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I have a H-bridge based on FAN7392 drivers. This bridge works at 350VDC to output a approximately 240V 50Hz waveform. This circuit is isolated from rest of the board and I would like to maintain the isolation. The bridge is fed by two complementary PWM signals from a microcontroller. The signals can be isolated easily with opto-couplers - however I'm worried that the timing difference between the opto-isolators might cause one side of the bridge to turn on both it's transistors simultaneously, for a short time. For instance, if one opto-coupler takes 2us to go high while the other takes 3us to go low there will be a 1us interval where one half of the bridge will be short-circuiting the power supply.

So my main question is, what is the best way to isolated timing critical signals?

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So my main question is, what is the best way to isolated timing critical signals?

For an application like this I'd consider sending only one PWM signal through one opto and, at the H-bridge side, recover that signal and apply the inversion and necessary timing differences to prevent shoot-through.

Here's a circuit that can avoid shoot thru by introducing a dead-band i.e. no overlap: -

enter image description here

Using a small time constant RC low pass filter and schmitt trigger OR and AND gates you can manufacture two PWM signals from one - all you should need to do is invert the output from the OR gate and you have dual, complementary PWM signals with shoot-thru protection.

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  • \$\begingroup\$ Won't the additional inverter add another propagation small delay into the signal? \$\endgroup\$ – Saad May 9 '14 at 14:07
  • \$\begingroup\$ It probably won't be significant but, if in doubt use an EXOR on both outputs and set one to invert and the other to not-invert \$\endgroup\$ – Andy aka May 9 '14 at 14:17
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You should adjust your timings to compensate for any possible delays. Many off-the-shelf gate drivers will have matched propogation delays.

Check a few of the irf gate driver ics. They often take care of many of these details for you, and simply design. IRF is ofcourse not the only manufacturer of such devices. https://www.infineon.com/cms/en/product/power/gate-driver-ics/

Another advantage, is that these companies want you to use their chips, the app notes on that page are a great resource; Even if you don't use the ic.

Note that these delays, and switching losses will affect your maximum frequency and efficiency at said frequency. Switching too fast will result in more switching losses, and possible "ringing". Designing/choosing your driver circuit should balance all of this dependent on desired operating parameters.

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