View Full Version : Base\Gate resistors when driving transistors\MOSFETs
12-12-2009, 11:20 AM
I'm planning to use some transistors (BC337\BC327) and MOSFETs (FQP7N10L)·to toggle LEDs from the prop.
I'm using common cathode RGB LEDs·with PNPs to drive the 3 colours for all the LEDs then NPNs to toggle each individual LED on/off.
I found this formula for base resistors at http://www.ermicro.com/blog/?p=423:
IC = hFE x IB
IB = IC / hFE
RB = (VPORT - VBE) / IB
I'm not sure exactly how many LEDs there will be in total yet, but going for the maximum collector current through the transistor (800ma) gives:
IB = 0.8 / 0.1 = 8ma
RB = (3.3v - 0.7v) / 0.008 = 325·Ohms (next highest value = 330Ohms)
The MOSFETs are used to drive IR LEDs at high current (unsure of exact value I'll use but maximum would be·1A)·at 56kHz.
General consensus seems to be that you don't need a gate resistor on the FET but that it is probably wise to put a low value resistor (~10ohms?) just to be safe.
Do these values sound about right? I think I'm good to go but figured it was better to get some confirmation now rather than after everything is soldered in place·http://forums.parallax.com/images/smilies/tongue.gif
12-12-2009, 11:38 AM
The formulas for junction transistors just don't apply for MOSFETs since junction transistors are current controlled and MOSFETs are voltage controlled. The particular MOSFET you mentioned no longer has a datasheet available from Fairchild and probably is obsolete. Make sure it can be turned on by the 3V available from the Propeller. Usually the datasheet will have a graph of Vgs (gate to source voltage) vs. Id (drain current). At 3V, the FQP7N10L may not turn on hard enough to conduct 1A. You might need a different MOSFET. You don't need a series gate resistor. You might want a high value pull-down (100K) to keep the MOSFET off when the I/O pin is set to input mode (like on reset).
12-12-2009, 11:52 AM
Yes the MOSFET is now obsolete, no doubt I will need to find another model when I need some more, or possibly if I don't need as much current I could just use those BC337s (NPN transistors).
The datasheet shows Gate threshold voltage is 2.0V (max), and the Vgs vs Id graph indicates that although it is not 'fully' on til 4.5-5V it will conduct over 1A at 3.3V
12-12-2009, 12:09 PM
In that case, you should be fine with it. Do add the pull-down resistor. I don't think you need the 10 Ohm resistor.
12-12-2009, 12:21 PM
Thanks for the help :) I haven't been using a resistor with the MOSFETs so far and haven't had any problems,·just came across something that said it was "recommended" to have them to reduce voltage spikes etc.
12-12-2009, 12:32 PM
The gate capacitance along with the internal gate resistance and your external gate resistor creates an RC filter that slows down the edges of pulses a little and attenuates very short noise pulses. It would also dampen reflections a little. If those were issues for you, you might want a larger resistor value.
12-12-2009, 05:52 PM
Mike is right to state that the gate sieries resistor is there to snub out ringing, not as a curent limiter although it will limit and smooth out the inrush to the gate capacitance. The ringing might cause spikes that exercise the Prop's protection diodes. so a low value (33 Ohm ish) seriesed and a high value (100K ish) pull down would be good if cost/space allows.
On the transistors it is more important to get it saturated, within the absolute base current ratings, to keep the heating down. Watch the earth currents and how the power is fed to the heavy load.
Style and grace : Nil point
12-12-2009, 06:21 PM
The particular MOSFET you mentioned no longer has a datasheet available from Fairchild and probably is obsolete. Make sure it can be turned on by the 3V available from the Propeller.
Post Edited By Moderator (Bean (Hitt Consulting)) : 12/13/2009 3:13:01 PM GMT
A datasheet can still be found at alldatasheet. What we would do without it ? http://forums.parallax.com/images/smilies/wink.gif
You will get a few 100s mA at 3V. But if you add a HC/HCT/AC/ACT gate as driver (like 74AC14 or similar) and you piower the gate with 5V, you will get some more goodness, i.e. current.
For LEDs you may use for example an IRF7103 while SMD they can handle 1A comfortably and are better suited for logic levels.
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12-13-2009, 08:27 AM
Bipolar current gain in common emitter configuration (Hfe) is usually rated as "typical" and can in fact be many times higher but not so at higher currents where it can be severely derated. But you want the transistor to act as a switch (much like a relay) and not like an linear amplifier (like variable resistance) so if you want 800ma drive with a Hfe of 100 you really need to drive the base with about double the current so around 16ma or more. This is one of the down sides with high current bipolar operation but since the actual drive voltage is not so important then it doesn't matter that the Propellers I/O voltage might drop down to even 2V rather then 3.3V as long as you have the current. Try a base value of 100 ohms.
The MOSFET you have is not really suitable and the fact that it shows you could drive 800ma with 3.3V means once again that it is operating in it's linear region and thus would be dissipating power and thus dropping voltage. You need to drive it harder so you could use the gate driver suggestions or simply a MOSFET more suited to this task. I have dual mosfets in 8-pin smd packs that can handle this very easily.
12-14-2009, 06:12 AM
Hmm, I'll be running on battery power·so perhaps·MOSFETs for everything could be better to keep overall current consumption down.
Forgive me (my background is not in electronics) but I don't see why the MOSFETs I have aren't usable, the VGS vs ID graph shows current over 1A at 3V gate voltage, and the RDS(on) vs ID shows resistance of ~0.3 ohms at 1A drain current
Another option I have found (though I'd prefer through-hole) is NTR3161NT1G, which looks like it is pretty well saturated at 3V (but again I can't see what's wrong with the ones I have so maybe I'm missing something simple).