Vicor Micro Family of DC-DC Converter - Manual (Page 45)

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 44 of 88
Apps. Eng. 800 927.9474 800 735.6200
FILTERING AND TRANSIENT PROTECTION
The ENMod system maintains the DC output bus voltage
between 250 and 370 Vdc over the entire input-voltage
range, which is compatible with all Vicor 300 V input con-
verters. 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 ENMod system is rated at 575 W 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.
FARM3 MODULE PIN DESCRIPTIONS
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 capaci-
tors (C1, C2) for proper (autoranging) operation.
Varistors V1 and V2 provide capacitor protection. The
bleeder resistors (R1, R2) discharge the hold-up capaci-
tors when power is switched off. Capacitors C7 and C8
are recommended if the hold-up capacitors are located
more than 3 inches (75 mm) from the output pins.
Enable (EN) Pin. The Enable pin must be connected to
the PC pin of all converter modules to disable the con-
verters during power up. Otherwise, the converters
would attempt to start while the hold-up capacitors
are being charged through the current limiting thermis-
tor, preventing the bus voltage from reaching the ther-
mistor bypass threshold, thus disabling the power
supply. The Enable output (the drain of an N channel
MOSFET) is internally pulled up to 15 V through a
150 k
Ω resistor. (Figure 9–11)
A signal diode should be placed close to and in series
with the PC pin of each converter to eliminate the pos-
sibility of control interference between converters. The
Enable pin switches to the high state (15 V) 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 convert-
ers by turning off the converters if the DC bus voltage
exceeds 400 Vdc. The thermistor bypass switch opens
if this condition occurs, placing the thermistor in series
with the input voltage, reducing 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 180 Vdc. (Figure 9–3)
CAUTION: There is no input to output isolation in
the ENMods, hence the –Out of the ENMods and thus
the –In of the downstream DC-DC converter(s) are at
a high potential. If it is necessary to provide an
external enable / disable function by controlling the
DC-DC converter’s PC pin (referenced to the –In) of
the converter an opto-isolator or isolated relay
should be employed.
9. Modular AC Front-end System (ENMod)
Figure 9–10a — Peak detection
Figure 9–10b — Quasi-peak detection
Figure 9–10c — Average detection