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FARM Datasheet PDF : 12 Pages
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Application Note (continued)
PRELIMINARY
Ripple (V p-p) π – θ θ
Hold-up Time
Power Fail
Warning
Ride-Thru Time
254V
205V
185V
Power
Fail
Figure 13—Hold-up time
B OK
Converter
Shutdown
The power fail warning time (∆t) is defined as the interval
between B OK and converter shutdown (EN) as illustrated
in Fig. 13. The Bus-OK and Enable thresholds are 205V and
185V, respectively. A simplified relationship between
hold-up time, operating power, and bus capacitance is
obtained by inserting these constants:
C = 2P∆t / (2052 – 1852)
C = 2P∆t / (7,800)
It should be noted that the series combination (C1, C2,
see Fig. 9) requires each capacitor to be twice the calculated
value, but the required voltage rating of each capacitor is
reduced to 200V.
Allowable ripple voltage on the bus (or ripple current in the
capacitors) may define the capacitance requirement.
Consideration should be given to converter ripple rejection
and resulting output ripple voltage. The ripple rejection (R)
of Vicor converters is specified as a function of the
input/output voltage ratio:
R = 30 + 20log(Vin / Vout)
(4)
For example, a converter whose output is 15V and nominal
input is 300V will provide 56dB ripple rejection, i.e., 10V p-p
of input ripple will produce 15mV p-p of output ripple
(see Fig. 17). Equation 3 is again used to determine the
required capacitance. In this case, V1 and V2 are the
instantaneous values of bus voltage at the peaks and valleys
(see Fig. 13) of the ripple, respectively. The capacitors must
hold up the bus voltage for the time interval (∆t) between
peaks of the rectified line as given by:
∆t = (π – θ) / 2πf
(5)
Where: f = line frequency
θ = rectifier conduction angle
The approximate conduction angle is given by:
( ) θ = Cos-1 V2/V1
(6)
40
35
1300µF
1600µF
2200µF (FARM2XXX)
30
25
1100µF
* 820µF
680µF (FARM1XXX)
20
15
10
5
*
*
0
250
500
750
1000
Operating Power (W)
Figure 14—Hold-up time vs. operating power and total bus
capacitance, series combination of C1, C2 (see Fig. 9)
100
90
80
70
60
50
40
30
20
10
0
250
Total
capacitance
820µF
90Vac
115Vac
500
750
1000
Operating Power (W)
Figure 15—Ride-through time vs. operating power
30
*
25
20
*
15
10
5
1100µF
* 820µF
680µF (FARM1XXX)
0
1300µF
1600µF
2200µF (FARM2XXX)
250
500
750
1000
Operating Power (W)
Figure 16—Ripple voltage vs. operating power and bus
capacitance, series combination of C1, C2 (see Fig. 9)
Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
FARM, Filter/Autoranging Rectifier Module
Set your site on VICOR at www.vicorpower.com
Rev. 4.7
Page 8 of 12
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