PRELIMINARY
Application Note
The Filtered, Autoranging Rectifier Module (FARM) provides an
effective solution for the AC front end of a power supply built
with Vicor DC-DC converters. This high performance power
system building block satisfies a broad spectrum of requirements
and agency standards.
In addition to providing transient/surge immunity and EMI
filtering, the FARM contains all of the power switching and
control circuitry necessary for autoranging rectification, inrush
current limiting, and overvoltage protection. This module also
provides converter enable and status functions for orderly power
up/down control or sequencing. To complete the AC front end
configuration, the user only needs to add hold-up capacitors, and
a few discrete components.
Functional Description
Power-Up Sequence.
(see Fig. 8):
1.1
Upon application of input power, the output bus capacitors
begin to charge. The thermistor limits the charge current,
and the exponential time constant is determined by the
hold-up capacitor value and the thermistor cold resistance.
The slope (dv/dt) of the capacitor voltage versus time
approaches zero as the capacitors become charged to the
peak of the AC line voltage.
The switch that bypasses the inrush limiting PTC (positive
temperature coefficient) thermistor is open when power is
applied, as is the switch that engages the strap for voltage
doubling.
(see Fig. 7).
In addition, the converter modules
are disabled via the Enable (EN) line, and Bus-OK (BOK)
is high.
2.1
If the bus voltage is less than 200V as the slope nears
zero, the voltage doubler is activated, and the bus voltage
climbs exponentially to twice the peak line voltage.
If the bus voltage is greater than 200V, the doubler is
not activated.
3.1
If the bus voltage is greater than 235V as the slope
approaches zero, the inrush limiting thermistor is
bypassed. Below 235V, it is not bypassed.
4.1
The converters are enabled 50 milliseconds after the
thermistor bypass switch is closed.
5.1
Bus-OK is asserted after an additional 50 millisecond
delay to allow the converter outputs to settle within
specification.
Power-Down Sequence.
(see Fig. 8)
When input power is
turned off or fails, the following sequence occurs as the bus
voltage decays:
1.2
Bus-OK is deasserted when the bus voltage falls below
210Vdc.
2.2
The converters are disabled when the bus voltage falls
below 190Vdc. If power is reapplied after the converters
are disabled, the entire power-up sequence is repeated. If
a momentary power interruption occurs and power is
reestablished before the bus reaches the disable threshold,
the power-up sequence is not repeated, i.e., the power
conversion system “rides through” the momentary
interruption.
+Out
90–132V
AC Line
Power
Up
Power
Down
PTC
Thermistor
L
Strap
Strap
Output
Bus
(Vdc)
Strap
PTC
Thermistor
Bypass
400
300
200
100
0
1.1
2.1
–Out
N
EN
Microcontroller
BOK
3.1
4.1
5.1
2.2
1.2
Converter
Enable
Bus OK
50ms
50ms
Figure 7—Functional
block diagram: autoranging rectifier
Figure 8—Timing
diagram: power up/down sequence
Vicor Corp.
Tel: 800-735-6200, 978-470-2900 Fax: 978-475-6715
FARM, Filter/Autoranging Rectifier Module
Rev. 4.7
Page 5 of 12
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