purpose of gate diode

[circuitmangler]

A control circuit I've got has the control (pin 3) of a 555
controlling an NPN transistor which itself controls a IRF710
MOSFET. In series with the MOSFET's gate is a James Bond
diode (last three digits "007" - can't make out the prefix).
The circuit appears to be intended for an inductive
load as there is a freewheeling diode in place across the
load terminals.

Question: is the diode there to limit transient gate
current from getting back to the transistor? Horowitz and Hill recommends using a resistor in series with the gate for this purpose. Does the use of a diode suggest anything about the kinds of inductive loads (current, power, switching
speeds) this circuit was designed for? Or, is using this
diode always preferable to using a resistor?

thanks...

 

[raquino6]

Pin 3 is the output pin 5 is the control.
The diode is probably a 1N4007.
Diodes are sometimes used to effect the on/off time of the FET, in this case maybe the duty cycle.
Resistors on gates can be there for protection or to alter switching transition times.

 

[circuitmangler]

Thanks for the info. Could you point me to a resource
which describes how a diode is used in this way?

Thanks,

circuitmangler

 

[raquino6]

International Rectifier Application Note AN-937.

http://www.irf.com