Application Note, continued
Hold-up Time
40
35
30
25
20
15
10
5
Power Fail
Warning
Ripple (V p-p)
π – θ
θ
254V
205V
185V
1300µF
1100µF
1600µF
820µF
2200µF (Arm-2)
(Arm-1)
Ride-Thru Time
680µF
Power
Fail
B OK
Converter
Shutdown
Figure 12—Hold-up time
The holdup time (∆t) is defined as the interval between power
fail warning (B OK) and converter shutdown (EN) as illustrated
in Fig. 12. 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:
0
250
500
750
1000
1250
1500
Operating Power (W)
Figure 13—Hold-up time vs. operating power and total bus
capacitance, series combination of C1, C2 (Fig. 8)
C = 2P∆t / (2052 – 1852)
C = 2P∆t / (7,800)
100
90
Total
80
70
60
50
40
30
20
10
0
capacitance
820µF
It should be noted that the series combination (C1, C2, Fig. 8)
requires each capacitor to be twice the calculated value,
but the required voltage rating is reduced to 200V.
90Vac
115Vac
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 many
Vicor converters is specified as a function of the input/output
voltage ratio:
250
500
750
1000
1250
1500
Operating Power (W)
R = 30 + 20log(Vin / Vout)
(4)
Figure 14—Ride-through time vs. operating power
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. 16) 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. 12) 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:
30
25
20
15
10
5
∆t = (π – θ) / 2πf
(5)
Where:
f = line frequency
(Ver. 12)
(Ver. 22)
820µF
680µF
1100µF
1300µF
θ = rectifier conduction angle
1600µF
2200µF
0
250
500
750
1000
1250
1500
The approximate conduction angle is given by:
Operating Power (W)
θ = Cos-1V2/V1
(6)
Figure 15—Ripple voltage vs. operating power and bus
capacitance, series combination of C1, C2 (see Fig. 8)
Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
ARM, Autoranging Rectifier Module
Rev. 2.4
Page 8 of 12
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