DN455 - Low Profile Synchronous, 2-Phase Boost Converter Produces 200W with 98% Efficiency

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Low Profile Synchronous, 2-Phase Boost Converter Produces
200W with 98% Efficiency – Design Note 455
by Victor Khasiev
Introduction
Automotive audio amplifiers require a high power boost
converter that is both efficient and compact. High
efficiency is essential to keep dissipated heat low and
avoid bulky and expensive heat sinks. The LT3782A
is a 2- phase synchronous PWM controller, making
it possible to produce a low profile, high power boost
supply that achieves 98% efficiency.
Output power is 200W continuous and 250W for short
pulse loads, corresponding to 8.5A continuous current
and 10.5A pulsed current.
This circuit comprises three major sections. Two are
the phase-interleaved power trains, and the third is
the control circuit.
Each power train includes an inductor, two switching
MOSFETs, a synchronous MOSFET and an output
capacitive filter. The output filters are connected
together in parallel. Schottky diodes D1 and D2 increase
efficiency during the dead time.
A 24V Output Boost Converter at 8.5A
(Continuous), 10.5A (Peak) from a Car Battery
Figure 1 shows a boost converter that generates
24V from an input voltage range of 8.5V to 18V.
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5V
8.5V to 18V
VIN
1
1μF
Q7
2
5.6V
SGATE1
10μF
×5
SGATE2
SGATE1 2
SGATE1
3
GND
4
GND
10Ω
5
SYNC
6
DELAY
7
DCL
8
SENSE1+
2.2nF
SENSE1–
9
SENSE1–
10
SLOPE
11
RSET
–
SENSE2
12
SENSE2 –
23.7k
13
SENSE2+
14
SS
2.2nF
60.4k
56.2k
0.1μF
330μF
×2
BST
GND
TG
IN
TS
5
VCC1 27
26
GND
25
GND
24
VEE1
23
BGATE1
22
GBIAS1
GBIAS2 21
BGATE2 20
19
VEE2
18
GND
17
RUN
FB 16
VC
D1
330μF
×2
1μF
4
LTC4440-5
VOUT 24V AT 8.5A
BG1
BG1
Q1
HAT2172H
Q2
0.006Ω
0.006Ω
SENSE1+
VIN
GBIAS
Q3
+
10μF
×3
6
28
2.2μF
SENSE1–
5V
HAT2166H
BG1
10μF
×3
Q6
2.2pF
1μF
BG2
U2
1
2
SGATE2 3
VCC
BST
GND
TG
IN
TS
1μF
6
D2
330μF
×2
+
5
4
LTC4440-5
RUN
VOUT
VIN
BG2
Q4
HAT2172H
15
475k
LT3782A
825k
RUN
15k
220pF
10Ω
VCC
L2
8.3μH
U1
SGATE2 1
22pF
3
L1
8.3μH
+
SENSE1+
HAT2166H
U3
2k
6.8nF
53.6k
402k
1nF
SENSE2
BG2
Q5
+
0.006Ω
0.006Ω
SENSE2 –
SENSE2 +
dn4vk F01
Figure 1. Synchronous Boost Converter Based on the LT3782A (VOUT = 24V at 8.5A, VIN = 8.5V to 18V)
11/08/455
The centerpiece of the control circuit is the LT3782A.
This 2-phase PWM controller features low side gate
signals and corresponding synchronous signals for
high side gate control. Control signals are interleaved
by running the two stages 180° out-of-phase. The
2-phase approach with accurate channel-to-channel
current sharing minimizes electrical and thermal stress
on power train components, and reduces EMI. Using
the LTC4440-5 as a high side driver allows for high
frequency switching.
Performance Results
This converter aims for high efficiency and low profile,
and it succeeds in reaching both goals with efficiency
reaching 98% (Figure 2) and a maximum component
height of 10.5mm. Output voltage regulation over the
full input voltage and output current ranges is better
than 2%. Figure 3 shows the transient response with
a 3A step load.
Basic Calculations and Component Selection
This section shows how to make a preliminary selection
of inductors and MOSFETs. Detailed calculations of
the losses and converter efficiency evaluation can be
found in Robert W. Erickson’s Fundamentals of Power
Electronics, 2nd edition.
For CCM operation, the maximum duty cycle at low line
can be found from the following expression:
DMAX =
VOUT − VIN(MIN)
VOUT
Average inductor current and peak current can be
calculated as follows:
IL(AVG) =
IOUT
ΔI
;I
=I
+
2 •(1− DMAX )• η L(PEAK) L(AVG) 2
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Linear Technology Corporation
The peak current through the switching MOSFET is
equal to IL(PEAK), and the RMS value of the MOSFET
current is:
2
1 I ISW(RMS) = IL(AVG) • DMAX • 1+ • 3 IL(AVG) The peak current through the synchronous MOSFET is
equal to IL(PEAK), and the RMS MOSFET current value is:
ISR(RMS) = IL(AVG) • 1 DMAX
2
1 I • 1+ • 3 IL(AVG) The MOSFETs should be rated to handle the output
voltage plus 20% to 30% of headroom.
98.5
18VIN
98.0
15VIN
97.5
EFFICIENCY (%)
The control circuitry can be further divided into three
parts: the PWM functionality based on the LTC3782A
(U1) and two high side drivers around the LTC44405 (U2 and U3). A linear pre regulator based on Q7
generates the required bias voltage for U2 and U3.
This approach allows the use of logic-level MOSFETs
to minimize gate losses.
12VIN
97.0
10VIN
96.5
96.0
95.5
8.5VIN
95.0
94.5
2
3
4
5
6
IOUT (A)
7
8
9
dn4vk F02
Figure 2. Efficiency vs IOUT (from Circuit in Figure 1)
IOUT
2A/DIV
VOUT
50mV/DIV
VIN = 14V, VOUT = 24V
IOUT = 2A To 5A
dn4vk F03
Figure 3. Transient Response of the Circuit in Figure 1 for
a 3A Loadstep
Conclusion
The LT3782A based 2-phase synchronous boost
converter provides high efficiency, excellent transient
response and excellent line and load regulation over a
wide input voltage range. High power, high efficiency and a
low component profile allow this converter to fit into tight
spaces commonly found in automotive environments.
For applications help,
call (408) 432-1900, Ext. 3161
dn455 LT/TP 1108 246K REV A • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2008
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