AN2864

Tips for reducing the standby consumption

Doc ID 15282 Rev 1

5/29

1

Tips for reducing the standby consumption

In this section, minimizing the standby power consumption in a flyback isolated converter is

analyzed, as well as how the new VIPER17L device can help to achieve this.

When light loaded or in a no load condition, most of the losses are switching losses that are

almost proportional to the switching frequency. It should be considered that even if no

external load is connected to the power supply, some power needs to be processed in order

to sustain the operation of the VIPER17L and the circuitry at the secondary side, which

senses the output voltage and closes the control loop (see Figure 2). The burst mode

operation of the VIPER17L significantly reduces the average switching frequency of the

converter, when light loaded or completely no-loaded. Basically, when the converter

switches to burst mode operation, the lower the load, the lower the average switching

frequency. So, in order to reduce the average switching frequency to as low as possible, the

control loop should be designed to minimize the required power so as to sustain the

operation of the circuitry which closes the control loop at the secondary side.

1.1

Reduction of circuitry consumption at the secondary side

control loop

Figure 2.

Circuitry consumption reduction at the secondary side control loop

Usually, in an isolated flyback converter where the output is directly sensed for control loop

input, the TL431 is used, which is a device that contains an error amplifier and a voltage

reference. For a TL431 the minimum bias current is 1 mA. In order to guarantee this bias

current in every operating condition, a 1 k

Ω resistor is connected in parallel with the photo-

diode of the opto-coupler (R15, see Figure 2). If, instead of the TL431, a device with a lower

TS431

R8

R9

R10

C11

R13

OPTO

R15

C8

VIPER17LN FB PIN

VOUT