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schematic

simulate this circuit – Schematic created using CircuitLab

Hi,

I want to make a Transistor based Push Pull Stage to drive a 30V Mosfet.

uC GPIO drives the transistor stage with 3.3V and GND to Turn On and Turn Off the Mosfet.

In datasheet, Mosfet has a 1.5V to 2V Threshhold voltage range.

The Problem is:

When GPIO turns the NPN Transistor ON to Turn ON the Mosfet, Gate has maximum 2.7V because of base-emitter diode and this 2.7V is not sufficient to drive Mosfet in Saturation region to allow sufficient current through Load.

I suspect it could also raise the Temperature of Mosfet being driven in Ohmic Region.

My question is:

how could I have more voltage on Gate ? or should I search another Mosfet with very low threshhold voltage ?

Thanks for your suggestions.

EDIT:

Mosfet replaced with that of one with lower threshold voltage.

NPN and PNP transistors changed with those having integrated biasing resistors as well.

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  • 1
    \$\begingroup\$ You have a nice 8V supply and you're not using it. Drive this P-P stage from a common emitter (NPN), collector pulled up to 8V. (Common emitter inverts, so invert the driving signal - ideally, in software. It looks like Q2 here electronics.stackexchange.com/questions/351837/…) \$\endgroup\$
    – user16324
    Jan 24, 2018 at 14:55

3 Answers 3

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Get hold of a MOSFET driver like the IR2110 and ditch the BJTs because that circuit will never perform well.

enter image description here

I'm sure you can find a single channel version of a similar device.

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  • \$\begingroup\$ TC4420 can be driven with up to 18V, goes high at 2.4V on the input, and is easy to get hold of. \$\endgroup\$
    – Dampmaskin
    Jan 24, 2018 at 17:11
  • \$\begingroup\$ Cost is the problem in my case. In the circuit that I shared has only NPN and PNP transistor and that is very cheap. I think this driver will be quite expensive in comparison to two bjts. \$\endgroup\$
    – HerrderElektronik
    Jan 25, 2018 at 13:35
  • \$\begingroup\$ @HerrderElektronik the TC4420 is less than £1 and of course that may be too costly. The MCP1415 is less than £0.50 and, if I scan through Farnell I see prices in the low £0.20 range for possible products. Farnell is not cheap and if your volume is high enough I suspect that you should find something in the £0.15 range. \$\endgroup\$
    – Andy aka
    Jan 25, 2018 at 13:51
  • \$\begingroup\$ @Andyaka I used instead Mosfet with lower threshhold voltage and it is turning completely on in simulation. However, I used transistors now with there internal biasing resistors RE and RB 10KOhm and I notice that my gate is not able to discharge completely. Is it because of higher base Resistance RB? If I lower RB like 1kOhm, it works, but on other way it works with RB 10KOhm too if RE is removed. Do you know what this RE and RB are together making the difference in system ? \$\endgroup\$
    – HerrderElektronik
    Jan 29, 2018 at 7:41
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    \$\begingroup\$ @HerrderElektronik I'm not in favour of push pull emitter followers because the output at the emitters will be (for a 3.3 volt IO) about 2.7 volts maximum and about 0.6 volts minimum when driving a square wave. You are never going to get anything remotely close to 8 volts gate drive so you might as well run it from 3.3 volts. In fact, there is a distinct possibility that with a series resistor of maybe 100 ohms you can directly drive your gate from a GPIO line. The BJTs are bringing nothing to the party. \$\endgroup\$
    – Andy aka
    Jan 29, 2018 at 11:06
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The problem with your schematic is that the buffer made by Q1 and Q2 cannot follow its input voltage properly. There will always be one Vbe lost across the Base-Emitter of Q1 when the input is 3.3 V.

A better solution is to make a "proper" levelshifter like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Do note that you will need to invert the GPIO signal as now a 0 Volt will turn the NMOS on and 3.3 Volt will turn it off.

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  • \$\begingroup\$ my Logic will be inverted and 470 Ohm will always have current to keep the Mosfet off. Therefore, I need to bring in a large value Resistor in place of 470 Ohm and this will reduce the turn On speed of Mosfet ! \$\endgroup\$
    – HerrderElektronik
    Jan 25, 2018 at 13:38
  • \$\begingroup\$ Yes, that is all true. I'm not saying that this is the optimal solution. It only addresses the Vgs issue you had with your circuit. If you want low power when off and no inversion you should consider using a proper level shifter IC like the IR2110 suggested by Andy. \$\endgroup\$
    – Bimpelrekkie
    Jan 25, 2018 at 14:07
  • \$\begingroup\$ Thank you. Yes I am sleeping on what andy said and meanwhile looking if I get any cheaper reliable option. \$\endgroup\$
    – HerrderElektronik
    Jan 25, 2018 at 15:12
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Use a totem pole driver:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ there can also be pull down 10K resistor with this circuit and bringing gate resistor in will also be better. what do you say ? \$\endgroup\$
    – HerrderElektronik
    Jan 25, 2018 at 13:40

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