APPLICATION NOTE | AN:016
Using BCM
®
Bus Converters
in High Power Arrays
Paul Yeaman
Director, VI Chip
®
Application Engineering
Contents
Introduction
Theory
Page
1
1
2
3
3
3
4
6
6
Introduction
This application note provides methods and guidelines for designing BCM bus converters into
high power arrays.
Symmetrical Input / Output
Resistances
R
OUT
Matching
Uniform Cooling
Arrays Powered
From Multiple Inputs
Design Example
General Guidelines
Conclusion
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 (R
OUT
) 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 R
OUT
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.
Figure 1
BCM Parallel Array
Block Diagram
+IN
+OUT
BCM
®
Module 1
–IN
–OUT
Common
Input Voltage
Source
+IN
+OUT
BCM
®
Module 2
–IN
–OUT
Isolated
Output
Bus
+IN
+OUT
BCM
®
Module 3
–IN
–OUT
AN:016
Page 1
RSV
TM
+IN
+OUT
Symmetrical Input / Output Resistances
-IN
-OUT
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 R
OUT
. This model can easily be expanded to represent larger arrays.
Figure 2
Simplified Model of BCM
Module Sharing
PRIMARY
SECONDARY
R
INPUT1
I
1
•
K
BCM
®
Module 1
R
OUTPUT1
R
OUT
I
1
V
IN
I
2
•
K
R
INPUT2
R
OUT
I
2
R
OUTPUT2
Load
BCM
®
Module 2
If R
INPUT1
= R
INPUT2
and R
OUTPUT1
= R
OUTPUT2
then the current through both legs will be equal. An increase
in R
OUTPUT1
will decrease I
1
proportionally. It is important to note, however, that an increase in R
INPUT1
will decrease I
1
to the square of the K factor. For BCM modules having a small K factor (<<1) the
BCM
®
Module 1
matching of the input impedance is less critical. For example, assume the following:
R
OUT
K = 1/32
R
OUT
= 10m
R
OUTPUT1
= R
OUTPUT2
= R
INPUT1
= 0.
R
INPUT2
= 1
I
1
V
IN1
I
1
I
2
R
OUT
BCM
®
Module 2
V
IN2
Solving for
I
2
:
I
1
• R
OUT
+ (I
1
• K• R
INPUT1
) • K = I
2
• R
OUT
+ (I
2
• K• R
INPUT2
) • K
R
INPUT1
= 0 so:
I
1
• R
OUT
= I
2
• R
OUT
+ I
2
• K
2
• R
INPUT2
Substituting values yields:
I
1
•
I
1
I
2
100
=
1
= I
2
•
(
100
1
+
1024
1
)
10
11
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.
AN:016
Page 2
-IN
-OUT
PC
V
OUT
RSV
TM
R
OUT
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 R
OUT
increases. This decreases the amount of current flowing through that
+IN
+OUT
BCM module in an array, reducing the module power dissipation.
R
OUT
Matching
-IN
-OUT
Uniform Cooling
Due to the positive temperature coefficient of R
OUT
, 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 R
OUT
may have a higher current when connected in an array (thus a higher power), but given that
SECONDARY
PRIMARY
dissipation is the same as neighboring units in an array, it will have similar MTBF characteristics.
its
BCM
®
Module 1
The power rating of an array of BCM modules is equal to the power rating of the individual module
R
INPUT1
times the
R
OUT
R
OUTPUT1
number of modules in an array. Even under the ideal circumstances, the current through each
I
1
•
K
V
IN
I
2
•
K
R
INPUT2
module will not be equal, so under full power conditions the current may not be perfectly balanced.
I
1
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
Load
others in the array. It is important never to exceed the maximum rated DC current of the module under
I
2
any circumstances.
Arrays
BCM
®
Module 2
Multiple Inputs
Powered From
Figure 3 addresses an arrangement in which the BCM modules are powered from separate inputs.
R
OUT
R
OUTPUT2
Figure 3
Parallel Arrays from
Separate Inputs
V
IN1
BCM
®
Module 1
R
OUT
I
1
Load
V
IN2
I
2
R
OUT
BCM
®
Module 2
In this example, input and output impedances are considered negligible. If V
IN1
= V
IN2
then the currents
in both legs are equal. However assume the following:
V
IN1
= 48V
V
IN2
= 49V
R
OUT
= 1m
K = 1/32
I
LOAD
= 100A
The two BCM modules must satisfy the following equation:
V
IN1
• K – I
OUT1
• R
OUT
= V
IN2
• K – I
OUT2
• R
OUT
AN:016
Page 3
Also,
I
OUT1
+ I
OUT2
= 100A
Solving the simultaneous equations for I
OUT1
and I
OUT2
yields:
I
OUT1
= 35A
I
OUT2
= 65A
The same technique can be extended to include arrays with a larger number of BCM modules.
If V
IN1
– V
IN2
> I
OUT1
• R
OUT
, then BCM
®
Module 1 will attempt to backfeed current through BCM
Module 2 to increase V
IN2
. 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.
AN:016
Page 4
BCM
®
Figure 4
Bus Converter Array
Using Seven Modules
L1
1.5µH
PC
RSV
TM
B352F110T30
+IN
-IN
PC
RSV
TM
B352F110T30
+IN
-IN
+OUT
-OUT
+OUT
-OUT
L2
1.5µH
F1
2A
F4
1A
L3
1.5µH
PC
RSV
TM
B352F110T30
+IN
-IN
PC
RSV
TM
B352F110T30
+IN
-IN
PC
RSV
TM
B352F110T30
+IN
-IN
PC
RSV
TM
B352F110T30
+IN
-IN
PC
RSV
TM
B352F110T30
+IN
-IN
+OUT
-OUT
+OUT
-OUT
+OUT
-OUT
+OUT
-OUT
+OUT
-OUT
+OUT
F2
2A
C1
400µF
F3
2A
L4
1.5µH
350V
DC
11V
DC
190A
L5
1.5µH
– OUT
L6
1.5µH
L1
1.5µH
PC
Table 1
Seven BCM Bus Converter Array
Current Sharing
Module #
48A Load
(6.86A / BCM)
I
BCM
% Deviation
14.0
3.4
2.4
7.9
3.4
5.0
0.9
5.9
7.1
6.7
7.4
7.1
7.2
6.8
95A Load
(13.6A / BCM)
I
BCM
12.6
13.2
13.6
14.4
14.0
14.0
13.5
% Deviation
7.4
0.0
2.9
0.7
2.9
5.9
2.9
143A Load
(20.4A / BCM)
I
BCM
19.2
19.9
20.6
21.3
20.8
20.9
20.4
% Deviation
5.9
1.0
2.0
0.0
2.5
4.4
2.5
192A Load
(27.5A / BCM)
I
BCM
27.6
27.3
27.7
27.4
27.5
27.7
27.2
% Deviation
0.4
0.7
0.0
1.1
0.7
0.4
0.7
U1
U2
U3
U4
U5
U6
U7
Worst‑Case
deviation
from
nominal (%)
14.0
7.4
5.9
1.1
AN:016
Page 5
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