I would like to PWM some LED strips I have running over a few aquariums - the PWM period will only last for 15 minutes in the morning and again in the evening so as to provide a gentle wake up for the fish. (The lighting will be on 100% for 12 hours each day)
I tried using some spare IRF540N I found but the gate was not fully on at 5v so heat was\would be an issue. I switched to a logic level (NDP6020P) but when given 5v at the gate the heat sink becomes extremely hot with 4.5 Amp load (12v)
The lighting consists of 3 x 1 metre lengths in parallel consuming almost 5 Amps (at 12V)
(Ideally I would like to control the MOSFET from a 3.3v IO but can use a different Arduino if no 3.3v logic level transistors can handle >5 Amps.)
What would be the most efficient way of providing this fade in\out functionality over the short windows from an Arduino to the high current LED's? This will be running for 12 hours a day for many years so a high level of efficiency would be required.
madlan:
I tried using some spare IRF540N I found but the gate was not fully on at 5v so heat was\would be an issue.
I switched to a logic level (NDP6020P) but when given 5v at the gate the heat sink becomes extremely hot with 4.5 Amp load (12v)
Not a logic level fet. Maybe just ok for small loads, but not for a 5A strip.
The NDP6020P is a P-channel fet. Can't use that with a LED strip.
You need an N-channel logic fet with a gate threshold of 1-2volt max if you want to drive it from 3.3volt logic.
What do you think of this one? (RFP30N06LE) - Seems to tick all of the requirements?
It has a maximum on-resistance of 45 milliohms at logic level drive, so at 5A it will dissipate a max of 1.1W
or so, so you will need a small heatsink.
[Find one with 20 milliohm or less on-resistance and no heatsink needed]
What PWM frequency are you going to use? If its a high frequency you will need to drive the MOSFET
gate properly with a MOSFET driver chip, not rely on a few 10's of miiliamps from an Arduino pin. At the
default Arduino PWM frequencies you are probably fine (use a 150 ohm gate resistor).
Note that switching losses get more important at higher switching rates, its best to ensure the switching
time is a percent or two of the switching period (no more). So switching at 1kHz you can get away with
a 5--10us switching time, but at 20kHz you'll need harder gate drive to keep switching to sub-microsecond
durations.
Instead of screwing around getting a logic level MOSFET, get some gate drivers (like MCP1406/07) so you can pump 12V onto the gate instead of screwing around with 3.3V. Every MOSFET benefits from more gate voltage, since it reduces the on resistance (which will in turn reduce your heat).
It gives you much more freedom in the types of MOSFETs you can use and the ways you can drive them.
There's no such thing as a 12V LED. Even a string of LEDs that add up to 12V forward voltage can't be directly connected to a 12V battery.
There may be some kind of constant-current driver inside the LED strip. That will react very badly to PWMed input power. If it's just a resistor, then that will work okay.
Jiggy-Ninja:
Instead of screwing around getting a logic level MOSFET, get some gate drivers (like MCP1406/07) so you can pump 12V onto the gate instead of screwing around with 3.3V. Every MOSFET benefits from more gate voltage, since it reduces the on resistance (which will in turn reduce your heat).
It gives you much more freedom in the types of MOSFETs you can use and the ways you can drive them.
Not true, some logic-level MOSFETs have a Vgs limit as low as 8V. They are usually SMT parts of course.
A 12V MOSFET is only useful if there is a 12V supply too, so if you are controlling 24V load, a logic-level
MOSFET is definitely the way to go to save having to add another power supply.
If you don't need a gate driver then a logic-level MOSFET saves on B.o.M., which is good practice.
However a gate driver is definitely requred for high speed PWM where switching time losses become all-important, or at high voltages and powers where logic-level MOSFETs are too fragile to handle the
dV/dt transients safely.
So select a MOSFET driver when appropriate, for a definite reason, not out of superstition. Match the
solution to the problem, not the other way round!
Seems my pithy quip was taken a bit more seriously than I intended.
Not true, some logic-level MOSFETs have a Vgs limit as low as 8V. They are usually SMT parts of course.
"Every MOSFET benefits from more voltage" doesn't mean "you can apply as much voltage as you want and nothing bad will happen". The absolute maximum rating must always be respected, but it is still true that the conduction properties of a MOSFET improve as you increase the gate voltage (until the point that the gate breaks of course).
A 12V MOSFET is only useful if there is a 12V supply too, so if you are controlling 24V load, a logic-level
MOSFET is definitely the way to go to save having to add another power supply.
There are MOSFETs and gate drivers that can handle 24V.
If you don't need a gate driver then a logic-level MOSFET saves on B.o.M., which is good practice.
BOM cost is usually not the biggest concern for low-volume hobby projects. An extra $ or 2 is nothing major compared to the time you spend dealing with it.
However a gate driver is definitely requred for high speed PWM where switching time losses become all-important, or at high voltages and powers where logic-level MOSFETs are too fragile to handle the
dV/dt transients safely.
I agree. And this project is using PWM, which is why I recommended it.
So select a MOSFET driver when appropriate, for a definite reason, not out of superstition. Match the
solution to the problem, not the other way round!
This is a reasonable position. It's just that gate drivers are hardly ever mentioned here even when they would be the most appropriate tool for the job so I got a bit overzealous. You have to know that they exist before you can evaluate if they're appropriate or necessary after all.
Your led strips, exactly what type are they and can they be wired /controlled as three separate lenghts ?
Have just been tying out the remaining 4mtr length of 5050 blue leds I used as moonlights on my tank.
They work just the same with a gate voltage of 5v and 3v3 with that mosfet I mention and at their full load of 2.1A at 12.9v and it only gets 9c above ambient, no heatsink.