# Power mosfet

Hi! I’m planning to use my Arduino to control a power mosfet with Vds = 325V DC, Id= 5A. I am thinking about using an IRF840 power mosfet. I am new to using mosfets, so that is the reason for asking these questions. Anyway, is the IRF840 a good choice or should I use another mosfet? Do I need a driver for the mosfet?

In case of 325V you definitely want to use an optocoupler to drive a MOSFET. And you will need a relatively big heatsink.

I also like to add a bit of safety factor on the components. You’re pretty close to current limit because it will get HOT. Sorry can’t suggest anything. I spend a lot of time searching for the correct MOSFET for my own projects. It’s always a compromise between pricing on state resistance and several other factors. You will have to do your own search. What is it for. That’s a lot of power.

I'm building a 120V battery charger (10*12V batteries). My plan was to convert 230V AC to 325V DC, and then use a mosfet for regulating the charging voltage.

So before we look at driving the MosFet, lets look at the power rating required.

1) your supply is ~ 300V ( after rectification and filtering etc.)

2) your load is ~ 10 X ( 12V + 2.5 (charge voltage)) = 145V

2) Lets say you only charge at 2 Amps

The power in the MosFet will be: Pwr = volts X amps = (300 - 145 ) X 5 = 775 watts !! It won't work with only one MosFet, you will need a whole array with special cooling.

What you have is called a Linear regulator. It simply absorbs the power between you input and output. This is the least efficient (and least practical at these power ranges) of all the regulators.

What you will need is a switching regulator. These regulators turn on fully then off. they do this very fast and store energy in an inductor and a capacitor so your load does not see the full on and off voltages.

Because (ideally) when the switch if ON the voltage across it is 0 so the power it dissipates is 0. When the switch is off the voltage is at max but the current is 0 so again the device dissipates 0 power.

Because we don't have the ideal case, there is some power dissipated but much much lower than the linear.

You may find such a device (as a board to be built into something else) on ebay.

Good luck

Type of battery is important when it comes to charging. 12V batteries sounds like lead/acid type, is that so?

Charging strategy depends on the battery type you want to charge. You have to have some circuit in place to prevent overcharging and overcurrent.

Disregarding the battery charging part (I don’t know much about that), let’s start with that MOSFET you selected. Max current 8A, that’s OK albeit a bit low compared to the requirement you have.

VGS should be at least 10V, that will require a separate power source (a 12V power supply would be fine), and a driver circuit so it can be switched from the Arduino’s 5V pin. If you can find a logic level one (IRL types should be logic level) you could switch it at 5V directly.

RDS, ON = 0.85Ω - that’s really high, there are much better MOSFETs around. The high voltage requirements may be an issue, though, check digikey or so to find alternative parts. In the ON state this MOSFET will dissipate 4.25W at 5A current, so needs a good heat sink.

alesam: In case of 325V you definitely want to use an optocoupler to drive a MOSFET. And you will need a relatively big heatsink.

That's a good idea, but the first thing to get right is the dV/dt protection for the gate, otherwise you'll probably blow the thing up immediately.

Use a gate driver chip to drive the gate, so it sees a low impedance that will hold the gate voltage steady as the drain voltage swings 300V - without a low impedance drive to the gate the capacitive coupling from drain back to gate will likely push the gate into overvoltage and fry it.

You can drive the gate driver's logic input through an opto coupler, and protect its 12V supply with a crowbar circuit should the MOSFET fail and take out the driver.

Another important issue is why a MOSFET? At 300V an IGBT is the preferred choice as they are more robust at high voltage. Standard IGBT's come in 600V and 1200V voltage ratings. You want something like double the supply voltage as the device rating.

wvmarle:
RDS, ON = 0.85Ω - that’s really high, there are much better MOSFETs around.

No its pretty average for a high voltage device - the higher a MOSFET’s voltage rating, the higher the
on-resistance has to be, due to the physics of the device. But yes there are better devices if this matters.

Since switching losses can often dominate at high voltage/power, the on-resistance may not be the only
issue for heat production, so you’d need to get some figures for switching losses if using PWM.

I'm just thinking this has gone way beyond your typical arduino project... I'm bowing out, don't like the liability of helping someone with a 300 volt high current application.