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mosfet based ups and inverter circuit.

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roykyn

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mosfet driver circuit

i need a mosfet driver circuit made up of transistors like bc547 and bc558. it should be like a schmitt trigger with very low output impedance.
 

FvM

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fet driver circuit

I suggest a circuit like below
21_1241372829.gif

BC547/558 has only limited current capabilities. Transistors, that can driver higher pulse currents and have sufficient current gain at
higher currents are recommended for gate drivers, e.g. ZETEX ZXTN2031/ZXTP2025.
 

roykyn

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mosfet driver circuits

i need resistor values.basically i need a schmitt trigger to drive the mosfet.can it be built using transistors.i am using mc34063 to generate 20v gate voltage.i can use 40XX series cmos buffer.but i want it to be simple and use transistors and diodes.
 

FvM

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mosfet circuit

You can't avoid to make some specifications for your driver. The presented circuit is a schmitt trigger with low output impedance, as requested. You won't achieve a much lower part count with transistors. You can set all resistors to 10k and R6 to 100k as a starting point. Adjust them according to your specification, using basic transistor properties as current gain and Vbe threshold voltage.
 

roykyn

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drive mosfet circuit

thank you.i was trying another circuit.
 

wizpic

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simple mosfet driver circuit

Try this one, I found it on the net and seen it many times
 

FvM

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p mosfet driver circuit

This is no schmitt-trigger as requested, also the discharge transistor is effectively disabled by R1 and C1, so switch-off is rather slow (several us). The relation of the driver supply to the MOSFET source isn't clear. It's important to keep maximum Vgs. I can't see, if it's achieved here. A Vb of 28V switched to the gate would damage the MOSFET.
 

roykyn

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4541+mosfet

ok ... are there any commercial switching regulators.i am tired of these mosfets.
i need 3.3v 2a switching regulators.i found lm1501 but it is not available at stores.
are there any famous(like7805) or widely used switching regulators.i was trying mc34063.
 

r2d2droids

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low side mosfet driver circuit

thank u wizpic.
i have tried to find similar high side driver for a long time.
thnx again
 

roykyn

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mos driver circuit

i finally used circuit using cd40106 6 cmos schmitt trigger not gates in parallel.
the cmos IC can work at maximum 15volts. you can shift one not gate to get a non-inverting buffer.
the npn and pnp pair described dissipated a lot of energy because at one point of time both the transistors will be in saturation region which result in wastage of power.
 

roddyalan

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fet drive circuit

I am also currently doing a H Bridge with MOSFETs. Each of my MOSFETs must carry a Starting Current of 40A with voltage of 12V

I thought that MOSFET was a "voltage controlled current source", so will using single transistor Pull up or Pull down transistors be enough?

How do you calculate the current requirement of the MOSFET - to fill the input capacitance of the MOSFET gate, before Vgs gets going.

Also correct me if I am wrong - my Vt is 2(min) or 4(max) volts (is it necessary to know exactly where Vt is?). For my P-MOSFET does my Vg = 0 to 8V to be turned on (since Vsource = 12V) and for my N-MOSFET Vg = 4 to 12V (since Vsource = 0V).

Thanks
AR
 

roykyn

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fet driver circuits

I am also currently doing a H Bridge with MOSFETs. Each of my MOSFETs must carry a Starting Current of 40A with voltage of 12V

is it the initial current or full saturated current..are you using smps as source..

I thought that MOSFET was a "voltage controlled current source", so will using single transistor Pull up or Pull down transistors be enough?

it can work..but while turning ON and turning OFF the gate voltage will slowly rise and fall compared to the driver circuit..this results in mosfet to be in higher resistance for some time..which is waste of energy..

How do you calculate the current requirement of the MOSFET - to fill the input capacitance of the MOSFET gate, before Vgs gets going.

may be by finding the output impedance of the driver circuit..

Also correct me if I am wrong - my Vt is 2(min) or 4(max) volts (is it necessary to know exactly where Vt is?). For my P-MOSFET does my Vg = 0 to 8V to be turned on (since Vsource = 12V) and for my N-MOSFET Vg = 4 to 12V (since Vsource = 0V).

i think Vg should be -ve for p-MOSFET. i tried irf5210 it did nt work.i think for n MOSFET Vg should be 12-15volts...
 
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roddyalan

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mosfet drive circuit

(1) I am not sure if it is starting or saturated current. I dont know the specs of the motor and have to go abt finding enough specs for the ckt. I used an ammeter and got 12A. But when the screw (that is driven by the motor) hits the end (a metal plate), the ammeter shows 40A. I don't know how to get I(start). I refer you to my other post () for help on that too if you can.

(2) I am using an auto battery 12V

(3) So basically, using transistors would give small current to initially fill the Gate Capacitance, while the Driver ckt would fill it quicker?

(4) I don't follow how the characteristics of the driver will give me the current req of the fet. I imagined to go backwards - if the FET takes too long to turn on your driver is not producing current fast enough.

(5) About Vth - as long as |Vs - Vg| > Vt, shouldn't it be on? If this assumption is correct, your statement that Vg = 12 to 15V with Vs at 0V would give Vgs = 12V, but the min Vgs when it will turn on is at Vg = 4V (|Vgs| = 4V).
 

bongkk

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mosfet drive circuits

I have had a lot of burnt Mosfets while prototyping an inverter. If found that the culprit was the drive. I read elsewhere in this forum to use an LM324 opamp (thanks to him or her). Sure enough it solved my problem. Having tried many other op-amps, i found only Lm324 works.
Attached is my generic circuit diagram to share with you all.
 

FvM

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n mosfet circuit

Can work for medium size FETs in slow applications and larger FETs in very slow applications.
 

walkura

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irf5210 circuit

Maybe i'm stating the obvious ,but why not just make a emiter coupled schmitt trigger followed by a totempole .
3 bc 337 + 1 bc327 & 3 resistors (input base resistor + 2 x collector resistor).
I used this setup for synchronous rectifiers and it works great .
 

roykyn

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high side mosfet driver circuit

i am still searching a good mosfet driver....
 

roykyn

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high side n mosfet driver circuit

I found out the solution. I have tested this and the resistance of the mosfet drops to less than 0.1ohms.
The mosfet acts as a thyristor. To switch ON a mosfet you need to create a isolated secondary supply like using a diode and a capacitor. Remember that this secondary supply should be connected to gate and the negative terminal of the mosfet. I have attached a figure that will help.
We have to note that the driver circuit has to be super efficient. when the mosfet switches ON the voltage drops to zero and the driver circuit is powered by capacitor alone. The capacitor should be large enough or the power consumed by the driver circuit should be very less. I suggest using CD40106 or low power cmos opamp used as Schmitt trigger.

Please reply back if I've missed some thing.
 

FvM

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fet drive circuits

Just a side remark. You never mentioned, that you intend to drive a high side N-MOSFET, although a similar bootstrap circuit has been already suggested by wizpic 3 months ago...

All these bootstrap circuits are limited in continuous on-time respectively duty cycle.
 

roykyn

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high side fet driver circuit

yes that is true. no one explained me that in simple terms.and The problem with wizpic circuit is that the transistors dissipate lot of energy hence it cannot be used.
Actually it is linked to .

i still dont understand why this circuit doesnt work ( high side N channel mosfet). here the mosfet is not switched ON properly.load is just 1k. I tried 10k and 100k also as load and IT doesnt switch ON properly.voltage across mosfet is 3v that means resistance is around 250ohms.
the power supply voltage is 14volts and not 12volts as shown.
 

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