# Single or multiple gate drive resistors?



## aeroscott (Jan 5, 2008)

I would like to know about shielding those wires as well as the resister question .


----------



## honn1002 (Nov 26, 2008)

ngrimm said:


> On my diy fifteen mosfet controller I have been using a single higher resistance resistor with all the gates tied together instead of a single resistor of a lower value for each gate. It seems to work ok but wondering if it matters. Incidentally, my design requires me to have ten inch wires from the gate driver to the mosfet gates. I know it's not ideal having that much length but I don't plan to change it. Just wondering what your thoughts are regarding shielding, twisting the wires, ferrite beads or cores etc. Thank you for any suggestions.


"a single higher resistance resistor" I hope you meant a single higher wattage resistor. The resistance value is depend on the specifications of the mosfet. The lower the resistance the faster the switching (rapidly turn on and turn off the mosfet gate) and the higher the V-spike. The higher the resistance the slower the switching, less V-spike but more heat build up on the mosfet. The gate resistor value has to be right for the mosfet to have the most efficient and reliable controller. What is the resistor value are you using? 

It's better to use a single low wattage like 1/4 watt and 1% tolerance resistor for each gate. Locate each gate resistor as close as posible to the mosfet gate. Try to group the mosfets, freewheel diodes and power capacitors close to each other to get the lowest inductance which means less ripple current.


----------



## biscayne (May 15, 2009)

Hi,
Gate ringing, is what you´re probably should be conncerned.
There´s a good tutorial http://www.fairchildsemi.com/an/AB/AB-9.pdf
which doesn`t suggest your methode.
Gate capacitance, resulting from the Mosfet itself and inductance from the gate pin and whats on it- after the resistor (which dampens) should be kept small. A lon wire to the resietor should not matter too much.
I saw improvemets using diodes in various arrangements before the resistor: Diode- resistor gatepin. The diode acts like a switch. To controll- match rise and fall times, every gate pin has 2 gareresistors , every one with a diode, one in one out. 
One single resistor would mean any lenght of copper leading to the gatepins- which is absolutely NO. see above appnote. 

My gatedrive is not a Pcb, its in the air, with matched lenght wires, the resistors directly on the short gate pin. SMD would be best. Every resistor 4.7ohm has 2 Mur1100 (parallel, kathodes opposite)- best results even better than wit a MUR 105 which is a little faster.
I`d appreciate any Ideas or expreimental results!


----------



## bga (May 25, 2009)

A worthwhile read is:
http://www.zeva.com.au/tech/controller_design/

Gate drivers on long leads will be tricky to make work decently. The gate lead should be considered to be a transmission line, so a twisted pair will work.

Depending on the device size, low value gate resistor are needed. 50Amp FETs need about 8 ohms to switch at a reasonable speed. Much more than this will cause poor turn-on/off performance and high dissipation.
Approximatelty 1.5 amps of drive is needed *per* FET.

A Zener or Tranzorb is needed "as-close-as-possible" to the FET component leads to limit the gate voltage to within safe limits, +/-20V typically. Practically, this should be across the device legs, one per FET. (see fairchild notes)

ZEVA used IXYS IXDD414 gate driver, which is good for 14 amps, can be parallelled, but with caution.
bipolar transistors can also be used to augment a gate driver. (need 15Amps at Ft of 250 MHz to work)
The gat driver power supply needed low ESR caps (Monolithic ceramic are good) and probably 2-3 watts of average power to switch at 20KHz. 

For a 500Amp controller, 10-12 FET devices will be needed. (on the money there)

Don't forget about the flyback diodes. These get a terrible time when under heavy load at low revs:

At stall, the transistors spend about 10% of thier time 'on', carrying the 500 amps and the rest of the time off. The other 90% of the time, the flyback diodes are carrying the 500 motor amps and can get really hot.

(500 amps peak at 10% on time is 50 amps average, about what is seen with battery vs motor amps at stall +/-)

I've been having a problem with line (bus) filter capacitors because of the high circulating currents. These caps have to handle 500A to be able to supply the transistors during the on-time so that the batteries don't see too much of the AC noise. A lot of aluminium electrolytics are needed to get the required current capacity.

Fairchild has some good docs on design.


----------

