Circuit precludes common-mode conduction
A crossover-control circuit prevents two or more transistors from powering on at once.
Ken Herrick, Oakland, CA; Edited by Martin Rowe and Fran Granville -- EDN, February 4, 2010
When driving an H-bridge or a similar circuit, you usually must ensure that two or more transistors are not on at the same time. Eliminating multiple transistors from turning on reduces power consumption and lowers EMI (electromagnetic interference). Crossover-delay circuits solve that problem. Figure 1 shows a simple, two-phase design that lets you adjust the crossover delays equally by changing the value of one component with minimal phase delay.
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Each Schmitt trigger inverter is driven during one half-cycle through a diode. The RC delay occurs during the alternate half-cycle. Equal-value resistors R1 and R2 serve alternatively as delay elements and gate-coupling resistors. The waveform in Figure 2 shows the result. For the two out-of-phase half-cycles, leading edges are delayed equally with respect to the input transition, and trailing edges are coincident with the transition within about one gate delay. If you need equal polarity “on” half-cycles, insert an inverter in one of the two phase outputs. Alternatively, if biphase drivers, such as those for driving coupling transformers, will follow this circuit, merely interchange the outputs of one of those drivers to effect the inversion.
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Ken, may we still wish a happy birthday, although it's now two days late?
I enjoyed the circuit. Good and simple. There are parallels to the discussion for the high side PMOS driver (I can't seem to include a URL here. Maybe one has to search for article CA6715771)
Even though I'm apparently fifty years greener than you, and I am a bit concerned that the art of figuring out nice and simple circuits will become forgotten...
Sebastian - 2010-8-2 12:39:00 PST -
I was pleased with it. Using it in a solid-state Tesla coil to drive 3 + 3 floating bridge-MOSFET drivers.
It may entertain you & others that while this is probably my last piece in EDN, the first one (I think the 1st) was in the July '67 issue. That's nineteen sixty seven. I turned 82 just yesterday. Still kickin'...
KCH
Ken Herrick - 2010-6-2 17:24:00 PST -
Amazing the variety of useful applications for the Schmitt Trigger. Ken, this one is outside the box. Nice concept!
Glen Chenier - 2010-6-2 15:57:00 PST
