Weird mosfet behavior o.O" - Page 1
Forum section: electronics / beginners

author: ice595
author_rank: Regular Contributor
timestamp: July 07, 2014, 04:51:01 am
content: I have been working on a circuit to dim a 12v string of LEDs.Im using the classic TL494 as my PWM signal source, the output waveform looked great on the oscilloscopeit had nice sharp rise and fall edges and stable frequency when I adjust the duty cycle.Of course my circuit would need an output stage to handle the current required for the LED string. (about 2A at 100% duty cycle)Im using the 2n3904/2n3906 small BJT in a totem pole configuration to drive the mosfet's gate.Here's my original schematic diagram of the output stage that had problems with high duty cycle.I thought this circuit was okay, since I used mosfets as switches in a similar fashion before.in fact, I used to connect the gate terminal directly to an Arduino I/O pin and source directly to ground then a load between Vcc and drain.Here's the waveform on the mosfet gate terminal with my old schematic above.Notice how there's an abundant amount of high frequency noise on the falling edge.It gets even worse at higher duty cycles, anything above 70% duty cycle it would all of a sudden jump to 100% duty cycle by it selfAnd this only happens when there's a load between Vcc and Drain, when there's no load its perfectly fine.I was scratching my head for days trying to figure out whats going on and where was the high frequency noise coming from.I thought the high frequency noise was from the inductance of the LED string, so I replaced it with a resistive load and the same thing happened.I was also wondering if it had any problems with gate drive current, so I disconnected the mosfet's gate and used a 0.1uF capacitor to model the mosfet's gate capacitance. The rising/falling edges still looked somewhat sharp on the output of the totem pole.That's when connected the gate back with the 0.1uF cap still connected to the output of the totem pole and ground by accident.And guess what??! IT WORKED PERFECTLY WHEN THERE A 0.1uF CAP CONNECTED TO GATE AND GROUND.It seems rather counter intuitive to increase gate capacitance by it worked A LOT better than having no added gate capacitance.anyways, here's a schematic of my working circuit and some waveforms on the gate terminal.What are your thoughts on my new circuit?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2014, 05:32:12 am
content: The feedback capacitance is causing it to oscillate during turn-off. Shunting that reduces AC gain enough to keep it from oscillating, but as you've noted, is hardly an ideal solution. Try a snubber on the drain, perhaps 10 ohm + 0.01uF. I'm guessing the LED string is on a long cable or something.Tim

author: poorchava
author_rank: Super Contributor
timestamp: July 07, 2014, 05:42:53 am
content: Try to avoid additional capacitance on the gare. I'd try maybe diode and resistor parallel to each other and in series with the gate. The mosfet would turn on via the capacitor, but turn off through the resistor.

author: ice595
author_rank: Regular Contributor
timestamp: July 07, 2014, 02:10:11 pm
content: ok, when I replaced the 10ohm gate resistor with a 470ohm resistor on my first circuit it also fixed the problemisn't it better to have a lower value gate resistor?

author: Andy Watson
author_rank: Super Contributor
timestamp: July 07, 2014, 02:32:19 pm
content: The gate capacitance and inductance of the wiring forms a tuned L-C circuit. To stop the oscillations you need to spoil the Q of this series L-C by puttingseriesresistance in the gate path. This also explains why putting a cap to ground is not a good solution (even if it sometimes works!)

author: nitro2k01
author_rank: Frequent Contributor
timestamp: July 07, 2014, 05:39:15 pm
content: How exactly are you interfacing the TL494? (Considering it's giving the output as a collector/emitter pair of a transistor as far as I can tell from the datasheet.)

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 06:11:21 pm
content: eventually you can try the attached schematic for driving n-channel mosfets. i'm using that circuit everywherejust make sure R1 is low enough for your mosfet's gate capacitance (to have nice raising edges). or eventually you can replace the single transistor input stage with a totem-pole.basically the PNP transistor is there to rapidly discharge the gate.edit: of course the resistor at the base of the input transistor should be lower... i just cloned the R2 and forgot to change the value

author: brabus
author_rank: Frequent Contributor
timestamp: July 07, 2014, 06:35:56 pm
content: Well... if you switch at5 kHz, you can forget everything you know about switching regulators. I would call it "slightly faster than my grandma toggling the lamp switch".Yes, adding the cap slows everything soooooo much, this is why it seems better.It is actually a lot worse if you look at the switching losses on the Mosfet: the signal fronts are very... very... slow.As already said, you should tune the Gate resistor to dampen the oscillation, but wait, there is more! Which path does the current follow during the discharge of Gate capacitance?Is the emitter of Q2 VERY close to the Mosfet Gate?If not, we found a very nasty culprit.

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 06:45:12 pm
content: Quote from: brabus on July 07, 2014, 06:35:56 pmare you talking about the discharge path of the circuit suggested by me ? then Q2 is quite close (unless you will route the trace around the world). the circuit works wery well and the switch off is fast with minimal oscillations.btw.. we're talking about dimming a led - a few kHz is more than enough for that purpose(basically it depends on the thermal mass of the leds - you need just few kHz to avoid rapid heating and cooling of the LEDs and therefore shortening the LED's lifespan).

author: ice595
author_rank: Regular Contributor
timestamp: July 07, 2014, 07:37:28 pm
content: as long as I can't see the LEDs flickering, im fine with it.also, isn't NPN/PNP totem pole gate drivers more edficient than single ended gate drivers?

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 07:45:46 pm
content: Quote from: ice595 on July 07, 2014, 07:37:28 pmyes it is, that's why i mentioned that you can replace the single transistor with a totem-pole. the point of the circuit proposed by me is the fast switch-off - the D2,Q2,R2 part of the circuit - D1 is there just to protect the gate if you use this on the high side and you're "dancing on the edge" with Ugsmax

author: lewis
author_rank: Frequent Contributor
timestamp: July 07, 2014, 08:22:46 pm
content: Quote from: rob77 on July 07, 2014, 06:11:21 pmThis circuit is much much faster. Here's a snippet from one of my designs.This was designed for a low side FET, but splitting the circuit at 'X' the driver can be adapted for high-side. R63 and lack of base resistor on TR17 keeps TR17 from saturating significantly speeding up transition time. Collector current is defined by R63 and TR17's base voltage.

author: rob77
author_rank: Super Contributor
timestamp: July 07, 2014, 09:02:49 pm
content: Quote from: lewis on July 07, 2014, 08:22:46 pmlike itsame principle for discharging the gate, but a much faster driver. however for a LED dimmer it might be a bit of overkilli usually tend to keep the part count low if possible and a single transistor does the job well enough at few kHz (1-3kHz) , and usually the few mA wasted is not a big deal (environmentalists will kill me now for this).