Hundreds of Watts, 60V In or Out: Synchronous 4-Switch Buck-Boost Converter is Easy to Parallel to Minimize Temperature Rise

Hundreds of Watts, 60V In or Out: Synchronous 4-Switch
Buck-Boost Converter is Easy to Parallel to Minimize
Temperature Rise
Keith Szolusha
The LT3790 is a 4-switch synchronous buck-boost
DC/DC converter that regulates both constant voltage
and constant current at up to 98.5% efficiency using
only a single inductor. It can deliver hundreds of watts
and features a 60V input and output rating, making it an
ideal DC/DC voltage regulator and battery charger when
both step-up and step-down conversion are needed.
A single LT3790 converter can deliver high
power due to its synchronous switching
topology, but eventually the switching
and/or conduction losses at higher power
can overwhelm a single converter with
excessive board heating. Although heat can
VIN
8V TO 56V
RIN
1.5mΩ
4.7µF
100V
×2
INTVCC
VIN
1µF
TG1
499k
M1
0.1µF
M4
M2
L1
10µH
M3
SWI
EN/UVLO
BG1
OVLO
INTVCC
LT3790
200k
4.7µF
50V
×2
BG2
SW2
TG2
ISP
ISN
FB
CTRL
CSS
33nF
1000pF
RT
RC
15k
CC
10nF
SGND
147k
200kHz
VOUT
24V
5A (12A*)
71.5k
1.37k
SNSN
SS SYNC VC
28 | January 2015 : LT Journal of Analog Innovation
ROUT
8mΩ
PGND
PWM
100k
COUT
220µF
35V
×2
RSENSE
2mΩ
ISMON
CLKOUT
PWMOUT
VREF
0.1µF
+
SNSP
SHORT
C/10
CCM
IVINMON
33nF
Figure 1. 120W, 24V, 5A output
buck-boost voltage regulator with
8V–56V input has up to 98.5%
efficiency and is easy to parallel.
CVCC
4.7µF
0.1µF
BST1
IVINP
100k
C1
47µF
80V
BST2
470nF
27.4k
+
D1 D2
IVINN
88.7k
The buck-boost converter shown in
Figure 1 regulates 24V with 0A–5A load
at up to 98.5% efficiency. It operates
from an input voltage range of 8V to
56V. Adjustable undervoltage and overvoltage lockout protect the circuit. It
has short-circuit protection and the
SHORT output flag indicates when there
is a short circuit on the output. It features DCM operation at light load for
lowest power consumption and reverse
current protection. The sense resistor
ROUT sets the output current limit during
be mitigated with bulked up heat sinks,
additional external gate drivers, and/or
forced airflow, it may be better to simply
tie together two or more converters in
parallel to spread the load. This is easy to
do with the LT3790 buck-boost regulator.
51Ω
499k
120W, 24V, 5A OUTPUT BUCK-BOOST
VOLTAGE REGULATOR
D1, D2: NXP BAT46WJ
L1: COILCRAFT SER2915L-103KL 10µH
M1, M2: INFINEON BSC100N06LS3 60Vds
M3, M4: INFINEON BSC032N04LS 40Vds
C1: NIPPON CHEMICON EMZA800ADA470MJAOG
COUT: SUNCON 35HVT220M ×2
*WITH VIN > 20V, CAN DELIVER 300W USING ROUT = 4mΩ, RC = 5k, CC = 22nF
3.83k
design features
Figure 3. Two LT3790 24V voltage regulators are easy
to parallel for double the output with limited discrete
component temperature rise.
VIN
8V TO
56V
1.5mΩ
499k
Figure 2. Single 24V, 5A converter shown in Figure 1
has a maximum of 20°C temp rise on any component
at 12V input (a) and 50°C at 9V input (b). Even at 8V
input (c), the hottest component reaches only 96.5°C
without forced airflow or heat sinking.
470nF
499k
IVINP
EN/UVLO
EN
51Ω
1µF
IVINN VIN INTVCC
27.4k
C/10
TG1
M1
SWI
SHORT
VREF
BG1
LT3790
PWM
M2
0.1µF
VOUT
24V
10A (25A*)
L1
10µH
ROUT1
8mΩ
M4
51Ω
M3
+
COUT1
220µF
35V
×2
+
COUT2
220µF
35V
×2
0.47µF
SNSP
2mΩ
100k
SNSN
PGND
SS
33nF
4.7µF
50V
×2
BST1
CTRL
VIN = 12V
VOUT = 24V
IOUT = 5A
SINGLE PCB
NO FORCED AIR
4.7µF
10V
D1 D2
0.1µF
200k
0.1µF
C1
47µF
80V
BST2
INTVCC1
SHORT
+
INTVCC1
CCM
OVLO
88.7k
4.7µF
100V
×2
BG2
IVINMON
CLKOUT
ISMON
SYNC
SW2
VC
(a)
1.37k
3.83k
147k
200kHz
5k
1000pF
SGND
RT
71.5k
TG2
ISP
ISN
FB
47nF
VIN
1.5mΩ
499k
VIN = 9V
VOUT = 24V
IOUT = 5A
SINGLE PCB
NO FORCED AIR
470nF
499k
IVINP
EN/UVLO
EN
51Ω
1µF
IVINN VIN INTVCC
27.4k
0.1µF
C/10
0.1µF
TG1
SHORT
VREF
BG1
LT3790
M6
0.1µF
L2
10µH
M8
M7
ROUT2
8mΩ
51Ω
0.47µF
SNSP
2mΩ
100k
33nF
M5
SWI
PWM
4.7µF
50V
×2
BST1
200k
SHORT
C2
47µF
80V
4.7µF
10V
D3 D4
BST2
INTVCC2
(b)
+
INTVCC2
CCM
OVLO
88.7k
4.7µF
100V
×2
SNSN
PGND
SS
BG2
1nF
VIN = 8V
VOUT = 24V
IOUT = 5A
SINGLE PCB
NO FORCED AIR
CTRL
IVINMON
ISMON
CLKOUT
SYNC
VC
470Ω
(c)
22nF
RT
SW2
TG2
ISP
ISN
FB
SGND
147k
200kHz
71.5k
14.0k
D1–D4: NXP BAT46WJ
3.83k
L1, L2: COILCRAFT SER2915L-103KL 10µH
M1, M2, M5, M6: INFINEON BSC100N06LS3 60Vds
M3, M4, M7, M8: INFINEON BSC032N04LS 40Vds
COUT1, COUT2: SUNCON 35HVT220M ×2
C1, C2: NIPPON CHEMICON EMZA800ADA470MJAOG
*25A FOR VIN > 20V AND ROUT1,2 =4mΩ
January 2015 : LT Journal of Analog Innovation | 29
The CLKOUT pin of the master can be directly tied to the SYNC input pin of the slave for
180° phase-interleaving of the two parallel converters. The 180° phase difference between
the converters reduces overall converter output ripple, instead of doubling it. If more than
two converters are connected in parallel, they can be synchronized to either operate
phase-shifted or in-phase with an external clock source, or daisy-chaining CLKOUT pins.
0.5
IL1(MASTER)
5A/DIV
IL2(SLAVE)
5A/DIV
IL1(MASTER)
2A/DIV
IL2(SLAVE)
2A/DIV
0.4
0.3
0.2
ISMON1
500mV/DIV
ISMON2
500mV/DIV
∆I (A)
0.1
ISMON1
500mV/DIV
ISMON2
500mV/DIV
0.0
–0.1
–0.2
–0.3
–0.4
VIN = 12V
VOUT = 24V
ILOAD = 10A
2µs/DIV
VIN = 24V
VOUT = 24V
ILOAD = 10A
2µs/DIV
–0.5
0
2
6
4
LOAD CURRENT (A)
8
10
Figure 4. Parallel converter inductor and output current matching
both a short-circuit and overload situations, making this a robust application.
The temperature rise of this 120W board
at 12V input is only 20°C on the hottest
component (a switching MOSFET) as shown
in Figure 2a. There is still margin for either
higher output power at 12V input, or the
same 120W from a lower VIN without
excessive component temperature rise—
note that higher output power requires a
correspondingly increased output current
limit. When operated down to 8V input
with 120W output, the components on
this standard 4-layer LT3790 PCB remain
below 97°C (at room temp) without
forced airflow or heat sinking. To deliver
significantly higher power with the same,
limited temperature rise and input voltage range, two or more LT3790 converters can easily be connected in parallel.
30 | January 2015 : LT Journal of Analog Innovation
PARALLEL CONVERTERS,
CONSTANT VOLTAGE MASTER,
CONSTANT CURRENT SLAVE
Ideally, paralleled switching converters
share the load equally throughout the
entire output range. The LT3790’s ability to
run in either constant voltage or constant
current operation allows one master converter to control the output voltage, while
its current monitor output (ISMON) tells
one or more slave converters how much
output current to regulate (CTRL input)
in order to match its own output level.
Current matching between multiple converters is nearly ideal using this technique.
The CLKOUT pin of the master can be
directly tied to the SYNC input pin of the
slave for 180° phase-interleaving of the
two parallel converters. The 180° phase
difference between the converters reduces
overall converter output ripple, instead of
doubling it. If more than two converters
are connected in parallel, they can be synchronized to either operate phase-shifted
or in-phase with an external clock
source, or daisy-chaining CLKOUT pins.
Figure 3 shows a 24V, 10A (or 25A under
certain conditions, see figure) voltage
regulator formed by running two
LT3790s in parallel. By using two parallel circuits, the maximum temperature
rise on any one discrete component is
only 20°C for the M3 and M7 MOSFETs
at 12V input and 50°C at 9V input.
The top converter (master) in Figure 3
regulates the 24V output voltage and commands the current level that is regulated
by the bottom (slave) converter. The
ISMON output of the master indicates
how much current the master is providing, and by connecting ISMON directly to
the CTRL input of the slave, the slave is
forced to follow the master. The LT3790
ISMON output level and CTRL input level
are identically mapped so that a direct
connection from one to the other is possible, and doing so forces the total output
current to be shared equally between the
design features
Ideally, paralleled switching converters share the load equally throughout the entire
output range. The LT3790’s ability to run in either constant voltage or constant
current operation allows one master converter to control the output voltage, while
its current monitor output (ISMON) tells one or more slave converters how much
output current to regulate (CTRL input) in order to match its own output level.
VOUT
1V/DIV
(AC COUPLED)
ISMON1
(MASTER)
500mV/DIV
ISMON1
(MASTER)
500mV/DIV
ISMON2
(SLAVE)
500mV/DIV
ISMON2
(SLAVE)
500mV/DIV
VIN = 12V
500µs/DIV
VOUT = 24V
ILOAD = 5A TO 10A
The constant current slave must have
its loop broken and signal injected in
the current loop feedback path instead
of the traditional voltage feedback
path since that is the feedback loop
in use during parallel operation. The
master bode plot in Figure 7 demonstrates the stability of the system.
500µs/DIV
VIN = 24V
VOUT = 24V
ILOAD = 5A TO 10A
CONCLUSION
The LT3790 synchronous buck-boost
controller delivers over 100W at up to
98.5% efficiency to a variety of loads,
and it is easy to parallel multiple converters for even higher power outputs. The
ability to control either output voltage
or current, combined with the levelmatching of the ISMON output amplifier
and the CTRL input amplifier, simplifies the connection of a master voltage
regulator and one or more slave current regulators. The result is high power
60V buck-boost regulation that can deliver
hundreds of watts at high efficiency. n
Figure 5. Parallel converter transient response evenly shares current
parallel converters, as shown in Figure 4.
Note that the output voltage of the slave is
set slightly higher (28V) so that the voltage
feedback loop of the slave is not in regulation, allowing it to follow the master.
demonstrates a properly compensated
converter and equally shared load current. Further analysis with the network
analyzer gives us the details of the separate
converters. The noise injection point and
measurement to generate control loop
bode plots is different for the constant
voltage regulator master and the constant
current regulator slave. Separately, each
loop can be measured by injecting the
LOOP ANALYSIS FOR STABILITY
Transient response and network analyzer
loop analysis can be used to measure
stability. A transient response of 50%
to 100% current, shown in Figure 5,
IOUT
ROUT1
+
+
100Ω
CH1
– +
CH2
–
CURRENT LOOP BODE PLOT
MEASUREMENT SETUP
FOR SLAVE
72.8k
IOUT
ROUT2
VOUT
+
COUT1
– CH1 +
NOISE
INJECT
ISP
– CH2 +
39Ω
VOUT
COUT2
NOISE
INJECT
60
180
50
150
40
120
30
90
PHASE
20
GAIN (dB)
VOLTAGE LOOP BODE PLOT
MEASUREMENT SETUP
FOR MASTER
30
0
3.83k
ISN
Figure 6. Loop response measurement of parallel converters
0
GAIN
–10
–30
–20
–60
–30
VIN = 18V
VOUT = 24V
ILOAD = 10A
–40
–50
FB
60
10
–60
PHASE (°)
VOUT
1V/DIV
(AC COUPLED)
perturbation signal and measuring the
loop response, as shown in Figure 6.
0.2
–90
–120
–150
PARALLEL
1
10
FREQUENCY (kHz)
50
–180
Figure 7. Bode plot shows measured results for parallel system.
January 2015 : LT Journal of Analog Innovation | 31