ESP8266 Community Forum

- Tue Jan 17, 2017 6:28 pm #61159 I understand the need for pull up / down on inputs, but is there any need on an output? You have to be sure not to draw too much current from the output, so maybe a resistor in series with the load, but as long as that is considered there's nothing else is there?

I'm using an STP36NF06L MOSFET to drive an LED strip.

The data sheet says..

Vgss = max +-100nA (Gate to body leakage current)

do I understand correctly that is the max current that the gate can pull from the GPIO pin? So it's pretty much negligible as far as the ESP9266 is concerned?

If I was using a BJT instead I'd (probably?) need a resistor to the base to limit the current. So if VCC=3.3 and I want a limit of 1mA pulled from the GPIO, i'd choose a 3.3K resistor? Roughly speaking, cause the voltage at the gate end of the resistor would be a bit above ground.

Ok, back to the STP36NF06L.

I want to drive about 20W of LED strips. So

20W / 12V = 1.66A of current draw (ignoring again that the FET end of the LED strip will be a bit above ground)

And the datasheet says...

Rds = 0.04Ohm, (tested at 10v 15A)

So heat dissipation by the FET would be approximately = I.66^2 * 0.04 = 0.1111W

Have I got that about right? Should be no problem for a FET to handle this (the three legged type with a metal tab).

I can ignore any heat from the gate current in this instance cause it's tiny?

thanks,

Chris

- Wed Jan 18, 2017 12:59 am #61168 Chris,

Regarding your question of resistors on outputs - it depends on what is connected to the output and if you need it to be in a defined state during reset (for example), as the GPIO will typically be an input during reset condition. For Mosfets, it is reasonably common to have a resistor (eg 10k) from Gate to Source to turn off the Mosfet when there is no gate drive signal present. In your case it may not be critical if you don't mind if the Mosfet is on or off during reset conditions.

For Mosfets, the main sources of losses are I^2 x R and switching losses.

For the I^2 x R losses - the on state resistance varies with gate voltage and the resistance will be higher with a gate drive of 3.3V than with 10V. Maximum gate threshold for this device is specified to be 2.5V, so you should be able to turn it on with 3.3V, but the resistance will be higher than 0.04Ohms (the data sheet does not specify on state resistance vs gate voltage, but looking at the curve of the Output Characteristics, it looks like the on state resistance is probably at least twice as high at a gate drive of 4V than it is at 10V.) Also the resistance increases with junction temperature - it is 50% higher at 100C than at 25C - so the losses will increase as the device heats up.

So you can probably conservatively assume a resistance of 0.15Ohm at 3.3V gate drive and elevated junction temperature, which would give I^2 x R losses of 0.41W.

The thermal resistance (junction/ambient) is given as 62.5 C/W max, so you should be ok without a heatsink assuming there are little/no switching losses.

Are you planning on applying a PWM signal from the GPIO to the gate of the Mosfet? It is unfortunately not very straightforward to estimate the switching losses, especially if the gate of the Mosfet is being driven by a relatively low current source like a GPIO.

- Thu Jan 19, 2017 5:06 am #61225 Hi Warwick,

Thanks for the explanations. Yes I plan to drive it with a PWM signal. Couple more questions...

Could I run two mosfets in parallel?

Could I determine the power wasted in the mosfet by measuring the voltage across it? Or rather if I assume the fixed current for the LEDs as 1.6A, i could figure the maximum voltage the mosfet could tolerate from drain to source before power used gets to high. So if I aimed for 0.1W,

0.1w = 1.6a * Vds
Vds = 0.0625

So if I hook it all up and keep an eye on Vds, I could know if it's using to much power? I mean I assume Vds will go up as it heats up, so I could see if it's getting too hot and turn it all off?

thanks,

chris