MOSFET question (Rds tradeoff / limiting current)

Hi all,

I’m slowly starting to use MOSFETs in my driver (a teeny tiny bit late to the party…) and stumbled upon 2 questions.

1.
Is a lower Rds always better or does it come with a tradeoff?

I just saw the NXP PSMN0R9-30YLD with stated Rds on of only 0.9 mOhm. Is this simply a better, cooler, higher current version of the 2.4mOhm and 3.0mOhm versions or is there a catch.
I mean, not that 1 or 2 mOhm would matter that much, but the PSMN0R9-30YLD is available locally which makes it even more interesting. And the same question about Rds arises for the FET selection for the 15mm boost driver we’re fiddling with.

2.
Can Rgate and/or Rpulldown be used to limit the current in full-on mode?

I’m in the process of building a BLF SRK FET v2 board as the stock driver on my SRK died. Only 13A and he called it quits … O:-)
Now it shows that the BLF SRK board and its FET deliver a bit more current than I render safe.
The highest mode shall be simply on/off without PWM involved.
The cells shall not be different (already only protected NCR18650B).
I know I could

- unbraid the springs

- use thiner LED-wires

  • use a less efficient MOSFET with higher Rds (doh…)
    But the results would be quite unpredictable and if I could test some Rgate / Rpulldown combo until I have the current I want that would be much better.
    Does that work? and how?

Thanks a lot
HQ

1) Comparing to a PSMN4R0-30YLD (4 mOhm), it seems to switch about 3-4 times more slowly. That appears to be the main compromise.

2) Sort of. See fig. 7 and 9 in the spec sheet. You’ll probably need the gate around 2V to 2.2V. But if you do it with a pair of resistors, that voltage with get lower as your cells drain and I expect you’ll be artificially limiting your power then to a much lower value. I’m not certain how it will behave but if I had to guess, I’d say it would be like adding resistance which increases in value when your cell is emptying (the opposite behavior of a regulator). I’m testing something similar but MCU-controlled to compensate for the undesirable behavior. Don’t know yet if it will be usable but it sure is tricky.

IMO, you’re better off messing with the wires or springs.

Thanks a lot for your informative answer.

1) So if lower Rds comes with higher switching times this shouldn’t be the problem. I recall that someone (maybe RMM) called the switching times in our drivers “pedestrian”. If that’s the case, we can quite freely choose among these PSMN mosfets (which is good to know).

2) I already understand now that mosfets are not really meant for regulating. I did build an SRK FET driver in the meantime and it is indeed higher in current than I want it to be. But I’ll unbraid the springs and see where this goes. This has the advantage that the surplus energy is not burnt in one single component in the head (the mosfet) but 4 in the tail (the springs).

Much obliged.
HQ

I always look at the Rds vs gate voltage graph because it shows more information about the behavior of the FET at battery voltage which is usually lower than the spec’d 4.5V or 10V specs. Not an issue with Zener modded drivers that fix the gate voltage to basically the Zener voltage but an FET with a low Rds at 4.5V might be higher at 3.5V than one with a higher 4.5V spec.