Optocoupler-driven triac - no gate asymmetry? - Page 1
Forum section: electronics / beginners

author: aneevuser
author_rank: Frequent Contributor
timestamp: May 07, 2022, 07:09:15 pm
content: I've been trying to understand the various analogue approaches to driving triacs for AC power applications, and a lot of the circuit effort seems to be aimed at eliminating asymmetrical triggering, which AFAICS is due to the asymmetrical gate current requirements when the triac is triggered in different quadrants. So we see diacs and whatever showing up in an effort to eliminate this.I've now started to look at driving a triac from a triac-based optocoupler, an example circuit of which can be seen in "Figure 8. Typical Application Circuit" in the datasheet linked below.https://www.farnell.com/datasheets/97984.pdfAm I right in understanding that this approach will essentially solve any gate drive asymmetry problems? My thinking is that the gate of the power triac in the circuit draws its current from AC line, and hence there will be unlimited current available to switch it on, as opposed to the analogue scenario where gate current comes from a discharging capacitor.Am I understanding this right?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 08, 2022, 01:44:53 pm
content: Proper gate drive (or any circuit actually) is not by using some magic component, but by proper design, engineering and testing.Although all these modern IC's do make it appear as if they are magic, the design / engineering / testing is still needed.

author: Alti
author_rank: Frequent Contributor
timestamp: May 08, 2022, 04:09:51 pm
content: Indeed triacs are not very symmetrical with respect to trigger current Igt. Wide tolerances in between quadrants, even in Q1/Q3. When you set some control setting with a pot then you want this setting and phase angle to hold in both polarities. Then this control must not be based on slowly ramping gate current or you get slightly different triggering angles in both polarities. If you use diac, this triggering is based on voltage accross diac, this works like a comparator (positive feedback). As soon as diac latches, the current via gate is set so high that even at freezing temperatures and worst polarity it is enough to decisively latch power triac. Diacs are more symmetrical because these trigger not through gate current but through exceeding their voltage rating (there are several ways to trigger SCRs, triacs and diacs).The random phase optocoupler approach allows decisively triggering at given phase angle, but only under some conditions. Obviously you cannot vary LED current to alter phase angle. To vary triggering angle you need an external (most likely uC based) circuitry to sense zero-crossing and generate the 100Hz pulsetrain that hits phase angle at precise timing. What is more, you must not turn LED on at very low angles or you get the asymmetry. This "low angle masking" is made in software. Random phase is quite tricky.

author: aneevuser
author_rank: Frequent Contributor
timestamp: May 09, 2022, 12:54:24 pm
content: Quote from: Alti on May 08, 2022, 04:09:51 pmTwo questions:1) Where does this particular asymmetry come from? I don't see any specific mechanism that would produce asymmetry at low angles in the optocoupler. (though I do see a potential problem produced by errors in zero-crossing detection - you may try to turn the opto-triac on before the end of a half-cycle if the zero-crossing detection is poor)2) What do you consider to be a "low angle" in this case? Can it be quantified?

author: Alti
author_rank: Frequent Contributor
timestamp: May 14, 2022, 10:55:05 pm
content: Ad 1. I do not know how optocouplers work internally but for sure these are highly unsymmetrical. Both random phase and zero crossing. Connect some AC load (i>il), then slowly raise diode current. You can easily notice that in between OFF and ON there is this intermediate state where consistently only one polarity is ON while the other is OFF. Same happens with triacs. Datasheet just gives the conditions where it is reliably OFF and conditions where it is reliably ON. But in between this is no man's land, anything can happen.Ad 2. Low angle.. Well with random phase you have chosen, the resistor that limits current flowing through coupler and gate has to be super high value, by design. This needs to be over 350V/1A (ballpark). And it has to withstand this pulse power. So using such big value means you won't be able to trigger main triac when there is 10V across mains because gate current is going to be super low. So you need to wait with triggering till the moment when i>Igt could flow. With zero crossing it is just super easy because the ZC circuitry inside coupler triggers only when there is low voltage present (5-15V). Usually 0R resistor does the job and you can trigger triac as soon as mains supply reaches about 5V.