AN1514 datasheet(10/17 Pages) STMICROELECTRONICS

10 / 17 page

VIPer application examples

AN1514

10/17

The two outputs are provided using a Buck-Boost converter for the -24 V output, named

"Vout1", and a coupled inductor in flyback mode for the -5 V output, named "Vout2". The

regulation feedback is connected to "Vout1" as well as the supply circuit of the Viper22A-E.

Doing so, only one high voltage diode and one capacitor are needed, i.e. D2 and C3 in

Figure 7, reducing the complexity and the cost of the circuit.

The output inductor, L, has two coupled windings on the same ferrite core, with a proper turn

ratio and coupling factor in order to get the correct output voltage. In particular, 1 mH

inductor "PANASONIC ELC08D102E" is used with a second winding (N2=45 turns) in order

to obtain the secondary output.

Zener diode Dz2 protects the Out2 against overvoltage, but this protection is not needed in

Out1. A burden resistor (Rb) is connected across Vout1 in order to perform the regulation on

Vout2 when Vout1 is in open load condition. The output rectifier diodes are both ultrafast

diodes: D3 is a high voltage diode since it has to sustain a reverse voltage given by the input

DC bus voltage while D4 is a low voltage diode. The part list of the proposed circuit is given

in Table 6.

Figure 7.

Converter schematic

Table 5.

Buck-boost converter specifications

Parameter

Value

AC input voltage Vinac

80 - 285 Vac

Output current Iout1

250 mA

Output current Iout2

70 mA

Output voltage Vout1

-24±10%V

Output voltage Vout2

-5 V±5%

Switching frequency

60 kHz

Output power

~ 7 W

-E

AN1514 Datasheet(PDF) 10 Page - STMicroelectronics