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Old 1st May 2005, 04:09 PM   #51
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Originally posted by Alme

I've found difficult to sense such low shunt drop of 40mV with LM393 comparator because comparator was more sensitive to voltage spikes which are produced by shunt parasitic inductance during converter switching operation. Transistor sensor proved itself as better choice for it.
how about a little hysterisis in the comparator circuit?
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Old 1st May 2005, 10:59 PM   #52
Alme is offline Alme  Ukraine
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Yes, a positive feedback is usually added into comparator circuit to help improve response time or make latching effect.
Looks like it is basically possible to add hysteresis into transistor sensor as well; I didn't try it though because used such sensor for current limiting type OCP, not latching shutdown. I think series diode plus resistor connected from Q3 collector to Q1/Q2 base can do such job. When thresholding, feedback resistor would be connected in parallel to R2, thus decreasing R2/R1 ratio. Certainly there also must be Q3 pull-up to allow this, actually pull-up will take part in R2 paralleling action, but feedback resistor is going to be of much higher value than pull-up (say, 470k vs. 10k).
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Old 2nd May 2005, 07:52 PM   #53
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Originally posted by Eva

At some power levels, EMI may be strong enough to interfere with the control circuit and cause asymetric drive to the transformer [clock or duty cycle interferences]

Improper ground PCB layout of the control IC may also cause ground loops and asymetric duty cycle [I've experienced this sometimes in prototypes]

Hi there ....that is why in SMPS designs I always use a driver ic rather than directly connecting the power mos/igbt to the control chip. YAnd you get far more stable operation.

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