AN1537 APPLICATION NOTE

14/18

Figure 22. 80W AC-DC adapter with PFC, flyback section: load transient 0.1

⇔ 4.5A @ 220Vac

Conclusions

A simple modification of the oscillator allows optimizing L5991-based converters to achieve a considerable de-

crease of the no-load consumption. This has been proved in a 45W AC-DC adapter and in an 80W power-factor-

corrected adapter, where the optimization here proposed has brought a reduction of the no-load input consump-

tion up to 400mW at high line.

This is the result of concurrent improvements. The modification to the oscillator allows very low switching fre-

headroom for overshoots and undershoots, so that it has been possible to enlarge the bandwidth of the control

loop without any frequency instability. In turn, the faster response due to the larger bandwidth has allowed the

reduction of the dummy load on the output, without compromising correct start-up when the adapter is unloaded.

The lower residual load and the loss reduction due to the very low oscillator frequency, results in a dramatic

decrease of the power absorbed from the mains at no load.

REFERENCES

[1]

"L5991/L5991A Primary Controller with Standby" Datasheet

[2]

"Minimize Power Losses of Lightly Loaded Flyback Converters with the L5991 PWM Controller" (AN1049)

[3]

"45W AC-DC Adapter with Standby Function" (AN1134)

[4]

"80W Power-factor-corrected AC-DC Adapter with Standby Using the L6561 and the L5991A" (AN1440)

Iout

Vout

COMP (pin 6 of L5991)