0
\$\begingroup\$

I'm using a mosfet for powering an Arduino mega 2560 on and off. The current drawn is less than 500mA with a 12V battery.

I chose the IRF7457Pbs (RS 543-0888), with these features:
VDSS - 20V
RDS(on) - 7.0mΩ
max ID - 15A

I measured the voltage drop on D-S:

  1. Vin=9.40, Vout=7.55
  2. Vin=12.15, Vout=10.24

This is too high for the mosfet specifications.

I'm using the mosfet driver UCC27517DBVT, (RS 774-1367), with Vdd=12V, IN+ TTL logic level from Arduino. The resistor at the gate is 5.1 ohm.

Why is the voltage drop too high?

\$\endgroup\$
  • 3
    \$\begingroup\$ What Vgs is that 7mohm gate resistance specified at? \$\endgroup\$ – Brian Drummond Nov 20 '14 at 12:08
  • \$\begingroup\$ Also - what VCC are you using on the UCC27517 gate driver? \$\endgroup\$ – Michael Karas Nov 20 '14 at 12:40
  • \$\begingroup\$ If you want to use an N-channel MOSFET as a high-side switch, you need to be able to drive its gate above the supply rail. Curiously, the data sheet never actually says "N-channel"; you need to infer it from the symbol they use. \$\endgroup\$ – Dave Tweed Nov 20 '14 at 13:54
1
\$\begingroup\$

You are using the MOSFET as a voltage follower. In other words you have drain connected to 12V and the source connected to the load. To switch on the device, the gate voltage needs to be a few volts higher than the source and your results demonstrate that i.e. Vin = 9.4 volts and Vout (source) = 7.55 volts i.e you have a volt drop of 1.85 volts trying to turn on the device.

Of course this isn't enough - if you read the data sheet, to turn the device on "properly" you need at least 2.7 volts (figure 1 page 3 of data sheet) and, to turn it on so that it produces 12 volts out means applying 14.7 volts to the gate.

I suspect you should be using a P channel device wired so that it can turn on hard with a gate voltage taken down to 0V: -

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.