Reliability Is No Flash in the Pan

Don t be so quick to pop the champagne when your new power supply prototype seems to work, Sanjaya wags a finger. Lest you want a lawsuit, ensuring reliability requires a disciplined effort around a second current limit. It s a long lonely trudge back to the drawing board, he says. 

But we also know that if this happens to be a high-voltage application (defined here as an input greater than about 40 V) we may have to size our inductor according to the current limit, not the load current. That number is 5 A in this case. But very roughly so  

Here we should actually check the IC datasheet to see the MAX value of the current limit range. For example, for a 5 A switcher the MIN value may be say 6 A (set high enough simply to guarantee full 5 A load capability with the usual 20% inductor design criterion mentioned above), but the MAX value of the current limit may be say 7 A over temperature and process variations, and depending upon how it may have been trimmed in production. 

For non-buck topologies, we now realize that say for a buck-boost application with an input of 15 V to 25 V and an output of 15 V output, the max duty cycle is 50% and this occurs at the minimum input of Vin = 15 V. So if the load current is 5 A, the average inductor current is 5/(1 - 0.5) = 10 A. With the 20% inductor criterion, the peak switch current will be 10 x 1.2 = 12 A. So the MIN of the current limit must be set higher than 12 A. And then depending on the available accuracy for current limit, the MAX may be as high as say 20 A. Clearly, a 5 A switcher is not going to suffice, nor an inductor rated only for 5 A  

A related issue is the case when a buck IC is used in a so-called inverting configuration . We should realize that in doing so actually the topology has in effect changed from a buck to a buck-boost. So now we just cannot get 5 A of load current from a declared 5 A IC. How much load current is possible depends on the specific input-output conditions. So again, our peak current is not close to 5 A, nor should the inductor rating be 5 A . Or we will certainly be frozen into a July 4th timeframe forever. 

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