Vicor Micro Family of DC-DC Converter - Manual (Page 37)

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 36 of 88
Apps. Eng. 800 927.9474 800 735.6200
8. Filter / Autoranging Rectifier Module (FARM)
The Filter / 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 (Figure 8–1)
Initial Condition.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. In addition, the
converters are disabled via the Enable (EN) line, and Bus
OK (BOK) is high.
POWER-UP SEQUENCE (Figure 8–2)
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. If the bus
voltage is less than 200 V as the slope nears zero,
the voltage doubler is activated, and the bus
voltage climbs exponentially to twice the peak
line voltage.
2.1 If the bus voltage is greater than 200 V, the
doubler is not activated.
3.1 If the bus voltage is greater than 235 V as
the slope approaches zero, the inrush limiting
thermistor is bypassed. Below 235 V, it is
not bypassed.
4.1 The converters are enabled ~150 ms after the
thermistor bypass switch is closed.
5.1 Bus OK is asserted after an additional ~150 ms
delay to allow the converter outputs to settle
within specification.
POWER-DOWN SEQUENCE (Figure 8–2)
When input power is turned off or fails, the following
sequence occurs as the bus voltage decays:
1.2 Bus OK is de-asserted when the bus voltage falls
below 205 Vdc (Typical).
2.2 The converters are disabled when the bus voltage
falls below 190 Vdc. 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 re-established
before the bus reaches the disable threshold, the
power-up sequence is not repeated, i.e., the power
supply “rides through” the momentary interruption.
Microcontroller
N
L
PTC
Thermistor
+OUT
–OUT
Strap
EN
BOK
Strap
EMI
Filter
EMI GRD
400
300
200
100
0
90–132 V
AC Line
Output
Bus
(Vdc)
Strap
PTC
Thermistor
Bypass
Converter
Enable
Bus OK
~150 ms
Power
Up
Power
Down
Timing Diagram, Power Up/Down Sequence
4.1
1.1
2.1
3.1
5.1
2.2
1.2
~150 ms
Figure 8–1 — Functional block diagram: FARM
Figure 8–2 — Timing diagram: power up/down sequence