IEEE Power Electronics Magazine - June 2022 - 49

dc rails as low as 1 V and sub-1 V. Very large-scale ICs
and ultra-large scale ICs proliferated after 2000, integrating
millions to billions of sub-micron feature transistors.
The ultimate result of this massive progress came with
the unique problem of vulnerability of modern electronics
to transient surges. A detailed background to this is
presented in Chapter 1 of reference [1].
Figure 1(a) provides a pictorial view of transients and noise
present on a 230 V 50 Hz utility ac power, and Figure 1(b)
depicts the difference between differential mode and common
mode transient voltage sources superimposed on the ac mains
and the associated ground connection.
Circuit Designer's View
to Surge Protectors
All surge protector devices (SPD) basically
work on the simple concept of voltage
divider [2]. Figure 2 illustrates this concept
in three steps. In Figure 2(a), the superimposed
surge appears on the load represented
by ZL based on the voltage division
network comprising the compound series
impedance ZS and the value of ZL. For the
transient high voltage surge to have a minimal
effect on the load represented by ZL,
ohmic value of the ZS should be very much
greater than ZL. On the other hand, for 50 Hz
power line frequency energy feed, value of
ZS at 50 Hz, should be very much smaller
than the ohmic value of ZL.
Figure 2(b) depicts how an additional
series non-linear resistance can come into
play in safeguarding the load. For the frequency
components of the surge,
;; Zblock
;;
should be very much higher than Z.L
Figure 2(c) shows how a non-linear shunt
device can help reduce the impact of transient
surge on the load, by Zshunt becoming
very small value due to the effect of the
transient on the inserted non-linear device.
Figure 3 shows a typical case of a traditional
surge protector designed to cater for
both differential mode and common mode
surges, where the simple concepts discussed
in relation to Figure 2 are applied,
using metal oxide varistors (MOV), bidirectional
break-over diodes (BBD) and
L-C filters combined. The MOVs and BBDs,
when fired due to high transient surge voltage,
they show very low shunt impedance,
as depicted in Figure 2(c). Series path
inductors act as high impedances at frequencies
associated with the surge pulse
in the order of microsecond duration,
based on 2πfL, where f represents the harmonic
frequencies of the surge transient
900
600
300
-300
-600
-900
Power
Entry
waveform. Shunt capacitances similarly act to show low
shunt impedances at harmonic frequencies of the surge
based on 1/2πfC. A detailed explanation is available.
Surge Protection Standards and Practices
In most countries, the equipment connected to power lines
has to be certified by nationally accredited testing laboratories.
Most of these national standards are based on the international
standards of the International Electrotechnical
Commission (IEC). In North America the international
standards of the IEC are not valid; instead, standards by
A
Transients
Av
T
A >Av
T >>τ
τ
(a)
Noise
CM
DM Differential-Mode
Voltage Source
Power
Return
CM
Common-Mode
Voltage Sources
Ground or
Reference
Plane
(b)
FIG 1 Transient and noise on the utility 230 V, 50 Hz power supply and the concept
of differential and common mode transients entering an electronic system
(a) transients and noise on utility power supply (b) common and differential
mode signals.
Electronic System
ZS
ZS
ZL
(a)
(b)
Zblock
ZL
(c)
FIG 2 Voltage division principle applied to SPDs (a) basic divider formed (b) a
series non-linear device helping the clamping voltage (c) shunt device action.
June 2022 z IEEE POWER ELECTRONICS MAGAZINE 49
ZS
Zshunt
ZL
IEEE Power Electronics Magazine - June 2022

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