mosfetswitches
I have a few n-channel MOSFETs (the irlb8721's) and I was able to use it as a switch to control an led with the gate at 5 volts.
I would like to change the source/drain voltage to 11.1v from 5v and current will be 20a with still having the gate at 5v (so I can control it with a microcontroller). Would this be possible? Or do I need to add additional transistors/resistors?
simulate this circuit – Schematic created using CircuitLab
but dims if there is no signal
It's hard to guess without a real schematic, but I'm making an assumption here: no signal means that it is left floating.
In that case, you need a pull down resistor from the gate to ground, to discharge the internal gate capacitance. Something like this:
simulate this circuit – Schematic created using CircuitLab
The value is not critical, it can be anything between 10 kΩ and 1 MΩ. You can also place it before R1 if you want.
The basic formula for smoking parts depends on the thermal resistance of temp rise per power lost in ['C/W] for the rating of the heatsink.
Pout = 50A*14.4V= 770 Watts
However when supplying a sine wave into a rectifier with a battery acting as almost a short circuit the peak currents can be be >>10x times as much, thus increasing the requirement now to >>500A pk
To minimize the Vds drop, AND have sufficient Vgs now you need a Pch switch with gate to ground control AND have accurate gate control.
Use a Pch and decide what threshold to disable the switch. I roughly chose 13.2V R divider with a poor-man's 1.1V Darlington comparator. ( it works) The threshold may need to be corrected slightly to 13.4V to as a threshold. Since the drain current is low the input threshold to the Darlington is also low at 1.1V @1mA
$2.20(1pc) 2.2mΩ Pch type 170A
simulate this circuit – Schematic created using CircuitLab
The Vbat threshold to Vgs voltage gain is > 1k so the threshold amplifies Vgs to quickly move past turn ON threshold from 1.2 to 2.2V with just a few mV change in V+.. the only drawback is the threshold is not very precise (5%) and has a NTC of -2.1mV/'C which translates to 21mV/'C of battery threshold shift or a drift of 13.2 +/-0.4V for +/-20'C so 13.5V is advised.
Best Answer
Let's see how bad this looks...
According to the datasheet Vds max = 30V, what is much higher than 11.1V, so you are good in this regard.
Let's check the power dissipation @ Id=20A. According to Fig. 12 of the datasheet, the Rdson degrades significantly when Vgs=5V instead of 10V. For Tj=125˚C (worst case), Rdson=16mΩ. So the power dissipated will be 0.016*(20A)^2=6.4W, what is pretty high.
According to the datasheet, the thermal resistance from junction to ambient with no heatsink is 62 ˚C/W (max). With Pd = 6.4W, assuming Tamb = 25˚C, we get Tj = 25 + 62 * 6.4 = 421.8˚C !!! It's clear you need a heatsink!
Let's see how big of a heatsink you need. Let's say Tj=125˚C (what is already pretty high) and Tamb=25˚C, so the delta T will be 100˚C. For Pd = 6.4W, the total thermal resistance will have to be less than 100˚C / 6.4W = 15.6 ˚C/W. The thermal resistance from junction to the heatsink is 2.3 + 0.5 = 2.8 ˚C/W, according to the datasheet. This means that the heatsink thermal resistance will have to be lower than 15.6 - 2.8 = 12.8 ˚C/W. You may be able to achieve that with a big heatsink or with a not-so-big heatsink with forced ventilation.
If you add a level shifter capable of driving the gate of the FET with 10V instead of 5V, the power dissipation will reduce significantly. According to the datasheet Rds max will become 8.7mΩ, what drops Pd to 3.48W, what will require a total thermal resistance of 28.7˚C/W, and 25.9˚C/W for the heatsink only. This translates into a much smaller heatsink.