Optimization of external oscillator

AN2052

10/13

DocID10909 Rev 2

4

Optimization of external oscillator

Targeted to lower down the average value of EMI noise, the PWM must go through the

whole frequency range within the testing time. In other words, the FM cycle is as short

as possible. On the other hand, since the PWM signal is modulated by the FM cycle

and switching cycle, to avoid audible noise, FM frequency should be lower than 2 kHz.

In fact the average data of configuration “B”, 700 Hz oscillator, are lower than the

configuration “A”, 50 Hz oscillator.

Another drawback of configuration “A” is the dependency on AC mains, which can be an

issue in the transformer design.