LED Driver based on UCC28180 PFC Controller - Page 1
Forum section: electronics / projects

author: RedNeckEngineer
author_rank: Contributor
timestamp: January 02, 2016, 04:32:58 pm
content: Hi,I need people that know what they are doing, critique this design. This is designed to run a string of 10 high powered LED COBs at an adjustable current. I'm not welded to anything but the UCC28180. The datasheet gives an application that seems to shows most of the main design. Any help would be great.Thankshttp://www.ti.com/lit/ds/symlink/ucc28180.pdf

author: Christopher
author_rank: Frequent Contributor
timestamp: January 02, 2016, 06:06:24 pm
content: What if P3 is shorted somehow and there is a massive voltage on U4B?I think your VSENSE is a little incorrect, as there is no feedback path back to the chip..? The over V & I should pull this pin down, possibly using a transistor

author: RedNeckEngineer
author_rank: Contributor
timestamp: January 02, 2016, 06:44:44 pm
content: thanks for the reply.What if P3 is shorted somehow and there is a massive voltage on U4B?- Fuse on the output and a MOV across R14 to limit the voltage?I think your VSENSE is a little incorrect, as there is no feedback path back to the chip..? The over V & I should pull this pin down, possibly using a transistor-If I understand the datasheet. VSENSE wants a analog value around 5 Volt to know it's PWM duty cycle is good. If VSENSE goes up to 5.25 volt it reduces the duty cycle. If VSENSE goes down to 4.75 volt it increases the duty cycle. IF VSENSE is over 5.35 it stops switching the mosfet. VSENSE receives feedback from the OP Amps in the feedback loop.

author: Christopher
author_rank: Frequent Contributor
timestamp: January 02, 2016, 08:15:09 pm
content: Unfortunately doing that with the Vsense will more than likely reduce your loop gain a bit too much,I would rather have the feedback going straight into the chip (or through a separate voltage and current amplifier) and have the OverVoltage and OverCurrent comparators pull the soft start (in this case Vsense) pin down.Is there a reason for using the opamps instead of a proper comparator?Others with more knowledge may have a better idea.

author: RedNeckEngineer
author_rank: Contributor
timestamp: January 02, 2016, 09:43:44 pm
content: Unfortunately doing that with the Vsense will more than likely reduce your loop gain a bit too much,- I'm not sure what you mean?I would rather have the feedback going straight into the chip (or through a separate voltage and current amplifier) and have the OverVoltage and OverCurrent comparators pull the soft start (in this case Vsense) pin down.- What I think your saying is? Use the voltage divider like TI application to measure the output voltage. Then use a comparator to activate a transistor that pulls down the voltage on VSENSE to limit the current, by making it think it's in soft start? I'm not sure there is a value in doing that over a OP Amp? Please explain.Is there a reason for using the opamps instead of a proper comparator?-The reason I'm using an OPAMP for the current limit, is it's the closest way, I can think of to simulate what the UCC28180 was designed to do. It's only designed to measure the output voltage using a voltage divider. Output voltage goes up, the voltage on VSENSE goes up too. The way I'm using the OPAMP is to boost the voltage drop on R14 (current sense) up to around 5 volts. If the current goes up on the on the current sense resister, the voltage output of the OPAMP goes up too. So, the UCC28180 thinks the output voltage is high and adjust the the duty cycle down or up if the voltage is low. The reason for using a OPAMP for the over voltage is, it was there and I might as well use it.I also added a resistor to the output to drain the output caps after the unit is turned off.

author: RedNeckEngineer
author_rank: Contributor
timestamp: January 03, 2016, 12:01:32 am
content: thanks for the reply.adding inrush limiter between U2 and D1http://www.digikey.com/product-detail/en/SG26/570-1161-ND/4862351Hope it's a good selection. This is where my idea on what to put in.http://powerelectronics.com/community/how-do-you-choose-right-type-ntc-thermistor-limit-inrush-current-capacitive-applications

author: RedNeckEngineer
author_rank: Contributor
timestamp: January 04, 2016, 01:45:44 am
content: I'm trying to figure out the best way to protect the circuit if their a short on the output. Any ideas? I not sure if want I have is going to work.

author: Zero999
author_rank: Super Contributor
timestamp: January 04, 2016, 09:20:16 am
content: I don't see how this circuit can reduce the voltage.Where are the LEDs going to be connected to? Presumably P3.This is a boost converter and will only limit the current if the forward voltage of the LEDs is above the peak mains voltage. For a maximum input voltage of 250VAC, you'll need a string of at least 118 LEDs, assuming minimum of 3V drop per LED, to regulate the current properly. If you try to connect a string of 10 LEDs to the output, you'll get a lot of magic smoke.

author: RedNeckEngineer
author_rank: Contributor
timestamp: January 04, 2016, 10:44:26 am
content: This is what I'll be driving or one like it.http://www.bridgelux.com/sites/default/files/resource_media/DS33%20Vero%2029%20Array%20Data%20Sheet%20Rev%20J%2020150908.pdf