AN1513

Standby

7/13

The VIPer50A-E is designed to work in burst mode automatically when the power delivered

to the load becomes very low. The burst frequency and its duty-cycle depend on the

transformer parameters and the power delivered. The burst mode takes place when the

power transferred during the minimum on time is greater than the power required by the

load. This should increase the output voltage, but instead the control loop reacts by missing

some cycles. It is important to point out that during this working mode, the output voltage is

always perfectly under control. The result is a working mode where the effective duty-cycle

is much lower than the minimum under normal operation.

To further decrease the consumption during standby operation an additional test has been

done. Of course the burst mode has efficiency proportional to its working duty-cycle and to

work or not in burst mode is also dependent on the primary inductance of the transformer.

Sometimes it is difficult to optimize the primary inductance of the transformer and obtain an

efficient burst. The result can be just a few cycles missed and most of the pulses are

present at switching frequency. It can be an advantage to work at a lower fixed frequency,

giving up the burst operation mode. The network Q1, R8÷R10 decreases the frequency

sinking current from R1 (timing resistor) when pin Comp (5) is lower than 1.6 V. In this way

the charging current of C13 (timing capacitor) decreases and according to the law

Equation 1

the frequency starts to reduce until the minimum frequency is at standby load. In steady

state condition at full load the switching frequency has been set to around 70 KHz, while in

standby it is around 30 KHz.

Table 3.

Standby measurements at 65 mA on output 1

Efficiency

Switching

frequency

Vdc

mA

Vdc

Vdc

W

W

%

KHz

276

2.98

6.51

19.5

0.42

0.82

53

30

325

2.69

6.51

19.6

0.42

0.87

48

29

374

2.48

6.51

19.8

0.42

0.93

45

28

T

CV

⋅

I

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