Hi treez
This could be a fun project. A bit different to the usual input + buck + LEDs.
I would like to draw your attention to the
Inductive Power Transfer research of the University of Auckland, carried out by John Boys and Grant Covic (and a lot of masters / PhD students). They have done some roadway lighting systems (very similar topology) using a 38.4kHz (I think) system.
Some safety comments:- Protection for open and short circuits of the transmission loop is critical.
- I suggest you replace L13 with an isolating transformer (you want a little leakage inductance). This way you can keep the 50 / 60 Hz mains out of the transmission loop. Also some safety standards allow higher safety limits for higher frequencies.
- Remember to check what happens if someone leaves some steel object (sheet metal, spanner, pipe etc.) next to the transmission loop. This includes electrical reliability (i.e. wild shifts in impedance shouldn't make the primary MOSFETs explode) and thermal hazards; we had some surprises by accidentally induction heating steel desk legs, sheet metal and once a whiteboard
- To protect against broken LED strings, you can add SCR clamps to the LED pods (i.e. broken wire -> over voltage -> short out) which vastly reduces the risk of fire.
Some more general comments:- EMC compliance is possible. I'm not sure if you'll be classed as an intentional or unintentional radiator.
- Adding an earthed center-tap to the transformer can help with EMC.
- Litz wire is your friend.
- The resonant capacitors are really important. Select carefully and check for things like derating with temperature and applied frequency.
- Test gear like voltage and current probes tend to derate as frequency goes up. Check their data sheets.
- Simple on/off control using M1 can be very effective. However, it will impact the operation of the whole system. This is because the power draw of D35-D44 is reflected back into the main loop via L6/L7 coupling as virtual resistance. Changing the resistance of the loop will change the loop current unless you have closed loop feedback
- The suggested primary side control scheme (per University of Auckland research) is to run the primary FETs at fixed frequency and vary their on times (also known as having more and less dead time between M13, M14) to regulate the 'track' current.
- Superjunction MOSFETs (e.g. CoolMOS) devices look very tempting with their fast switching and low on resistance. However, their body diodes have terrible reverse recovery properties. CoolMOS half bridges can be good for soft switching applications (where reverse recovery does not occur) but have a tendency to blow up (sometimes literally) if you ever move into a hard switching region (as might happen when something goes open circuit). If you value robustness (and time to market) over maximum efficiency, I would suggest you look at using ultrafast IGBTs (with co-packaged diode) instead.
Advanced operation: Selective Harmonic EliminationIf you run the primary switches near 50% duty cycle, you will get maximum power output. But you will also get lots of 3rd harmonic content, which can be difficult to filter out. 3rd harmonic content may be subject to tighter emission limits and can make your life more difficult.
It may complicate the soft switching considerations, but if you set the primary switch duty cycles to 33.3% (vs approx. 50% for maximum power), you have a conduction angle of 120 degrees (electrical degrees). This reduces the amplitude of the 3rd harmonic to near zero (performing the Fourier Transform is left as an exercise for the reader) which can help with EMC. This technique is called Selective Harmonic Elimination and can be very helpful.
You would adjust the duty cycle up and down a bit to regulate the primary loop current.
Advanced operation: Working with an Air GapI expect your current design concept requires quite high magnetic coupling factors. This implies that you need closed ferrite cores (you mention a clip on ferrite part) with a very small air gap. This is bad news for waterproofing and assembly.
Ideally, you want the only ferrite
inside the LED pod. This suggests a big air gap between the pickup and the loop, and therefore a low coupling factor. Which will wreck your power transfer capability.
But all is not lost! The approach developed by the University of Auckland involves adding a resonant capacitor (either series or parallel, there are properties and tradeoffs) to the output winding.
If you add capacitors to the pickup inductors you can form resonant circuits which vastly improve the available power. This enables you to operate with very low magnetic coupling factors (e.g. k = 0.05 to 0.3) and could mean you don't need the clip-on ferrite structures. Maybe you could just glue a ferrite plate to the top of the secondary PCB winding and then overmold the whole LED pod for waterproofing. But it requires quite different circuit design principles. For a start, series resonant and parallel resonant have very different behaviours.
If you want to go down this path, I strongly suggest you lobby your management team to get a little budget to buy some IEEE papers. Don't laugh - there are some excellent ones which include a lot of the design information you will need. I expect you could get a good set for < US$500 which is orders of magnitude less than the time taken to learn it the hard way.