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EN 152
9.
EM6E AB
9.3.3
DC/DC Converter (diagram B12)
A DC/DC converter has the following advantages:
•
The DC/DC converter is directly on the SSB near the
circuits that needs to be powered with the 3.3 V.
•
Circuits in the FBX and the DW part need some high
current by low voltage, so, there is no risk to have power
dips or voltage loss in connections between the LSP and
the SSB panel.
The circuit used is a so-called "synchronous buck converter".
Some characteristics:
•
Input parameters: 12.5 V / 330 mA.
•
Output parameters: 3.42 V ± 0.15 V / 1.2 A.
•
Switching frequency: approx. 60 kHz.
•
Efficiency: approx. 90%.
•
Inhibit (+5V) and fast turn-off (3V3_FAULT) control inputs.
Block diagram
T 800mA
12.5V
Voltage booster
+5V
G
2
PWM generator
and drivers
G
1
Voltage control feedback
Standby current reduction
Line feed-forward
+5V
12V
3V3_FAULT
1
Figure 9-5 Block diagram DC/DC converter.
Advantage to use a "synchronous buck converter" instead of a
"classical buck converter" is a better efficiency (about 90%).
The difference between the two is that in a synchronous buck
converter the "low -side" diode is replaced by a MOSFET (item
7U03). This, because the resultant voltage drop across the
MOSFET is smaller than the forward voltage drop of the diode.
This second MOSFET conducts current during the "off" times
of the first MOSFET (at the input side). The upper MOSFET
conducts to transfers energy from the input to the inductor and
load, while the lower MOSFET conducts to circulate the
inductor current. The synchronous PWM control block
regulates the output voltage by modulating the conduction
intervals of the upper and lower MOSFETs.
PWM Generator and Drivers
This circuit is build around transistors 7U01, 7U02, 7U08,
7U09, and the associated components. MOSFETs T7U04 and
T7U03 are the switching transistors, they are conducting
alternatively.
•
Time sequence 1: T7U04 is conducting; energy is stored in
coil 5U01. The current is flowing from the +11V power
supply source.
•
Time sequence 2: T7U04 is blocked; energy is stored in
coil 5U01.
•
Time sequence 3: T7U03 is conducting, and the current
circuit is now closed via T7U03, Coil 5U01, C2U08, and the
load. So the energy stored in the coil during time sequence
T1 is consumed during sequence T3. The signal on the
gate T7U03 is 180 degrees turned compared with the
signal on the gate T7U04.
The principle of operation of the PWM generators and drivers
is an astable circuit, whose frequency is mainly given by the
values of capacitor 2U32 and resistor 3U03. Capacitors 2U05,
2U27, and 2U28 form a capacitive voltage divider to limit the
Circuit Descriptions, Abbreviation List, and IC Data Sheets
5U05
+
-
D
10uH
7U04
2U23
S
3
4
100uF
5U01
+
-
47uH
5U06
2U08
+
-
D
470uF
10uH
7U03
S
2U31
100uF
12V
12V
1A
CL 36532008_149.eps
2
3
4
negative peak voltage on the base of transistor 7U09 at about
3 V. The same role have the capacitors 2U09 and 2U12 for the
transistor 7U08.
The resistor 3U14 and the capacitor 2U30 introduce a small
dead time between switching "off" of transistor 7U03 and
switching "on" of transistor 7U04 to prevent cross-conduction
through them.
The capacitors 2U07, 2U16, and 2U18 speed-up the switching
of the astable circuit. This circuit is blocked in "off" state (7U02
and 7U08 saturated and 7U01 and 7U09 blocked) when the
transistor 7U11 is conducting.
This happens when either +5V has a low value (below 2 V), the
boost voltage drops below its minimum value, or the
3V3_FAULT signal is low (below 1 V) for more than 20 ...30 ms.
Voltage Booster
This circuit is build around capacitors 2U03, 2U04, 2U05,
resistor 3U51, and diodes 6U03 and 6U04.
It generates the boost voltage (approx. +20 V) for supplying the
"high-side" driver transistor for the power MOS-FET 7U04. The
voltage is generated only during normal operation of the
converter; therefore, any drop in its value means an internal
fault condition, which is sensed by the internal protection
circuit.
+3V3_SIM
The A.C. component of the voltage on the source of transistor
7U04 is rectified by diodes 6U04 and 6U03 and added to the
input voltage, resulting into the boost voltage. The resistor
+3V3_DCDCFBX
3U51 limits the peak current through the rectifier diodes to
about 300 mA.
Voltage Control Feedback
This circuit keeps the output voltage constant when the input
voltage and/or the output current vary. It is build around
transistors T7U06-A, T7U06-B, T7U05, diode D6U07, and
associated components.
A 6.9 V reference voltage is provided by zener diode D6U07
120503
(via T7U15). Transistors T7U05 and T7U06A are comparators.
E.g. if the voltage on the base of T7U06-B increases, T7U06-B
and T7U08 will conduct more, and via T7U08 the conducting
time of T7U04 will be reduced, resulting in a decreasing output
voltage.
Linefeed Forward
This circuit (resistor 3U46) improves the line regulation factor.
Stand-by Current Reduction
This circuit is build around transistors 7U14 and 7U15. It cuts
the quiescent current through the reference zener diode 6U07
in stand-by mode.
Adapter
This circuit is necessary for having proper start-up/shut-down
operation of the converter at various shapes of the +5V voltage.
Fault detection
In "on" mode, transistors 7U12, 7U10, 7U07, and 7U13 are
conducting and the transistor 7U11 is blocked.
In protection mode, transistors 7U11, 7U07, and 7U13 are
conducting (provided that the "+5V" voltage is higher than 4.5
V) and transistors 7U10 and 7U12 are blocked.
During start-up, the protections are bypassed for a short time
(tenths of ms) by capacitors 2U22 and 2U20.
+20V Detection
This circuit detects the boost voltage, and is build around
transistor 7U12, diode 6U10, and associated components. If
the boost voltage is lower than 15 V, transistors 7U12 and
7U10 are blocked, 7U11 is conducting, so via diodes 6U08 and
6U09 the PWM and Driver circuit is switched "off".
It is also the case when the input voltage "+11V" is less than
about 10 V (the under-voltage lock-out function).
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