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Circuit prevents common-mode conduction

20 Aug 2015  | Ken Herrick

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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 electromagnetic interference (EMI). 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.


Figure 1: 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.


Figure 2: 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.


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.


About the author
Ken Herrick is from Oakland in California, United States.


This article is a Design Idea selected for re-publication by the editors. It was first published on February 4, 2010 in EDN.com.




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