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I'm currently using a Mosfet BS170 to drive LEDs from a 3.3V 1Khz PWM output. The problem is I would like to use more current (something around 2A max at 12V Max) than the 500mA allowed by the BS170. Which Mosfet could I use ?

There is tons of models available, and I must admit I'm pretty confused by all the specs. But if I understand well, I should choose one with a Max Gate Threshold Voltage below 3.3V am I right ?

Something like.

(max of 2.35V)

Or should I use a cheaper one, like. (max of 4V) and use another mosfet (BS170/2N7000) and connect it to the gate of the power mosfet in order to reach the requested 4V?

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  • \$\begingroup\$ Notice the V(GS,TH)=4V you mention is rated for I(D)=250µA. You'll need a much higher V(GS) to ensure I(D)>=2A. \$\endgroup\$ – jippie Aug 23 '15 at 10:55
  • \$\begingroup\$ How can I calculate/determine this gate voltage ? If I understand well, I can use my 3.3V PWM signal to output a higher voltage (but very low current) through another mosfet and use this output as the gate signal of the irfz44n ? \$\endgroup\$ – dsagilles Aug 23 '15 at 11:09
  • \$\begingroup\$ Consider to use transistor to drive the MOSFET, i.stack.imgur.com/jYZSf.png \$\endgroup\$ – Oka Aug 23 '15 at 11:10
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IRFZ44N is really made for around 10 V V_GS, at 3.3 V it will conduct a little bit, but conduction below 4.5 V_GS is not even specified. IRLB8721 is made 5 to 10 V V_GS operation. 3.3V at the gate would turn it on more than the IRFZ44N but it would still be rather ineffective. RDS_on would be around 50 mOhm. You would use a huge transistor capable of switching tens of amperes to switch 2 A. And because the transistor is huge, the miller charge is huge and switching speed using a microcontroller output will be rather slow, because at the currents that the outputs of the microcontroller can supply, recharging the gate will take a long time. For using such as transistor you would typically use a MOSFET driver IC or two transistors as a push-pull configuration to supply several A to quickly recharge the gate. You should use a transistor that is made for 3.3 V operation and has an even lower threshold voltage. Gate threshold voltage is the voltage where the FET starts to turn on, but a significantly higher voltage will be needed until the transistors conducts well and can be used efficiently anywhere near it is peak current handling capabilities.

IRLML2502 should be a good choice and would even fit into the same SOT-23 footprint as your BS170. It is also specified at 2.5 V gate to source voltage with 50 mOhm RDSon. At 2 A 0.1 V would drop at the transistor and 200 mW of power would dissipate at the transistor which is well within the specification. Gate to Source (miller) charge is around 1.8 nC. That means you can charge or discharge the mosfet gate using a typical 20 mA MCU GPIO in around 100ns and no external gate driving circuit will be required.

http://www.irf.com/product-info/datasheets/data/irlml2502.pdf

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  • \$\begingroup\$ Thanks a lot, the IRLML2502 seems to be the component I was looking for. You said 'That means you can charge (...) using a typical 20 mA MCU GPIO', does it means it is possible that the gate pin consume that much ? \$\endgroup\$ – dsagilles Aug 24 '15 at 8:47
  • \$\begingroup\$ The MOSFET gate is like a capacitor, you need to charge or remove a charge from the gate to switch the FET on or off. Power from the GPIO pin is only consumed during switching. Large FETs such as the IRLB8721 have large gate charges and the limited current of the GPIOs pin limit how fast you can switch off or on the FET. While the FET is transitioning from switched off to switched on or vice versa, high power dissipation can occur. The transition should thus happen as fast as possible. Large FETs can require a dedicated FET driver to supply enough current into the gate during switching. \$\endgroup\$ – Jan Lucas Aug 24 '15 at 12:34
  • \$\begingroup\$ Okay I think I get it. Thanks for theses clarification, I'll try the IRLML2502. \$\endgroup\$ – dsagilles Aug 25 '15 at 9:11

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