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Topic: DC dimmer circuit (Read 3 times) previous topic - next topic

MarkT

The 1k resistor can be reduced, indeed, but the 10k resistor is sensible - it stops the MOSFET gate floating when the Arduino is powered down.
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dc42


The 1k resistor can be reduced, indeed, but the 10k resistor is sensible - it stops the MOSFET gate floating when the Arduino is powered down.


If you are referring to the 10K resistor on the mosfet gate, it isn't needed, because the mosfet is driven from the TC4429, not the Arduino. Where you need the 10K pulldown is on the output of the Arduino, because the TC4429 has a mosfet input.

The 1K resistor MUST be reduced, after all the whole point of using a mosfet driver is to get increased gate drive to reduce the mosfet turn on and turn off times. With 1K, I estimate the turn on/turn off times will be in the region of 20 us.
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BenF

#12
Jan 30, 2012, 10:26 pm Last Edit: Jan 30, 2012, 10:28 pm by BenF Reason: 1
MOSFET turn on (and off) time is determined by drive current (Ig) and MOSFET gate charge (Qg). Based on datasheet specifications for IRF5210, we can calculate drive current required for a target Ton (50ns) as follows:

Ig = Qg / Ton =  180nC / 50ns = 3.6A

Assuming a nominal voltage of 13.7, this dictates a total gate resistance as low as 3.8 ohm (13.7V / 3.6A). Output resistance for the TC4429 is specified as 2.5 Ohm and so the gate resistor should be 1.4 Ohm (3.8 - 2.5).

To get to 80ns (as quoted in the datasheet) and using TC4429/IRF5210, Rg should be 3.7 Ohm.

Another useful calculation is to check if a heat sink is required. With a load of 8A, the MOSFET will drop about 3.84W (8A * 8A * 0.06 Ohm).  Max wattage for a TO-220 package is about 1W and so a heat sink will be required.


dc42

Why did you choose a target time of 50ns? We're not talking about switching at several MHz here, just a few tens of kHz.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

Pokey

I have ordered the parts--they should be here in a few days.

I will use a heatsink, connect the 10k pulldown to the TC4429 input, and use a resistor value below 50 ohms in between the TC4429 and the mosfet.

In theory, if I did pull the gate of a p-channel mosfet to ground, would that not turn it on rather than off?

I am doing my best to grasp all of these details but sometimes my head hertz! :D

I will report back when the whole thing is installed.

Thanks everyone.

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