8-bit Microcontrollers中的主题 Re: Damage Freescale microcontroller by driving switch-mode FETs directly from microcontroller pins?

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

googer — Sun, 23 Jan 2011 22:35:17 GMT

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

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

celsoken — Mon, 24 Jan 2011 07:09:53 GMT

Since your switching frequency is quite low, I suggest adding a series resistor in every output to the fets.

You could actually observe current peaks and drive them to a safe level.

And you would decrease emission as a bonus.

Although I use simulation I never trust it entirely since models sometimes are not good enough.

I hope it helps,

Celso

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

bigmac — Mon, 24 Jan 2011 13:15:01 GMT

Hello, and welcome to the forum.

I might suggest placing, say a 100 ohm resistor between each output and the MOSFET gate. Then to give additional protection from high voltage spikes finding their way back to the output pins, I would also suggest connecting a Schottky diode between each output pin and Vdd (cathode to Vdd). This would reduce any transient current that might be injected into the pin, since the Schottky diode will conduct at a lower voltage than the internal parasitic diode within the MCU. You might actually consider using 3-pin Schottky diode packages, containing two series connected diodes. The anode of the second diode would connect to ground, to additionally afford protection against negative transients.

The bulk capacitance value connected to the Vdd rail should be sufficient to absorb the transients without significant Vdd voltage increase. An additional 5.6 volt Zener diode at the Vdd rail would provide additional protection against over-voltage.

I presume that the switching process is to control the average solenoid current, by manipulating the duty cycle. The solenoid time constant is about 6 milliseconds, so continuous current would be maintained with 27 kHz switching rate (37 us period).

With direct drive from the MCU pin, the switching delay plus risetime is likely to be about 1 us. This will result in higher power dissipation within each MOSFET, compared with using higher current drive circuits, i.e. increased switching losses. You will need to determine whether this is acceptable.

Since the transient drive current associated with each circuit is likely to persist for only 1 us, the switching control of each output might be staggered by at least this amount, so that simultaneous transients do not occur for more than one output.

Regards,

Mac

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

TurboBob — Fri, 28 Jan 2011 22:15:51 GMT

Another advantage to adding series resistance and clamp diodes is the case of a FET failure. With the additional components, the micro will not get destroyed if the FET shorts.