Boost 12V to 140V with a Single Converter IC

Boost 12V to 140V with a Single Converter IC
Victor Khasiev
Generating a high voltage from a much lower voltage
presents a number of challenges for the classical single
stage boost topology. For instance, the maximum
duty cycle limitation of a boost controller may not
allow the required step-up ratio. Even if it does, there
is often a sharp decrease in efficiency at high duty
cycles. The duty cycle can be shortened by choosing
discontinuous mode of operation, but this leads to high
peak input current, higher losses and EMI challenges.
An alternative to a single boost converter
is a 2-stage boost converter, where the
first stage produces an intermediate
voltage and the second stage boosts
to the final high voltage. A 2-stage
converter can be produced with a single
controller IC, such as the LTC3788, a
high performance 2-phase dual output
synchronous boost controller, which
drives all N‑channel power MOSFETs.
The LTC3788 can be configured such that
the first boost stage takes advantage of its
synchronous rectification feature, which
maximizes efficiency, reduces power losses
Figure 1. Block diagram of LTC3788-based 2-stage boost converter
CINT
40V ABS MAX
TG1
SW1
PGOOD1
ILIM
SS1
ITH1
FB1
SENSE1+
Q1
RS1
Q2
SENSE1–
VIN
GND
BOOST1
FREQ
BG1
PHASMD
VBIAS
CLKOUT
PGND
LTC3788
BG2
RUN2
BOOST2
SS2
ITH2
FB2
SENSE2
TG2
RUN1
SW2
INTVCC 5V
PGOOD2
SGND
+
EXTVCC
SENSE2–
PLLIN/MODE
GND
L1
U1
30 | November 2015 : LT Journal of Analog Innovation
VOUT
COUT
D1
L2
DR
7V TO 10V
MOSFET
DRIVER
Q3
GND
RS2
GND
and eases thermal requirements. The
maximum output voltage of this controller
is 60V, when using synchronous rectification. If greater than 60V is required,
the second stage can be designed to run
non-synchronously, as described below.
2-STAGE BOOST PRODUCES 140V
FROM 12V
The block diagram in Figure 1 shows
the LTC3788 in a 2-stage boost configuration. This block diagram also reveals a
few caveats that must be observed in this
design:
•The output of the first stage (Q1,
CINT) is connected to the input of
second stage (RS2, L2). The output
of the first stage should not exceed
40V, because the maximum absolute
rating of the SENSE pins is 40V.
•The gate drive voltage of 5V is suitable
for logic level MOSFETs, but not for high
voltage standard MOSFETs, with typical
gate voltages of 7V to 12V. The external
gate driver DR, controlled by the BG2
signal can be used as shown here to
drive high voltage standard MOSFETs.
•To generate an output voltage
above maximum limit of 60V, the
synchronous rectification MOSFET
is replaced by a single diode D1.
Figure 2 shows the complete solution.
Transistors Q1, Q2 and inductor L1
compose the first stage, which generates an intermediate bus voltage of 38V.
The first stage employs synchronous
rectification for maximum efficiency.
The output of the first stage is connected
as input to the second stage, comprised
design ideas
12.1k
V_INT
2.21k
V_INT
PLLIN/MODE
EXTVCC
SGND
INTVCC
RUN1
BG2
SW2
SS2
PGOOD2
ITH2
FB2
SENSE2+
SENSE2–
GND
12.1k
DRIVER BIAS
CIRCUIT
5.1k
Q4
MMBTA42LT1G
8.2V
SNS2+
SNS2–
1μF
100pF
2.7k
6.98k
+
Q2
BSC028N06LS3G
VIN
82μF/50V
50HVH82M
GND
V_INT
VOUT 140V AT 1.0A
2x0.47μF/450V
C4532X7T2W474M
+
22μF/200V
EEVEB2D220SQ
VOUT
GND
D1
SBR10U200P5130
0.1μF
15nF
VIN 3V TO 36V
4x4.7μF
4.7μF
BOOST2
RUN2
309k
SENSE1– SENSE1+
L1, 6.8μF
SER2915H682 RS1, 0.002Ω
0.1μF
PGND
LTC3788EUH
82μF/50V
50HVH82M
Q1
BSC067N06LS3G
VBIAS
VBIAS
CLKOUT
VBIAS
BAS140W
BG1
PHASMD
+
4x4.7μF
0.1μF
BOOST1
FREQ
42.2k
TG1
SW1
ILIM
SS1
ITH1
SENSE1–
U1
TG2
SNS1–
FB1
SENSE1+
0.1μF
INTERMEDIATE BUS
15nF
PGOOD1
8.66k
SNS1+
100pF
374k
U2
VBIAS
LTC4440
BST
VCC
GND
TG
IN
TS
L2, 100μH
PCV210405L
SENSE2–
SENSE2+
RS2, 0.01Ω
Q3
BSC320N20NS3
806k
Figure 2. Full schematic of 2-stage 140V output, 1A boost converter
of Q3, D1, L2. The output of the
second stage produces 140V at 1A.
Transistor Q3 is a standard level
MOSFET, driven by the LTC4440.
Here, an LDO, based on transistor Q4,
biases the gate driver, but a switching
94
regulator can be employed instead
(such as one built around the LTC3536)
to further increase overall efficiency.
This solution features an input voltage
range from 3V to 36V, nominal 12V. To
decrease components’ thermal stress, the
VOUT = 140V
EFFICIENCY (%)
CONCLUSION
VIN = 12V
IOUT = 1A
92
LTC3788 is a high performance 2-phase
dual output synchronous boost controller, suitable for high power, high voltage
applications. Its dual outputs can be
used in tandem to achieve extremely
high step-up ratios to high voltages. n
90
VOUT
20V/DIV
88
86
84
output current should be reduced
when the input voltages fall below 10V.
Figure 3 shows measured efficiency,
and Figure 4 shows the start-up waveforms. A 93% efficiency is shown with
VIN = 24V and with the 140V output
loaded from 0.4A to 1A. This converter
can operate at full load with no airflow.
VIN = 24V
VIN = 12V
VIN = 8V
0
0.2
0.6
0.4
LOAD CURRENT (A)
0.8
Figure 3. Efficiency of the 2-stage
converter in Figure 2
1
5ms/DIV
Figure 4. Start-up waveforms
November 2015 : LT Journal of Analog Innovation | 31
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