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Is it a benifit to use a mosfet driver with a logic style mosfet? — Parallax Forums

Is it a benifit to use a mosfet driver with a logic style mosfet?

Capt. QuirkCapt. Quirk Posts: 872
edited 2009-04-05 17:57 in General Discussion
Do Logic style (IRL)·Mosfets··need or benifit from a mosfet driver to open the gate fast, like a·Non-Logic (IRF)Mosfet.

Thank you

Bill

Comments

  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-04-04 21:10
    There's no way to answer this question in general. Whether to use a MOSFET driver will still depend on the MOSFET's gate capacitance, the drive capability of the naked logic, and the required switching speed.

    -Phil
  • Capt. QuirkCapt. Quirk Posts: 872
    edited 2009-04-04 22:33
    Is this like filling a capacitor? and if you want to fill it faster than 1mhz it may need some help? The math and terms in my mosfet basics data sheets are above me, but I am working on it.

    Can an SX48·@20 - 50mhz drive a logic mosfet to 4mhz or above (I just picked that #, it has no significance) without a driver, if a mosfet has a certain charactristic? Perhaps a certain range of input or output capacitance?
  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-04-05 00:42
    Capt. Quirk, yes, driving a mosfet on/off is very much like charging/discharging a capacitor. The outputs of any logic device have a fairly high impedance when you are talking about charging something that looks like a short at high frequency. If you are switching the mosfet at high speeds(>1kHz), the logic may not be able to supply enough current, causing the mosfet to turn on slowly. When it turns on slowly, alot of power is dissipated as it turns on. This will get the mosfet very hot.

    The other thing to consider, is the logic device itself. A propeller can supply a fair amount of current, but if you have a mosfet with fairly high capacitance, you might blow out an IO pin due to the large inrush current.

    So, yes, a mosfet driver is generally recommended unless you are simply "powering on" a device with the mosfet, i.e. very infrequent switching. Reason one, preventing the mosfet from getting hot, and reason two - protecting the logic device that is supplying the signal.

    As for the SX, I'm not TOO familiar with the chips, but 4mHz is pretty high even for my 9A driver! You should DEFINATELY use a high current driver for those kinds of frequencies. If I may ask, what are you making? I've always wanted to make an inductive heater with high freq's like that... [noparse]:)[/noparse]
  • Capt. QuirkCapt. Quirk Posts: 872
    edited 2009-04-05 01:19

    Like many people, I have about a half dozen projects going all at one time, lol. Many of them are inductive projects like Ignition systems, DC-DC converters and H bridge motor control. There have been a few articles in Nuts n Volts in the last year, one of them was for adding a micro-controller into a slot car. In that article they mentioned the motors made noise (and noise was bad) above 1Mhz and below 4Mhz when the controllers were operating at those frequencies.

    Somebody (on this forum), possibly Mike Green, suggested adding a transistor as a bridge between a microcontrollers and the source voltage as an alternative. I could see where that may help turn on a mosfet but would it work as well to shut the mosfet off. Or does a mosfet driver work the best for on and off?

    Thanks

    Post Edited (Capt. Quirk) : 4/5/2009 2:23:56 AM GMT
  • PhilldapillPhilldapill Posts: 1,283
    edited 2009-04-05 17:57
    A mosfet driver has a half-bridge internally. A single transistor could turn the mosfet on, but when it came time to turn it off, the transistor could not sink the current. A half-bridge is two transistors. One sinks current, and the other, sources current.

    I have no clue why anyone in their right mind would be running a DC motor at >100kHz, let alone, 1 Mhz! That's in the RF range, and you're right - that WOULD cause a bunch of noise. Not just in the circuit, but totally external devices absorbing RF energy.

    If you want isolation between the motor and your circuitry, I'd suggest using an ISO721. It's an isolator that totally isolates circuits. You can have completely different supplies and grounds with this thing, meaning your uC can be the input with the 5V supply and digital ground, and the output would be on the other side(isolated) with it's own voltage and ground. The chip works up to 150megabits/second. PROBABLY fast enough... Oh, and you can get a few free as samples from TI.com to try out. Yes, I rape and pillage their samples department when I am trying out something new... But I do eventually buy some parts when my design is finished. Just go easy on them. [noparse]:)[/noparse]
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