Noway2
Electrical
- Apr 15, 2005
- 789
All,
I have been dragged into a problem at work that is a bit over my head and I am hoping that some of you could help shed some light on it for me. The problem relates to our motor soft start controllers. The production test department has learned that there is an apparent problem with getting SOME OF the SCRs to fire. I have been dragged into the situation to see if I can figure it out.
Over the past couple of days, I have been trying to come up to speed on the gate drive circuit and how it relates to the typical methods of triggering the SCRs. The firing circuit, originally designed circa 1980 generates periodic pulses to the SCR gate coupled via a pulse transformer. The circuit is an analog design that relies on the B-H curve of the transformer for timing and a bunch of other analog tricks.
So far, from measurements I have made I am seeing that the circuit is generating a firing pulse about every 24uS. These firing pulses are about 4uS wide. I am NOT seeing any 'back porch' portion to the pulse as at 4uS it is gone. The voltage amplitude applied to the gate is about 5V with a ~24V open circuit voltage. Readings with a current probe are indicating that the peak gate current is about 340mA. The anode - cathode voltage is 460Vac with a resistive load, which should be relatively easy to trigger due to the higher voltage and resistive load providing a fast latching current.
From app notes I have seen and the SCR data sheet, the voltage and current figures look like they are correct for a static (DC) scr firing, but I suspect that there isn't enough a$$ in these pulse to trigger dynamically like this. Specifically I believe that the 4uS pulse is too narrow and it should be more like 10uS. I believe that for dynamic triggering the gate current should be a couple of amps rather than a lousy 350mA.... Though I may be way off base here.
The data sheet says that the gate trigger voltage is a max of 3V with 200mA trigger current. The instantaneous gate power rating is 150W, 10W RMS - which I am nowhere approaching. Again, if I were looking at this from a DC perspective, ie load line equation I think I would be in the trigger region - but I don't think this applies to my case.
Can anyone shed some additional light on this situation and perhaps confirm or dismiss my analysis?
I have been dragged into a problem at work that is a bit over my head and I am hoping that some of you could help shed some light on it for me. The problem relates to our motor soft start controllers. The production test department has learned that there is an apparent problem with getting SOME OF the SCRs to fire. I have been dragged into the situation to see if I can figure it out.
Over the past couple of days, I have been trying to come up to speed on the gate drive circuit and how it relates to the typical methods of triggering the SCRs. The firing circuit, originally designed circa 1980 generates periodic pulses to the SCR gate coupled via a pulse transformer. The circuit is an analog design that relies on the B-H curve of the transformer for timing and a bunch of other analog tricks.
So far, from measurements I have made I am seeing that the circuit is generating a firing pulse about every 24uS. These firing pulses are about 4uS wide. I am NOT seeing any 'back porch' portion to the pulse as at 4uS it is gone. The voltage amplitude applied to the gate is about 5V with a ~24V open circuit voltage. Readings with a current probe are indicating that the peak gate current is about 340mA. The anode - cathode voltage is 460Vac with a resistive load, which should be relatively easy to trigger due to the higher voltage and resistive load providing a fast latching current.
From app notes I have seen and the SCR data sheet, the voltage and current figures look like they are correct for a static (DC) scr firing, but I suspect that there isn't enough a$$ in these pulse to trigger dynamically like this. Specifically I believe that the 4uS pulse is too narrow and it should be more like 10uS. I believe that for dynamic triggering the gate current should be a couple of amps rather than a lousy 350mA.... Though I may be way off base here.
The data sheet says that the gate trigger voltage is a max of 3V with 200mA trigger current. The instantaneous gate power rating is 150W, 10W RMS - which I am nowhere approaching. Again, if I were looking at this from a DC perspective, ie load line equation I think I would be in the trigger region - but I don't think this applies to my case.
Can anyone shed some additional light on this situation and perhaps confirm or dismiss my analysis?