LED current driver oscillation - Page 1
Forum section: electronics / projects

author: oilburner
author_rank: Contributor
timestamp: March 27, 2014, 03:18:21 pm
content: Hi All-Having a problem with a voltage to current converter driving a LED. The signal is ringing at 250 KHz! I tried putting a 0.01uF capacitor all around the circuit and can change the ringing, (lower frequency a bit, higher p-p voltage a bit) but can't kill the oscillation.The scope plot is the voltage at the sense resistor.Any hints?Thanks!Greg

author: dannyf
author_rank: Super Contributor
timestamp: March 27, 2014, 03:27:11 pm
content: QuoteA ccs will necessarily have negative feedback, and as such, run the risk of oscillation. There is no way around it, other than not using it.In your case, you can try to use a mosfet with low gate capacitance, or a bjt, or a larger gate stopper - use ac analysis to find a more suitable value, or put a low-pass filter on the sensor / inverting input, etc.

author: dannyf
author_rank: Super Contributor
timestamp: March 27, 2014, 03:28:17 pm
content: One more, try to use SLOWER parts whenever you can.High speed parts are evil and should be avoided as much as you can.

author: oilburner
author_rank: Contributor
timestamp: March 27, 2014, 03:36:47 pm
content: Thanks dannyf, I have a IRF510 I will give it a try.Greg

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 27, 2014, 03:41:42 pm
content: You need to do it like this,Ballpark, 10k and 100pF for R and C should be a good starting point.Tim

author: Marco
author_rank: Super Contributor
timestamp: March 27, 2014, 04:00:27 pm
content: If he just moved the load to the drain it would probably be stable ...

author: Vgkid
author_rank: Super Contributor
timestamp: March 27, 2014, 04:25:16 pm
content: You are going to have a devil of a time using your original opamp. Sorry I couldnt resist it.

author: oilburner
author_rank: Contributor
timestamp: March 27, 2014, 04:56:37 pm
content: Quote from: Vgkid on March 27, 2014, 04:25:16 pmyou had me there for a sec, I double checked the opamp and its not too fast, but then saw the part number...

author: oilburner
author_rank: Contributor
timestamp: March 27, 2014, 05:02:50 pm
content: Quote from: Marco on March 27, 2014, 04:00:27 pmThanks, I can try this easy change. What is the theory?

author: dannyf
author_rank: Super Contributor
timestamp: March 27, 2014, 05:13:16 pm
content: Long wires on the feedback loop equals troubles.

author: Marco
author_rank: Super Contributor
timestamp: March 27, 2014, 05:57:35 pm
content: Quote from: oilburner on March 27, 2014, 05:02:50 pmVds has a smaller influence on the current than Vgs, so by putting it on the drain you effectively have a smaller inductance in the control loop.

author: oilburner
author_rank: Contributor
timestamp: March 27, 2014, 09:51:50 pm
content: Hi, back from the lab- Changing the LED (at the end of long wires) up to the Drain didn't kill theoscillations. I then put in a IRF510 and it looks good so far. I am confused at the specs of theseFETs though, the STP looks pretty slow. The IRLZ24N oscillates similar to the STP. Why is the IRF510 good?Td(on) Tr td(off) TfIRLZ24N 7.1 74 20 29STP141NF55 30 150 125 45IRF510 6.9 16 15 9.4I can also try T3sl4co1l and dannyf's suggestion to put a filter (pole?) in the feedback.Thanks Guys-Greg

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 27, 2014, 10:08:24 pm
content: Quote from: oilburner on March 27, 2014, 03:18:21 pmYour circuit is oscillating cause the "sum of phase shifts" introduced by the parasitics amount to 360* with a Gain of 1, its the gate capacitance resonating with the Rg the drain with series resistor, the feedback node is unstable and needs to be compensated. with a pole as suggested.Throwing random parts at the problem wont help. even if you used a slower op-amp the problem is still there, when you use a different FET parasitic capacitance channels between gate to drain will have different characteristics, thus altering the characteristics.Also added a 10k between the gate and ground, your gate with low Vgs threshold in this case, is much more sensitive noise immunity under DC AND AC conditions are an important.

author: dannyf
author_rank: Super Contributor
timestamp: March 27, 2014, 10:15:22 pm
content: QuoteSuper low, for a power mosfet, gate capacitance.IRF510 is one amazing mosfet from that perspective.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 28, 2014, 12:36:38 am
content: Quote from: Marco on March 27, 2014, 04:00:27 pmStable? You're making me horse.