IEEE Power Electronics Magazine - March 2018 - 49

Live

ac Supply

HVDC 400 V

Rectifier

1-kW Converter
Power
Factor
Correction

Filter

+
C1

N
0V

Neutral

Earth

ac Input
dc Output
C1 Voltage

400 V
300 V

Hold Up
FIG 6 The capacitor for a ride through (hold up). HVDC: high-voltage dc.

An electrolytic at 180 µF, 450 V might have a ripplecurrent rating of only roughly 3.5 A rms at 60 °C, including frequency correction (EPCOS B43508 series). Thus,
for 80 A, 23 capacitors would be required in parallel,
producing an unnecessary 4,140 µF with a total volume of 1,200 cm3 (i.e., 73 in3). This complies with the
sometimes-quoted 20 mA/µF ripple-current rating for
electrolytics. If film capacitors are considered, now,
just four in parallel from the EPCOS B32678 series give
a 132-A rms ripple-current rating in a volume of 402 cm3
(i.e., 24.5 in3). If the temperature is restricted to, e.g.,
less than 70 °C ambient, then a smaller case size can
still be chosen. Even if we choose electrolytics on other
grounds, the excess capacitance could cause other problems, such as controlling the energy in inrush current.
Of course, if transient overvoltages could occur, then the
film capacitors would be far more robust in the application. An example of this would be in light traction, where
an intermittent connection to a catenary causes overvoltage on the dc-link connection.
This example is typical of many environments today,
such as in uninterruptible power supply systems, wind and
solar power, welding, and grid-tied inverters. The cost differences between film and Al electrolytics can be summarized in figures published in 2013 [2]. The typical costs for a
dc-bus from rectified 440 Vac can be found in Table 1.

Film Capacitors Are Good for Decoupling
and Snubbing
Other applications are for decoupling and snubber circuits in converters or inverters. Here, film/foil construc-

Table 1. Typical costs for a dc-bus from
rectified 440 Vac.

Film
Electrolytic

Per Joule

Per ripple-current
ampere

US$0.20-0.50
US$0.5-0.10

US$1
US$3

tion should be used if size permits, as metalized types
require special design and manufacturing steps. As decoupling, the capacitor is placed across the dc bus to provide a low inductance path for circulating high-frequency
currents, typically 1 µF per 100 A switched. Without
the capacitor, the current circulates through higher-inductance loops, causing transient voltages (Vtr) according to
the following:
Ldi
Vtr = - dt .
With current changes of 1,000 A/µs being possible,
just a few nanohenries of inductance can produce significant voltages. Printed-circuit-board traces can have
an inductance of around 1 nH/mm, providing, therefore,
roughly 1 Vtr/mm in this situation. Thus, it is important
for connections to be as short as possible. To control
dV/dt across switches, the capacitor a nd a resistor/
diode network are placed in parallel with an IGBT or
MOSFET (Figure 7).
This slows ringing, controls electromagnetic interference (EMI), and prevents spurious switching due to high
March 2018

z IEEE PowEr ElEctronIcs MagazInE

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