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discrete MOSFET driver

I would like to drive an N-channel MOSFET that switches a bypass resistor on an LiFePO4 battery cell. The balancing process is done at around 3.5 V cell voltage.

A microcontroller, working on 3.3 V, measures the cell voltage and has to switch the bypass MOSFET by PWM.

The scetched circuit works with the indicating LED2 but when I add the resistor ZR-Load the MOSFET heats up and LED2 stays off.

What did I miss?

Did I select suitable components?

As I understand the datasheet of the LR3103 at 3.5 V Vgs the MOSFET could drive up to 15A?

Edited next day: R1 is a 12 kΩ resistor.

At first I started with the circuit here: Non-inverting MOSFET switch circuit and alternatives?

under "High flexibility N-Channel MOSFET driver" but I had an inversion of my signal. So I swapped Q2 and R1 and increased R1 to 12 kΩ. Now it's non-inverting but I guess the driver does not open the gate fully?

We find also the Dturnoff at the base of Q2 in the original post, which is strange.

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  • \$\begingroup\$ Is that a 12 kOhm pull-down there? Or 12 Ohm? (Pulling down on the emitter-follower's emitter, I mean.) \$\endgroup\$
    – jonk
    Commented Dec 21, 2021 at 3:54
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    \$\begingroup\$ Q2 and R1 are swapped If you do not want inverted logic find another way because you do not want a source follower there. You also seem to be treating Q2 like a MOSFET the diode and R3. It's not. That kind of thing normally goes on the power MOSFET, not the driver. \$\endgroup\$
    – DKNguyen
    Commented Dec 21, 2021 at 4:09
  • \$\begingroup\$ its a 12 kOhm resistor at first i started with the circuit here: electronics.stackexchange.com/questions/261868/… under "High flexibility N-Channel MOSFET driver" but I had an invertation of my signal. So I swaped Q2 and R1 and increased R1 to 12 kOhm. Now its noninverting but I guess the driver does not open the gate full? \$\endgroup\$
    – MyMOSFET
    Commented Dec 22, 2021 at 2:23

1 Answer 1

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You only need a driver if you want a large gate current to turn the FET on and off quickly. If you use low frequency PWM like 500Hz, you can have slow switching speed, and you can drive the gate directly from a MCU output.

The driver schematic in the question won't work at all because it drops two Vbe (Q1,Q2) which means the maximum gate voltage will only be about 2.1V if the micro outputs 3.3V.

What you need is a FET that is guaranteed to turn on at Vgs=3.3V or less, that means RdsON is specified for this Vgs in the datasheet, and a gate resistor like 330R. That's all, no need for extra components.

If you already have a bunch of MOSFETs that won't turn on properly with 3V3 and want to use them, you can make a voltage doubler with diodes and power that from a MCU output to make a higher voltage.

Here's a quick and dirty inspiration:

enter image description here

Basically you use one of the PWM timers to output some high frequency like 20kHz on a pin, and a diode charge pump to make about 5V from your 3V3 power supply. You can use any fast diode like 1N4148, but a dual Schottky like BAY54S will lose less voltage. Add a smoothing cap on the output, and a 74HCT logic gate to translate your micro's PWM from 3V3 logic levels to 5V logic level, and that drives the MOSFET.

This cheap quick and dirty power supply won't deliver a lot of current, it is limited by the MCU pin's current capability, but you don't need much to drive a MOSFET gate if the PWM is only at a few kHz.

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  • \$\begingroup\$ Hello, your answer helped me a lot how to select the right MOSFET, thanks. Some MOSFETS, like SiSH106DN, are mentioned in the datasheet as "impoved for PWM" what does that mean? I did not find the place in the datasheet where I can lern what this improvement is? I may like to take 1000Hz PWM..... You are right, I got lots of LR3103 MOSFETs here and liked to use them, will try with voltage doubler until RdsON @ 2.5V arrive... \$\endgroup\$
    – MyMOSFET
    Commented Dec 23, 2021 at 17:42
  • \$\begingroup\$ From the voltage and very low RdsON, I guess this MOSFET is intended for PC motherboard CPU Vcore buck regulator, which is sometimes called "PWM" although it has nothing to do with the PWM you'll use in your application... Note if you use a voltage doubler, then you should use it to make some stable voltage around 5-6V, which you then use to power a 5V MOSFET "driver", basically a 74HCT logic gate, that will translate your 3V3 into 5V for a price that is difficult to beat... \$\endgroup\$
    – bobflux
    Commented Dec 23, 2021 at 18:05
  • \$\begingroup\$ Hello, thanks for your fast reply, I will try the voltage doubler but I may not have enough free PWM pins left on my MCU. I intended to try Vishay SiSH106DN and DIODES DMN2005UPS, both MOSFETs rated as load switches. As the LiFePo cells are about 100Ah each and charging current is up to 25 A per cell I need bigger dumpload/bypass resistors to do the balancing job. I tried and found bypass currents of about 9 to 12 A start to become usefull in the balancing process. So the bottom line is: I need to switch 9 to 12 A with 3.3V gate voltage coming from my cell controller. cheers MyMOSFET \$\endgroup\$
    – MyMOSFET
    Commented Dec 24, 2021 at 2:07
  • \$\begingroup\$ 12A with 8 mOhms RdsON means your FET will burn 1W, that's fine with this package if the drain is on a large enough copper plane. But you're going to need high power resistors... BTW, how can you run out of PWM pins? With the schematic above, you'll need one microcontroller per cell \$\endgroup\$
    – bobflux
    Commented Dec 24, 2021 at 8:00
  • \$\begingroup\$ Good evening, The upper circut shows only a part of the concept used, each LiFePo batterie cell has a cell module and these modules are daisy chained to comunicate with the central controller.The cell modules carry an ATtiny 841 as MCU. There are no available PWM pins left. So I would need to switch to an other MCU or wait for the 2.5V MOSFET. I cant bring extern power to the modules as they are all electricaly insulated. I am bound to use the cell voltage (3.3 to 3.6 V @ balancing procedure) to switch the bypass MOSFET. So in the end your first sugg. to get the right MOSFET is best \$\endgroup\$
    – MyMOSFET
    Commented Dec 26, 2021 at 2:03

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