MOSFETS II

[Will]

Hi you guys,
I've been following the 'Shorted MOSFETS' thread so I know the expertise is out there/here ?
My brother has built a ciruit using VMOS whereby he wants to control a power level to a 12 volt motor by Pulse Width Modulation - he has presently set it to a base frequency of about 40 Hz. He cannot get the FET's to switch off in other words if he tries to run his circuit and puts a scope on it he can see that the gate is getting the correct Mark/Space modulation as desired but that the device is remaining switched on - Any ideas what might be wrong ? He says he says scoured the internet but can not yet get an alternate circuit to try.
Will appreciate any help

[Joseph]

Some things to consider are whether the mosfet is N or P channel, what value the gate resistor is, and whether the gate voltage with respect to the source is dropping below the gate threshold level. Other considerations are how much current is the device rated for, and has that figure along with the power dissipation rating been exceeded, and for how long. Maybe the mosfet is damaged also. That could have happened if the gate voltage has ever exceeded 20 volts with respect to the source for any reason including static electricity for even the shortest length of time.

[Chris Smith]

Put a light load, a light bulb on the out put and check it. Diagnosis comprises of making changes to all the parameters, and finding out when the failure occurs. Capacitance, saturation, load, voltage, timing every thing needs to have a substitute of opposites values to "find" when and where the failure occurs. Also adjustable controls/values are nice.

[haklesup]

Check to make sure no current can flow into the gate terminal. Unless you know there is some kind of diode protection on the gate, any current flowing indicates gate oxide punch through which will often render a FET permenantly on or off (Depending on type and extent of damage). <p>What FET are you using (tell us the part number) What is Vt, VDSmax and what voltage are you putting across it (supply V). <p>If it is failed, it may be due to improper handling (static zap) or due to improper selection of the device for the application.<p>If the Fet is not failed it may be an improper device or faulty assembly (D and S reversed?). With a part number and schematic (even partial) we could help much more.

[Joseph]

Is the device a 2N7000? You may want to choose something like an IRF510 or something robust like that.

[toejam]

way too slow to work. Tell him to go up to 25 or so khz.

[Bob Scott]

Will,<p>08-15-03 Please disregard this message. I stand corrected.<p>I could be wrong in my recollection because it was about 22 years ago that I was servicing high end audio. I seem to recall that a VFET is a Sony part, that it is not a MOSFET but a depletion mode JFET, requiring a "reverse" voltage on the gate in order to turn it off.<p>We had a few Sony VFET amps come in for service which had blown outputs due to the picky power-up sequencing involved in using these devices.<p>Someone please correct me if my recollection is wrong, but if it is not, it would explain why your VFETs are blowing.<p>Bob<p>[ August 15, 2003: Message edited by: Bob Scott ]</p>

[Joseph]

Here are some links which may help.<p> http://semiconductorglossary.com/defaul ... EF%2C+VMOS<p>http://www.powerdesigners.com/InfoWeb/d ... sfets.shtm

[Will]

thanks for the replies guys. Im waiting for my brother to respond - will get back to y'all when he does

[Bill Lahan]

This problem is not unknown!!<p>I am assuming an inhancment mode type FET.<p>One cause is the large miller effect capacitance between the gate and drain. Simply switching off the gate drive may not be adiquate to bring the FET into the off mode.<p>I have found that a bipolar drive +/- 12V maybe necessary to discharge the miller capacitance and drive the FET to the OFF state.