Design Sometimes Compensates for Lower-quality Switchers

Loop Design Sometimes Compensates for Lower-quality Switchers 

Sanjaya Maniktala is back with a new installment of his popular power supply design column. The question he asks is how to make up for the erratic quality of the 3842 and 3844 switching power supply controllers now on the market. An adjustment of the RC components in the hysteresis loop allowed a power supply company to safely utilize the batch lot of jittery components they were stuck with — some 50,000 of them. 

The 3842/43/44/45 series of pulse width modulators are possibly the most popular controllers for off-line applications for several years now. Originally from Unitrode (now Texas Instruments, TI ), I can find clones from at least 12 more semiconductor companies on the web (see if you can better the count). The list may in fact be quite endless. Their quality is however often questionable, though I do admit that their Application Information seems all rather well written (hmmm, but did I read that somewhere else ). 

Practically speaking these equivalents can differ quite a bit. At one time we even had basic functionality problems due to an insurmountable jitter from an alternate source part. 

I did manage to band-aid the problem shortly after my arrival at the Singapore-based company I worked for, and they did manage to slip the bad parts into their high volume production, with no future ppm issues either. Though understandably they didn t return to the previous vendor to renew their learnings on what constitutes a guaranteed spec and what is not. 

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