PRELIMINARY
Application Note (continued)
Hold-up Time
Ripple (V p-p)
π
–
θ
θ
Power Fail
Warning
40
254V
205V
185V
35
Power Fail Warning (ms)
1300µF
30
1100µF
25
20
15
10
5
0
250
1600µF
820µF
2200µF
(FARM2XXX)
Ride-Thru Time
*
680µF
(FARM1XXX)
Power
Fail
B OK
Converter
Shutdown
Figure 13—Hold-up
time
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 / (205
2
– 185
2
)
C = 2P∆t / (7,800)
*
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
Ride-through Time (ms)
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)
90
80
70
60
50
40
30
20
10
0
250
Total
capacitance
820µF
90Vac
115Vac
500
750
1000
Operating Power (W)
P-P Ripple Volts (V)
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, V
1
and V
2
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
Where:
f = line frequency
θ
= rectifier conduction angle
(5)
Figure 15—Ride-through
time vs. operating power
30
25
*
*
20
15
10
5
1100µF
820µF
1600µF
1300µF
500
The approximate conduction angle is given by:
θ
= Cos
-1
(
V
2
/V
1
)
(6)
0
250
680µF
*
2200µF
750
(FARM1XXX)
(FARM2XXX)
1000
Operating Power (W)
Figure 16—Ripple
voltage vs. operating power and bus
capacitance, series combination of C1, C2 (see Fig. 9)
FARM, Filter/Autoranging Rectifier Module
Rev. 4.7
Page 8 of 12
Vicor Corp.
Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
Set your site on VICOR at www.vicorpower.com
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