PWM MOSFET RGB driver problem. - Page 1
Forum section: electronics / beginners

author: M@rkBot
author_rank: Newbie
timestamp: January 06, 2014, 03:39:54 pm
content: hi all,firstly apology's for the brain dump and grammatical pain in the following sentences.iv been playing around with what should be a simple circuit and after lots of learning and research im kinda at a wall as to my limited electrical engineering knowledge and experience.im currently designing a circuit too drive a high powered RGB led from a li-ion battery but am having some problems and was wondering if someone can point me in the right direction or offer there 2cents on how i might solve my issue's.the circuit is a micro controller running logic through a gate resistor into some NPN mosfets, which have a lowside resistor of 1 to 1.8ohm, im seeing ringing when the fets sink the current and this is reflected as a voltage spike on any of my other currently ON fets, i reduced this by increacing the gate resistor but its still high and the fet's pwm signal gets super wack when i get up to something that limits it enough.im currently playing with a number of different fets some of which handle the ringing differently but all the fets which have a wider current bandwidth are effected by this ringing more and the more tightly rated components limit the current when fully on. my moduals are 650ma per channel.another more serious issue that i cant seem too understand is that im seeing the ring of one fet momentarily turn on the output of another fet that's logic off at the gate and i dont see any spikes or pops on its gate so im wondering is its possible that having them really close to each other could be causing some sort of emi making its brothers turn on. iv tried pulling the gates low with 10k'sthanks in advance for any help.bot.

author: Kremmen
author_rank: Super Contributor
timestamp: January 06, 2014, 04:07:23 pm
content: Let's start with the usual because someone is going to say it anyway: a circuit diagram and possibly a photo of you circuit would be nice. Then we would actually know what we are speaking of.Let's assume when you say NPN mosfet you actually meant N-channel mosfet.When you drive a fet switch in a hard switching circuit the gate is prone to ringing at high frequencies (100 MHz+) if the drive is fast enough. While you should turn a swtich on and off quickly to avoid high switching losses it is possible to overdo it. Depending on the exact physical layout of your circuit, nearby switches can experience high enough dv/dt in their gate circuits to cause spontaneous turn-on, at least momentarily. This would be due to ground bounce or similar effect in the actual gate circuit traces.To do: limit the gate drive to a reasonable maximum. This can be done in both directions separately by putting 2 gate resistors in parallel and a diode in series with one blocking in turn-on, passing in turn-off. that way you can fine tune the turn-on and turn-off currents. Also an efficient way to suppress gate ringing is to use a ferrite tubelet in the gate lead of the switch. There are flexible tubes you can cut to length oir then the more traditional solid types.If you can provide more accurate info on the circuit then maybe the quality of the advice will improve as well

author: M@rkBot
author_rank: Newbie
timestamp: January 06, 2014, 04:24:09 pm
content: yeh sorry should have posted up schematic and photo, little bit scared of being judged for my sloppyness etc......yes n-channel fet, also im very restricted by space in the pcb.thanks for the reply.

author: M@rkBot
author_rank: Newbie
timestamp: January 06, 2014, 04:47:03 pm
content: so yes, i see the ringing like i stated and also i can see the ground bounce also but this isnt he source of the false trigger it happens always on the first oscillation of the ring.i will try the blocked resistor setup you suggested and also play with a ferrite tube or an inductor.thanks.

author: Kremmen
author_rank: Super Contributor
timestamp: January 06, 2014, 04:56:46 pm
content: Sloppy? You aint seen nothing yetThe ferrite tube won't work for those sot23 devices. For some reason i thought of to220 or similar.Maybe a small chip ferrite could do something similar...Anyway, do strengthen the common traces from the fet sources as much as practical. Ona PCB the thing to do would be a copper pour but here maybe solder a wire to augment the trace and reduce the ground bounce at least a little. Other than that try playing with the gate resistors to find a sweet spot for turn on and off.Edit: hit send before reading your previous.

author: M@rkBot
author_rank: Newbie
timestamp: January 06, 2014, 05:22:14 pm
content: yeh, the on the actual PCB board im using every piece of spare copper for a ground as the design creates the circuit through its housing, i did tin the track with this in mind but these are all easy things for me too try.ill order a selection of inductive stuff and see if i cant fudge a solution given my limited understanding of filtering.thanks for your time, it is much appreciated.

author: kaindub
author_rank: Regular Contributor
timestamp: January 06, 2014, 11:27:19 pm
content: A long shot here, but have you tried debouncing the button, either with an RC filter on the input or in software?The ringing of the FET would be in the order of a few 100 millivolts . It depends on the inductnace of the load, and LEDs do not have much inductance.I think the output is changing becuase the software is seeing multiple switch clsoures.Robert

author: M@rkBot
author_rank: Newbie
timestamp: January 18, 2014, 07:16:41 pm
content: so i finally got some parts and sat down today with this circuit and i have solved some problems and made some new ones bigger.so as suggested the ferrite beads helped immensely with the ringing and affected the overall shape of the square wave very little which was great!!! thanks!!!after experiencing lots of cross triggering and wiggyness i decided my breadboard was banjaxed and this has introduced a lot of noise and significant voltage drop when mosfets are triggered in time with each other. after much messing about and testing i settled on the following fet, DMN2041L-7http://www.diodes.com/datasheets/ds31962.pdf.im running the circuit from an arduino mini now just in case something was up with my micro setup and it hasn't changed very much, 10k on the gates and i beefed up the ground on the source of the fet's all helped immensely!! thanks again!!so the thing im unsure about is why i get a voltage drop when another fet fires??and what might be causing all the noise on the line?thanks in advance.

author: M@rkBot
author_rank: Newbie
timestamp: January 18, 2014, 07:21:25 pm
content: sorry just noticed i had attenuation set wrong on ch2.also i ment my breadboard was wiggy and i made a PCB and its crated noise.