LED PWM dimming straight from PFC circuit? - Page 1
Forum section: electronics / projects

author: jmole
author_rank: Regular Contributor
timestamp: December 04, 2014, 04:52:44 pm
content: So I want to design a little lamp for myself. I have a lot of experience with designing LED drive circuits that run off regulated DC, but I'd like to have as little PSU in this one as possible, with as much power output as possible. I also want high-resolution PWM dimming from a micro.After doing some research, it seems like all (or almost all) offline LED dimming ICs on the market are geared toward TRIAC dimmer compatibility, which I could care less about.So I'm thinking, why not use a front-end PFC circuit to generate a HVDC supply at ~400V, then string together a bunch of LEDs in series, and put a CCR and MOSFET at the bottom to dim them.Problem is, I don't know much about PSU design, and if a PFC is really the right tool for the job. I do want to get a good power factor, and a solid DC rail, which is why I'm not running straight from a rectifier/capacitor off the mains, but I don't really want or need an isolated supply.Anyone have advice in this arena?

author: jmole
author_rank: Regular Contributor
timestamp: December 04, 2014, 05:10:56 pm
content: Looking at something like this for the PFC circuit:http://www.ti.com/tool/ucc28070evm

author: johansen
author_rank: Super Contributor
timestamp: December 04, 2014, 10:30:14 pm
content: don't forget you can vary the pfc voltage.as the 400 volts drops towards 1.41X240vac (340vdc), the power factor will decrease however.also, you can boost higher than 400 if you want to.i don't know what the impedance of today’s high efficiency LEDs are, you might be able to get sufficient dimming by sending 330 volts into a string of leds that reach full rated current at 450 volts, idk.

author: tom66
author_rank: Super Contributor
timestamp: December 04, 2014, 10:35:15 pm
content: One thing to be aware of is most PFC circuits are terribly inefficient under light or low load. I've seen 15W quiescent power from one design. How well will your PFC circuit handle rapidly changing load? I've seen a plasma TV power supply pull 6A peak from the 230V wall when the load changed transiently (from black to white to black again), this is because the circuit was optimised for occasional changes in picture brightness only.Wouldn't an isolated design work better? You could modulate the drive current to be proportional to line voltage, if you want a high PF. Maybe you could use a TRIAC-capable PF dimmer, but with your circuit anyway?How much power do you need?

author: Zero999
author_rank: Super Contributor
timestamp: December 04, 2014, 11:43:00 pm
content: I've had a very quick glance at the marketing blurb.It's boost regulator. The transient response will be poor but that's the point. It's designed to be used before a properly regulated SMPS. The current is integrated and the duty cycle varied to keep the waveform sinusoidal. It should be fine to add a current regulator on the output and dim the LED by varying the current, not PWM which is actually less efficient than simply reducing the current.It's true some PFC stages suffer from inefficiency under light loads but I don't know about this one. The efficiency of LEDs increases as the current it lowered so perhaps this may help to counteract this? Either way, don't have a standby mode when the LEDs aren't powered but the PFC is still powered. Design it with a minimum brightness setting which is bright enough to be seen.

author: Zero999
author_rank: Super Contributor
timestamp: December 05, 2014, 10:05:05 am
content: http://www.nxp.com/documents/data_sheet/SSL2129AT.pdfhttps://www.fairchildsemi.com/datasheets/FL/FL7701.pdf

author: jmole
author_rank: Regular Contributor
timestamp: December 05, 2014, 07:48:44 pm
content: Quote from: tom66 on December 04, 2014, 10:35:15 pmActually, the more I think about it, the more I think this approach is more suited to a high-wattage light (which I'm also interested in making), something from 200-400W.This seems like the regime where the straight-PFC solution will actually save money over a traditional PSU, as compared to a small desk lamp which could be easily driven from a small external switcher.If I use a PWM frequency of 2kHz+, I might be able to avoid the transient response issues mentioned by Hero999 while still maintaining a useful range of dimming.The big issue with current-based dimming, as opposed to PWM dimming, is that you end up with color shift in the LEDs. Another reason is that some manufacturers (like OSRAM) specify a minimum LED current. I've always been skeptical of the minimum current ratings, but I don't know that much about the physical characteristics of the LEDs that would necessitate including a spec like that in a datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2014, 01:25:04 am
content: Nice thing about PFC is the voltage is stabilized (or can be..), give or take ripple. Which is a whole hell of a lot better than, say, putting the LEDs right across the line (plus a resistor, and a rectifier or whatever).I would suggest simply using a current limiting resistor: it needs to drop, oh, 5% is probably pretty marginal, but less than 20% would be no problem, of DC link to account for all the ripple and stuff. That's not a bad deal (>80% efficiency from a resistor, times whatever the PFC is doing).I wouldn't suggest controlling it from the PFC loop; it's very slow and not very consistent. Also, if the PFC is self powered (aux winding), you don't want it throttling down too much or it will brown itself out.Tim