SWITCH MODE POWER SUPPLY

摘要

A switch mode power supply comprises a transformer (T1). A switch (Q4) alternately opens and closes a current path through a primary winding (L1) of the transformer in response to a pulse switching signal. An oscillator (110) generates the switching signal and synchronises the switching signal to a synchronisation signal (HSYNC') from an external source. A pulse width modulator (140) varies the pulse width of the switching signal in dependence upon a feedback signal indicative of a load on the power supply. A current limiter (150) turns off the switch to close the current path in response to current in the primary winding exceeding a threshold. Compensation means (160) is provided for varying the threshold in dependence upon the frequency of the synchronisation signal.