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'[EE] Logic Level Mosfet 2 PIC'
2006\11\27@053151 by Markus Rohe

picon face
Hello everyone,

it must have been an about 4 or 5 years since I backed out of the
piclist and the world of PICs. Now, a few weeks before Xmas I decided to
do a new PIC project again.

I have a rather simple question, however, some extensive search on the
web and http://www.piclist.com did not delivered enough information for me. I
plan to drive a few lightbulbs (12V) by a PIC and to minimize a loss of
energy this should be done with a logic level mosfet. The IRLZ 34N seems
a good chice for me (Ron=35mOhms, rather cheap and _available_ at my
local dealer). Now: How to attach this device to a PIC port. An approach
like this sould do the work:


                            bulb
                             |
------                       D
RBx  |--------------*-----G
------              |        S
                    Z        |
                10k Z        |
                    Z        |
                    |        |
                    |        |
                   GND      GND


I am not very familliar with this sort of device (In fact I am only
hobbyist) something is very confusing to me: In some schematics the
authors use some low-valued resistors directly befre the gate. I assume
this is to protect the driving device from some high current pulses when
the gate has to be charged. Would it be necessary for the PIC as well like


                                   bulb
                                    |
------                   100Ohms    D
RBx  |--------------*----|/\/\|--G
------              |               S
                    Z               |
                10k Z               |
                    Z               |
                    |               |
                    |               |
                   GND             GND

Any hints or web-pointers iwth similar schematics are welcome. If you
think this generates too much noise over the list, please email me
privately.


Best Regards
Markus

2006\11\27@092639 by Richard Prosser

picon face
Markus
The resistor in series with the gate is typically used to prevent
oscillaton at RF frequencies. It may not be required or it may be
essential. Layout is a major factor as well as the type of load and
mosfet characterisics. For the cost, I'd add it. 100ohms is a
reasonable figure but if your switching frequency is high you may want
to drop it somewhat (10 ohms minimum ?)

It does provide some protection to the PIC in the event of device
failure, but that is more a benifit than a reason.

Richard P

On 27/11/06, Markus Rohe <spam_OUTpiclist.roheTakeThisOuTspamgmx.net> wrote:
{Quote hidden}

> -

2006\11\27@094620 by Wouter van Ooijen

face picon face
> The resistor in series with the gate is typically used to prevent
> oscillaton at RF frequencies.

IIRC it is also used to prevent the high dI/dt in the load and hence in
the FET, capacitivly coupled back to the gate, from harming the uC.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2006\11\27@134923 by Markus Rohe

picon face
Hello everyone,

thank you for the responses so far. I was a little bit confused because
today I saw some schematics with and without a resistor in serial to the
gate and I was wondering about this purpose.

The resistor from the RAx or RBx to GND are used suppress voltage spikes
caused by HF noise since the gate has a quite high impedance.

The circuit will consist of 12 MOSFET drivers all independently driven
in PWM mode such that a good noise reduction is essential.
Unfortunatelly, I do not have an oscilloscope such that I simply have to
"guess" some "good" value and to hope that everything works well enough.
So, what are your suggestions? Is this resistor value critical. Do you
have some "empirical values"? 10 Ohms, 50 Ohms, 100 Ohms?

Best
Markus

2006\11\27@141028 by William Chops Westfield
face picon face

On Nov 27, 2006, at 10:49 AM, Markus Rohe wrote:

> The resistor from the RAx or RBx to GND are used suppress voltage
> spikes
> caused by HF noise since the gate has a quite high impedance.
>
The PIC output, however, has low impedance.  I wouldn't think the
resistor to ground would be necessary unless you planned to float
the PIC outputs for some reason (fail safe?  Power on transient
protection?)

BillW

2006\11\28@070805 by Markus Rohe

picon face
William Chops Westfield schrieb:
> On Nov 27, 2006, at 10:49 AM, Markus Rohe wrote:
>
>> The resistor from the RAx or RBx to GND are used suppress voltage
>> spikes
>> caused by HF noise since the gate has a quite high impedance.
>>
> The PIC output, however, has low impedance.  I wouldn't think the
> resistor to ground would be necessary unless you planned to float
> the PIC outputs for some reason (fail safe?  Power on transient
> protection?)
>
> BillW

No, the port pins will constantly used as outputs for driving the gates
and never be set as inputs.

Ok, since designing such a mosfet driver seems to depend a lot on the
given mechanical and electrical paramenters (board, wires, current of
the loads, switching frequency) such that I dcided to start like this
and see (and measure) what happens.

                              o +12V
                              |
                              \/     bulb
                              /\     12V/10W
                              |
 ------         47 Ohms       D
 RBx  |---------|/\/\/|-----G
 ------                       S
                              |
                              |
                              |
                              |
                             GND


Markus



2006\11\28@074138 by Alan B. Pearce

face picon face
>No, the port pins will constantly used as outputs
>for driving the gates and never be set as inputs.

but don't forget that for a few microseconds while the unit boots up and
initialises the pots they are defaulting to inputs, which is why the
resistors to source are wise - to ensure you don't end up with something
accidentally turning on when it shouldn't.

2006\11\28@084405 by Spehro Pefhany

picon face
At 07:41 AM 11/28/2006, you wrote:
> >No, the port pins will constantly used as outputs
> >for driving the gates and never be set as inputs.
>
>but don't forget that for a few microseconds while the unit boots up and
>initialises the pots they are defaulting to inputs, which is why the
>resistors to source are wise - to ensure you don't end up with something
>accidentally turning on when it shouldn't.

Or drifting to half-on (which can cause power devices to automatically
unsolder themselves) if the micro is socketed and is removed from the
socket.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff


2006\11\28@102303 by Richard Prosser

picon face
On 28/11/06, Alan B. Pearce <A.B.PearcespamKILLspamrl.ac.uk> wrote:
> >No, the port pins will constantly used as outputs
> >for driving the gates and never be set as inputs.
>
> but don't forget that for a few microseconds while the unit boots up and
> initialises the pots they are defaulting to inputs, which is why the
> resistors to source are wise - to ensure you don't end up with something
> accidentally turning on when it shouldn't.
>

And during programming if you're using an ICP.

RP

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