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EN1M11 Datasheet PDF : 12 Pages
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Application Note (continued)
PRELIMINARY
Off-Line Power Supply Configuration
The ENMods system maintains the DC output bus voltage
between 250 and 370Vdc over the entire input voltage range,
which is compatible with all Vicor 300V input converters.
Autoranging automatically switches to the proper bridge or
doubler mode at startup depending on the input voltage,
eliminating the possibility of damage due to improper line
connection. The ENMods system is rated at 575W output power.
These modules can serve as the AC front end for any number
and combination of compatible converters as long as the
maximum power rating is not exceeded.
Pin Descriptions (see Figure 1a)
Strap (ST) Pin. In addition to input and output power pin
connections, it is necessary to connect the Strap pin to the center
junction of the series hold-up capacitors (C1, C2) for proper
(autoranging) operation. Varistors V1 and V2 provide capacitor
protection. The bleeder resistors (R1, R2) discharge the hold-up
capacitors when power is switched off. Capacitors C7 and C8
are recommended if the hold-up capacitors are located more
than 3 inches from the output pins.
Enable (EN) Pin. The Enable pin must be connected to the PC or
Gate-In pin of all converter modules to disable the converters
during power-up. Otherwise, the converters would attempt to start
while the hold-up capacitors are being charged through the current
limiting thermistor, preventing the bus voltage from reaching the
thermistor bypass threshold, thus disabling the power supply. The
Enable output (the drain of an N channel MOSFET) is internally
pulled up to 15V through a 150kΩ resistor. (see Figure 11)
A signal diode should be placed close to and in series with the
PC or (Gate-In) pin of each converter to eliminate the
possibility of control interference between converters.
The Enable pin switches to the high state (15V) with respect
to the SR pin to turn on the converters after the power-up inrush
is over. The Enable function also provides input overvoltage
protection for the converters by turning off the converters if the
DC bus voltage exceeds 400Vdc. The thermistor bypass switch
opens if this condition occurs, placing the thermistor in series
with the input voltage, which reduces the bus voltage to a safe
level while limiting input current in case the varistors conduct.
The thermistor bypass switch also opens if a fault or overload
reduces the bus voltage to less than 180Vdc. (see Figure 9)
Bus-OK (BOK) Pin. (see Figure 12) The Bus-OK pin is
intended to provide early-warning power fail information and is
also referenced to the SR pin.
Caution: There is no input to output isolation in the ENMods. 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 destroy the unit.
L, N Pins. Line and neutral input.
+, – Pins. Positive and negative outputs.
SR Pin. Signal return for BOK and EN outputs
Filter (see Figure 1b)
The input EMI filter consists of differential and common mode
chokes,Y– rated capacitors (line-ground) and X– rated
capacitors (line-line). This filter configuration provides
sufficient common mode and differential mode insertion loss in
the frequency range between 100kHz and 30MHz to comply
with the Level B conducted emissions limit, as illustrated in
Figures 8a thru 8c.
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 (see Figure 13). 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
in order to allow time to perform an orderly shutdown.
The energy stored on a capacitor which has been charged to
voltage V is:
ε = 1/2(CV2)
(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:
Where:
ε = P∆t = C(V12–V22) / 2
(2)
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 / (V12–V22)
(3)
Vicor Corp. Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
ENMods System: FARM3 and MiniHAM
Set your site on VICOR at www.vicorpower.com
Rev. 1.3
Page 8 of 12
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