Hello,
We are doing a 6W Constant off time controlled Buckboost LED driver…
(schematic and LTspice simulation attached).
Vin = 18-137.5VDC
VLEDs = 45V
Power = 6W
Constant off time control (off time = 3.5us)
No electrolytic capacitors allowed (no large films either)
Note: It actually has to work down to Vin = 8V , but only at 1.2W for that case.
The wide Vin range means constant off time control is needed for control. The problem is, there are virtually no offtheshelf constant-off-time controllers available. (except the HV9910B, but that has no internal error amplifier, and only a weak gate drive).
Therefore, we had to use a “hacked” UCC28C43 controller.
The problem is that the controller, with all the auxiliary components needed with it, take up a lot of space, and space is very constrained. The PCB is only 22mm by 200mm. We are only allowed double sided PCB, no more layers. Driver components go on the bottom, LEDs on top.
The biggest problem, is that our lack of space means that we can’t fit much input capacitance on the PCB.
We only have 2uF of ceramic input capacitance, -that’s all we could fit on.
The result is that there is fairly significant ripple in the input current. This may fail EMC, (though we havent had the standard limits given to us yet).
Another point is that in some products, several of these 6W PCBs will be used together…..and as you know, this means that there will be beat frequencies as they interact with each other. This could well occur at the resonant frequency of the EMC input filter, and that would mean unwanted resonance there.
What we really need is a “dual threshold hysteretic controller”. -By that I mean a controller which allows one to set a maximum and minimum threshold for power inductor current….(and these would be varied ‘on the fly’ by a micro)…we could then simply get a micro to read the actual LED current, and then simply increment the thresholds gradually until the desired LED current was achieved.
Do you know if such a controller exists?
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