How does the zvd circuit work

[cambazz]

Hello All,

How does this "ideal diode" circuit work?



How can I determine max current and max voltage tru it?

Can I use it to feed motor drivers that cause noise in my mcu power bus? Or would there be a limitation?

Best Regards,
C.

[Ian.M]

The dual transistor is configured as a comparator, sensing the voltage difference between its emitters. It will have a linear transition between low and high on [U14:4] over a small range centred on a point where V[U14:5] is fractionally lower than V[U14:6].  When V[U14:5]>>V[U14:6], [U14:4] is pulled up to near v[U14:1], turning the MOSFET off.  When V[U14:5]<<V[U14:6], R3 pulls the MOSFET gate down, turning it on.

Max current is set by the MOSFET's Id_max and dissipation due to I²R losses in its Rds_on, though if there is insufficient gate drive voltage, losses in its body diode may take over.

Max voltage has four constraints - the ideal diode cathode voltage is limited by  U14 Vcbo_max and Q3 Vgs_max,  and the maximum reverse voltage across the diode is limited by U14 reverse Vbe breakdown and Q3 Vds_max.  None may be exceeded.

I would not expect it to do well at eliminating transient over-voltage on its supply  when feeding motor drivers as it is unlikely to react to spikes fast enough to make a difference.  It certainly wont stop the supply dipping and browning out the MCU during sudden acceleration of the motor.

[timb]

If that’s the circuit I’m thinking of, it’s used on the Raspberry Pi. As Ian.M said, it most likely won’t work well to prevent back EMF from a motor causing transients on your supply rail.

For that an ideal diode would be pointless anyway.