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Continuation of prior question: Logic level mosfet to switch on 3.7v battery

I am a 'mature' entrepeneur Arduino level electronics noob, apologies. Want to build a flashing alert using 3W LEDs in parallel powered from a 3P 18650 (3.7V) battery pack. I think I have crude solutions using LEDs in series and a 3S battery pack, but not appealing due to uneven LED performance in series configuration, and problems with charging a 3S (balancing, access to 5V/USB charging solutions).

The last answers/comments recommended then un-recommended a P-channel MOSFET, specifically the FQP27P06, because the max Vgs(th) was too high at 4V. In 'fun with sparks' websites I found the Fairchild NDP6020P as an alternative. Finally my question. The Vgs(th) for this MOSFET is -0.4 min to -1 max. Seems like this would be exceeded when I'm using 3.7V source. Consequence, could be MOSFET destruction up to small burst of shrapnel, counterproductive for a safety invention. I read about protecting with a Zener diode and series resistor, but that seems complicated and also primarily designed for surges and polarity reverses.

Kinda yes or no, am I going down the right road with the Fairchild NDP6020P? Bonus points for continuing my education, and much appreciated.

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  • \$\begingroup\$ How do you want LED power to be regulated as 3.7 drops to 3.1V and LEDs only need what 2.9V? Give LED link to datasheet. Otherwise this question may be irrelevant \$\endgroup\$
    – Tony Stewart EE75
    Jan 24, 2018 at 21:43
  • \$\begingroup\$ Sorry. Chinese LEDs. 2-2.4V forward voltage, 400-500 mA. 6 in parallel with low ohm resistor. \$\endgroup\$
    – Gerbe
    Jan 24, 2018 at 21:51
  • \$\begingroup\$ hmm so 6x 1W in parallel = 6W not 3W but is that at 3.7V Vbat or 3.1V \$\endgroup\$
    – Tony Stewart EE75
    Jan 24, 2018 at 21:56
  • \$\begingroup\$ Noobie replies, no, 6 ea 3W LEDs in 1 at a time parallel. Made up a 3P battery pack using 18650 batteries, so nominally 3.7V but could go to 4.1V fully charged. \$\endgroup\$
    – Gerbe
    Jan 24, 2018 at 21:59
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    \$\begingroup\$ erhm no . 2~2.4V forward voltage, 400-500 mA. 2x0.4 = 800mW , 2.4x0.5=1200mW avg 1W and LiPo drain to 3.1V what then? \$\endgroup\$
    – Tony Stewart EE75
    Jan 24, 2018 at 22:01

2 Answers 2

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The threshold voltage is where the MOSFET is mostly off (a couple hundred uA) so you definitely want to exceed that whenever the MOSFET is supposed to be 'on'.

It can withstand Vgs of +/-8V so you're fine on a 3.7V supply, and it's rated with 2.5V drive for Rds(on) so you're fine for drive down to below destructive voltage for the battery.

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If you apply on the gate less than your voltage - 1V (in this case 3.7V - 1V = 2.7V, so anything under 2.7V), your P-Mosfet will be ON. Usualy, for voltage lower than 18V, we just to turn on a P-Mosfet by driving the gate to 0V. As long as voltage is not 20V lower (or 8V in your case) , it's fine, (hence the 18V limit with some safety). To turn it OFF, it's more complicated because you must make sure that the gate is at same voltage as the source (3.7V). According to the data sheet, a drop of only 0.4V could already turn it ON (-0.4V min, -1V max, that's what it means). To be safe the voltage should be really the same (+- 0.1V) as the source. I don't know who recommended a P-Mosfet, but N-Mosfet are easier and have less heat dissipation, perform better. For 6*3W I'd choose a N-Mosfet. But that's your choice.

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  • \$\begingroup\$ "If you apply on the gate less than your voltage - 1V ..., your P-Mosfet will be on." - Barely. -1V is Vgsthmax, which is where the Mosfet just starts conducting (defined as a drain current of 250µA). That is not "on" in any useful sense of the word. "As long as voltage is not 20V lower, it's fine" - No, it's not. The maximum gate voltage of this mosfet is ±8V; 20V is likely to kill it. Read the datasheet before making recommendations. "To turn it on, ... that the gate is at same voltage as the source" What? No. Setting the gate voltage to the source voltage turns the mosfet off! \$\endgroup\$
    – marcelm
    Jun 17, 2018 at 1:59
  • \$\begingroup\$ Sorry I mixed up the words "on" and "off" when I was typing. I corrected my reply now. About the "just starts conducting", it depends what you mean by "ON". If you consider the logic level, it will be ON or "1" at the specified voltage. However if you want to power some load, you need to be a few volts beyond that value to get minimum impedance. In that sens, it's neither ON nor OFF, it's between two states, and it should be avoided. Yet, in this case, it's more ON than OFF because it started conducting. But that's just my opinion. \$\endgroup\$
    – Fredled
    Jan 23, 2019 at 19:16
  • \$\begingroup\$ About the 8V gate voltage limitation, it's unusual. I would recommend a mosfet with the classical 20V limitation. \$\endgroup\$
    – Fredled
    Jan 23, 2019 at 19:16

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