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   I built a power-factor-corrected battery charger using Fairchild's FAN=
7529 PFC chip and Onsemi's UC2843 constant frequency PWM chip. Basically =
it's a boost converter driven by FAN7529 followed by 2843 chip driven fly=
back.

   It worked fine on my bench for hours. But the customer made a test on =
a very deeply discharged battery set (somehow discharged to 8V for 24V ba=
tteries, near dead ones to me) and they said something was not ideal.=20

    When I re-test the returned unit, there was something weird. It worke=
d fine for 2 minutes or so, then the 2843 chip gate drive frequency rampe=
d up from 58k Hz to about 68k very fast in seconds ( remember it's a cons=
tant frequency chip programmed by RC components outside), thus giving mor=
e load power. Its duty cycle was not changed, however, thus only the load=
 current increased. The input power increases linearly thus the efficienc=
y is similar. When restarted, it repeats the whole process. I can't allow=
 this, since overheating might occur.

    I probed the oscillator pin of that chip and apparently the oscillato=
r frequency (should be constant) varies along with the changing gate driv=
e. Also the power factor stage always works.

     Any suggestion on what could be wrong before I replace that chip?

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