HV9910 and sub-harmoic oscillation - Page 1
Forum section: electronics / projects

author: Vicus
author_rank: Regular Contributor
timestamp: March 07, 2017, 11:52:46 am
content: Hi, I did design a new driver board for a RGB led light. The RGB led is 18 in total, 2 strip of 9 leds in parallel. The psu is a AC/DC from main to +36V slightly tunable. The orginal driver was linear and it suffer of over heating and reliable issue. The mcu generate a pwm signal (200Hz if I am not wrong) into a LPF then compared with the votage across a shunt resistor.Then I did decide to build a new one. I found that in modern led light similar to mine they use a HV9910 (also called AL9910, IL9910 and other) so I copy the schematic and modify the value to match my specification. I also some features like max current tunable via trimmer and bypass cap. I order the pcb and they works. My problem now is that I can't measure the voltage across the shunt resistor to set the right avg current. I don't know it's due to my crappy oscilloscope (an Owon sds something) of the subharmonic oscillation due to the duty cicle <0.5http://skory.gylcomp.hu/alkatresz/HV9910_led_driver.pdfAt the end of page 6 is write:Quote9 blue (or green) led in series need about 30V and my psu can reach 40V max so I have to found a solution. I don't understand what problem it gives. Can I use a cap in parallel with the shunt to prevent sub harmonic oscillation?As soon as possible i will post the complete schematic and some screenshot of the oscilloscope.Thanks.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 07, 2017, 12:35:22 pm
content: Subharmonic oscillation (or instability) is a common pathology ofcurrent mode control. It manifests as alternating wide and narrow pulses from the PWM comparator. The quick-n-dirty fix is to feed a little bit of the oscillator's sawtooth waveform into the current sense input. See the datasheets for the venerable UC3842/3/4/5 series current mode controller ICs for more details on how to do this and/or do a search on the phrase, "slope compensation."

author: Vicus
author_rank: Regular Contributor
timestamp: March 07, 2017, 03:18:37 pm
content: Ok, in an application like mine what concrete problem will I have?

author: bktemp
author_rank: Super Contributor
timestamp: March 07, 2017, 03:45:24 pm
content: Adding the fix to the HV9910 is a bit more difficult because injecting a current into the current sense loop changes the avarage LED current. For UC384x it doesn't care, because there is another control loop regulating the output voltage.Unless you really need the high input voltage range, I would use a different ic. There are many buck mode LED driver ics (AL880x are great for up to 36V input voltage, AL8860 works up to 40V). They operate using hysteretic mode. This gives a much better avarage current control even with almost no voltage drop.

author: Vicus
author_rank: Regular Contributor
timestamp: March 07, 2017, 10:34:21 pm
content: I did not say that the anode of the tree color are linked together and cannot modify. This is due by the pcb with a substrate of alluminium. It cannot be modify.

author: MagicSmoker
author_rank: Super Contributor
timestamp: March 08, 2017, 11:40:46 am
content: Quote from: bktemp on March 07, 2017, 03:45:24 pmYes, that appears to be the case here. I wasn't familiar with this specific chip so I searched on the keywords "slope compensation hv9910" and one of the first results is an application note by Supertex (now part of Microchip) describing a clever hack for adding slope compensation to this IC. The OP is strongly encouraged to search for this AN on his own.Quote from: Vicus on March 07, 2017, 10:34:21 pmThere is a specific solution to this problem (for the HV9910, that is), but if you can't modify the circuit then you can't fix it. Period.

author: Vicus
author_rank: Regular Contributor
timestamp: March 14, 2017, 06:50:00 am
content: Quote from: MagicSmoker on March 08, 2017, 11:40:46 amOnly the pcb with the led cannot be modify. PSU, control and driver as in differents pcbs.

author: Vicus
author_rank: Regular Contributor
timestamp: April 01, 2017, 01:36:18 pm
content: Hi volks, I try to usa constant off time instead of constant freuency and it works better. Still some problem triggering che voltage across the shunt resistor but I suppose it is due to my crap oscilloscope. Thanks.