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I'm trying to understand this circuit but I can't understand purpose of R4 and R6. Can someone explain their purpose?

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2 Answers 2

up vote 4 down vote accepted

D2 keeps the MOSFET gate voltage from getting too high. R6 drops the voltage that doesn't appear across D2 and Q2. The MOSFET gate is rated for 20V so 6.8V is very conservative for the zener voltage.

R4 pulls up the drain of Q2 to close to the input voltage even if there is a bit of leakage in Q2. Maximum leakage allowable is about 1.2V/13.3K = 90uA, which again is conservative (50uA maximum at 125C for the ZVN4106, but at 48V rather than the 60V it could see).

A similar thing could be accomplished by reducing R5 and R6 to 11K and 2.2K respectively, but it would waste as much as another 3-4mA under normal operating conditions.

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Thank you for such a detailed answer! if I don't need zener diodes (voltage will never be higher than max gate-source voltage) then I can remove zener diodes, R4, R6 and change value of R5 to 10k. Correct? –  user46347 Jun 27 at 18:06
    
Yes, that should be fine- but the voltage must not exceed the max for even an instant. I'm not sure that LMV431 part of the circuit is bulletproof- it may not pull the gate of Q2 down far enough under worst-case conditions, which would be bad. –  Spehro Pefhany Jun 28 at 12:55

R4 is redundant; it just causes extra current to flow through Q2 when it's on.

R6 serves to limit the current through the zener diode D2 once it starts to conduct. The purpose of D2 is to prevent high input voltages from causing the maximum VGS of Q1 from being exceeded.

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