Vicor Micro Family of DC-DC Converter - Manual (Page 39)

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 38 of 88
Apps. Eng. 800 927.9474 800 735.6200
8. Filter / Autoranging Rectifier Module (FARM)
Bus OK (BOK) Pin. (Figure 8–5) The Bus OK pin is intended
to provide early-warning power-fail information and is also
referenced to the negative output pin.
CAUTION: There is no input-to-output isolation in
the FARM. It is necessary to monitor Bus OK via an
optoisolator if it is to be used on the secondary
(output) side of the converters. A line-isolation
transformer should be used when performing
scope measurements. Scope probes should never
be applied simultaneously to the input and output
as this will damage the module.
Filter. (Figure 8–6) An integral input filter consists of a
common-mode choke and Y-capacitors (line-ground) plus
two X-capacitors (line-line). This filter configuration
provides common-mode and differential- mode insertion
loss in the frequency range between 100 kHz and 30 MHz.
Hold-up Capacitors. Hold-up capacitor values should be
determined according to output bus voltage ripple, power
fail hold-up time, and ride-through time. (Figure 8–7)
Many applications require the power supply to maintain
output regulation during a momentary power failure of
specified duration, i.e., the converters must hold up or
ride through such an event while maintaining undisturbed
output voltage regulation. Similarly, many of these same
systems require notification of an impending power failure
to allow time to perform an orderly shut down.
The energy stored in a capacitor which has been charged
to voltage V is:
ε
= 1/2(CV 2 ) (1)
where:
ε
= stored energy
C = capacitance
V = voltage across the capacitor
Energy is given up by the capacitors as they are
discharged by the converters. The energy expended
(the power-time product) is:
ε
= P
∆t = C(V
1 2
– V
22
) / 2 (2)
where: P = operating power
∆t = discharge interval
V1 = capacitor voltage at the beginning of
∆t
V2 = capacitor voltage at the end of
∆t
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 Bus OK and converter shut down (EN) as illustrated
in Figure 8–7. 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 8–3) requires each capacitor to be twice
the calculated value, but the required voltage rating
of each capacitor is reduced to 200 V. 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.
For example, a converter whose output is 15 V and
nominal input is 300 V will provide 56 dB ripple rejection,
i.e., 10 V p-p of input ripple will produce 15 mV p-p of
output ripple. (Figure 8–11) 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 8–7) 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 8–7)
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 = total output power
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
8–8, 8–9, and 8–10, respectively.