AN2934

Circuit description

Doc ID 15394 Rev 1

5/27

1

Circuit description

The VIPer17 has two switching frequencies: the VIPer17HN switches at 115 kHz and the

VIPer17LN at 60 kHz. The choice of frequency is left up to the designer. The "HN" version

makes the transformer smaller, whereas the "LN" version makes it easier to optimize EMI.

This document focuses on the VIPer17HN, switching at 115 Hz. The following description

refers to the circuit shown in Figure 1. The board has been designed so that it can be used

with either single or double outputs, the only difference being the extra output parts, the

transformer and the voltage divider for the feedback loop. For the board’s operation, we have

taken as example the dual output. The input is connected to the line input, which operates

from 85 to 264 Vac. It is fused for safety and has 6.8 ohms ¼ W. A carbon resistor is better

for inrush than carbon or metal film. If a surge is required, a 2 to 3 W wire-wound resistor

should be used to pass the 6 kV ring wave test. C1 is a 0.1 µf X capacitor and L1 forms an

EMI filter to reduce line-conducted emissions. BR1 is a bridge rectifier and the C3 filter is the

input line to a DC level.

The topology used is of a discontinuous flyback type with an isolated output. The regulation

comes from the output (as opposed to a stable TL431, which uses an optocoupler for

isolation) to feed the information back to the VIPer17HN.

Table 1.

Dual output board specifications

Parameter

Limits

Input voltage range

85 Vac to 264 Vac

Input frequency

47 Hz to 63 Hz

Temperature range

0° to 85° Celsius, 105°C possible

Output voltage and current # 1

5 V at 0.5 A

Output voltage and current # 2

12 V at 0.25 A

Load and cross regulation #1

+/-1%

Load and cross regulation #2

+/-10%

Output power

5.5 W

Line regulation

+/- 0.2%

Efficiency

80% typical at 12 V output

Safety

Overvoltage, overcurrent, brownout

EMI

EN55022 Class “B”

Table 2.

Single output board specifications

Parameter

Limits

Output voltage and current

12 V at 0.5 A, 6 W

Load regulation

+/- 1%