IEEE Power Electronics Magazine - December 2020 - 16

K = 1/16
Fixed-Ratio
Converter

IHIGH-SIDE
ZPDN

+

RESRIN
770 V

K .ILOW-SIDE

VHIGH-SIDE
CIN

-

CIN = COUTK 2

ILOW-SIDE ROUT
+
+
-

RESROUT
VHIGH-SIDE . K

RESRIN =

ZOUT = (ZCINZPDN) K 2 + ROUT
COUT

48 V
Motor
Drive

-

RESROUT
K2

FIG 6 Impedance reflection can reduce the effective source impedance-and therefore the needed capacitance-by a factor of K2.

are typically driven using high-frequency pulses with
large instantaneous changes in current, significant source
impedance will distort the voltage and current present
at their terminals. Similarly, parasitic inductances within
an extensive PDN can limit the current available to the
motor windings, limiting torque.

with the Middlebrook Criterion of stability analysis. So while
optimizing wire gauge for ampacity, it's AC impedance can
be reduced with appropriately sized capacitors at the input
of the converters, also reducing ac current losses and interference in longer wire runs.

Efficiency and Battery Life

Application Considerations in Robotics
Lightweight Low Impedance Harnessing,
Stability of Power Distribution Network
The above bring us to applying simple principles for power
distribution routing and harnesses as power needs increase,
exploring higher voltage distribution converting to the load
voltage near the load with discussed converters so lower
currents reduce distribution losses, (dynamic) voltage drops
and EMI interference. In addition low inductance layout and
wiring utilizing field-cancellation with tight loops, twisted
wires or routing on adjacent pcb planes may also help. Converters generally need their source's AC impedance 10x
smaller than the load impedance up to the bandwidth of
their control loop, particularly with dynamic loads to limit
voltage drops as shown in the example with Fig 8, in line

0.25 lbs

98% Efficiency

2.5 lbs

91% Efficiency

The losses of DC-DC converters may seem negligible in
regards to battery life as they tend to be an order of magnitude lower than their loads, but they can deceptively add up
in form of no-load losses when the associated payload is in
sleep mode. As any datasheet review reveals transformerbased DC-DC converters tend to draw substantial power
when enabled to operate their controls and magnetize/
demagnetize the main switching transformer and can easily
add up to 0.5-1% of their full power capability. Some regulated converters consume even more power at no-load,
requiring or building in a pre-load of a few percent of the
maximum load to stabilize output.
Disabling these converters along with their loads when
not needed may be a good option, but even disabled, power
dissipation can be substantial.
Choosing as few transformer-based converters as possible, ideally one per isolation barrier needed, followed by
Buck or Buck/Boost converters for additional outputs to
the same return can reduce idle-losses proportionally.
The quiescent current of many Buck/or Buck-Boost converters is in milli-amperes due to the utilization of techniques
such as pulse-skipping, or more advanced techniques.

Fixed-Ratio or Regulated Conversion?

7.7 lbs

87% Efficiency

FIG 7 (From top to bottom) K = 1/16 fixed-ratio converter with
heat sink, a commercially available regulated DC-DC converter
array with heat sink, and a generator-driven ac-dc converter
(fan-cooled).

16	

IEEE POWER ELECTRONICS MAGAZINE	

z	December 2020

If the input voltage range of the load is equal to or wider
than that of the source, a fixed-ratio converter may be the
best option due to its size, efficiency, and performance.
A 770-to-48 V 1.5 kW fixed-ratio converter (Figure 7) has
about 1/2-1/3 of the losses of a regulated DC-DC forward
converter as the latter has additional losses in the transformer and due to the regulation stage. A less fair but practical comparison is to the AC-DC converter that previously
fed the same drive from the vehicle's AC generator with
additional losses generated by the rectifier and typical PFC
IEEE Power Electronics Magazine - December 2020

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - December 2020
