Logic level mosfet to switch on 3.7v battery

Question

Can anyone recommend a logic level mosfet(2v-5v gate) that can switch on/off a 3.7v lipo battery. Battery load will be 2-3 ampere on 3.7v-4.2v. I tried something like FQP30N06L but will only work on Drain Source Voltage of 5v and above.

I've been reading some mosfet datasheet but im not sure how to look for my requirements. All of the datasheet just mention the maximum Vds(drain source voltage). How do you know from the datasheet the current it can support from a particular Vds?

Will appreciate any input. Thank you!

Answer 1

Shopping questions are off-topic. But since several people have already answered, I will pretend you asked "how can I choose a good MOSFET for this application."

Most likely your best choice is going to be to use a P-channel MOSFET (PMOS). You want it to turn on reliably with a voltage as low as, say, 3V, and have a voltage drop of less than, let's say 0.1V when passing 3A.

1) Rds(on). The first thing to consider is Rds(on). When it is on, a MOSFET is like a low-value resistor. The resistance value is given in the datasheet as Rds(on). We want the voltage drop in the PMOS to be less than 0.1V at 3A, so we use Ohm's law to calculate the maximum resistance.

V=I*R R = V/I R = 0.1V / 3A = 33mOhm

So we want Rds(on) to be less than 33mOhm. But there is more to it than that. The voltage between gate and source is what determines if the PMOS is on or not. We want to make sure that the 33 mOhm figure is given when gate voltage is 3V or less.

2) Power. We can also calculate power dissipation. The relevant formula here is: P = I^2 * R

If the Rds(on) really is 33 mOhms, the calculation is as follows: P = 3A * 3A * 0.033 Ohms = 300 mW (approximately). You can probably use a SOT-23 package transistor, especially if you don't plan to use 3A very often.

PMOS will turn on when the gate voltage is lower than the source voltage. So the source terminal is your power input, the drain terminal is your power output, and the gate is the control terminal. Here is a circuit that shows how to switch the PMOS on and off.

^{simulate this circuit – Schematic created using CircuitLab}

You just have to supply the control signal.

Answer 2

Since heat loss from RdsOn*I² may need extra copper surface area (>6sq.cm/W) try to make it 1/4W thus RdsOn is R=P/I²=0.25/3² or roughly in the <200mΩ range. This often implies the part may be rated for 5~10x the current you need but that needs a big heat sink.

So search for ones in the 100mΩ range. In order that get this Vgs must be at least 3x the threshold Vgs(th) . So for 3.1V when weak Vgs(th) = < 1V

These 2 parameters will give you hundreds of choices at Digikey or Mouser. Then sort by lowest cost for qty entered

Answer 3

You should use a P-channel MOSFET such as an FQP27P06, which is off at 5 V and on at 0 V gate voltage. N-Channel MOSFETs need to have, as you have sort of stated, Vgs above the power rails, which usually adds complexity. The trade off is that N-MOSFETs have lower \$R_\mathrm{DS}\$ than comparable P-MOSFETs, and so will drop less voltage for a given \$I_\mathrm{D}\$. In the datasheet, the \$R_\mathrm{DS}\$ is a figure of merit and will be quoted under optimum conditions.