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BasicESCs voltage drop


(Ben) #1

Hi,

I’m part of a team building a 5s LiFePO4system (16V nominal, 18.25 max), and we’re using the non-BEC Afro ESCs (link) to control 7 T200 thrusters, which will be operating at about 4.5A each, max (generally). We need to be able to cut them all off at once manually at any give moment, so we’re using a magnetic reed switch (and a magnet) connected to a single very-low R_{DS,on} transistor, the NXP PSMN0R7-25YLD (link to datasheet) to control it. The transistor is a low-side N-type MOSFET; the source will be grounded, the gate will be attached to the reed switch (which in turn is attached to a voltage divider and a 12V rail so we have the V_GS=10V), and the drain will run to the black wire of the motor controller. The red wire will run back to the battery.

My question concerns two things. First, any general comments on this design would be appreciated. Second, I’m concerned about the V_DS across the transistor. Will the voltage drop sufficiently over a T200 in series with one of these ESCs so that the transistor doesn’t have to drop it all?

Thanks.


(Rusty) #2

Ben,

This sounds like a good design. Low-side, N-fet are good design decisions.

There will be plenty of voltage drop over the ESCs so that the MOSFET doesn’t have to absorb much power. You should only get about 0.7W heat dissipation at full 7 x 4.5A current draw. This MOSFET will handle that just fine.

-Rusty


(Ben) #3

Thanks! I appreciate that you believe it to be fine, but I’m going to have to present this design, so I need more than assumptions. Do you have any numbers to back that up? The best I can find is the phase resistance of the thrusters themselves.

Ben


(Rusty) #4

Ben,

We connect the ESCs directly to a 4s lipo battery and it only draws about 40mA at idle. Your FET will only have to handle that current. When the ESC is active the current will vary based on speed but it will always handle the full battery voltage.

I might be misunderstanding your question so let me know if you need something else.

-Rusty


(Ben) #5

Hi Rusty,

When you say “it will always handle the full battery voltage”, do you mean that the voltage on the black lead of the ESC will be 0V when the ESC is active?

Now my concern is that there will be too little voltage on the N-FET drain, which will not keep it in saturation mode.

Sorry if I’m missing something.

Ben


(Rusty) #6

Ben,

That’s correct, you’ll see 0V relative to the battery ground on the black lead of the ESC when active. When the ESC is not active the MOSFET will be turned off and will not allow any current through. The 5V regulator onboard the ESC will continue to draw about 20-40mA to supply the microcontroller.

-Rusty


(Ben) #7

That sounds like the FET will not pass current then when used as a low side switch even when the ESC is active, since the drain-to-soure voltage with be zero. The gate-to-source will be high enough above the threshold voltage to pass current, but at V_DS=0, the maximum passing current is also zero.

It sounds like I’d need a high side switch for these ESCs.


(Rusty) #8

Hi Ben,

I’m no electrical engineer so I’m getting confused thinking about this. This StackExchange post explains it pretty well: http://electronics.stackexchange.com/questions/18885/mosfet-as-a-switch-when-is-it-in-saturation

-Rusty