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Damage microcontroller by driving FET?


Aug 3, 2009
Aug 3, 2009

Will we damage our Freescale MC9S08AC60CFGE microcontroller by driving a FET directly from a microcontroller output pin, with no series resistor, (and no gate driver circuitry)?

We are powering solenoids via switch-mode solenoid drivers which comprise FETs being driven directly from a microcontroller.
-as stated , the FET gates are being driven directly from the microcontroller pins, with no series resistor used -and no specific gate driver circuit used. (-the microcontroller is supplied from 5V)

(there are actually four solenoids and so four of the same microcontroller's pins directly drive four different FETs)

Here is the basic switching solenoid driver schematic:


...but i think the uC pin output resistance is more like 100R(?)

-solenoid current is 220mA and switching frequency is 27KHz.

Page 304 of the MC9S08AC60CFGE microcontroller datasheet states that the maximum instantaneous current allowable in a microcontroller pin is 25mA

Page 304 of MC9S08AC60CFGE microcontroller datasheet:

I make it that the total power_fet gate capacitance (including miller effect) is around 2.2nF for a 48V bus.........C = Qg/V (values from datasheet, below)

The actual gate (Cgs) capacitance is Ciss - Crss = 316pF.

Our Bus is 36V , so we will have a total gate capacitance (Cgs) of around 1.8nF.

This is one of our sold products, but we had to stop producing it shortly after launch because a connector was not made of flame retardant plastic.
-we intend to re-launch in 6 months with the new connector.

I am told by colleagues that this product survived and passed "quite a lot" of qualification testing.

....but how can potential microcontroller damage be prooven?