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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Application Note, continued

*

Operating Power (W)

0

5

10

15

20

25

30

35

40

2200µF

1600µF

1300µF

1500

1250

1000

750

500

250

1100µF

820µF

680µF

*

*

(Arm-2)

(Arm-1)

Figure 13—Hold-up time vs. operating power and total bus

capacitance, series combination of C1, C2 (Fig. 8)

Operating Power (W)

0

5

10

15

20

25

30

2200µF

1600µF

1300µF

1500

1250

1000

750

500

250

*

1100µF

820µF

680µF

*

*

(Ver. 12)

(Ver. 22)

Figure 15—Ripple voltage vs. operating power and bus

capacitance, series combination of C1, C2 (see Fig. 8)

Operating Power (W)

0

10

20

30

40

50

60

70

80

90

100

90Vac

115Vac

1500

1250

1000

750

500

250

Total

capacitance

820µF

Figure 14—Ride-through time vs. operating power

Figure 12—Hold-up time

π – θθ

205V

185V

Power

Fail

Power Fail

Warning

B OK

Converter

Shutdown

Hold-up Time

254V

Ride-Thru Time

Ripple (V p-p)

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

C = 2P

∆t / (7,800)

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)

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

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

(5)

Where:

f = line frequency

θ = rectifier conduction angle

The approximate conduction angle is given by:

θ = Cos

(6)