Application Note, continued
Hold-up Time
Ripple (V p-p)
π
–
θ
θ
Power Fail
Warning
40
AC Fail Warning Time (ms)
254V
205V
185V
Ride-Thru Time
35
30
1300µF
25
1100µF
20
15
10
5
0
250
820µF
1600µF
2200µF
(Arm-2)
*
680µF
(Arm-1)
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:
C = 2P∆t / (205
2
– 185
2
)
C = 2P∆t / (7,800)
*
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)
100
90
Total
capacitance
820µF
90Vac
60
50
40
30
20
10
0
250
115Vac
Hold up Time (ms)
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.
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:
R = 30 + 20log(Vin / Vout)
(4)
80
70
500
750
1000
1250
1500
Operating Power (W)
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, V
1
and V
2
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:
∆t
= (π –
θ)
/ 2πf
Where:
f = line frequency
θ
= rectifier conduction angle
(5)
30
25
*
*
P-P Ripple Volts (V)
20
15
10
5
1100µF
1300µF
820µF
1600µF
1000
680µF
(Ver. 12)
*
2200µF
(Ver. 22)
1250
1500
0
250
500
750
The approximate conduction angle is given by:
θ
= Cos
-1
V
2
/V
1
(6)
Operating Power (W)
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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