AN2961

Circuit description

Doc ID 15559 Rev 1

7/14

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Circuit description

The demonstration board is based on a non-isolated buck converter designed and

implemented with STMicroelectronics’ VIPER22A-E low power offline SMPS primary

switcher.

AC input section

The input section for AC voltage is composed of a fuse resistor R7, a bridge rectifier D3 and,

in order to reduce the conduced EMI, an input filter consisting of C7, L1 and C3.

Buck converter

The buck converter is based on the VIPER22A-E, which integrates a power MOSFET with

the logic parts, and is composed of the VIPER22A-E device, freewheeling diode D4, main

inductor L2, and a circuit to supply the VIPER22A-E composed of a D2 diode and C1

capacitor.

The buck converter works in continuous conduction mode with a frequency of 60 kHz.

Constant current circuit

Current feedback is provided by sensing the voltage drop across R5. This voltage is filtered

by R4 and C6. When the voltage drop exceeds the VBE of the NPN transistor, both Q2 and

Q1 turn on, adding additional current on the FB pin of the VIPER22A-E, and causing the

device to switch off.

In this way the average inductor current is controlled simply and accurately. Capacitor C5 is

the filter for the output current.

Resistor R6 provides a minimum load to ensure correct operation at zero load.