Vicor PFC MegaPAC-HPEL Power Factor Corrected AC-DC Switchers - Manual (Page 21)

UG:119 vicorpower.com Applications Engineering: 800 927.9474 Page 21
Example:
±10% Trim adjust on a 12 V nominal output.
Figure 11 shows a typical variable Trim circuit. Using a 10 k trimpot (R7), the resistor
values for R6 and R8 can be calculated as follows:
V1= V
ref
+ 10% = 2.75 V Given: V
ref
= 2.5 V (see Table 1)
I
R5
= (2.75 V - V
ref
)/R
TH
= (2.75 V - 2.5 V)/10 kW = 25 mA
Setting the bottom limit:
V
R6
= 2.5 V - 10% = 2.25 V
And since I
R5
= I
R6
= 25 mA,
R6 = V
R6
/I
R6
= 2.25 V/25 mA = 90 kW
V
2
= V
1
+ V
R6
= 2.75 V + 2.25 V = 5 V
I
R7
= V
2
/R7 = 5 V/10 kW = 500 mA
I
R8
= I
R7
+ I
R6
= 525 mA
V
R8
= (V
nom
+10%) - V
2
= 13.2 V - 5 V = 8.2 V Given: V
nom
= 12 V
R8 = V
R8
/I
R8
= 8.2 V/525 mA = 15.62 kW
Using the above resistor combination, a 12 V output can be trimmed externally up
to 13.2 V and down to 10.8 V. For further information on external trimming, refer to
Chapter 5 of the VI-200/VI-J00 Design Guide and Applications Manual or consult the
factory for assistance.
CONSULT APPLICATIONS ENGINEERING WHEN TRIMMING OUTPUTS BELOW 5 V.