AVR-Chat

At 01:50 AM 7/20/2006 +0000, Ryan wrote:
>Wait a sec....
>
>Looking at the datasheet for the IRL520, it shows a zener across the
>drain & source.  It says the diode forward voltage is 1.3 volts.  Does
>this mean anything more than that will be shorted to ground (in my
>schematic)?

It's not really a zener.  At least no more than any other diode.  The 
forward voltage means that when gnd rises above Aout by that it will 
conduct significantly.

>If so, the polarity protection will work, and I don't
>need the extra diode (D2)....  Although it does say the D-S breakdown
>voltage is 100V, and I don't know that the solenoid won't create
>spikes above that...

That depends what the parasitic paths are and the breakdown voltage of 
various insulators along the way.  Also the inductance of solenoid 
coil.  Theoretically with an ideal circuit when you open the circuit in 
current flowing through an inductor the voltage will approach infinity as 
the current drops to 0 in 0 time.  You will need D2 or a snubber of some sort.

I don't know about the 520 in particular but some power FETs are avalanche 
rated.  If you are pinching pennies you can dispense with D2 provided you 
know the energy stored in the inductor won't exceed the repeatable 
avalanche rating.  Personally, I'll take the extra cost for most designs.

BTW if the +12 is diode protected against feedback into the source then you 
probably should have a cap across the output to absorb the snubbed 
current.  That will also act to decouple the supply when you turn the 
solenoid on.

Robert


http://www.aeolusdevelopment.com/

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