andrew massey

Damage Freescale microcontroller by driving switch-mode FETs directly from microcontroller pins?

Discussion created by andrew massey on Jan 23, 2011
Latest reply on Jan 28, 2011 by Robert Bailey
Hello,

Will we damage our Freescale MC9S08AC60CFGE microcontroller by driving  four  separate FETs  directly from  four  microcontroller output pins, with no series resistors, (and no gate driver circuitry)?




We are powering solenoids via  27KHz , 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)

(as said, 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: (just one FET is shown, and the microcontroller output resistance is wrongly shown as 5R...my apologies)

SCHEMATIC:
http://i53.tinypic.com/1zm1j08.jpg


-solenoid current is 220mA and switching frequency is 27KHz in all 4 cases


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:

http://i51.tinypic.com/4rtj00.jpg


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?
-This microcontrollers output resistance appears to be 100R,  (actually  anywhere from 83R to 125R) from looking at page 310 of its datasheet
Graph showing microcontroller pin output resistance (IOH)
...from pg 310 of MC9S08AC60CFGE microcontroller datasheet

http://i51.tinypic.com/4r9apv.jpg

Now here is one  microcontroller pin's  current when driving the FET  directly, with a 100R series resistance added to the simulation to simulate the pin output resistance (this LTSpice simulation  uses the STB16NF06LT4   FET .model)

FET gate current with 100R series resistance (100R represents the microcontroller's suggested output resisistance):-

http://i53.tinypic.com/o60bkn.jpg


do you consider that this current is unnaceptable? ..(it is going above the instantaneous maximum current of 25mA  stated on page 304 of the uC datasheet)

-Please bear in mind that its FOUR  pins of the same microcontroller that are each supplying this current. (switching frequency is 27KHz)


MICROCONTROLLER DATASHEET (Freescale MC9S08AC60CFGE):
http://cache.freescale.com/files/mic...f?fsrch=1&sr=5

FET DATASHEET (STB16NF06LT4):
http://www.st.com/internet/com/TECHN...CD00002847.pdf  

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