Easiest: use a low frequency like 1kHz to avoid flicker, and a switching speed that's ponderously slow like 10us to avoid much radiation. Maybe still put an LC across it (with a cutoff of some 100s kHz), with damping so it doesn't ring, to be sure.
Slow switching can be done by putting an R+C across the MOSFET D-G and a relatively generous series gate drive resistor. The R's prevent oscillation (capacitance from G to D/S is a no-no, you'll make an oscillator at 100s MHz that way) and the feedback network sets bandwidth as if it were an amplifier (which it clearly is during switching edges :) ).
Best efficiency: use an adjustable DC converter. Filtered input and output, high switching frequency, steady output. Typically you'd use a buck or boost converter (depending on supply, you might need a flyback or SEPIC configuration rather than boost, but, close enough) and either control the current setpoint node (external compensation type) or tease the voltage feedback pin with a resistor (thus shifting Vout).
In the first case, you try to avoid generation of harmonics; in the second case, you go all in, then filter them as close as possible to the source.
Additional filtering may still apply, on account of the first case, the impedances between all the nodes mean that the ~5ns edge you get from the MCU pin driver or IO expander or whatever it is, can feed forwards through everything and get into the wires. An RC in front of the gate R can help with that, for example. Or in both cases, there may be ground-loop noise, due to return currents from logic signals, or switching currents.
Tim
--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/
"Tim Watts" wrote in message
news:h1d7cdF8c75U1@mid.individual.net...
> Hi,
>
> This year's project is to drive 3 strings of 30V DC LED outdoor fairy
> lights from Raspberry Pi's[1]
>
>
> I've been on here before last year and took some greet advice with respect
> to H-Bridges and found some, along with DC-DC converters that will enable
> me to interface a 30V DC supply to the Pi and the Pi back to the lights.
>
>
> The only thing I am mindful of is not to inadvertantly blat out a ton of
> EM noise.
>
> So I was wondering if anyone (without guarantees of course) might be able
> to offer any rule of thumb advice please?
>
>
> 1) I'll keep the PWM frequency as low as possible without causing visible
> flickering to passing motor cars (which is sometime I can test by driving
> by, road speed limit is 30mph so not super fast).
>
> 2) I'm thinking to pop a suitable choke inline with each string of lights
> after the H-Bridge to block the higher harmonics. This is the bit I'm not
> sure of. What would be a suitable number of Henrys as a function of drive
> current and base frequency of the PWM?
>
> 3) Are there any inexpensive ways of doing a quick and dirty verification
> of the amount of noise being emitted? I don't have a 'scope, though I
> could get a picoscope type unit if sticking an antenna of some sort on it
> would provide useful tests???
>
>
> Many thanks,
>
> Tim