Another Mosfet Question

[tazntex]

Good Morning to All,
I am driving a solenoid with a 12V coil, the coil is 34Ohms, off the top of my head thats about 1/3 of an Amp, I am also powering a 12VDC lamp that pulls about 2.5 Amps. I hear that I can use an IRF3205 Hexfet to be driven directly from my output on a 16F628A. The next question I have, on most datasheet for the FET's I've looked shows Vcc then FET then the load one is wanting to control (connected to drain) and then the Ground (connected to the Source).

Because of the solenoid and the lamp housing are ground, does it make a difference if I use the IRF3205 connected as: gate directly to output pin from 16F628A, Vcc 12VDC to Drain,
Source connected to load, and through the housing the load will get its ground.

Thanks

[mackrackit]

With an NPN mosfet, the Source is connected to "ground" 0 volts, and the Drain is connected to the load. The other side of the load (lamp) will be connected the positive rail.

The gate connects to the PIC pin with a pull down resistor. I normally use a 100K.

Check the gate saturation voltage if you plan to run the PIC at a lower voltage than 5.

If you are needing switching speed, you may want to look at IGBTs. Some mosfets are pretty slow if PWM is the goal.

[sayzer]

PIC gives you 5V, but your load is 12V; you can not ignite the gate.

To drive NPN mosfet with the load going to GND, consider the following way.


<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2860&d=1221917652" >

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[rmteo]

Another solution to drive an N-channel MSOFET with the load going to GND is to use a hi/lo gate driver such as IRS2001, LM5101, FAN7842 (all come in an 8-pin packages). They provide sufficient gate drive current and will allow you to PWM at pretty high frequencies - up to 1MHz depending on the application. I typically use them at 5-50KHz. Also, because they have hi and lo side drivers, they are very useful in bridge type situations using N-channel MOSFETs only.

[Acetronics2]

Hi,

Or output 50% PWM signal to drive the fet ... via a simple voltage doubler - or more - ( diodes + capacitors )


Hi, Sayzer

not so good for high current load ...

you'll drop at least 3 v in your Mosfet ( Vgs on + Vbe BC 337 + V diode ! ) ...

Bootstrap only works at "firing" the Mosfet ... not while steady conduction ...

Alain

[sayzer]

Hi Alain,

This way of "driving" is an issue that I always encounter with.

I have the following idea but not fully developed it yet.

Any comment is appreciated.


<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2865&d=1222017290" >

[rmteo]

I would just use a dedicated gate driver such as FAN7371 (less than US$1).

The FAN7371 is a monolithic high-side gate drive IC, which can drive high-speed MOSFETs and IGBTs that operate up to +600V. It has a buffered output stage with all NMOS transistors designed for high pulse current driving capability and minimum cross-conduction.

* Floating Channel Designed for Bootstrap Operation to +600V
* 4A/4A Sourcing/Sinking Current Driving Capability
* Common-Mode dv/dt Noise Canceling Circuit
* 3.3V and 5V Input Logic Compatible
* Output In-phase with Input Signal
* Under-Voltage Lockout for VBS
* 25V Shunt Regulator on VDD and VBS
* 8-Lead Small Outline Package (SOP)

[Acetronics2]

Hi Alain,

This way of "driving" is an issue that I always encounter with.

I have the following idea but not fully developed it yet.

Any comment is appreciated.


>

Hi, Sayzer

Same problem ... the gate must be 5 + volts MORE than V+ for the mosfet to be fully ON.

and no existing turnaround ... except voltage multipliers ( dedicated drivers also have their own voltage multipliers inside !!! )

Alain

[rmteo]

Ace, is correct. A driver IC, like the ones that I mention above typically apply 12V above V+. So if your circuit is running from say 20V, the voltage applied to the gate of the MOSFET is 32V - this will ensure that is turned on fully.