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Request for feedback on LED PWM driver output circuitry

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daviegravee:
I am looking for feedback, thoughts and general input on a design I am starting out on. I am designing a 24 channel LED driver (capable of driving 6 x RGBW strips). I am designing the output stage now. I have attached a screenshot of my LTSPICE file that simulates one single output stage. There will be 24 of these in total.

The key points of the current design are:

* PWM, constant voltage driver;
* Each channel must be able to handle 4A, at 48V (24V will be more common in practice);
* Needs to operate in ambient environments of up to 55C;
* Switching MOSFET is the DMNH6021SPDQ : https://www.diodes.com/assets/Datasheets/DMNH6021SPDQ.pdf;
* MOSFETs are driven by MCU pins (no gate driver IC);
* Minimum switching frequency will be in the range 200-400Hz;
* Maximum switching frequency will be in the range of 15-20kHz;
* The only heatsinking method that will be used in this design is copper of the PCB (see page 2 of the datasheet for thermal characteristics). I’m not up to layout yet, but I will be using the 2oz copper, 1-inch square copper plate to achieve a Junction to Ambient thermal resistance of 54C/W;
Thank you in advance!

daviegravee:
It looks like some of my attachments got missed. Here's a zip containing the simulation file and the datasheet for the switching MOSFET.

ArdWar:

--- Quote from: daviegravee on September 20, 2024, 01:38:11 am ---MOSFETs are driven by MCU pins (no gate driver IC);

--- End quote ---

Pick logic level MOSFET with lower VGS, or get better gate drive. 3V max VGS(TH) is too high for comfort. Just look at the curves with VGS=3.0V!
Or maybe just get better gate drive anyway. Depending on your MCU's output drive strength, the switching losses may be quite noticeable with even with low QG.

PCB.Wiz:

--- Quote from: daviegravee on September 20, 2024, 01:38:11 am ---... There will be 24 of these in total...
...Each channel must be able to handle 4A, at 48V (24V will be more common in practice)

--- End quote ---

That's significant power being turned on and off, you will need to consider parasitic inductances of a practical wiring system.
Some clamping may be needed on the FET, and to manage RFI you may need to carefully slow the gate drive slew rate, and trade off some loss for lower RFI.
An increased gate resistor is sometimes enough.

As already mentioned, 3v3 is a bit feeble for MOSFET power drive, and with that many amps & channels, you need a gate driver with some ground bounce difference tolerance.


eg a newer gate driver like UCC44273 (etc) Single-Channel Low-Side Driver has added features like
• 5-V UVLO
• Ability to handle negative voltages (–5 V) at inputs
• Fast propagation delays (13 ns typical)
• Fast rise and fall times (9 ns and 7 ns typical)
• 4.5-V to 18-V single supply range
• Outputs held low during VDD UVLO (ensures glitch-free operation at power up and power down)
• TTL and CMOS compatible input-logic threshold (independent of supply voltage)
• Hysteretic-logic thresholds for high-noise immunity

You might even consider opto isolation, to keep the noisy power signals completely away from your MCU.

daviegravee:
Thanks!


--- Quote ---Some clamping may be needed on the FET
--- End quote ---

Voltage clamping between source and drain? Is this due to overshoot that might occur from wiring inductances?


--- Quote ---That's significant power being turned on and off,
--- End quote ---

I should clarify: any single channel needs to be able to handle 48V of LED strip, taking in 4A (192W of strip) but the design will not be rated for 24 x 192W simultaneous drive, and certainly not 192W dissipated by the MOSFET. Your point still stands, but I think my original wording of the spec might be misleading.

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