Datasheet

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. Ta b l e 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
(V
AUX
column) and transformer design.
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
V
OUT
(V)
nominal value
R5 (kΩ)R6 (kΩ)
V
OUT
(V)
no load
V
OUT
(V)
full load
I
OUT
(A)
at load
V
AUX
(V)
P
IN
(mW)
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