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LM5104 FET driver and the Prop, gate resistor help — Parallax Forums

LM5104 FET driver and the Prop, gate resistor help

JonathanJonathan Posts: 1,023
edited 2008-09-25 18:36 in General Discussion
Hi All,

I am working on a Prop based MPPT/Battery charger for a solar array. I am using the LM5104 buck converter fet driver chip, and have a mock up working. Before I solder together a proto version, I need a little help figuring out the correct value for the series resistor from the driver to the FET.

I will be using an IRFP250 FET, which has an Rdson of .08 Ohms. I will be supplying the LM5104 with 12V for VDD. The input voltage to be bucked will range from 16 to 14VDC.

Any help would be great, I really want to solder this thing up and see what I can do with it.

Jonathan

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Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2008-09-25 17:15
    Jonathan,

    On a FET you don’t usually need a resistor on the gate unless you want failure protection. Unlike a BJT the signal pin doesn’t need to supply a proportional current. The gate is instead voltage driven. Some MOSFETs have TTL compatible inputs, although at 3.3V you’ll just need to check the datasheet for the gate on voltage. Take care.

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    Chris Savage
    Parallax Engineering
  • LeonLeon Posts: 7,620
    edited 2008-09-25 17:57
    It is a good idea to have a resistor to ground on the gate, to prevent it floating when the MCU is reset. In some circumstances that can be rather dangerous.

    Leon

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    Suzuki SV1000S motorcycle
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2008-09-25 17:58
    Quite right Leon...I should have mentioned that.

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    Chris Savage
    Parallax Engineering
  • JonathanJonathan Posts: 1,023
    edited 2008-09-25 18:15
    What value for the gate pulldown? And this is needed even with a FET driver like the LM5104?

    Also, another problem just cropped up. I am using some RFP10n12 FETS on the breadboard. I decided to put a heatsink on them so I could try putting a little more power through the system. They got hotter than without the sink very quickly, and smoked my LM5104 before I could pull the plug. Is this due to increased gate capacitance? I only have one LM5104 left, so I'm reluctant to play with it now. [noparse]:([/noparse]

    Thanks VERY much for the help!

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    www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
  • LeonLeon Posts: 7,620
    edited 2008-09-25 18:36
    100k should be fine. You might not need one with a driver chip.

    Leon

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Amateur radio callsign: G1HSM
    Suzuki SV1000S motorcycle
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