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Electronics => Beginners => Topic started by: bryce1 on February 13, 2014, 05:03:07 pm

Title: First try at designing a 5A/12V multi purpose mosfet switch
Post by: bryce1 on February 13, 2014, 05:03:07 pm

Hi guys,

I need your input on this since it is the first time doing something like this.  :o

Basically, I want to design a (low side-) "switch" that allows an AVR (5V) to safely switch a variety of 12V loads. This includes LEDs (PWM), SSRs, DC pumps and things I haven't thought of yet (up to 5A).

VCC=12V
SWITCH1=Atmega pin
R5=depends on the LED
R10=pull down
R9=limits the current to the MOSFET gate when PWMing (1/8W enough?)

The mosfet will be something like this:
http://www.irf.com/product-info/datasheets/data/irlb8748pbf.pdf (http://www.irf.com/product-info/datasheets/data/irlb8748pbf.pdf)

It will probably need proper cooling.

I am not sure yet on how to calculate R9 and R10 and I have not chosen D2 and C3 yet.
Can you give me advice/criticism on the schematic? Maybe some layout advice? This is all new to me, so I'd like to learn as much as I can (and hopefully end up with something great ;-) ).

Thanks!

Title: Re: First try at designing a 5A/12V multi purpose mosfet switch
Post by: madworm on February 13, 2014, 05:13:55 pm

The gate resistor should be high enough to limit the peak current on the AVR's pin to about 30-ish mA. Consult the datasheet. I think for some AVR chips, the absolute maximum is 40mA for GPIO pins.

So 160Ohm would work for 5V.

Using the GS-capacitance, you can calculate the RC-time and see how long it takes for V(GS) to be reasonably high for the mosfet to turn on fully. That will also give you the upper limit for the PWM frequency.

For really fast switching you need a mosfet-driver ic, that can charge the gate really fast and avoid thermal loss in the transistion from off to on.

The pull-down resistor (bleeder) is not that critical. If you like to conserve power, 10k seems a bit low. Again, calculate the RC-time if you think it matters.