LED boost converter - final design  - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 15, 2016, 06:40:07 am
content: Hi I'm building small boost converter to power LED from 12V to 33-40V 2AIt basically works but at turn off time there is one short turn on and small oscillator cycleWhat could cause it ? Any layout issue with induced currents to oscillator ?Did you ever see it ?Scope screens: yellow channel is Gate voltage, green channel is Output voltage / Oscillator voltageI can clearly see that oscillator start new cycle but then end it soon and continue to discharge WHY ?

author: batteksystem
author_rank: Regular Contributor
timestamp: April 15, 2016, 06:47:56 am
content: you mean 33-40V?

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 15, 2016, 06:59:37 am
content: Quote from: batteksystem on April 15, 2016, 06:47:56 amyes, edited

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2016, 07:14:53 am
content: Switch mode supplies can be tricky, a PCB layout problem is a real possibility here. Can you post the layout image, or at least a high-res photo of the converter? A schematic would be nice to look at, too.

author: bktemp
author_rank: Super Contributor
timestamp: April 15, 2016, 07:22:14 am
content: Looks like subharmonic oscillations. When using >50% duty cycle this is a common problem. You need to add a slope compensation.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 15, 2016, 07:37:53 am
content: Definitely use current-mode controller for a LED driver. Voltage feedback loop should be absolutely out of question. Current mode makes feedback easier and removes any need for any compensation and is the silver bullet for any control loop instabilities. Then you only need to think about layout and EMI related things like false triggering.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 15, 2016, 07:45:40 am
content: It have current feedback, it is current source to 2ABut it not reach this current because of this switch glitch kill efficiency

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 15, 2016, 07:50:43 am
content: Here is layout and schematicsJust oscillator capacitor have longer leads and are used two parallel because it was tricky to get desired frequencyPAD3 and PAD4 are boost coil

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 17, 2016, 02:21:19 pm
content: Solved by adding decoupling capacitors around UC3843 Ucc and Uref, there was spikes which disrupt oscillatorNow switching "nice and clean"Green channel Input current 10A/divYellow channel Output current 4A/divJust need bigger transistor I had bad calculated power dissipation

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 17, 2016, 09:55:38 pm
content: Inductance is way too high.What's the LM358 for? It looks like you're force-driving the internal error amp, but then doingexactly the same thing that the internal error amp does: negative feedback with a 2.5V offset!Ed: well, it's probably not 2.5V, given that you didn't specify resistor values....... it's also a current feedback circuit, I guess. In that case, an amp is still helpful, but it's probably better to simply amplify the current shunt signal, rather than force the internal amp.The transistor has a considerable amount of stray inductance around it. There should at least be an RCD clamp snubber across drain-source, for example like this:(the 10n, 15 ohm, and BYV diode; best suited values will differ)But it would be very much better to change the layout to fix the stray inductance. Adding ground plane is a must.Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: April 18, 2016, 05:27:10 am
content: Yes, the layout is wrong on so many levels that you really should try to redraw it, thinking about current flow from the supply capacitors, through the parasitic inductances.

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 18, 2016, 06:51:21 am
content: It was just a testing if I can drive this LED because it is powered from 200Hz PWM remote controlled source and layout was to be easy hand drawI must use bigger transistor package and higher voltage ratting diode in final versionBoost inductor is calculated to 13uHI agree layout is bad and wire wound shunt resistors don't help itBut biggest problem is to choose inductor core to not make that 200Hz noiseTo use shunt amplifier and smaller value resistors is good idea//edit:Do you thing that Kool Mu / Sendust will solve audible noise ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 18, 2016, 09:16:37 am
content: 200Hz???Can you put component values in the schematic?Tim

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 18, 2016, 09:21:23 am
content: Quote from: T3sl4co1l on April 18, 2016, 09:16:37 amThis circuit works at 230kHz but is powered from remote controlled supply designed for common led strip 12V and have dimming capability > 200Hz PWM

author: Miyuki
author_rank: Frequent Contributor
timestamp: April 19, 2016, 11:17:48 am
content: HiI had some time to make final design of this boost converterReplace transistor with TO220 package to keep temperature and other semiconductors change to surface mount, replace choke core from iron powder to sendust to make it silent and replace sense resistors with much smaller with amplifierThru hole capacitors and resistors I use because want to keep board single sided and I have them in stockWhat do you think about this layout ?