lot more turns, increasing capacitance that you don't need in a hard switching forward converter. Half bridge is great at line voltages because it only sees full rated voltage and needs one winding. Tim -- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
From: John Fields on 26 Jul 2010 15:18
On Mon, 26 Jul 2010 07:50:22 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >From the OP's loose description (he says a few amps, which I took as >2A), I got 36uF :-( > > ...Jim Thompson --- I originally misread the OP's post WRT frequency, and the work I did was based on 100Hz. The only "concrete" spec's he had were a triangular 300Hz, 40VRMS waveform across across a 3�F cap. At 300Hz, the reactance of a 3�F cap is about 177 ohms and would require about a 94 millihenry choke for resonance. Switching the supply through the choke would result in a sinusoidal voltage across the cap, and since the OP wanted a triangle, Larkin's suggestion of using a larger choke to increase its reactance and more nearly approximate a "lossless" current source for the cap was a good one. I reworked the circuit and increased the inductance to 1 henry, which will bring the choke's reactance up to about 1900 ohms and lower the circuit's resonance to about 92Hz. Hammond has a nice 1 henry, 300 mA, 40 ohm choke, its 158T, which is a little pricey (?) at $18.49 from Mouser, but they have at least 1 in stock. It's starting to look like this thing might be able to be built! (And work) ;) Version 4 SHEET 1 1228 680 WIRE 32 -16 -48 -16 WIRE 176 -16 112 -16 WIRE 336 -16 256 -16 WIRE 448 -16 336 -16 WIRE 576 -16 448 -16 WIRE 336 48 336 -16 WIRE 576 48 576 -16 WIRE 992 48 912 48 WIRE 288 64 272 64 WIRE 640 64 624 64 WIRE 288 112 240 112 WIRE 672 112 624 112 WIRE -48 160 -48 -16 WIRE 912 176 912 48 WIRE 336 192 336 128 WIRE 416 192 336 192 WIRE 576 192 576 128 WIRE 576 192 480 192 WIRE 864 192 576 192 WIRE 336 240 336 192 WIRE 864 240 336 240 WIRE 336 272 336 240 WIRE 576 272 576 192 WIRE 288 288 272 288 WIRE 640 288 624 288 WIRE 64 336 64 304 WIRE 176 336 176 304 WIRE -48 432 -48 240 WIRE 64 432 64 416 WIRE 64 432 -48 432 WIRE 176 432 176 416 WIRE 176 432 64 432 WIRE 240 432 240 112 WIRE 240 432 176 432 WIRE 288 432 288 336 WIRE 288 432 240 432 WIRE 336 432 336 352 WIRE 336 432 288 432 WIRE 576 432 576 352 WIRE 576 432 336 432 WIRE 624 432 624 336 WIRE 624 432 576 432 WIRE 672 432 672 112 WIRE 672 432 624 432 WIRE 912 432 912 256 WIRE 912 432 672 432 WIRE -48 512 -48 432 FLAG -48 512 0 FLAG 64 304 A FLAG 272 64 A FLAG 640 288 A FLAG 176 304 B FLAG 272 288 B FLAG 640 64 B FLAG 992 48 Vcap FLAG 448 -16 Sup SYMBOL voltage -48 144 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 12 SYMBOL sw 336 368 M180 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S1 SYMBOL sw 336 144 M180 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S2 SYMBOL voltage 64 320 R0 WINDOW 0 -53 5 Left 0 WINDOW 3 -242 110 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(1 0 0 1E-6 1E-6 .0016666 .003333) SYMBOL sw 576 144 R180 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S3 SYMBOL sw 576 368 R180 WINDOW 0 32 15 Left 0 WINDOW 3 32 44 Left 0 SYMATTR InstName S4 SYMBOL cap 416 208 R270 WINDOW 0 32 32 VTop 0 WINDOW 3 0 32 VBottom 0 SYMATTR InstName C2 SYMATTR Value 3e-6 SYMBOL ind 16 0 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L2 SYMATTR Value 1 SYMBOL voltage 176 320 R0 WINDOW 0 -53 5 Left 0 WINDOW 3 -242 110 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V3 SYMATTR Value PULSE(0 1 0 1E-6 1E-6 .0016666 .003333) SYMBOL e 912 160 R0 SYMATTR InstName E1 SYMATTR Value 1 SYMBOL res 160 0 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 0 56 VBottom 0 SYMATTR InstName R1 SYMATTR Value 40 TEXT -40 480 Left 0 !.model SW SW(Ron=0.1 Roff=1E6 Vt=0.5 Vh=0) TEXT -32 456 Left 0 !.tran .1 TEXT 168 8 Left 0 ;/ TEXT 56 40 Left 0 ;INDUCTOR TEXT 56 64 Left 0 ;SERIES R JF |