Burst mode

AN1344

10/14

Figure 7.

Bad burst mode

start up current source.

As shown in Figure 7 the reoccurrence of this cycle is about 300ms. The worst output

voltage swing is 2.4 V, which occurs at 20mA. At no-load condition, the output voltage swing

becomes negligible (45 mV).

This mode of operation leads to the following drawbacks:

1.

Because the start up current source is turned on to supply the capacitor from a high

voltage rail, efficiency is dramatically reduced.

2.

The recurring period leads to as much as 13% variation in the output voltage. For this

audio application it does not matter, but the designer should review all aspects of

operation.

3.

Below the minimum current of 40 mA, the dynamic behavior is very poor which is

typical of all power supplies. If the demand of current occurs during the recharging

phase, the output capacitor is discharged and normal operation returns only at the next

starting phase.

In conclusion for this design a 40 mA minimum load is needed, 0.6% of maximum load, to

keep the unit in optimal performance. However, below this range, the output voltage is still

under control and no stresses are applied to the unit.

The transformer was optimized for the current scheme and not for Blue Angel.

Table 4.

Stand-by input power

Input voltage

Input wattage at no-load

Input wattage at 40 mA

0.85 W

1.77 W

1.1 W

1.8 W

1.3 W

1.86 W