I love the background, but it doesn't seem to illuminate the present
project. How do they relate?
For low voltage inputs, push-pull is often better. You don't spend the
voltage drop of two switches in series, as you do with an H-bridge, and the
extra winding loss is quite tolerable as the primary halves are only a few
turns, often of copper strap or something like that.
Note your schematic is missing an inductor. You can put it on the primary
(like L3, but larger, and /after/ C2), or on the secondary (after the
diodes, before C1). Without, turn-on dI/dt is limited by LL only, most
likely defeating any subsequent attempts at current mode control and current
limiting.
Don't follow the trap of the automotive inverter. They use such a circuit,
typically an open loop TL494. The only reason the poor things start up at
all, is a big fat capacitor on the soft-start pin. They're a house of cards
on a wagon... start it moving slowly enough and it might not fall, but
that's a far sight from being reliable.
Also, being open loop, they aren't regulated, and can't be controlled to
regulate (at least not in any reasonable manner).
Story warning:
Watch out for parasitic winding capacitance. The last high voltage forward
converter I made had a combination of ills:
- Poorly wound transformer. Ran out of space on the bobbin, so the primary
and secondary weren't well coupled. Lots of leakage.
- The leakage caused large voltage overshoot at the rectifier (FWB, choke
input filter). This necessitated 2kV rectifiers -- which is to say, pairs
of UF4007s in series.
- The diodes still got very hot. R+C dampers were added to the winding, and
caps piggybacked on the diodes. An awful hack, at this point, and still
insufficient. (Diodes tend to behave poorly when connected in series,
because they don't recover at quite the same time, thus the early bird wins
the high voltage worm, and burns power in avalanche.)
This was for about 100W and +/-200VDC. Quite some years ago. It would've
definitely benefitted from a more holistic design process. Hacking doesn't
work out very well!
For the project, I later smashed the whole converter section, and rebuilt it
(point to point, like a real man!) with flyback topology. The new
transformer was wound on a similar sized core, but it was taller, so had
more winding width. I got the primary and split secondaries in single
layers, same number of turns, phased so that no AC voltage appears between
layers, only DC. (HV flyback transformers are designed this way -- single
layers, with a diode into the next layer, stacked all the way up. It's the
only way to get HV, at variable-up-to-100kHz operation, in a high resolution
CRT monitor!)
Result? Much more efficient. All the passive stuff (transformer, diodes
and filtering) ran quite cool. (I also tossed in some 1200V 4A SiC
schottkys, just because.) The switching transistor had one flame-out, which
was solved by buffering its gate drive with a proper driver chip (the UC3842
controller wasn't quite powerful enough on its own).
So, that's 100W. At 500-1000W, you will be better off with a forward
converter. At that ratio, you certainly won't be able to use a transmission
line transformer approach (which, after all, is kind of what I constructed
above), so you'll still need to mind the windings. 500V 1A is 500 ohms,
pretty high, so the parasitic capacitance will easily show up. Try to
balance LL and Cp so that 500 ohms ~= sqrt(LL/Cp), secondary referred that
is.
Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
Website: http://seventransistorlabs.com
"P E Schoen" <paul@pstech-inc.com> wrote in message
news:npltb0$3od$1@dont-email.me...
> First, some background.
>
> I have designed an SCR trigger board using an IRS24541 self-oscillating
> FWB driver and two dual MOSFETs FDS6930B (30V, 5A, 0.055 RdsOn, 2W). I
> made a couple of transformers using EPCOS E187 or E16-8-5 cores with N27
> ferrite having an Al of 950 nH. I used about 20 turns of #32 AWG wire on
> both primary and secondary, to get 12 VDC out with 12 VDC input. Running
> at 65 kHz I get about 11.5 VDC output into a 250 mA load (about 3W). The
> windings measure about 400 uH with 25 uH leakage inductance, and they seem
> to run very cool.
>
> I ordered some samples from www.customcoils.com with target price of about
> $10 each, and they measure about 500 uH with only 2 uH leakage inductance.
> My transformers used a split bobbin for high isolation (for 480 VAC mains,
> so 4kV test), while his are layer wound with heavy insulation. They also
> work fine.
>
> The drive is 50% duty cycle with only a couple hundred nSec dead time, so
> the rectified output is essentially solid DC with just a tiny droop during
> the crossover. I used 220 uF in parallel with 1 uF ceramic. I was somewhat
> concerned that there might be high current spikes on the square wave
> transitions, but that does not appear to be the case. Some waveforms:
>
> http://enginuitysystems.com/pix/electronics/SCR_DC-DC_Waveform_2993.jpg
>
> http://enginuitysystems.com/pix/electronics/SCR_DC-DC_Waveform_2997.jpg
>
> My first prototype transformer:
> http://enginuitysystems.com/pix/electronics/SCR_Transformer_2981.jpg
>
> A second prototype with heavier wire #28:
> http://enginuitysystems.com/pix/electronics/SCR_Transformer_2989.jpg
> http://enginuitysystems.com/pix/electronics/SCR_Trigger_Board_2998.jpg
>
> The board with the sample transformers:
> http://enginuitysystems.com/pix/electronics/SCR_PCB_3034.jpg
>
> Now for the present project. I want to generate 320 or 640 volts nominal
> at 500 to 1000 watts or more, from batteries of 12, 24, or 48 VDC nominal.
> I would not expect any more than 500 watts from the 12V source. I plan to
> make a transformer using an E55/28/21 core pair of N27 material, for which
> Al=5800 nH. At 50 kHz, I think 3 turns on each of four primary windings
> would be about right, with about 50 uH inductance, and about 16 ohms. The
> 24V P-P is about 12 volts RMS and about 1.2 amps no load. The secondary
> would have 38 turns each on four windings which would provide about 150
> volts each, connected in series or parallel for 600 or 300 volts.
>
> I ran a simulation that seemed to show about 95% efficiency. I will
> probably use a PIC to provide the 50 kHz square wave PWM, and a pair of
> FAN7382 hi-lo drivers. The LTSpice file is at the end of this post. I'd
> appreciate any opinions on this design. It just seems too simple but seems
> to work well in simulation and actual circuit.
>
> Thanks,
>
> Paul
>
> ====================================================
> Version 4
> SHEET 1 880 840
> WIRE -160 16 -464 16
> WIRE 0 16 -160 16
> WIRE -560 80 -640 80
> WIRE -464 80 -464 16
> WIRE -464 80 -480 80
> WIRE -160 112 -160 16
> WIRE -224 144 -384 144
> WIRE 0 144 0 16
> WIRE -224 192 -224 144
> WIRE -208 192 -224 192
> WIRE 240 192 176 192
> WIRE 320 192 304 192
> WIRE 368 192 320 192
> WIRE 416 192 368 192
> WIRE -384 208 -384 144
> WIRE -160 208 -320 208
> WIRE -464 224 -464 80
> WIRE -464 224 -544 224
> WIRE -48 224 -256 224
> WIRE -256 240 -256 224
> WIRE 176 240 176 192
> WIRE 224 240 176 240
> WIRE 320 240 320 192
> WIRE -160 256 -160 208
> WIRE 48 256 -160 256
> WIRE 112 256 48 256
> WIRE 176 256 176 240
> WIRE -640 272 -640 80
> WIRE -544 272 -544 224
> WIRE -464 272 -464 224
> WIRE 416 272 416 192
> WIRE -320 288 -320 208
> WIRE -320 288 -384 288
> WIRE 320 288 320 240
> WIRE 0 320 0 240
> WIRE 0 320 -256 320
> WIRE -320 352 -384 352
> WIRE -80 352 -256 352
> WIRE 176 352 176 336
> WIRE 256 352 256 240
> WIRE 256 352 176 352
> WIRE 0 368 0 320
> WIRE 48 368 0 368
> WIRE 112 368 112 336
> WIRE 112 368 48 368
> WIRE 224 368 224 240
> WIRE 256 368 224 368
> WIRE 320 368 320 352
> WIRE -544 384 -544 336
> WIRE -464 384 -464 336
> WIRE -464 384 -544 384
> WIRE -384 384 -384 352
> WIRE -256 384 -256 352
> WIRE 0 384 0 368
> WIRE -160 400 -160 256
> WIRE 176 432 176 352
> WIRE 256 432 176 432
> WIRE 320 432 320 368
> WIRE 416 432 416 352
> WIRE 416 432 320 432
> WIRE -80 464 -80 352
> WIRE -48 464 -80 464
> WIRE -320 480 -320 352
> WIRE -208 480 -320 480
> WIRE -80 480 -80 464
> WIRE 416 496 416 432
> WIRE -640 528 -640 352
> WIRE -464 528 -464 384
> WIRE -464 528 -640 528
> WIRE -384 528 -384 464
> WIRE -384 528 -464 528
> WIRE -320 528 -384 528
> WIRE -256 528 -256 464
> WIRE -256 528 -320 528
> WIRE -160 528 -160 496
> WIRE -160 528 -176 528
> WIRE 0 528 0 480
> WIRE 0 528 -160 528
> WIRE -320 608 -320 528
> FLAG -320 608 0
> FLAG 416 496 0
> FLAG 368 192 out
> FLAG 48 256 p1
> FLAG 48 368 p2
> SYMBOL ind2 96 240 R0
> WINDOW 0 -21 38 Left 2
> WINDOW 3 -49 83 Left 2
> SYMATTR InstName L1
> SYMATTR Value 100�
> SYMATTR Type ind
> SYMATTR SpiceLine Rser=1m
> SYMBOL ind2 192 240 M0
> WINDOW 0 -26 34 Left 2
> WINDOW 3 -40 69 Left 2
> SYMATTR InstName L2
> SYMATTR Value 5000�
> SYMATTR Type ind
> SYMATTR SpiceLine Rser=10m
> SYMBOL nmos -208 400 R0
> SYMATTR InstName M1
> SYMATTR Value IRLH5036
> SYMBOL nmos -48 384 R0
> SYMATTR InstName M2
> SYMATTR Value IRLH5036
> SYMBOL voltage -640 256 R0
> WINDOW 123 0 0 Left 2
> WINDOW 39 7 147 Left 2
> SYMATTR SpiceLine Rser=50m
> SYMATTR InstName V1
> SYMATTR Value 48
> SYMBOL cap 304 288 R0
> WINDOW 3 24 64 Left 2
> SYMATTR Value 5�
> SYMATTR InstName C1
> SYMATTR SpiceLine V=500 Irms=1.01 Rser=0.01359 Lser=0
> SYMBOL res 400 256 R0
> SYMATTR InstName R1
> SYMATTR Value 100
> SYMBOL nmos -208 112 R0
> SYMATTR InstName M3
> SYMATTR Value IRLH5036
> SYMBOL nmos -48 144 R0
> SYMATTR InstName M4
> SYMATTR Value IRLH5036
> SYMBOL voltage -384 192 R0
> WINDOW 123 0 0 Left 2
> WINDOW 39 -17 112 Left 2
> WINDOW 3 -289 379 Left 2
> SYMATTR SpiceLine Rser=10
> SYMATTR Value PULSE(0 10 0 40n 40n 9.5u 20u)
> SYMATTR InstName V4
> SYMBOL voltage -256 224 R0
> WINDOW 123 0 0 Left 2
> WINDOW 39 -25 113 Left 2
> WINDOW 3 -433 374 Left 2
> SYMATTR SpiceLine Rser=10
> SYMATTR Value PULSE(0 10 10u 40n 40n 9.5u 20u)
> SYMATTR InstName V5
> SYMBOL ind -576 96 R270
> WINDOW 0 32 56 VTop 2
> WINDOW 3 5 56 VBottom 2
> SYMATTR InstName L3
> SYMATTR Value 10n
> SYMATTR SpiceLine Rser=10u
> SYMBOL polcap -480 272 R0
> WINDOW 3 24 64 Left 2
> SYMATTR Value 1000�
> SYMATTR InstName C2
> SYMATTR Description Capacitor
> SYMATTR Type cap
> SYMATTR SpiceLine V=63 Irms=185m Rser=50m Lser=0
> SYMBOL cap -560 272 R0
> SYMATTR InstName C4
> SYMATTR Value 1�
> SYMATTR SpiceLine Rser=50m
> SYMBOL res -272 544 R270
> WINDOW 0 32 56 VTop 2
> WINDOW 3 0 56 VBottom 2
> SYMATTR InstName R3
> SYMATTR Value 10m
> SYMBOL voltage -256 368 R0
> WINDOW 123 0 0 Left 2
> WINDOW 39 -95 126 Left 2
> WINDOW 3 -424 321 Left 2
> SYMATTR SpiceLine Rser=10
> SYMATTR Value PULSE(0 10 0 40n 40n 9.5u 20u)
> SYMATTR InstName V2
> SYMBOL voltage -384 368 R0
> WINDOW 123 0 0 Left 2
> WINDOW 39 -59 103 Left 2
> WINDOW 3 -307 280 Left 2
> SYMATTR SpiceLine Rser=10
> SYMATTR Value PULSE(0 10 10u 40n 40n 9.5u 20u)
> SYMATTR InstName V3
> SYMBOL diode 240 176 M90
> WINDOW 0 0 32 VBottom 2
> WINDOW 3 32 32 VTop 2
> SYMATTR InstName D1
> SYMATTR Value RF1005TF6S
> SYMBOL diode 256 224 M90
> WINDOW 0 0 32 VBottom 2
> WINDOW 3 32 32 VTop 2
> SYMATTR InstName D2
> SYMATTR Value RF1005TF6S
> SYMBOL diode 320 352 R90
> WINDOW 0 0 32 VBottom 2
> WINDOW 3 32 32 VTop 2
> SYMATTR InstName D3
> SYMATTR Value RF1005TF6S
> SYMBOL diode 320 416 R90
> WINDOW 0 0 32 VBottom 2
> WINDOW 3 32 32 VTop 2
> SYMATTR InstName D4
> SYMATTR Value RF1005TF6S
> TEXT 16 392 Left 2 !K1 L1 L2 0.98
> TEXT -712 544 Left 2 !.tran 200m startup
>