Application recommendations

UM0984

22/25

Doc ID 17812 Rev 2

5

Application recommendations

5.1

Output voltage operating range

The demonstration board STEVAL-ISA081V1 was originally designed for output

specification 12 V / 1 A supplied from wide input range 85 to 305 VAC. With minimal

changes of components it is possible to set a different output voltage in the nominal output

voltage range of 10 to 18 V, keeping the output power capability. The only changes are in the

values of voltage divider R5, R6. The other components including the transformer remain

the same. Table 3 shows applicable values of the voltage divider for different nominal output

voltages. There is also visible an indicative value of output voltage at full and no load,

standby consumption and efficiency at full load measured on one sample.

The major limitation regarding the setting of the output voltages is the VIPer26 supply range

The supply voltage of VIPer26 has to be in the range of 11.5 to 23.5 V which consequently

limits the output voltage range using the same transformer.

There are theoretically several possible limiting factors caused by the transformer due

primarily to the turns ratio. The consequence of using a transformer designed for 10 V

output for an 18 V output voltage level is a higher voltage stress on the primary side switch.

Thanks to the VIPer26 which has implemented an 800 V MOSFET, there is still enough

margin even at 18 V on the output. The maximum drain source voltage measured at

305 VAC with 18 V set on the output was 700 V.

Table 3.

Settings and performance of demonstration board for different output

voltages

nominal value

R5 (k

Ω)R6 (kΩ)

no load

full load

at load

no load

Eff. (%)

10

150

56

10.90

9.70

1.20

12.2

100

83.0

12

150

43

13.10

11.80

1.00

15.0

112

84.1

15

150

33

15.90

14.75

0.80

18.6

170

83.9

18

91

16

18.90

17.80

0.67

21.8

270

83.7