AN:016 Page 1
Using BCM® Bus Converters
in High Power Arrays
APPLICATION NOTE | AN:016
Paul Yeaman
Director, VI Chip® Application Engineering
Introduction
This application note provides methods and guidelines for designing BCM bus converters into
high power arrays.
Theory
BCM modules current share when their respective inputs and outputs are connected in parallel.
Sharing accuracy is a function of a) input and output interconnect impedance matching, b) the output
impedances (ROUT) of the BCM modules and c) uniform cooling.
In theory, a very large number of modules can be paralleled. In practice arrays larger than ten become
difficult due to a) and c) above. Please contact Vicor Applications Engineering if you are designing an
array with more than 10 modules.
Since bus converters are isolated transformers, their outputs may be paralleled with inputs powered
from different sources. The lower the ROUT of the module, the more closely input voltages must
match to avoid excessive current imbalance. As such, the input voltages must be equal to ensure
evenly‑distributed sharing.
+IN
–IN
+OUT
–OUT
+IN
–IN
+OUT
–OUT
+IN
–IN
+OUT
–OUT
Isolated
Output
Bus
Common
Input Voltage
Source
BCM® Module 1
BCM® Module 2
BCM® Module 3
Contents Page
Figure 1
BCM Parallel Array
Block Diagram
Introduction 1
Theory 1
Symmetrical Input / Output
Resistances 2
ROUT Matching 3
Uniform Cooling 3
Arrays Powered
From Multiple Inputs 3
Design Example 4
General Guidelines 6
Conclusion 6
AN:016 Page 2
Symmetrical Input / Output Resistances
The primary design concern for a high power array is the layout of a symmetrical input and output feed.
Figure 2 represents a simplified model of BCM® bus converter sharing for an array of two.
In this case, the circuit has been reduced to its core elements and each BCM module is represented as a
resistor with resistance ROUT. This model can easily be expanded to represent larger arrays.
If RINPUT1 = RINPUT2 and ROUTPUT1 = ROUTPUT2 then the current through both legs will be equal. An increase
in ROUTPUT1 will decrease I1 proportionally. It is important to note, however, that an increase in RINPUT1
will decrease I1 to the square of the K factor. For BCM modules having a small K factor (<<1) the
matching of the input impedance is less critical. For example, assume the following:
Substituting values yields:
This indicates that BCM Module 1 carries approximately 10% more current with a 1Ω impedance in
series with the input of BCM Module 2 for K = 1/32. However, if K were equal to 1, then BCM Module 1
would carry essentially 100% of the current.
+IN
-IN
+OUT
-OUT
PC
TM
RSV
+IN
-IN
+OUT
-OUT
PC
TM
RSV
C1
400 µF
F4
1 A
VIN
VOUT
V
IN
BCM® Module 1
ROUT
BCM® Module 2
Load
I1 • K
VIN1
Load
I1
I2
VIN2
ROUT
ROUT
ROUT
ROUTPUT2
ROUTPUT1
RINPUT2
RINPUT1
I1
I2
PRIMARY SECONDARY
I2 • K
BCM® Module 1
BCM® Module 2
Figure 2
Simplified Model of BCM
Module Sharing
K = 1/32
ROUT = 10mΩ
ROUTPUT1 = ROUTPUT2 = RINPUT1 = 0.
RINPUT2 = 1Ω
Solving for
I1 • ROUT + (I1 • K• RINPUT1) • K = I2 • ROUT + (I2 • K• RINPUT2) • K
RINPUT1 = 0 so:
I1 • ROUT = I2 • ROUT + I2 • K2 • RINPUT2
I1
I2
:
I1 •
1
100
1
100
1
1024
= I2 •
()
+
I1
I2
11
10
=
AN:016 Page 3
ROUT Matching
ROUT is specified as a range in the BCM® bus converter data sheet and has a positive temperature
coefficient with the specified range that reinforces sharing. As the modules temperature increases due
to increased dissipation, the ROUT increases. This decreases the amount of current flowing through that
BCM module in an array, reducing the module power dissipation.
Uniform Cooling
Due to the positive temperature coefficient of ROUT, BCM modules mounted close to each other and
cooled equally will tend to equalize power dissipation.
The true power limitation on the module is based on dissipation. Therefore, the module that has a
lower ROUT may have a higher current when connected in an array (thus a higher power), but given that
its dissipation is the same as neighboring units in an array, it will have similar MTBF characteristics.
The power rating of an array of BCM modules is equal to the power rating of the individual module
times the number of modules in an array. Even under the ideal circumstances, the current through each
module will not be equal, so under full power conditions the current may not be perfectly balanced.
However, assuming that the module array is cooled equally, and the input and output impedances
are matched, a current imbalance is acceptable if the dissipation of this BCM module is the same as
others in the array. It is important never to exceed the maximum rated DC current of the module under
any circumstances.
Arrays Powered From Multiple Inputs
Figure 3 addresses an arrangement in which the BCM modules are powered from separate inputs.
In this example, input and output impedances are considered negligible. If VIN1 = VIN2 then the currents
in both legs are equal. However assume the following:
The two BCM modules must satisfy the following equation:
+IN
-IN
+OUT
-OUT
PC
TM
RSV
+IN
-IN
+OUT
-OUT
PC
TM
RSV
C1
400 µF
F4
1 A
VIN
VOUT
VIN
BCM® Module 1
ROUT
BCM® Module 2
Load
I1 • K
VIN1
Load
I1
I2
VIN2
ROUT
ROUT
ROUT
ROUTPUT2
ROUTPUT1
RINPUT2
RINPUT1
I1
I2
PRIMARY SECONDARY
I2 • K
BCM® Module 1
BCM® Module 2
Figure 3
Parallel Arrays from
Separate Inputs
V
IN1
= 48V
V
IN2 = 49V
R
OUT = 1mΩ
K = 1/32
I
LOAD = 100A
V
IN1
• K – I
OUT1
• R
OUT
= V
IN2
• K – I
OUT2
• R
OUT
AN:016 Page 4
Also,
Solving the simultaneous equations for IOUT1 and IOUT2 yields:
The same technique can be extended to include arrays with a larger number of BCM modules.
If VIN1 – VIN2 > IOUT1 • ROUT, then BCM® Module 1 will attempt to backfeed current through BCM
Module 2 to increase VIN2. To prevent reverse current in this situation, diodes can be added in series
with +IN of each BCM module.
Design Example
Figure 4 shows an example array of seven high‑voltage input 300W BCM bus converters to provide a
total power of 2.1kW. Table 1 illustrates the measured currents for the laboratory layout shown in
Figure 5. Even with less than ideal layout conditions (long wires, separate boards, use of standoffs to
carry current), the overall sharing of the array is within 5%.
BCM modules switch at >1MHz and have an effective output ripple of two times the switching
frequency, so output filtering is provided using a small point‑of‑load capacitor in conjunction with trace
inductance. The use of the input inductors confines the high‑frequency ripple current of each module.
Some input inductance between the modules inputs is necessary to minimize interactions between
parallel connected modules and allow for proper operation for the array. Input inductance also reduces
EMI and promotes the overall stability of the system by reducing (or eliminating) beat frequencies
caused by the asynchronous switching of the BCM modules.
Connecting the PC pins of the BCM modules in the array allows all units in the array to be enabled and
disabled simultaneously. Simultaneous startup is required in cases where the array will start up into
more current than one BCM module is sized to handle.
I
OUT1
+ I
OUT2
= 100A
I
OUT1
= 35A
I
OUT2 = 65A
AN:016 Page 5
Module # 48A Load
(6.86A / BCM)
95A Load
(13.6A / BCM)
143A Load
(20.4A / BCM)
192A Load
(27.5A / BCM)
IBCM % Deviation IBCM % Deviation IBCM % Deviation IBCM % Deviation
U1
U2
U3
U4
U5
U6
U7
5.9
7.1
6.7
7.4
7.1
7.2
6.8
14.0 3.4
2.4 7.9
3.4 5.0
0.9
12.6
13.2
13.6
14.4
14.0
14.0
13.5
7.4 2.9
0.0 5.9
2.9 2.9
0.7
19.2
19.9
20.6
21.3
20.8
20.9
20.4
5.9 2.5
1.0 4.4
2.0 2.5
0.0
27.6
27.3
27.7
27.4
27.5
27.7
27.2
0.4 0.7
0.7 0.4
0.0 0.7
1.1
Worst‑Case
deviation
from
nominal (%)
14.0 7.4 5.9 1.1
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
B352F110T30
+IN
-IN
+OUT
-OUT
PC
TM
RSV
L1
L1
L6
L5
1.5µH
L4
1.5µH
L3
L2
PC
C1
400µF
350V
DC
11V
DC
190A
F1
2A
F3
2A
F2
2A
F4
1A
1.5µH
1.5µH
1.5µH
1.5µH
1.5µH
+OUT
– OUT
Figure 4
BCM® Bus Converter Array
Using Seven Modules
Table 1
Seven BCM Bus Converter Array
Current Sharing
AN:016 Page 6
General Guidelines
1. Always ensure that the BCM® bus converters are fused according to safety agency requirements.
2. PC pins of BCM modules should be connected together to enable and disable the modules
simultaneously.
3. All signal and power traces should be laid out on the PCB to minimize noise coupling and
impedance. For more details on PCB layout guidelines, please see AN:005.
4. An inductor should be placed in series with the +IN of each BCM bus converter in the array to
minimize high frequency circulating currents in the primary as well as beat frequencies caused by
asynchronous switching.
5. BCM modules fed from different sources with outputs in parallel must have appropriately matched
inputs as the input voltage matching plays a critical role in current sharing.
6. In large arrays, routing issues may cause mismatching input and output impedances to each BCM
module. In that case, varying trace widths should be used to equalize impedances between close
and distant modules.
7. In large arrays, it may be difficult to match cooling for each BCM module in the array. In that case,
heat sink design or airflow routing should be adjusted to equalize module cooling as much as
possible. To optimize reliability, overall temperature should be as low as possible.
8. Load capacitors should be placed near the load. Refer to the BCM datasheet for the maximum
output capacitor value in an array. In cases where the load bypassing capacitance must be placed
near the BCMs, they should be created with individual capacitors distributed across each BCM
output, rather than lumped on a single BCM output.
Conclusion
High power arrays can be created using the bus converters in parallel provided that care is taken
in designing the input and output connections. BCM modules share inherently with inputs and
outputs connected in parallel, with the positive temperature coefficient of ROUT reinforcing sharing.
Assuming equal cooling, an array can operate at full power with accurate sharing and no derating.
The array should be designed based on guidelines that optimize protection, efficiency, reliability, and
minimize noise.
Figure 5
Laboratory Demonstration
of the Seven BCM
Bus Converter Array
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10/17
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