Driving Dual MOSFETs From A NPN Transistor
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Posted by Michael on December 12, 2006, 6:49 pm
Please log in for more thread options Hi, I'm working on a circuit that uses a NPN transistor to drive (i think that's the right word) two MOSFETs. Could someone please explain how to work out the theory behind it? For example what value resistors I need between the gates and emitter, etc. (i presume I need some). I'd like to be able to work it out myself as opposed to asking you guys for a 'solution' but sadly I'm struggling..... So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet introduction, transistor introduction but to no avail....If it really just a case of wiring the MOSFETs' gates together and connecting them directly to the emitter? Any 'rules' to doing this sort of thing? Thanks In Advance, Michael | |||||||||||||
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Posted by Phil Allison on December 12, 2006, 6:54 pm
Please log in for more thread options "Michael" ** You must supply more info. Are the MOSFETS wired in parallel to increase current ? Are the MOSFETS switching or linear types ? Is you circuit switching or linear ? Etc. ........ Phil | |||||||||||||
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Posted by jasen on December 13, 2006, 2:04 pm
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On 2006-12-12, Michael > Hi,
> I'm working on a circuit that uses a NPN transistor to drive (i think that's
> the right word) two MOSFETs. Could someone please explain how to work out > the theory behind it? For example what value resistors I need between the > gates and emitter, etc. (i presume I need some). I'd like to be able to work > it out myself as opposed to asking you guys for a 'solution' but sadly I'm > struggling..... what you describe suggests that you have the NPN transistor manipulating the voltage on the gates of the mosfets, that effects their resistance. not much more can be said without seeing the types of transistors and how it's all hooked up. > So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
> introduction, transistor introduction but to no avail....If it really just a > case of wiring the MOSFETs' gates together and connecting them directly to > the emitter? Any 'rules' to doing this sort of thing? plenty of rules, the more info you give the better. -- Bye. Jasen | |||||||||||||
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Posted by Michael on December 15, 2006, 3:53 pm
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> On 2006-12-12, Michael
>> Hi,
>
>> I'm working on a circuit that uses a NPN transistor to drive (i think
>> that's >> the right word) two MOSFETs. Could someone please explain how to work >> out >> the theory behind it? For example what value resistors I need between the >> gates and emitter, etc. (i presume I need some). I'd like to be able to >> work >> it out myself as opposed to asking you guys for a 'solution' but sadly >> I'm >> struggling..... >
> what you describe suggests that you have the NPN transistor manipulating > the > voltage on the gates of the mosfets, that effects their resistance. > > not much more can be said without seeing the types of transistors and how > it's > all hooked up. > >> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
>> introduction, transistor introduction but to no avail....If it really >> just a >> case of wiring the MOSFETs' gates together and connecting them directly >> to >> the emitter? Any 'rules' to doing this sort of thing? >
> plenty of rules, the more info you give the better. > > > > -- > > Bye. > Jasen Hi, Sorry for not getting back sooner, I've drawn you a schematic and uploaded it to: http://www.mhims.co.uk/MOSFET%20Schematic.png I'm trying to work out a suitable transistor (Q3) to use and it's base resistor (R1) value... From what I've read in the mosfets datasheet (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be applied to the gates so that's where I get the 3V value from and the 36V figure is from the onboard batteries (it's for a robot). Where should I go from here? Thanks, Michael | |||||||||||||
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Posted by PeteS on December 15, 2006, 4:47 pm
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Michael wrote: >> On 2006-12-12, Michael
>>> Hi,
>>> I'm working on a circuit that uses a NPN transistor to drive (i think >>> that's >>> the right word) two MOSFETs. Could someone please explain how to work >>> out >>> the theory behind it? For example what value resistors I need between the >>> gates and emitter, etc. (i presume I need some). I'd like to be able to >>> work >>> it out myself as opposed to asking you guys for a 'solution' but sadly >>> I'm >>> struggling..... >> what you describe suggests that you have the NPN transistor manipulating
>> the >> voltage on the gates of the mosfets, that effects their resistance. >> >> not much more can be said without seeing the types of transistors and how >> it's >> all hooked up. >> >>> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
>>> introduction, transistor introduction but to no avail....If it really >>> just a >>> case of wiring the MOSFETs' gates together and connecting them directly >>> to >>> the emitter? Any 'rules' to doing this sort of thing? >> plenty of rules, the more info you give the better.
>> >> >> >> -- >> >> Bye. >> Jasen >
> Hi, > > Sorry for not getting back sooner, > > I've drawn you a schematic and uploaded it to: > http://www.mhims.co.uk/MOSFET%20Schematic.png > > I'm trying to work out a suitable transistor (Q3) to use and it's base > resistor (R1) value... From what I've read in the mosfets datasheet > (http://www.rapidonline.com/netalogue/specs/47-0530.pdf) 2-4V needs to be > applied to the gates so that's where I get the 3V value from and the 36V > figure is from the onboard batteries (it's for a robot). > > Where should I go from here? > > Thanks, > > Michael > > Looking at this, you could do the following: 1. Replace Q3 with a logic level MOSFET (the venerable VN2222LL would be perfect), and change the sense completely. As shown, the circuit really won't work that well. Either use a PNP with emitter to positive rail or use a P-channel MOSFET (Source to positive rail). If you need to drive with 3V or 3.3V logic, then there are solutions. The simplest fix here (assuming 3.3V logic) and the fact you need 5V for gate drive (see 3) is to a. Remove Q3 emitter from the gates and tie it to ground b. Remove Q3 collector from supply, and pull it to the supply via a 10k resistor c. Connect a 2N3096 with collector to gates, base to Q3 collector via a 1k resistor, emitter to 5V 2. Add a pulldown from the MOSFET gates of 10k or so. There has to be a DC return, and a resistor to ground is the simplest. 3. You need to provide a minimum of 4V to the gates of Q1 and Q2. This is the only way you are guaranteed to exceed Vgs(th). Better would be 5V, so change the power supplied to the collector [drain if you take the advice in 1] to that level. Note your TTL circuitry could be operating on 3V and the VN2222LL would work fine as it would if it were a bipolar device. 4. I assume M1 and M2 are motors. As inductive loads, you need to put diodes across them; anode to the MOSFET drain, cathode to positive power for the motor. If you don't, the MOSFETS will operate once, and at the first turn off, they will be toast (literally). Cheers PeteS | |||||||||||||
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> I'm working on a circuit that uses a NPN transistor to drive (i think
> that's the right word) two MOSFETs. Could someone please explain how to
> work out the theory behind it? For example what value resistors I need
> between the gates and emitter, etc. (i presume I need some). I'd like to
> be able to work it out myself as opposed to asking you guys for a
> 'solution' but sadly I'm struggling.....
>
> So far I've googled: driving mosfet, switching mosfet, mosfet, mosfet
> introduction, transistor introduction but to no avail....If it really just
> a case of wiring the MOSFETs' gates together and connecting them directly
> to the emitter? Any 'rules' to doing this sort of thing?