Voltage drop over IRF740 MOSFET

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	Voltage drop over IRF740 MOSFET « on: February 04, 2011, 08:38:40 am »

Hi Everyone

I pulse my PI coil with a IRF740 mosfet that is driven by a MC34151-D MOSFET GATE DRIVER. The gate resistor is 10 ohm and drive voltage is 12V so I drive the gate with 1.2 Amps.

Coil pulse voltage is also 12 Volt. There is a 4V voltage drop over the IRF740 so I only get 8V on the coil!

Any ideas on what I am doing wrong?

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	Re: Voltage drop over IRF740 MOSFET « Reply #1 on: February 04, 2011, 12:38:03 pm »

If this is your own design, there may be an error in the RLC timing network of your overall circuit impedance. If an existing design, there may be a component that is incorrect or even (heaven forbid) labeled wrong. I now measure suspect components like resistors having received a batch of 1000 with incorrect color codes.

The internal resistance in a MOSFET is very low and the drop shouldn't be that great.

Would you post a copy of your design schematic? It would surely help.

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	Re: Voltage drop over IRF740 MOSFET « Reply #2 on: February 05, 2011, 08:32:48 am »

Thanks GoldDigger.

It is my own design. I resolved the MOSFET problem and got it to switch properly.

Now I have a new challenge to overcome. I need to switch the mosfet driver with a negative voltage from the PIC TTL (0 to 5V) signal.

Most simple method to do this is obviously an optocoupler but I have never tried this. The one I have has a rise and fall time of approximately 5us. I can live with this slow reaction time but my concern is the consistency of switching time.

Anyone with practical experience of this method?

*** I use the mosfet driver since it is a totem pole driver that is very fast and consistent. But maybe there are other methods of driving the mosfet that is good enough for a metal detector. So I am also open to any other ideas as well.

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	Re: Voltage drop over IRF740 MOSFET « Reply #3 on: February 05, 2011, 01:25:55 pm »

	Quote:	Posted by levettp
Thanks GoldDigger. It is my own design. I resolved the MOSFET problem and got it to switch properly. Now I have a new challenge to overcome. I need to switch the mosfet driver with a negative voltage from the PIC TTL (0 to 5V) signal. Most simple method to do this is obviously an optocoupler but I have never tried this. The one I have has a rise and fall time of approximately 5us. I can live with this slow reaction time but my concern is the consistency of switching time. Anyone with practical experience of this method? *** I use the mosfet driver since it is a totem pole driver that is very fast and consistent. But maybe there are other methods of driving the mosfet that is good enough for a metal detector. So I am also open to any other ideas as well.

Well done!

There are MOSFET driver chips out there which are faster than an optocoupler. You can request samples from TI on this one.

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	Re: Voltage drop over IRF740 MOSFET « Reply #4 on: February 06, 2011, 01:56:07 am »

I already use a Mosfet gate driver (MC34151-D)

The question is how do I level shift my PIC's TTL level signal from 0/5V to -12/0V to trigger the Mosfet gate driver.

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	Re: Voltage drop over IRF740 MOSFET « Reply #5 on: February 06, 2011, 10:01:35 am »

Could you use a Darlington or are they to slow?

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	Re: Voltage drop over IRF740 MOSFET « Reply #6 on: February 06, 2011, 12:16:13 pm »

I really need to see a schematic. The gate voltage is only 4 volts. The output voltage should be isolated via a capacitor to the coil and on the input side, two diodes are normally used as level shifters or clippers to bring the voltage on the coil back down to a reasonable level.

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	Re: Voltage drop over IRF740 MOSFET « Reply #7 on: February 07, 2011, 01:51:47 am »

A Darlington pair will be fast enough but how does that resolve the level shift problem?

Posted on: February 07, 2011, 01:33:00 AM

Here is the schematic so far. Ignore the second IRF740 ... this will be only for experimental purposes. My problem now is to level shift the signal between R3 and MC34151 pin 2 from TTL (0 to 5V) to 0 to -12V.

Posted on: February 07, 2011, 01:35:47 AM

I found this schematic with an arrangement for level shifting that looks OK to me. Instead of driving the Mosfet directly as in this design, I will drive the Mosfet gate driver with the same technique.

What do you think of this?

I also found 10MHz optocouplers (6N137) with rise time of 50ns and fall time of 12ns ... also a possibility that I consider. Vcc max however is 7V meaning that I must now add a voltage regulator as well.

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	Re: Voltage drop over IRF740 MOSFET « Reply #8 on: February 07, 2011, 02:50:44 am »

The diode and resistor arrangement on the gate of the MOSFET seem to me to be counter intuitive. What is your thinking behind that design? The most you can get across that circuit is 3v no matter what you do and the resistors are going to limit only current, not change the voltage.

Normally, that configuration is used on the input of a high impedance op amp to limit the voltage seen at the input. Too high a voltage and the op amp dies a quick death. The coil circuit can build up a static charge of up to 400v momentarily. These diodes limit the voltage spike and keep the levels suitable for the first stage of amplification.

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	Re: Voltage drop over IRF740 MOSFET « Reply #9 on: February 07, 2011, 03:12:27 am »

Diode and resistor arrangement on the gate of the MOSFET is to have a mechanism to control turn-on and turn-off times. I can of course leave this out or replace with single gate resistor. It is only to experiment with.

I fully understand the use of the typical 1N4148 voltage limiting diodes that must be used prior to feeding the signal to the amplifier circuit. The whole amplifier circuit is not yet shown on the schematic. But I have experimented with an LF357 pre-amplifier and achieved good results at a gain of 2,500. My problem is that the output signal of the amplifier "drifts" by up to 50mV (as read by PIC A/D). After I have ensured that my +/-5V power supply on the amplifiers are 100% stable, I can only think that the cause is related to inconsistent switching of the MOSFET. That is why I am re-designing the MOSFET switching part of the schematic.

So my problem still remains ... how do I switch the MOSFET gate driver with TTL level

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