IEEE Power Electronics Magazine - December 2020 - 14

Low-Voltage Sources: Higher-Efficiency Wide
Input-Range Buck and Buck-Boost Converters
When powered from an extra-low voltage source like a 24 or
60 V battery (Figure 2), all loads are often tied to the battery

(a)
42.3 W Loss

(b)
46.4 W Loss

(d)
11.8 W Loss

(e)
14.9 W Loss

FIG 3 600 W, 48-to-12 V solutions to scale, including required
external components. (a) 36 - 75 V, 320 W isolated, regulated
modules x2. (b) 43 - 154 V, 240 W wide-range isolated, regulated modules x3. (c) 30 - 60 V, 216 W, 18 A Buck converter x4.
(d) 40 - 60 V, 750 W fixed-ratio converter x1. (e) 40 - 60 V, 750 W
buck-boost + fixed-ratio x1. Power dissipation measured using
production units.

+
~57 V
-

Current
Multiplier

Regulator
-

-
48 V
15 A

12 V
60 A

FIG 4 Diagram of a 720 W (1 kW peak) 48-to-12 V buck converter, consisting of two conversion stages.

IEEE POWER ELECTRONICS MAGAZINE

z December 2020

negative, making isolated DC-DC converters unnecessary. A
much better design would employ a modern high-voltage
Buck converter offering 96-97% efficiency with low standby
power, enhancing battery life. If the input-to-output voltage
ratio were to allow the Buck converter to operate close to
its " sweet spot " in terms of the duty cycle there would be
very little common-mode EMI noise. For this example, optimal Buck operation would require stepping the ~60 V battery voltage down to ~12 V.
Many hard switching MOSFET based Buck converters overheat when powered from > 24 V as opposed to the
lower VIN at which their " 97% efficiency " is specified due to
switching losses. The switching losses scale exponentially
proportionally to VIN (1) generating significantly more heat
when upgrading from a 24 V platform to a 48 or 57 V platform
for example. Reducing switching frequency reduces losses
and minimum on-time issues; however, this increases the
size of output inductors and capacitors.
Here, the rapid adoption of 48 V backplanes in other
high-power computing and automotive applications provides a model for similarly improving robotic systems. As a
result, some manufacturers have improved Buck converter
efficiencies to a true 96-97% for > 48-to-12 V outputs, and
with similar results for outputs as low as 2.5 V.
For perspective on available choices, Figure 3 shows
typical efficiencies, losses and sizes for several 600 W,
12 V converters using a 40 - 60 V input measured under
same conditions at 80% load:
■■Solution A: a ZVS isolated Forward converter, a common
first choice for many designers during development.
■■Solution B: another ZVS isolated Forward converter but
with higher voltage transistors for wider input voltage
range. This can be useful for covering multiple input voltage platforms.
■■Solution C: a synchronous ZVS Buck converter with low
switching losses and no transformer losses
■■Solution D: a Sine Amplitude Converter (SAC) a type of
fixed-ratio DC-DC converter stepping VIN down by a factor of ¼. This solution requires very little storage elements due to its high-bandwidth and no regulation
■■Solution E: a SAC™ as in Solution D co-packaged with a
Buck-Boost converter adding in losses of a regulator but
still rivaling a ¼-brick DC-DC in efficiency with 1/16th the
size albeit at a narrower 40-60 V input.
For larger voltage steps than what typical Buck converters can handle without lowering their switching frequency,
increasing their size, or compromising performance too
much, a modular two-step DC-DC approach that is commonly used in data center applications (2), (3) can be used
(Figure 4). A 36-75 V Buck-Boost regulator sets an accurate
48 V at 96-98% efficiency at the input of a 97.8% 4:1 current
multiplier (fixed-ratio converter discussed below), achieving smaller space and high dynamic performance, reliability, and efficiency. For improved voltage regulation, the
regulator's feedback can be taken from the output of the
current multiplier. The 75 V rating was chosen over 60 V as

IEEE Power Electronics Magazine - December 2020

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