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I am working on a home project that uses a micro controller to switch a set of transistors on and off rapidly(msecs). The current through the last transistor is about 300-400 mA to drive a high power LEDs. The Micro output pins produces ~25mA @ 5v. I currently use this to turn on/off a 3A capable NPN by connecting it to the base.

  1. I would like to switch over to using MOSFETs. Can someone help me understand the important symbols and their values on a MOSFET data sheet so that I can order the correct MOSFETs for my application. (First time user of MOSFETs)
  2. I am wondering about the gate/base resistor and how I determine that value also.
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  • \$\begingroup\$ look at MOSFET N Channel at www.digikey.com | Vgsth say <= 2V (<= 3Vmaybe). Low allows 3V3 logic to be used when desired | Rdson < 0.1 Ohm but much lower better, | IDS rated > to >> Iload max - 5A+ fine. | Vds rated > to >> Vmax - 20V + OK. | No or low bse drive R ok - gate draws zero current except when switching when !!! 1 nF gate cap must be charged or discharged. | Power dissipation on static = i_load^2 x Rdson x say 2 x for safety. Check c/W thermal R to see what if any heatsink needed. | More ... ask ... \$\endgroup\$ – Russell McMahon Jan 31 '15 at 13:38
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You need a low gate threshold mosfet (logic level), Vgt must be lower than the MCU supply voltage.

Vds must be higher that the supply voltage the LEDs are connected to.

Rds ON should be low enough, likewise so for Rth_junction2ambient.

The temperature of the junction will be:

Tj = Tcase + Pmosfet * Rth

Make Pmosfet small and the temperatures will be low.

Another parameter you need to look at is the total gate charge. The larger it is the harder to turn the mosfet on, but for your circuit (low frequency) this is not a problem.

The gate resistor does not matter that much in your case, since you will be driving the mosfet directly with the MCU. The MCU output is "limited to 25mA anyway" - this is not actually true, but if you want to be on the overingeneering side, calculate:

Rg = Vsupply_MCU/25mA

I recommend you show the circuit before trying.

The VN0104 transistor has for instance low enough Vgs(th) to work with a 3.3V MCU. It has a relatively high Rds ON. The lowest dissipation for the current you mentioned is:

P = 0.4*0.4* 3 Ohm = 0.48W

This is a max power, that is to big to be sustained by such a small device. For 0.5W I would recommend a TO-220 device.

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  • \$\begingroup\$ Thank you for the information. I was looking at this part: ww1.microchip.com/downloads/en/DeviceDoc/VN0104%20C081913.pdf. - Is Vgs (Gate threshold voltage) listed here the same as Vgt you mentioned? \$\endgroup\$ – Bud Jan 31 '15 at 14:28
  • \$\begingroup\$ - Vds on this part looks to be 40v so that seems to be in line with your comment. I will post the circuit soon. \$\endgroup\$ – Bud Jan 31 '15 at 14:43
  • \$\begingroup\$ Updated above. Watch the power, get a bigger transistor. \$\endgroup\$ – iggy Jan 31 '15 at 17:50
  • \$\begingroup\$ Get a MOSFET with lower Rds_on. \$\endgroup\$ – rioraxe Feb 1 '15 at 0:27
  • \$\begingroup\$ Assuming that he really saturates the mosfet a lower Rds would help. Lets hope he saturates the mosfet :) \$\endgroup\$ – iggy Feb 1 '15 at 9:56

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