Hi all,
I have a very large number of 12V LED's I'm trying to control via PWM.
Currently I'm using 16 80NF70 MOSFET's via PWM from a 3.3v GPIO.
I'm using the below circuit for this, but adding multiple 80NF70's causes the output voltage from the BC640 to lower meaning the 80NF70 not activating at lower PWM's. (cumulative)
The BC640 to 80NF70 connection ranges from 30cm to 5m.
Is there a solution other than having multiple BC547\BC640's supplying the 80NF70's?
The 80NF70 is not a logic level mosfet, and won't be fully 'on' with 5volt at the gate.
If you want to use these fets, then you should consider a different drive circuit (mosfet driver chip), powered from 12volt.
What sort of LED current are we talking about.
Leo..
I have 7x 300w power supplies running batches of LED strip lights (Drawing about 240w a batch), the MOSFET\motor controller PCB is used to ramp up\down the brightness for 30 minutes at the beginning and end of each day.
The 80NF70's are in pairs on a motor controller board (x7). I've removed the 555 timer and I'm feeding 5v PWM directly to the 80NF70 pairs.
Once PWM is at 65536 (16 bit for smooth transition at low levels) a 30A relay switches on taking over from the MOSFET.
It's been working excellently for the past year with one setup for testing, the heat sinks get slightly warm to the touch when running for extended periods.
16-bit PWM, even at a low PWM frequency, requires fast pulses.
Your circuit can't provide the current to charge/discharge the gate capacitances at a high frequency.
You can try to mod the circuit, or use a mosfet driver chip.
Try lowering the gate to ground resistor to 100ohm, and the BC640 base resistor to 1k.
The 10k base to ground resistor of the BC547 should be at the GPIO side of the 4k7 resistor.
Leo..
Great, I'll try the resistor changes - is this correct?
Regarding using a driver - Would the IR2125 work for the 80NF70? Could it drive all 16 or would it be best to spread the load?
I found a tutorial on using this chip but I cannot find any examples of it being used as a low side switch: How to use the IR2125 MOSFET gate driver – Krakkus.com
RDS(on) is 10V according to the data sheet, Gate-source voltage is ± 20V and the IR2125 Gate drive supply range is from 12 to 18V.
Each 80NF70 in the pair has a 300 Ohms in series which I've left intact.
I would try the circuit you have first.
Leo..
Hi Leo,
Certainly an improvement, but the 100ohm resistor gets extremely hot.
Would supplying the BC640 with 12V instead of 5V improve the situation?
A hot resistor is normal in that kind of circuit.
You could try increasing it's value to 220ohm.
Or use two 220ohm resistors in parallel, to spread the heat.
12volt on that drive circuit would only benefit those non-logic level mosfets.
A mosfet driver chip could be a better solution.
I think the TC4427 is suitable.
Two drivers in one package, so each driver could switch eight mosfets.
Could power the chip with 5volt or 12volt.
Make sure you have good decoupling (ceramic cap) close to the chip.
Leo..
Would I need to make any changes to the circuit if replacing the 5V with 12?
The transistor circuit?
The resistor would get much hotter.
A different circuit could be needed to stop that.
TC4427 chips are designed for ~12volt (4.5-18volt).
Leo..
Why no use a 3.3V logic level MOSFET?
Granted only available in SOT23 so hard to solder but those tiny packages happily handle 4A currents or even more (especially with large heat plane in the PCB).
Saves a lot of components.
The gate capacitances add up of course - add a resistor between the gate and the GPIO pin as protection. Assuming you're using a ESP8266 processor that should be 270-330R, in case of Arduino 150-180R.
The TC4427 arrived today, I've set it up for testing as below:
Does this setup look ok?
Unfortunately I have no control over the MOSFET's used on the setup, out of interest are there any logic level devices that can handle as much current as the 80NF70? I couldn't find any in a TO-220 package.
The TC4427 needs good decoupling of the supply, very close to the chip.
The datasheet shows 4.7uF+100n.
Don't think it's wise to connect two outputs together.
What do you mean with "no control over the mosfets"
Do you want to control/PWM every LED strip?
Then a PCA9685 breakout board (16-channels of 12-bit PWM) might be needed.
And 16 of those mosfet drivers.
Leo..
madlan:
are there any logic level devices that can handle as much current as the 80NF70? I couldn't find any in a TO-220 package.
Never seen them for 3.3V levels - you may find them for 5V levels. For 3.3V logic levels I've got them on hand for 4-6A continuous. For 5V there are types in TO220 packages that can handle 20-30A, maybe more.
That particular MOSFET you link to appears to need 10V V(GS) to switch on properly. That's the only value I see in the datasheet.
Should both of the decoupling capacitors be electrolytic (The data sheet appears to indicate this on the test circuit but the notes say ceramic)?
I have 0.1µF ceramics but only electrolytic 4.7µF capacitors - I can order some MLCC 4.7µF capacitors if required.
3.4 Supply Input (VDD)
The VDD input is the bias supply for the MOSFET driver
and is rated for 4.5V to 18V with respect to the Ground
pin. The VDD input should be bypassed with local
ceramic capacitors. The value of these capacitors
should be chosen based on the capacitive load that is
being driven. A value of 1.0 µF is suggested.
I've split the load across the MOSTFET's and added some protection for the MCU outputs:
Sorry, that response was for wvmarle - I am stuck with the 80NF70 MOSFET's unfortunately, replacing them with better logic level devices would be a significant amount of work.
I am PWM'ing a whole room of high power LED lighting (57 metres) that has 8 power supplies at key points. The only requirement is to PWM the LED's for a short period in the morning and evening (fade in\out) so no need for any more granular control.
does anyone know the Rds of 80NF70 @ 5v? It doesn't look that bad.
wolframore:
does anyone know the Rds of 80NF70 @ 5v? It doesn't look that bad.
3.3volt processor, and 16 mosfets with long cables on one output.
Leo..
Why the 4.7ohm and 4k7 resistors...
The caps should be able to provide high peak currents.
Normal 4.7uF electrolytic caps can't.
Leo..
Sorry, should be 4.7k on both - would resistors on the outputs not protect the MCU?
I've ordered some 4.7µF MLCC capacitors.
Thanks Leo.
You're driving the input of a Cmos chip (virtually no load), not a power mosfet gate (high capacitance).
Loose 4R7/4k7 series the resistors.
Wise to leave the 10k resistors to ground.
That will keep the drive chip inputs LOW in case the processor is off or still booting.
They could be 100k though.
Leo..