MOSFET Question

[Farlander]

Hi guys,

I researched MOSFETS a little more and came across a term I can't explain.... Gate to Source Voltage, (VGS) in this case +/-20V.  Does this mean that the difference between the Gate voltage and the Source voltage cannot be more than 20V??

I was planning on using a low voltage circuit to open and close the FET, but pass 100V from a different source through the FET.... would a diode in series with the PWM and FET gate control enable higher pass-through voltage?

Thanks

[turbo]

Hi 

If you want to switch with real low voltage like 2.5 to 10 Volts you could use Trench Fet's.
These are especially designed to open and close on TTL and CMOS compatible levels.
This works very much beter then when using traditional Fet's and low voltage as they will respond slow and won't open all the way producing a lot of heat and not passing the voltage completly.

If it says Gate to Source Voltage +/-20Volt it will be closed completly at 20 volts but they need some amps to switch fast as well.

Marco.

[insane4evr]

That value of +/-20 volts Vgs is usually the maximum value it can tolerate without damage. Most datasheets have two sections absolute maximum ratings and typical operating conditions. Look at tke operating conditions, you will see that it is much lower. What mosfet part number are you looking at?

[Farlander]

IRF3710 has the Vgs +/-20... but I have a fairly huge supply of all different kinds of Mosfets...

Thanks for the info about trench Fets.  That might work out well...

As for the Vgs, are you saying that is the maximum voltage the GATE can handle?  Because it says the gate threshold is 2Vmin and 4V max...  however it should be able to handle 100V at the source...

One other thing -- what is the result of wiring multiple Fets in series and in parallel?

Thanks!~

[insane4evr]

Gate threshold is the voltage at which the mosfet starts to turn on. It varies from device to device of the same part number. The manufacturer is saying you might get an IRF3710 with a threshold (starts to turn on) of 2V or 4V or any value in between. Look at its datasheet figure 3. For this part number I will apply a gate drive voltage of 10 volts to 15 volts.

If you have matched mosfets, you can parallel them for increased total load current. Just make sure your PCB/wiring/soldering is well designed to balance load current among the mosfets. This approach is common with remote controlled battery operated hobby cars, etc.

You have to be creative on individual gate drives to wire them in series. Or take the simple route of getting a device with higher Vds rating.

Just my 2 cents.