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AN1523 APPLICATION NOTE

decrease the switching losses during light load operation, thus improving the stand-by efficiency. For reference,

also the Vcomp and the Vcc are captured. In detail, the Vcc shows that there is still margin when working at

light load respect to the Vccoff value (which is 6.5V typ. and 7Vmax.). This guarantees that even with a different

transformer batch, delivering may be a bit lower Vcc, the converter will still work correctly, without showing any

irregular behaviour at start-up or inopportune missing start-up due to a Vcc too low, unable to power correctly

the primary controller.

Output voltage ripple @full load

In Figure 9 the output voltage ripple at switching frequency have been measured. As per the previous measures,

the probes have been connected on test points after the output connector. The ripple and the spikes are very

low making this design suitable to power sensitive loads. In Figure 10, the residual ripple on the output voltages

at mains frequency is measured. The low frequency residual ripple compared with the 100Hz undulation across

C1 (input Elcap), demonstrates an excellent rejection of the circuit (~66dB) at 115V. Obviously the low frequen-

cy rejection becomes even higher when the circuit is working at 220Vac (figure 10). At that voltage, the rejection

becomes 76dB and this means a residual line ripple on the 5V output of 3mV only.

Figure 9. HF RIPPLE

Figure 10. LINE RIPPLE REJECTION

Vin = 115 Vrms - 50 Hz @FULL LOAD

Vin = 115 Vrms - 50 Hz @FULL LOAD

REF1:

CH1:

CH3:

CH2:

CH3: