Look what I ´ve found about Mosfet´s ringing.
http://files.engineering.com/getfile.aspx?folder=7f8dbdbe-0015-4979-b82a-4e5ee52b8cfb&file=avoid_mosfet_ringing.pdf
Hi Lionel, thanks for your reply.
Those are Msim files.
Overshoot is caused, at some point by gate drive from ne555 pwm and because of the lack of snubber across
mosfet.
That is an issue that I can deal with.
IRF is working good now and heating is moderate...
Thats right, I may even get a working dimmer for pennies,
but the fact is that i want learn something in the way about switching mosfets,
scrs and the like.
You are very kind to help me, thanx a lot![wavey].
Heres a picture of the Drain-Source oscillogram, at 280vdc,
tested during a couple of seconds to avoid mosfet overheating.
Motor is no loaded, but running free.
Would like to know, if rise and fall time are ok, or must be improved.
IRF is working without snubber by...
A more accurate oscillogram, using another test probe.http://files.engineering.com/getfile.aspx?folder=fde33ac5-a8e3-4831-a70d-c2eb52ba5b64&file=150vdc_18useg_50%..JPG
Yes, the thyristor approach was left aside by now, until I find a reliable
way to detect zero cross, and be sure that unwanted firing is 100% unlikely.
May be, I will try something with a PIC to avoid fake zero cross detections.
i like SCR because of its incredible current withstand capability...
dv/dt is like a cloud thats waiting to rain over me, any time a turn a switch off[tongue]
This time is not that bad, because its inside IRF840 SOA.
look:http://files.engineering.com/getfile.aspx?folder=421e5727-8ca9-48bc-bb53-dedf1d117225&file=dv_Dt_1.5volts_x_nanosec..gif
That is not reassuring at all.
Now i am a bit concerned about SCR and Triac erratic temper.
An spurious low angle firing, may cause the load to receive a lot of current.
Guess will try to control my motor, with a PWM signal driving an IRF840 at 310Vcc! [sadeyes].
Talking about things that should not be, look this Triac commutation failure.
After a couple of positive half cycles, triac cannot block negative half cycles,
So load (DC motor), stops turning and starts oscillating in its place under AC current.
(red window)...
A picture of dV/dt between anode n cathode.http://files.engineering.com/getfile.aspx?folder=6a31eeaa-12f8-4015-80ac-d7c510b83745&file=dV_dt_across_scr..png
Eureka ppl![thumbsup2]
As ITSMOKED and BRIANE22 already pointed, voltage rising rate across SCR anode-cathode,
was re-firing my SCR.
Snubber was there, but no really working...
20 ohms resitor (1 watt,sorry), in series with .1uf capacitor, was indeed open because
of heat.
Will keep testing...
Kind ppl, thanks for your replies.
Heres another video, where fault is more obvious:
Link
Snubber is already there, as u can see in the attached circuit diagram.
Load is 500 watts a hand driller, with 100uf capacitor in parallel...
Hi ppl, need advice to find out, what is causing this unwanted SCR firing.[mad]
Look:
Link
There u can see the signal at my BT152 SCR gate, that sometimes shows spurious firing,that
looks like a bridge, between two firing points.
If some one can help to discover the culprit, i may upload my...
