Discrete Astable Multivibrator

287

04:25:03

Schmitt Trigger based Relaxation Oscillator. I have used this circuit in my discrete SMPS project. It's coming soon, Provide EC stops crashing on me and I figure out a way to make a proper latch.

published 6 years ago

hurz

6 years ago

schmitt trigger? Its a comparator with changing switching level, and as you said a relaxation oscillator. So ehat have you found fundamental to avoid CFS?

PrathikP

6 years ago

CFS?

PrathikP

6 years ago

Yes it does work on the principal of a Schmitt trigger, doesn't it, with a positive feedback and a changing voltage level at the non-inverting pin. Our input is the voltage on the capacitor, instead of some external signal that we are trying to compare with a reference voltage.

hurz

6 years ago

CFS = Cant Find Solution

PrathikP

6 years ago

Oh your talking about the SMPS. Actually, adding an interference cap between pri and sec increases the run time, but it still fails. Let's see. So far, throwing in capacitors or RC has helped. This circuit is quite complex and also has a relaxation Oscillator in the mix, so I'd be lucky to get it going forever. Also, the EC BJTs can't handle a bit of dV/dt, I'll post about that a lil later.

PrathikP

6 years ago

Actually now it looks like I got the BJTs to behave. The latch is working. I'm over the roof right now. Electronics really gives me a kick! But now that the latch is working, Murphy decided CFS is in order so I'll have to fix that somehow.

PrathikP

6 years ago

Randomly throwing in capacitors really seems to do the business here

hurz

6 years ago

No, you said "EC stops crashing" and you found a way to make a proper latch. So think the oscillator was the root cause for CFS in your SMPS?

PrathikP

6 years ago

Did i say EC stops crashing? I still crashes, just reduced it a bit. Actually, may latch was proper, it was just EC's BJT that was causing the problem. My latch is a 2 transistor equivalent of an scr. It works fine on LTspice.

PrathikP

6 years ago

The latch on EC couldn't handle a 10khz sq wave (the 3904&6 on LT could). The undesirable (over here at least) dV/dt triggering happened. It doesn't after I put a cap somewhere in the latch

PrathikP

6 years ago

Ok it looks like the cause of the crash is the voltage spike at the drain of the main Switching transistor. Even though there is a snubber, the spike is sharp (but doesn't exceed 400V). I added a capacitor between drain and ground. It causes oscillations, but it delays the crash. I'll try to figure something out

hurz

6 years ago

Yes, exactly this "EC stops crashing" But this is not just an BJT issue. Sometimes little caps here and there can help, but sometimes they cause the opposite, because of to higher currents. When EC does recognize convergence issues it does trap in CFS.

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