Pc817 optocoupler amd igbt high side switching  decay - Page 1
Forum section: electronics / beginners

author: elektronJack
author_rank: Contributor
timestamp: September 13, 2019, 02:08:43 am
content: In the attached circuit I get the result that the globe initially lights brightly then after1-3 seconds the brightness diminishes in a 0.5 -1sec pulsating manner until eventually there is no light emitted. If I switch it off then on again it will pulsate immediately until nil emission. The igbt also gets quite hot. Without the pc187 the light emits continously with no diminishing.Any suggestions on how to overcome this problem is appreciated.I have attached a slightly altered schematic showing a resistor and an LED connected to the pc817.

author: ledtester
author_rank: Super Contributor
timestamp: September 13, 2019, 02:22:23 am
content: How much current is going through pins 1 and 2 of the PC817? That's the opto-coupler's LED, and usually a resistor is used to limit the current to around 10 mA. More than 50 mA for an extended period of time can burn it out.Also, you usually want a resistor between the IGBT gate and emitter - otherwise it can stay on. This video explains why and also has some other good info on using IGBTs:Update: Also try putting your load on the high side of the IGBT - i.e. between Vcc and C.

author: krish2487
author_rank: Frequent Contributor
timestamp: September 13, 2019, 07:40:45 am
content: You need a high side driver. I am assuming the output side of the PC817 which is pulled up through the resistor is a seperate isolated supply from the rest of the circuit?Also, are you trying to run the IGBT in a linear mode or switching it?If you are trying to run the IGBT in a linear mode then it will get hot and burn out.And what is the supply voltage on the Collector of the IGBT?IGBTs are most useful at high-ish voltages... they suck for low to medium voltages. :-)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 13, 2019, 04:36:41 pm
content: It's only getting 3V Vgs(on), which isn't enough. Get rid of R1.The supplies are specified differentially, but are they really isolated? Voo has to be isolated.Are they really constant-voltage supplies? Is the voltage drooping under load or anything?There's no current limiting resistor on the LED?Tim

author: elektronJack
author_rank: Contributor
timestamp: September 14, 2019, 08:08:31 am
content: Why dispose of R1?The supply Voo is isolated as i understand "isolated ".If I'm incorrect can u enlighten me?I have measured Voo when the circuit is operating and there is no v droop.

author: floobydust
author_rank: Super Contributor
timestamp: September 14, 2019, 07:20:55 pm
content: R3 and R2 make up a voltage divider, when the opto is on. In your circuit, the IGBT is starved for gate drive. It needs at least 9V of gate-emitter drive to fully turn on. Otherwise it is partially on and heating up, causing the weird effects you are seeing.Voo is 9.7V, subtract opto VCE 0.2V and then voltage drops of R3, R1, R2 give only about 3.15V of gate drive. To fix that, I would delete R3, it looses 6.3V.R1 does nothing unless you have high speed high current drive for the IGBT.The circuit would not be suitable for PWM because the opto could turn on the IGBT somewhat fast but R2 means a slow turn off time, discharging the gate capacitance. The PC817 optocoupler is slow around 20usec rise or fall time.

author: elektronJack
author_rank: Contributor
timestamp: September 16, 2019, 10:27:13 am
content: Thanks floobydust for ur reply, i can see becoz u made a schematic that u spent some effort .I tried ur suggestions, however it doesn't work. I removed R3 and made R2 = 1k ohm, bulb wouldnt even light up. When i replaced 1k with 10k the bulb lights up but still pulsates as described in my initial submission.

author: Jwillis
author_rank: Super Contributor
timestamp: September 16, 2019, 11:56:21 am
content: What is the value of -Voo. Are you sinking the 12 v across the IGBT to a negative voltage ?

author: floobydust
author_rank: Super Contributor
timestamp: September 16, 2019, 07:27:36 pm
content: Quote from: elektronJack on September 16, 2019, 10:27:13 amI'm saying remove R3 and replace with a piece of wire. Otherwise, your power supplies are not isolated from each other as you think.

author: elektronJack
author_rank: Contributor
timestamp: September 17, 2019, 09:28:12 am
content: I did replace the resistor with a wire. No change, unfortunately