Autoranging rectifier module (arm) – Vicor Micro Family of DC-DC Converter User Manual
Page 36
Design Guide & Applications Manual
For Maxi, Mini, Micro Family DC-DC Converters and Configurable Power Supplies
Maxi, Mini, Micro Design Guide
Rev 4.9
vicorpower.com
Page 35 of 88
Apps. Eng. 800 927.9474
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7. Autoranging Rectifier Module (ARM)
Rearranging equation 2 to solve for the required capacitance:
C = 2P
∆t / (V
12
–V
22
)
(3)
The power fail warning time (
∆t) is defined as the interval
between (BOK) and converter shutdown (EN) as illustrated
in Figure 7–7. The Bus OK and Enable thresholds are 205 V
and 190 V, respectively. A simplified relationship between
power fail warning time, operating power, and bus capac-
itance is obtained by inserting these constants:
C = 2P
∆t / (205
2
– 190
2
)
C = 2P
∆t / (5,925)
It should be noted that the series combination (C1, C2,
Figure 7–3) requires each capacitor to be twice the
calculated value, but the required voltage rating is
reduced to 200 V.
Allowable ripple voltage on the bus (or ripple current in
the capacitors) may define the capacitance requirement.
Consideration should also be given to converter ripple
rejection and resulting output ripple voltage.
For example, a converter whose output is 15 V and nominal
input is 300 V will provide typically 56 dB ripple rejection,
i.e., 10 V p-p of input ripple will produce 15 mV p-p of
output ripple. (Figure 7–11) 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 (Figure 7–7) 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
(4)
where: f = line frequency
θ = rectifier conduction angle
(Figure 7–7)
The approximate conduction angle is given by:
θ = Cos
-1
V
2
/ V
1
(5)
Another consideration in hold-up capacitor selection is
their ripple current rating. The capacitors’ rating must be
higher than the maximum operating ripple current.
The approximate operating ripple current (rms) is given by:
Irms = 2P / Vac
(6)
where: P = operating power level
Vac = operating line voltage
Calculated values of bus capacitance for various hold-up
time, ride-through time, and ripple voltage requirements
are given as a function of operating power level in Figures
7–8, 7–9, and 7–10, respectively.
EXAMPLE
In this example, the output required at the point of load
is 12 Vdc at 320 W. Therefore, the output power from the
ARM would be 375 W (assuming a converter efficiency of
85%). The desired hold-up time is at least 9 ms over an
input range of 90 to 264 Vac.
Determining Required Capacitance for Power Fail
Warning. Figure 7–8 is used to determine capacitance
for a given power fail warning time and power level,
and shows that the total bus capacitance must be at
least 820 µF. Since two capacitors are configured in
series, each capacitor must be at least 1,640 µF.
NOTE: The warning time is not dependent on line
voltage. A hold-up capacitor calculator is available
on the Vicor website, at
Determining Ride-through Time. Figure 7–9 illustrates
ride-through time as a function of line voltage and output
power, and shows that at a nominal line of 115 Vac, ride-
through would be 68 ms. Ride-through time is a function
of line voltage.
Determining Ripple Voltage on the Hold-up
Capacitors. Figure 7–10 is used to determine ripple
voltage as a function of operating power and bus
capacitance, and shows that the ripple voltage across
the hold-up capacitors will be 12 Vac.
Determining the Ripple on the Output of the DC-DC
Converter. Figure 7–11 is used to determine the ripple
rejection of the DC-DC converter and indicates a ripple
rejection of approximately 60 dB for a 12 V output. If
the ripple on the bus voltage is 12 Vac and the ripple
rejection of the converter is 60 dB, the output ripple of
the converter due to ripple on its input (primarily 120 Hz)
will be 12 mV p-p.