Any technique that accelerates the cutting of the transistor, of the CNY17-2 opt - Page 1
Forum section: electronics / projects

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 12, 2023, 10:37:09 pm
content: I'm starting the tests of my new Electronic Load. I would like some suggestions to work around a problem.I'm using my Signal Generator to inject a 500Hz square wave, with the duty cycle set to 10%, thus generating a 200us pulse. I'm using an optocoupler, not only to isolate my Signal Generator, but also to insert a delay on the rising edge, which is between 6 and 10us. The problem here is the excessive slowness of the wave's falling edge, as can be seen in image 2. I started the tests with the PC817B, where I detected the problem. I replaced it with CNY17-2 which gives access to the base of the transistor. I inserted a schottky diode from the base to the collector, hoping to speed up the transistor cut-off but it didn't produce the expected change. Any idea how I could speed up transistor cutting?

author: moffy
author_rank: Super Contributor
timestamp: May 12, 2023, 10:44:00 pm
content: The slow fall time is due to the value of R21, 100k, and the recovery of the optocoupler, reduce its value and you will reduce the fall time. The other way is to connect a resistor from the base to the emitter of the optocoupler to remove base current, but that will also effect the CTR(current transfer ratio) of the optocoupler.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: May 12, 2023, 11:23:09 pm
content: Quote from: Avelino Sampaio on May 12, 2023, 10:37:09 pmDoes that really matter at 500Hz ?Quote from: Avelino Sampaio on May 12, 2023, 10:37:09 pmYour circuit shows a darlington optocoupler, but your part codes show single transistor opto couplers ?simple Optos have a large area transistors which gives large C-B capacitance, and the turn off time is direct proportional to load resistance.The next class of optos, use aseparate base photo-diodeand those are faster - part codes like 6N136/7/9 are like that, as too are M501 family parts.Look for 1Mbd+ etc speeds, and sometimes these photodiode+transistor optos are called 'logic output' .Or you may choose to jump straight to a Logic output opto coupler if speed is an issue, parts like H11L1 are low cost and very easy to use.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 12, 2023, 11:34:02 pm
content: Did you read the datasheet? Turn-off time is >20us, so this is expected.It will fall faster with lower resistor values (More current).Instead 100K/18K/2K, try 10K/1K8/200R or 1K/180R/20R.Or use a faster device like the PC123.

author: Benta
author_rank: Super Contributor
timestamp: May 12, 2023, 11:46:02 pm
content: Erm, R21 has nothing to do with this. The CNY is operating as common-collector, so the 10k (R22 I think, hard to read) emitter resistor is the relevant one.But I can absolutely second the opinion about reading the datasheet.

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2023, 12:02:46 am
content: Quote from: Benta on May 12, 2023, 11:46:02 pmGood point.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 13, 2023, 12:19:22 am
content: Quote from: moffy on May 12, 2023, 10:44:00 pmIt worked with a base-to-emitter resistance. After testing some resistance values, I realized that it would be more interesting to use a potentiometer for a correct adjustment.Thank you very much!

author: moffy
author_rank: Super Contributor
timestamp: May 13, 2023, 12:46:21 am
content: Glad to hear it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2023, 06:35:04 am
content: Nice performance from a part not meant to be so fast

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 13, 2023, 09:53:29 am
content: Quote from: PCB.Wiz on May 12, 2023, 11:23:09 pmThe frequency doesn't matter as much but the slope of the rising edge does. I want to set the gate resistors as high as possible. As the Mosfet will only work in the linear region, I will only consider the IRLZ44 gate load (Qgs=8.6nC) to calculate the maximum current.Imax = Qgs/dt :::> 8.6nC/8µs =~1mARg = Vgs / Imax ::> 2.5v/1mA ::> 2k5At first I performed tests with the gate resistor set to 1K and it worked without a problem.- image 1 - Pulse on the 0.22R resistor- image 2 - Transient test result of one of my PSUs.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2023, 10:23:36 am
content: It's an electronic load, so why the 500Hz in first place?You need to make a continuous load, not making a hell of current spikes.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 13, 2023, 07:35:32 pm
content: Quote from: DavidAlfa on May 13, 2023, 10:23:36 amDavid,One of the functions of an Electronic Load is the possibility of carrying out PSU's transient tests. This is the reason for generating a 200µs pulse. See the test of some sources with Carga Eletrônica on the page below. Please use Chrome's page translator as the page is in Dutch.https://www.circuitsonline.net/forum/view/130792/2

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 13, 2023, 08:36:09 pm
content: I completely understand!Using +-12v would switch them much faster, but anyways don't expect lighting speed driving the fets with op-amps.

author: Avelino Sampaio
author_rank: Frequent Contributor
timestamp: May 13, 2023, 09:06:39 pm
content: David,Opa364 is doing the job. The project is working as planned. The only problem was related to the optocoupler.Thank you for your participation