dc/dc soft start

Here's my soft start circuit. Thanks for suggestions from Tilmann and
Jeroen.

The idea is to drive the ends of the transformer with in-phase square
waves and then slowly walk them to 180 degrees, to achieve a soft
start. An FPGA can do that easily. We might take a second... but that
would simulate forever.

What works well is to use two spice "modulator" elements and FM the
frequency of one of them for 50 ms, until they are 180 degrees out of
phase.

Spice BV elements with sin(time*freq) type expressions seemed to
overload or something.

Version 4
SHEET 1 2568 1220
WIRE -512 -944 -576 -944
WIRE -400 -944 -512 -944
WIRE 16 -944 -32 -944
WIRE 48 -944 16 -944
WIRE 256 -944 192 -944
WIRE 304 -944 256 -944
WIRE -1104 -912 -1136 -912
WIRE -1040 -912 -1104 -912
WIRE -400 -912 -400 -944
WIRE -32 -912 -32 -944
WIRE 192 -912 192 -944
WIRE -128 -896 -144 -896
WIRE -80 -896 -128 -896
WIRE -848 -880 -896 -880
WIRE -800 -880 -848 -880
WIRE -128 -848 -144 -848
WIRE -80 -848 -128 -848
WIRE -576 -816 -576 -944
WIRE -784 -800 -848 -800
WIRE -720 -800 -784 -800
WIRE -624 -800 -720 -800
WIRE -400 -800 -400 -832
WIRE -32 -800 -32 -832
WIRE 192 -800 192 -832
WIRE -848 -768 -848 -800
WIRE -624 -752 -656 -752
WIRE -576 -688 -576 -736
WIRE -448 -688 -576 -688
WIRE -368 -688 -448 -688
WIRE -208 -688 -304 -688
WIRE 160 -688 -32 -688
WIRE 208 -688 160 -688
WIRE 416 -688 272 -688
WIRE 480 -688 416 -688
WIRE 544 -688 480 -688
WIRE 656 -688 544 -688
WIRE -848 -656 -848 -688
WIRE -576 -640 -576 -688
WIRE -656 -624 -656 -752
WIRE -624 -624 -656 -624
WIRE -720 -576 -720 -800
WIRE -624 -576 -720 -576
WIRE 208 -576 112 -576
WIRE 416 -576 416 -688
WIRE 416 -576 272 -576
WIRE -1024 -560 -1104 -560
WIRE -880 -560 -944 -560
WIRE -848 -560 -880 -560
WIRE -1104 -544 -1104 -560
WIRE -848 -544 -848 -560
WIRE -656 -528 -656 -624
WIRE -576 -528 -576 -560
WIRE -208 -528 -208 -688
WIRE -32 -528 -32 -688
WIRE 416 -512 416 -576
WIRE 544 -512 544 -688
WIRE 656 -512 656 -688
WIRE -1104 -448 -1104 -464
WIRE -848 -448 -848 -480
WIRE -528 -400 -576 -400
WIRE -480 -400 -528 -400
WIRE -576 -368 -576 -400
WIRE 160 -368 160 -688
WIRE 208 -368 160 -368
WIRE 416 -368 416 -448
WIRE 416 -368 272 -368
WIRE -784 -352 -848 -352
WIRE -720 -352 -784 -352
WIRE -624 -352 -720 -352
WIRE -848 -320 -848 -352
WIRE -624 -304 -656 -304
WIRE -576 -240 -576 -288
WIRE -448 -240 -576 -240
WIRE -368 -240 -448 -240
WIRE -208 -240 -208 -448
WIRE -208 -240 -304 -240
WIRE -32 -240 -32 -448
WIRE 112 -240 112 -576
WIRE 112 -240 -32 -240
WIRE 208 -240 112 -240
WIRE 416 -240 416 -368
WIRE 416 -240 272 -240
WIRE 544 -240 544 -432
WIRE 544 -240 416 -240
WIRE 656 -240 656 -432
WIRE 656 -240 544 -240
WIRE -848 -208 -848 -240
WIRE -576 -192 -576 -240
WIRE 416 -192 416 -240
WIRE -656 -176 -656 -304
WIRE -624 -176 -656 -176
WIRE -1072 -160 -1152 -160
WIRE -880 -128 -928 -128
WIRE -848 -128 -880 -128
WIRE -720 -128 -720 -352
WIRE -624 -128 -720 -128
WIRE -1152 -112 -1152 -160
WIRE -656 -80 -656 -176
WIRE -576 -80 -576 -112
FLAG -400 -800 0
FLAG -512 -944 +48
FLAG -32 -800 0
FLAG -128 -896 HA
FLAG -128 -848 HB
FLAG 16 -944 BRO
FLAG -448 -688 HA
FLAG -448 -240 HB
FLAG -576 -528 0
FLAG -656 -528 0
FLAG -576 -80 0
FLAG -528 -400 +48
FLAG -656 -80 0
FLAG 416 -192 0
FLAG 480 -688 OUT
FLAG -784 -800 A
FLAG 256 -944 SS
FLAG 192 -800 0
FLAG -1104 -912 SS
FLAG -1152 -112 0
FLAG -784 -352 B
FLAG -1104 -448 0
FLAG -880 -560 AB
FLAG -848 -448 0
FLAG -848 -208 0
FLAG -880 -128 K
FLAG -848 -656 0
FLAG -848 -880 J
SYMBOL voltage -400 -928 R0
WINDOW 0 53 46 Left 2
WINDOW 3 53 78 Left 2
SYMATTR InstName V3
SYMATTR Value 48
SYMBOL e -32 -928 R0
WINDOW 0 45 50 Left 2
WINDOW 3 49 78 Left 2
SYMATTR InstName E5
SYMATTR Value 1
SYMBOL sw -576 -720 M180
WINDOW 0 58 74 Left 2
WINDOW 3 61 47 Left 2
SYMATTR InstName S2
SYMATTR Value Sj
SYMBOL sw -576 -544 M180
WINDOW 0 58 74 Left 2
WINDOW 3 61 47 Left 2
SYMATTR InstName S3
SYMATTR Value Sj
SYMBOL sw -576 -272 M180
WINDOW 0 58 74 Left 2
WINDOW 3 61 47 Left 2
SYMATTR InstName S4
SYMATTR Value Sj
SYMBOL sw -576 -96 M180
WINDOW 0 58 74 Left 2
WINDOW 3 61 47 Left 2
SYMATTR InstName S5
SYMATTR Value Sj
SYMBOL cap -304 -704 R90
WINDOW 0 79 30 VBottom 2
WINDOW 3 86 29 VTop 2
SYMATTR InstName C1
SYMATTR Value 20µ
SYMBOL FerriteBead -336 -240 R90
WINDOW 0 -37 -1 VBottom 2
SYMATTR InstName L1
SYMATTR Value 2.5µ
SYMATTR SpiceLine Ipk=3 Rser=0.0023 Rpar=475 Cpar=1.75p
SYMBOL ind2 -224 -544 R0
WINDOW 0 -49 37 Left 2
WINDOW 3 -61 76 Left 2
SYMATTR InstName L2
SYMATTR Value 72µ
SYMATTR Type ind
SYMBOL ind2 -48 -544 R0
WINDOW 0 61 40 Left 2
WINDOW 3 50 76 Left 2
SYMATTR InstName L3
SYMATTR Value 112µ
SYMATTR Type ind
SYMBOL schottky 208 -672 R270
WINDOW 0 42 40 VTop 2
WINDOW 3 -14 54 VBottom 2
SYMATTR InstName D1
SYMATTR Value RB168VAM150
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL cap 400 -512 R0
WINDOW 0 -58 36 Left 2
WINDOW 3 -70 67 Left 2
SYMATTR InstName C6
SYMATTR Value 100µ
SYMBOL res 528 -528 R0
WINDOW 0 -63 51 Left 2
WINDOW 3 -61 83 Left 2
SYMATTR InstName R5
SYMATTR Value 5K
SYMBOL schottky 208 -560 R270
WINDOW 0 39 33 VTop 2
WINDOW 3 -14 55 VBottom 2
SYMATTR InstName D2
SYMATTR Value RB168VAM150
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky 272 -384 R90
WINDOW 0 -10 31 VBottom 2
WINDOW 3 45 28 VTop 2
SYMATTR InstName D3
SYMATTR Value RB168VAM150
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL schottky 272 -256 R90
WINDOW 0 -16 31 VBottom 2
WINDOW 3 43 32 VTop 2
SYMATTR InstName D4
SYMATTR Value RB168VAM150
SYMATTR Description Diode
SYMATTR Type diode
SYMBOL current 656 -512 R0
WINDOW 0 54 63 Left 2
WINDOW 3 19 107 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName I1
SYMATTR Value PULSE(0 0.5 75m 1m 0 9)
SYMBOL voltage 192 -928 R0
WINDOW 0 58 36 Left 2
WINDOW 3 41 74 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(0 1 0 1u 1u 50m)
SYMBOL SpecialFunctions\modulate -1040 -912 R0
WINDOW 0 56 29 Left 2
SYMATTR InstName A1
SYMATTR SpiceLine space=250K
SYMATTR SpiceLine2 mark=250.01K
SYMBOL SpecialFunctions\modulate -1072 -160 R0
WINDOW 0 56 31 Left 2
SYMATTR InstName A2
SYMATTR SpiceLine space=250K
SYMATTR SpiceLine2 mark=250K
SYMBOL bv -1104 -560 R0
WINDOW 0 46 98 Left 2
WINDOW 3 30 135 Left 2
SYMATTR InstName B1
SYMATTR Value V=V(A) * V(B)
SYMBOL res -928 -576 R90
WINDOW 0 66 56 VBottom 2
WINDOW 3 65 57 VTop 2
SYMATTR InstName R1
SYMATTR Value 1K
SYMBOL bv -848 -336 R0
WINDOW 0 -89 48 Left 2
WINDOW 3 -185 94 Left 2
SYMATTR InstName B2
SYMATTR Value V=sgn(V(K))
SYMBOL bv -848 -784 R0
WINDOW 0 -89 59 Left 2
WINDOW 3 -183 96 Left 2
SYMATTR InstName B3
SYMATTR Value V=sgn(V(J))
SYMBOL cap -864 -544 R0
WINDOW 0 -53 40 Left 2
WINDOW 3 -66 68 Left 2
SYMATTR InstName C2
SYMATTR Value 100n
TEXT 216 -136 Left 4 ;P970 DC-DC Converter
TEXT 288 -88 Left 3 ;JL  Jan 15  2025
TEXT -200 -80 Left 2 !.model Sj SW(Ron=50m Roff=1g)
TEXT -200 -128 Left 2 !.tran 0 100m 0 20n
TEXT -192 -568 Left 2 ;PL300-101L
TEXT -160 -488 Left 2 ;4T
TEXT -96 -488 Left 2 ;5T
TEXT -200 -720 Left 2 !K1 L2 L3 0.999
TEXT -392 -192 Left 2 ;163-5001
TEXT -392 -152 Left 2 ;2.7 uH 5A
TEXT -168 -600 Left 2 ;Coilcraft
TEXT 232 -816 Left 2 ;50 ms soft start