For universal inputs none of Pi devices switch at a frequency > 150 kHz. This is to avoid the fundamental swicting component to appear on the EMI spectrum. Between 5 and 20 W I'd reccomend that you try the Tinyswitch-III family (132 kHz) with on off control. If you'd like to use PWM based chips you can try the Topswitch-HX family (also 132 kHz operation)