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Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
What might be causing the bridge current to increase incredibly (even with no load) when i increase the dead time control pot (R1, adds more dead time) and any suggestions to fix it? I have connected a scope across Hin and Lin (with the IR2110 chips pulled out) and i see the 'tristate' square wave as expected, checked it on both chip sockets. R1 in the diagram is a dual gang pot.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
What might be causing the bridge current to increase incredibly (even with no load) when i increase the dead time control pot
Shoot-through down the bridge legs caused by introducing overlap in the top and bottom MOSFET conduction times.
The way your circuit is set up increasing R1 will slug the rising edge of the signals into the inverters, therefore delaying the falling edge of the drive signal to the IR21xx. Delaying the falling edges relative to the rising edges introduces conduction overlap.
You need to reverse the polarity of the 1N4148 diodes to make it introduce dead-time instead of overlap. You always want turn-on to be slower than turn-off.
Registered Member #580
Joined: Mon Mar 12 2007, 03:17PM
Location: Melbourne, Australia
Posts: 410
oh, i can see the problem now using 2 scope traces! hm, if i reversed the diodes wouldn't that just feed the signal directly from one inverter to the other with no delay and cause the pot to do nothing?
Edit: i simulated it with the reverse diodes, it seems to have fixed the problem, but I'm still about unsure of the working. Looking at it further reveals that in-fact the signal does pass straight from one inverter to the other during to off to on transition (at its input), and then when it turns off, the diode disconnects it from the previous inverter and the capacitor discharges at a slower rate through the pot directly to the previous inverter, thus delaying the on to off transition (input side).
after removing and re-soldering the diodes the other way! ah much better
if anyone is interested: file>import
$ 1 5.0E-6 10.391409633455755 50 5.0 50 I 592 448 640 448 0 0.5 r 544 448 544 496 0 200.0 c 544 496 544 528 0 1.0E-5 5.929016351897253 g 544 528 544 544 0 w 592 448 592 496 0 w 592 496 544 496 0 I 592 576 640 576 0 0.5 r 544 576 544 624 0 200.0 c 544 624 544 656 0 1.0E-5 2.377706454596996 g 544 656 544 672 0 w 544 624 592 624 0 w 592 624 592 576 0 I 512 448 512 576 0 0.5 w 512 576 544 576 0 w 512 448 544 448 0 I 464 448 512 448 0 0.5 R 464 448 432 448 0 2 160.0 5.0 0.0 0.0 0.5 150 656 448 704 448 0 1 0.0 w 656 448 640 448 0 w 704 448 720 448 0 150 656 576 704 576 0 1 5.0 w 656 576 640 576 0 w 704 576 720 576 0 w 720 576 736 576 0 I 736 224 784 224 0 0.5 150 800 208 864 208 0 2 0.0 w 784 224 800 224 0 w 720 192 800 192 0 I 736 272 784 272 0 0.5 I 720 304 784 304 0 0.5 w 720 304 720 192 0 w 736 224 736 272 0 150 784 288 864 288 0 2 0.0 I 720 336 784 336 0 0.5 w 720 448 720 336 0 w 720 336 720 304 0 150 784 352 864 352 0 2 5.0 w 784 368 736 368 0 w 736 272 736 368 0 150 768 464 864 464 0 2 0.0 w 768 448 720 448 0 w 736 576 736 480 0 w 736 480 768 480 0 w 736 480 736 368 0 162 864 208 912 208 0 1.0 0.0 0.0 162 864 288 912 288 0 1.0 0.0 0.0 162 864 352 912 352 0 1.0 0.0 0.0 162 864 464 912 464 0 1.0 0.0 0.0 w 912 208 912 288 0 w 912 288 912 352 0 w 912 352 912 464 0 g 912 464 912 496 0 d 544 448 592 448 0 d 544 576 592 576 0 x 929 218 1010 221 0 12 High side only x 927 298 960 301 0 12 All off x 928 360 1005 363 0 12 Low side only x 926 473 1098 476 0 12 High and low side on (BOOM!) o 19 32 0 34 5.0 9.765625E-5 0 -1 o 22 32 0 34 5.0 9.765625E-5 1 -1
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Dead time control increase in current mystery.
