AN2450 datasheet(20/35 Pages) STMICROELECTRONICS

20 / 35 page

Operation under overload and short-circuit condition

AN2450

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DocID12784 Rev 6

converter will no longer be able to keep the output voltage regulated and the output voltage

will fall despite the reduction of the operating frequency (feedback reversal).

Limiting the minimum operating frequency (e.g. at the frequency value corresponding to the

intersection of M = Mx with Q = Qmax) is not enough to prevent the converter from entering

the capacitive region of operation. In fact, as the minimum frequency is reached, from that

point onwards a further load increase will make the operating point move along the vertical

line f = fmin and eventually cross the borderline.

Limiting the minimum operating frequency is effective in preventing capacitive mode

operation only if the minimum (normalized) frequency value is greater than 1. This suggests

that, in response to an overload/short-circuit condition at the output, the converter operating

frequency must be pushed above the resonance frequency (it is better if well above it) in

order to decrease power throughput.

It is worth noticing that, if the converter is specified to deliver a peak output power (where

output voltage regulation is to be maintained) greater than the maximum continuous output

power for a limited time, the resonant tank must be designed for peak output power to make

sure that it will not run in capacitive mode. Of course, its thermal design will consider only

the maximum continuous power.

In any case, whatever the converter specified, short-circuit conditions or, in general,

overload conditions exceeding the maximum specified for the tank circuit, need to be

handled with additional means, such as a current limitation circuit.

AN2450 Datasheet(PDF) 20 Page - STMicroelectronics