spread-spectrum model

I'm designing a switching power supply module and could reduce EMI by
going spread-spectrum on the switching frequency. The simple one below
reduces things by 20 dB. Probe the SS node and FFT.

The ss inside switching reg chips is no doubt more sophisticated. In
an FPGA, we could do some sort of pseudo-random thing.

On a multi-channel power supply, there may be some small advantage to
have a separate spread per channel. That would be easy.


Version 4
SHEET 1 880 680
WIRE 144 80 80 80
WIRE 240 80 144 80
WIRE 80 112 80 80
WIRE 432 112 384 112
WIRE 464 112 432 112
WIRE 80 224 80 192
FLAG 80 224 0
FLAG 144 80 MOD
FLAG 432 112 SS
SYMBOL voltage 80 96 R0
WINDOW 0 43 80 Left 2
WINDOW 3 12 111 Left 2
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR InstName V1
SYMATTR Value PULSE(0 1 0 1m 1u 1u 1m)
SYMBOL SpecialFunctions\modulate 240 80 R0
WINDOW 0 48 -48 Left 2
SYMATTR InstName A1
SYMATTR SpiceLine mark=220K  space=280K
TEXT 462 54 Left 2 !.tran 2m
TEXT 400 144 Left 2 ;Basic spread-spectrum
TEXT 408 176 Left 2 ;for P943 8-ch supply
TEXT 432 208 Left 2 ;JL Apr 18 2024