Hello,
my master power supply provides me with synchronously rectified, but not filtered (!) AC current. For the used 12V toroidal transformer the output waveform is thus 17V*abs(sin(50Hz)), up to 40A. I want to use this supply to power a bank of 10 SEPIC switchers. Issume 20A load in the worst case. It means I must post-rectify the input current in order to feed a big filter capacitor. The capacitor cannot be moved to the point directly after the primary rectifier, because its control circuit monitors the transformer's voltage, not the input-output difference as a diode would do (a design feature + I really don't want it that way).
A simple big-enough Schottky diode would solve the problem, but it would produce too much heat at 20 amps. Alternatively, I think about a secondary synchronous rectifier based on a PMOS. Below is what I came up with. The circuit basically simulates OK, but:
- Is the real performance of LT1716 anywhere close to the SPICE model, especially if it comes to its advertised "over the top" capability?
- The rising edge of the current on R6 contains a 40 mA bump with negative flow direction (you need to magnify the "plateau" to see it). It's not much, but I don't like it. Should I care? Its origin is a kind of mystery, because the JFET-based gate discharge circuit seems to work like a charm.
- How can I solve this problem better? The volume of the final circuit will be whopping 1 or 2 instances (it's a hobby project), so I am not repelled by exotic parts. The reliability is, however, very important.
Best regards, Piotr
Version 4 SHEET 1 1880 708 WIRE 896 -16 640 -16 WIRE 992 -16 976 -16 WIRE 1280 -16 992 -16 WIRE 1408 -16 1360 -16 WIRE 1504 -16 1488 -16 WIRE 1280 0 1280 -16 WIRE 640 16 640 -16 WIRE 1056 32 1024 32 WIRE 1024 64 1024 32 WIRE 1024 64 992 64 WIRE 1408 64 1408 -16 WIRE 1408 64 1328 64 WIRE 112 96 -128 96 WIRE 320 96 112 96 WIRE 336 96 320 96 WIRE 464 96 416 96 WIRE 560 96 560 64 WIRE 560 96 544 96 WIRE 656 96 656 64 WIRE 704 96 656 96 WIRE 768 96 704 96 WIRE 832 96 768 96 WIRE 1024 96 832 96 WIRE 1056 96 1024 96 WIRE 1280 96 1056 96 WIRE 128 160 112 160 WIRE 224 160 208 160 WIRE 320 160 304 160 WIRE 112 176 112 160 WIRE 320 176 320 160 WIRE 656 176 656 96 WIRE 656 176 592 176 WIRE 464 208 464 96 WIRE 624 208 464 208 WIRE 592 240 592 176 WIRE 624 240 624 208 WIRE 1024 240 1024 96 WIRE 576 272 544 272 WIRE 704 272 704 96 WIRE 704 272 640 272 WIRE 544 288 544 272 WIRE 608 336 608 304 WIRE 1024 336 1024 320 WIRE 1024 336 608 336 WIRE 1504 336 1504 -16 WIRE 1504 336 1024 336 FLAG 112 240 0 FLAG 320 240 0 FLAG -128 176 0 FLAG 768 160 0 FLAG 832 176 0 FLAG 544 288 0 SYMBOL schottky 128 160 R180 WINDOW 0 24 64 Left 2 WINDOW 3 42 38 Left 2 SYMATTR InstName D1 SYMATTR Value MBR745 SYMATTR Description Diode SYMATTR Type diode SYMBOL voltage 224 160 R90 WINDOW 0 -32 56 VBottom 2 WINDOW 3 32 56 VTop 2 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value SINE(0 17 50) SYMBOL res 320 144 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 1m SYMBOL schottky 336 160 R180 WINDOW 0 -31 10 Left 2 WINDOW 3 241 179 Left 2 SYMATTR InstName D2 SYMATTR Value MBR745 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 336 240 R180 WINDOW 0 -32 63 Left 2 WINDOW 3 -103 34 Left 2 SYMATTR InstName D3 SYMATTR Value MBR745 SYMATTR Description Diode SYMATTR Type diode SYMBOL schottky 128 240 R180 WINDOW 0 24 64 Left 2 WINDOW 3 49 24 Left 2 SYMATTR InstName D4 SYMATTR Value MBR745 SYMATTR Description Diode SYMATTR Type diode SYMBOL res 816 80 R0 WINDOW 3 57 76 Left 2 SYMATTR Value 10 SYMATTR InstName R2 SYMBOL cap 752 96 R0 SYMATTR InstName C1
SYMBOL res -144 80 R0 SYMATTR InstName R18 SYMATTR Value 10 SYMBOL res 432 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 1m SYMBOL Comparators\\LT1716 608 272 R90 SYMATTR InstName U1 SYMBOL pmos 560 16 M90 SYMATTR InstName M1 SYMATTR Value Si4427DY SYMBOL res 992 -32 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 1m SYMBOL zener 1040 32 R0 WINDOW 0 49 24 Left 2 WINDOW 3 18 -13 Left 2 SYMATTR InstName D5 SYMATTR Value BZX84C12L SYMATTR Description Diode SYMATTR Type diode SYMBOL njf 1328 0 M0 SYMATTR InstName J1 SYMATTR Value U309 SYMBOL res 1504 -32 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R10 SYMATTR Value 1m SYMBOL res 1376 -32 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 1k SYMBOL res 976 -32 R0 SYMATTR InstName R14 SYMATTR Value 1m SYMBOL res 560 80 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 1m SYMBOL res 1008 224 R0 SYMATTR InstName R5 SYMATTR Value 1K TEXT -32 264 Left 2 !.tran 0.15