I've finished up the PCB design of my new class-D power amp, so now the kids have to change the FPGA and ARM code to match. One problem is current limiting. We have a current shunt on the amp output, but we don't sense the voltage across the load.
We were brainstorming and simulating current limit schemes that we can implement in the FPGA, which drives the DAC into my amp, and sees the sensed load current. A hard clipper needs a lot of gain, and the customer load is unknown, so a high-gain loop might be unstable.
So I was wondering (about 3 AM) how bad a low-gain clipper would be, when it occurrs to me that a class-D amp has current gain from its power supply into the load, so we can have a higher current limit at low voltage out, for instance when the customer shorts our output. It's an anti-foldback current limit in a system that can't sense the output voltage.
This seems to work in an analog model. At any rate, it makes a cool waveform at Isense. The current limit goes up when the load resistance goes down. I'm still trying to get my brain comfortable with that. More coffee is indicated.
At the extreme implementation of anti-foldback, a graph of current limit vs output voltage would be really strange, uuuuuu with spikes to infinity.
Version 4 SHEET 1 1224 724 WIRE 1120 16 432 16 WIRE 432 64 432 16 WIRE -64 80 -128 80 WIRE 0 80 -64 80 WIRE 176 80 80 80 WIRE 288 80 176 80 WIRE 384 80 288 80 WIRE 1072 112 1024 112 WIRE 1120 112 1120 16 WIRE 1120 112 1072 112 WIRE 384 128 352 128 WIRE -128 144 -128 80 WIRE 176 160 176 80 WIRE 352 176 352 128 WIRE 432 176 432 144 WIRE 1120 176 1120 112 WIRE 176 288 176 224 WIRE 272 288 176 288 WIRE 432 288 352 288 WIRE 512 288 432 288 WIRE 672 288 592 288 WIRE 720 288 672 288 WIRE -128 304 -128 224 WIRE 720 352 720 288 WIRE 272 368 256 368 WIRE 304 368 272 368 WIRE 880 368 768 368 WIRE 976 368 880 368 WIRE 1120 368 1120 256 WIRE 1120 368 1056 368 WIRE 304 400 304 368 WIRE 880 400 880 368 WIRE 432 416 432 288 WIRE 432 416 336 416 WIRE 784 416 768 416 WIRE 176 432 176 288 WIRE 272 432 176 432 WIRE 432 448 336 448 WIRE 480 448 432 448 WIRE 496 448 480 448 WIRE 1120 448 1120 368 WIRE 720 464 720 432 WIRE 784 464 784 416 WIRE 784 464 720 464 WIRE -96 480 -128 480 WIRE -80 480 -96 480 WIRE 64 480 32 480 WIRE 80 480 64 480 WIRE 720 496 720 464 WIRE 880 496 880 464 WIRE -128 512 -128 480 WIRE 32 512 32 480 WIRE 272 512 256 512 WIRE 304 512 304 464 WIRE 304 512 272 512 WIRE 432 512 432 448 WIRE -128 640 -128 592 WIRE 32 640 32 592 WIRE 432 640 432 592 WIRE 1120 640 1120 528 FLAG -128 304 0 FLAG 432 640 0 FLAG -128 640 0 FLAG 32 640 0 FLAG -96 480 +5 FLAG 64 480 -5 FLAG 272 512 -5 FLAG 272 368 +5 FLAG 352 176 0 FLAG 432 176 0 FLAG 880 496 0 FLAG 1120 640 0 FLAG 720 496 0 FLAG -64 80 DDS FLAG 288 80 CLIP FLAG 480 448 LIM FLAG 672 288 Isense FLAG 1072 112 VOUT SYMBOL res 96 64 R90 WINDOW 0 69 57 VBottom 2 WINDOW 3 74 59 VTop 2 SYMATTR InstName R1 SYMATTR Value 1K SYMBOL diode 160 160 R0 WINDOW 0 -62 30 Left 2 WINDOW 3 -84 62 Left 2 SYMATTR InstName D1 SYMATTR Value 1N914 SYMBOL voltage -128 128 R0 WINDOW 0 37 102 Left 2 WINDOW 3 17 145 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName Vdds SYMATTR Value SINE(2.5 2.5 1K) SYMBOL voltage 432 496 R0 WINDOW 0 57 38 Left 2 WINDOW 3 62 70 Left 2 SYMATTR InstName Vlim SYMATTR Value 2.5 SYMBOL Opamps\\UniversalOpamp2 304 432 M0 WINDOW 0 -57 -53 Left 2 SYMATTR InstName U1 SYMBOL res 368 272 R90 WINDOW 0 -38 56 VBottom 2 WINDOW 3 -33 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 100K SYMBOL res 608 272 R90 WINDOW 0 -40 56 VBottom 2 WINDOW 3 -33 56 VTop 2 SYMATTR InstName R3 SYMATTR Value 100K SYMBOL voltage -128 496 R0 WINDOW 0 51 37 Left 2 WINDOW 3 55 75 Left 2 SYMATTR InstName V3 SYMATTR Value 5 SYMBOL voltage 32 496 R0 WINDOW 0 60 33 Left 2 WINDOW 3 59 69 Left 2 SYMATTR InstName V4 SYMATTR Value -5 SYMBOL e 432 48 R0 WINDOW 0 51 43 Left 2 WINDOW 3 68 76 Left 2 SYMATTR InstName Eamp SYMATTR Value 10 SYMBOL res 1104 160 R0 WINDOW 0 -101 36 Left 2 WINDOW 3 -90 74 Left 2 SYMATTR InstName Rload SYMATTR Value {RL} SYMBOL res 1104 432 R0 WINDOW 0 -108 64 Left 2 WINDOW 3 -89 93 Left 2 SYMATTR InstName Rshunt SYMATTR Value 1m SYMBOL e 720 336 M0 WINDOW 0 74 48 Left 2 WINDOW 3 62 81 Left 2 SYMATTR InstName E2 SYMATTR Value 1000 SYMBOL res 1072 352 R90 WINDOW 0 -41 53 VBottom 2 WINDOW 3 -35 54 VTop 2 SYMATTR InstName R4 SYMATTR Value 1K SYMBOL cap 864 400 R0 WINDOW 0 51 22 Left 2 WINDOW 3 49 51 Left 2 SYMATTR InstName C1 SYMATTR Value 1n TEXT 640 312 Left 2 ;1 v/a TEXT 968 416 Left 2 ;160 KHz TEXT 696 560 Left 2 !.tran 2m TEXT 752 88 Left 2 ;P902B TEXT 680 136 Left 2 ;current limit concept TEXT 704 184 Left 2 ;JL Nov 15 2020 TEXT 696 600 Left 2 !.param RL = 10 TEXT 696 640 Left 2 !.step param RL list 10 2 0.1 TEXT 448 48 Left 2 ;class-D power amp