Electronics > Beginners
How to deal with EMI and wire inductance on a LED PWM application?
Faranight:
Hello, doing a custom LED lighting circuit with high-power LED's (up to 1A each) that are dimmable by 1.5 kHz PWM. The controller will drive multiple LED's that will be spread out by long wires up to 15 meters away (1 LED per wire). I'd like to minimize any potential issues with this design like current spikes and EMI. I've already done some research and tried to implement countermeasures, but I'd like someone to review my design to see, if I'm doing this correctly. I will show the circuit for a single LED channel. If any modifications are needed, I will replicate the them on other channels.
Current spikes
I've seen numerous circuits on online forums where people are using PWM on N-MOS switches to regulate speed of a DC motor (an inductive load). Most of these circuits strongly suggest to add a flywheel diode in parallel to the motor to reduce any current spikes. I know that long wires can also act like an inductive load, so I added a diode D2 in a similar manner. Is this ok/required?
Reducing EMI
Second issue I'd like to address is EMI. Long wires can act like antennas, and the fast switching of a mosfet causes ringing that can span up into the VHF/UHF regions. Though, many people teach us that it's not about the switching frequency, but rather the signal rise and fall times that determines the frequency domain we're operating in. Most solutions I've found deal with this by increasing the mosfet switching time by adding some extra components like a gate series resistor and shunt capacitor. I've added C2 to increase the gate capacitance, R1 series resistor to slow down gate dis/charging, D1 to speed up turn-off time when needed, and R2 to keep the mosfet turned off while the controller PWM pin is tristated. Component values are not yet optimized and can be adjusted, if needed.
Will this approach be sufficient for my application?
Thanks in advance.
jonpaul:
Stock controllable LED ballasts are easily accessible commodity items.
Dimming via industry standard DMX digital or 0..1V analog.
Safety and EMI compliance 100..240V mains
no need for DIY
jon
Faranight:
I am sorry, I don't mean to sound rude, but it's just that your post fits into the category of "mildly infuriating". First, you're giving off the impression that you only posted in this thread as a pity for having 0 replies, your post does not answer any of my questions, and "no need for DIY" is not really the spirit of this forum.
I already have the circuit design mostly finished, and I am working on finalizing the PCB layout. Just need to sort out some final details. Microcontroller firmware is also mostly done. I am thinking of adding an additional LC filter at the board LED outputs (inputs since I use a low-side switch). That should slow down the rise/fall times some more. Worst case I'll have to design a small test PCB with all the components and measure the EMI myself.
Regards.
MrAl:
--- Quote from: Faranight on December 06, 2022, 09:23:17 am ---Hello, doing a custom LED lighting circuit with high-power LED's (up to 1A each) that are dimmable by 1.5 kHz PWM. The controller will drive multiple LED's that will be spread out by long wires up to 15 meters away (1 LED per wire). I'd like to minimize any potential issues with this design like current spikes and EMI. I've already done some research and tried to implement countermeasures, but I'd like someone to review my design to see, if I'm doing this correctly. I will show the circuit for a single LED channel. If any modifications are needed, I will replicate the them on other channels.
Current spikes
I've seen numerous circuits on online forums where people are using PWM on N-MOS switches to regulate speed of a DC motor (an inductive load). Most of these circuits strongly suggest to add a flywheel diode in parallel to the motor to reduce any current spikes. I know that long wires can also act like an inductive load, so I added a diode D2 in a similar manner. Is this ok/required?
Reducing EMI
Second issue I'd like to address is EMI. Long wires can act like antennas, and the fast switching of a mosfet causes ringing that can span up into the VHF/UHF regions. Though, many people teach us that it's not about the switching frequency, but rather the signal rise and fall times that determines the frequency domain we're operating in. Most solutions I've found deal with this by increasing the mosfet switching time by adding some extra components like a gate series resistor and shunt capacitor. I've added C2 to increase the gate capacitance, R1 series resistor to slow down gate dis/charging, D1 to speed up turn-off time when needed, and R2 to keep the mosfet turned off while the controller PWM pin is tristated. Component values are not yet optimized and can be adjusted, if needed.
Will this approach be sufficient for my application?
Thanks in advance.
--- End quote ---
Hello there,
Well after looking over a few things in your schematic i can see some things that need asking about.
For one, what is the reason for C2 the gate to ground capacitor? The MOSFET already has significant gate capacitance and you usually work it with the gate resistor to get the switching speed to change.
Second, why asymmetrical gate drive?
If you want slower rise and fall times, the resistor would mean slower fall time but the diode would mean faster rise time at the drain. A single resistor with no diode would mean both rise and fall times would be shorter and thus reducing EMI.
If you have a good reason for doing this though perhaps you can mention what that is.
Third, using a microcontroller output pin to drive a mosfet probably means the rise and fall times will be slow already and that is because of the significant gate charge. Designs that want maximum switching speeds use gate driver ICs that are made just to drive MOSFETs. The typical output current for one of these guys is 1 amp. They act to charge the gate as fast as possible, or at least as fast as needed. A microcontroller output pin will most likely have maybe a 20ma limit (or something like that) so you may want to check the switching times before adding more components, although a resistor is a good idea to help prevent overloading the uC output pin. Since your uC has a 5v output for a logic high, a 100 Ohm resistor means 50ma max into the gate, but that quickly drops as the gate starts to charge up. At 1v the max is 40ma, at 2v 30ma, at 3v 20ma, and at 4v 10ma, and if your uC pin really gets up to the full 5v then 0ma at 5v.
I think the back emf diode is a decent idea because what the two parasitic inductors end up forming along with the MOSFET is a boost converter. The output at the drain will have a tendency to overshoot the power supply source voltage like boost converters are by design meant to do. Thus the diode should be able to shunt that back into the power source with only a small increase in drain voltage. It will however cause some high current spike at the time the diode has to switch off, so it is a good idea to use a very fast diode like a Schottky or a zero recovery diode.
The most important part of these kinds of designs is the testing after the first phase of the design is completed. You scope it out. This tells you exactly what you've got and what you haven't got. You then make modifications until you get what you really need.
You can start with spice of course, but then take it to the bench with a prototype and scope and waveform generator or the actual uC chip programmed as needed.
Of course you are probably thinking about efficiency too, which means you dont want to slow down the rise and fall times any more than necessary.
As to a little theory on the switching speed in case you havent read this yet, the frequency components in an ideal rectangular wave go up approaching infinity, but the energy of each comes down as frequency increases. Lucky we dont even have an ideal rectangular wave. As you decrease the rise and fall times the wave ends up having ramping up and down portions and a ramp has less energy in the higher frequency components. Transmitters with lower power dont put out as much energy so it's a good idea to keep the power low when possible.
It would be interesting to hear your answers to these questions and any other ideas you might have.
inse:
I would like to understand how you are driving the LED.
The supply voltage is 24V, the current 1A, what is the forward voltage of your LED?
EMI, PWM frequency and switching losses all depend on each other.
If you drive the LED with a lower frequency, you can control the MOSFET with slower slopes while still maintaining reasonable power dissipation and thus reduce EMI and also the freewheeling diode would be obsolete.
Either do a simulation or a practical measurement on how to dimension the switching cell.
Also for 5V gate voltage the asymmetrical driving is not necessary.
It makes sense for driving it at high frequency with high gate voltage to symmetrize delay times.
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