Would appreciate an explanation of how this circuit works. It is a MOSFET H-bridge that takes a square wave input and either passes it through unchanged or changes its phasing 180 degrees. The “unchanged” or “changed” is controlled by a microcontroller (not shown). The square wave input is 28V p-p with 0V average so varies -14V to +14V. Its frequency can be as low as 80Hz and as high as 10kHz. This is the switch part of an autoreverser used on a model railway using the “Digital Command Control” (DCC) system which is an industry standard.

Autoreverser Switch Schematic

For simplicity the overall arrangement is shown in block form and one of the blocks is then shown in detail. In operation, either M1 and M3 or M2 and M4 is turned on, depending on “unchanged” or “changed”. Vgs of the relevant MOSFETs are controlled via an optoisolator. The schematic is a drawn from an actual operating device so there’s no question that the circuit works but I am struggling to understand how.

If we assume that the controller is causing output C to follow input A and output D to follow input B than modules M1 and M3 will be conducting. I can see how M3 will conduct because Vgs will be powered through D3 which is forward biased for both positive and negative half cycles of the input square wave. But how will M1 conduct as D1 is always reverse biased so there is no voltage source to set Vgs?

  • \$\begingroup\$ You say this is MOSFET based, but the transistors you've drawn aren't MOSFET symbols. Nor are they any type of transistor I recognise. You might want to fix that. \$\endgroup\$ – Hearth Mar 9 at 22:00
  • \$\begingroup\$ Sorry about that. I will fix the diagram but, regardless these are N-channel enhancement mosfets and the existing diagram shows gate, source and drain. I should also have indicated they are FDB8445. \$\endgroup\$ – Jan Carr Mar 10 at 0:08

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