IEEE Power Electronics Magazine - December 2015 - 49

end-to-end efficiency from 65% to about 75%.
Table 1. A comparison of the total power dissipation
While this represents a significant step forand thermal removal capability required for three generations
-
ward, it is still inadequate for systems where
of communication satellites.
a higher end-to-end efficiency is required.
centralized
Intermediate
Intermediate
To enable next-generation electronic
required thermal
Power
Bus architecture, Bus architecture,
payloads, improved power-distribution arremoval (kw)
Distribution
Unregulated
regulated
chitectures are needed. One cost-effective
Total payload power (kW)
10
10
10
approach is to further develop the bus con65
75
85
End-to-end efficiency (%)
verter to produce regulated intermediate bus
End-to-end power
5.38
3.33
1.76
voltage while maintaining high efficiency
dissipation (kW)
and circuit simplicity. Since it regulates
twice, it is referred to as a double-regulated
electronic payload units. This means more mass allocated
intermediate bus.
for payload bandwidth and channel capability. In summary,
With tightly regulated intermediate bus voltage, the
with reduced dissipation, there are three system-level benPOL switching converter can be further optimized for efefits: 1) reduced thermal removal requirement, 2) reduced
ficiency and device and component integration. Ultralow
battery storage capacity, and 3) reduced solar panel size.
dropout linear regulators can then be used effectively. It
is projected that, with the regulated intermediate bus,
the end-to-end efficiency can be improved to be greater
The Adiabatic Region of the Efficiency Curve
than 85%, a total of 20% improvement over the state of the
To appreciate the effectiveness of efficiency improvement,
art. Table 1 shows the technological characteristics for
the efficiency curve is revisited. The efficiency curve is nonthree generations of example 10-kW electronic payloads.
linear. To gain more insight, we can use some approximaWith current end-to-end electrical efficiencies of 65%, the
tion. Refer to Figure 2, where the efficiency curve is plotted
total thermal power to be removed is 5.38  kW. If the efin a logarithmic scale. The horizontal axis is the conversion
ficiency is improved to 75%, the thermal power is reduced
efficiency as a percentage, and the vertical scale is relative
to 3.33 kW with an unregulated intermediate bus. If the
loss, also a percentage. The relative loss is the ratio of the
efficiency is further improved to 85%, the thermal power
total loss over the output power. The curve is normalized,
is reduced to 1.76 kW. It is shown in [10] that the doublestarting from an efficiency of 75%. If we use the piecewise
regulated intermediate power bus can reduce losses by
linear approach, the curve, to the first order, can be modeled
more than 67%.
by three intersecting lines, dividing the curve into three
Furthermore, with approximately US$2,000/W loss as a
regions: region 1, region 2, and region 3 (adiabatic region).
common thermal performance metric for space systems,
For each region, the line has a different slope. To apprethe cost savings from a simpler thermal design could apciate the usefulness of the piecewise approximation, we
proach US$4 million on a new spacecraft. More importantcan look into the percentage of efficiency improvement it
ly, the weight savings could easily approach tens of kilotakes to reduce the loss by a half (50%). Figure 2(b) illusgrams due to the repetitive use of converters throughout
trates the trend.

Relative Loss Versus Efficiency (%)
100

70

75

80

85

90

95

100

Relative Loss (%)

Adiabatic Region 3 Slope: 0.97
10

74.78
80.41
84.66
89.38
92.83
95.6
97.4
98.84
99.45

Region 1 Slope: 0.65
Region 2 Slope: 0.85

1.0

0.1

Efficiency
(%)

Efficiency
(a)

Efficiency Delta (%)
to Cut Loss by a
Half
11.4
9.0
7.3
5.3
3.7
1.9
1.3
0.6
0.3
(b)

FIG 2 (a) The adiabatic region of the efficiency curve is defined as where the efficiency is greater than 98.5% (region 3, to the far
right of the graph). (b) The table shows the percentage of efficiency improvement it takes to reduce the loss by a half.

December 2015

z	IEEE PowEr ElEctronIcs MagazInE

49



Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2015

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