Circuits and Systems, 2011, 2, 249-259
doi:10.4236/cs.2011.23035 Published Online July 2011 (http://www.SciRP.org/journal/cs)
Analysis and Design Aspects of a Series Power
Semiconductor Array with Digital Waveform
Control Capability for Single Phase AC Voltage
Regulators and Other Applications
Nihal Kularatna, Chandani Jindasa
The University of Waikato, Hamilton, New Zealand
Received May 17, 2011; revised June 15, 2011; accepted June 22, 2011
Abstract
A series connected power semiconductor array, with digital control capability could be used for developing
single phase AC regulators or other applications such as AC electronic loads. This technique together with an
ordinary gapless transformer could be used to develop a low cost AC voltage regulator (AVR) to provide
better or comparable specifications with bulky ferro-resonant AVR types. One primary advantage of the
technique is that digital control can be used to minimize harmonics. Commencing with a review of AC volt-
age regulator techniques for single phase power conditioning systems, an analysis and design aspects of this
technique is presented with experimental results for AVRs. Guidelines on how to utilize the technique in a
generalized basis is also summarized together with a summary of a technique for achieving harmonic con-
trol.
Keywords: Power Conditioners, AC Voltage Regulators (AVR), Power Semiconductors, Digital Control,
Electronic AC Loads
1. Introduction
With the proliferation of electronic systems based on
submicron feature transistors, power quality (PQ) has
become a major concern for end users as well as distri-
bution authorities around the world [1-6]. Voltage sags
and surges are a very common phenomenon in many
distribution circuits, and it is particularly so in over-
loaded distribution systems. Voltage sags and surges
seem to be distributed and tend to follow the daily load-
ing patterns of the utility [2]. Extreme voltage fluctua-
tions in many developing countries [7] and the need for
lower cost products with easy manufacturability moti-
vated the preliminary R & D work related to this spe-
cific technique. Figure 1 indicates a typical pattern of
voltage fluctuations in an urban location sub-circuit in
Sri Lanka [7].
To improve the quality of power at the end user prem-
ises, three major considerations are voltage sags and
surges, transient surges such as lightning or inductive
energy dumps, and harmonics and flicker etc. An AC
Voltage Regulator (AVR) is a particularly useful power
conditioning equipment. The common AVR techniques
used are: 1) motor driven variacs; 2) transformer tap
changers; 3) ferro resonant regulators; 4) thyristor based
Figure 1. An example of a voltage variation in an over-
loaded distribution line (Source: Lanka Electricity Com-
pany).

250 N. KULARATNA ET AL.
systems (v) solid state AC regulators. Table 1 summa-
rizes the performance of these commercial families in a
practical stand point in order to compare with the new
technique we introduce in the paper. The information is
particularly applicable to the single phase systems used
by the end users and with output ratings from few
100Watts to few kilowatts. Magnetic amplifier tech-
niques [8] usable in large capacity single or 3 phase sys-
tems etc are not discussed here.
Given the above variety of techniques used in com-
mercial circumstances [9], a consistently popular tech-
nique has been the ferro-resonant type. Invented by Jo-
seph Sola in the 1930s, this technique is popular due to its
operational reliability, simple construction, and the ability
to ride through a couple of AC cycles. However, this line
frequency tuned LC resonant circuit based technique has
the following disadvantages: 1) dissipates approximately
150 - 250 watts per each KVA of its output, due to the
gapped transformer operating near saturation; 2) output
regulation is dependant on the power factor of the con-
nected load [10]; 3) various malfunctions when powered
by a standby generator where the output frequency fluc-
tuates with the load (due to LC circuit operating beyond
resonance). Item 3) was a common occurrence in Sri
Lanka during the drought periods with long power out-
ages, where the first part of the work presented in the
paper was carried out. In these cases frequent malfunc-
tions of some commercial line interactive type UPS sys-
tems with ferro based AVRs were a common occurrence.
Apart from the above approaches used in commercial
AVR techniques, there are few other published ap-
proaches to achieve AC voltage regulation in single
phase power conditioners. Many of these are based on a
series AC voltage component generated by a switching
PWM scheme [11,12], or electronic transformers, which
are also based on a PWM switching technique [13,14].
Another variation of a PWM based series connected AC
regulator technique is described in [15,16]. The common
problem in these are RFI/EMI due to PWM switching
schemes, and the complex cost and manufacturing issues
of adapting them to output power levels from few 100W
to few kilowatts in single phase environments.
Given the above summary, key requirements in de-
veloping AVR techniques suitable for modern power
conditioning requirements could be summarized as:
a) Reliability of operation in surge-prone PQ envi-
ronments;
b) Efficiency under all load levels and load power
factor situations;
c) RFI/EMI minimization;
d) Output harmonic minimization;
e) Speed of the control loops within the regulator;
f) Operability within the extremes of input line voltage
limits;
g) Energy storage for short duration ride through re-
quirements.
Table 1. Comparison of AC voltage regulators.
Family Basic Technique used Advantages Disadvantages
Motor driven
variacs
A servo motor based auto
transformer with a voltage
feedback loop
 Simple construction
 High capacity
 Simple electronics
 High efficiency
 Bulky
 Slow response
 Can get stuck at the lowest input voltage and create an
over-voltage when the line voltage returns to normal
Transformer tap
changers
A transformer with multiple taps
and a feedback loop to
automatically change the taps
 High efficiency
 Easy to design
 Simple construction
 Low cost
 If input voltage fluctuates frequently “tap dancing”
could occur
 Arcing in taps can create problems, with inductive loads
 Voltage transients may appear at the output during tap
changes
Thyristor based
designs
A series secondary winding or an
auto transformer is used with a
thyristor phase controlled
technique to maintain the
RMS voltage constant
 Compact
 Low cost
 Efficient
 Fast response
 High harmonic content at the output
 Could cause problems with inductive loads
 Filtering at output may be necessary for reducing
RFI/EMI issues
Ferro-resonant
regulators
A precisely gapped transformer is
used in resonance with a capaci-
tor to create a resonant circuit,
while core saturation is used for
regulating the output voltage.
 Very reliable
 Simple design
 Can withstand a fractional or
few cycle outage at the input
side
 Differential mode transients
can be tolerated
 Non sinusoidal output with flattened top
 Power factor dependant load regulation
 Extremely sensitive to frequency fluctuations on the
input side (i.e.: when a small standby generator is used
as the AC input)
 Low efficiency and no load power consumption of 20% -
30% of the VA rating
Solid state types
Either linear amplifier based
technique or switching technique
based compensation is used
 Wide input range is possible
 Compact design may be possi-
ble (with a switching tech-
nique for voltage buck or
boost)
 Complex circuitry
 RFI/EMI problems (in switching technique based ones)
 Reliability issues in environments with high common
mode transient surges

N. KULARATNA ET AL.
251
2. AC Power Control Technique Based on a
Series Power Semiconductor Array
Given the above introduction to the commercial design
approaches with key design requirements in AVRs, fol-
lowing section introduces the primary design concepts
used in this technique. The technique discussed in this
paper was developed for single phase end user equip-
ment where the line voltage could fluctuate widely and to
achieve a design without any ferro-resonant transformers
for easy manufacturability.
This technique based on a series connected power
semiconductor array [17-20] is suitable for situations
where 230V AC rms voltage fluctuates widely between
160 V to 260 V. The technique has the following advan-
tages and useful features:
1) Fast response and high efficiency at worst case sags
2) Gapless 50/60 Hz transformer;
3) True RMS output control;
4) Electrical isolation between low voltage control
circuits and the power stage;
5) Minimum RFI/EMI issues;
6) Lower harmonic distortion at output and less de-
pendence on the load power factor;
7) Equal power dissipation and voltage distribution
among transistor elements.
This technique works for both sags and swells without
any transformer configuration changes, compared to the
technique described in [21]. The technique in [21] with a
claim for a high efficiency near 96% is only for a limited
range of regulated output which should be lower than the
incoming rail with a minimum input voltage of just 4
volts above the regulated output.
The same design approach in [17-22] could also be
used in developing an electronic AC load with digital
control for harmonic minimization [23,24]. The rest of
the paper describes the fundamentals and two applica-
tions of the technique, with an in depth discussion on the
technique as applied to a single phase, low power AVR
with the potential to minimize the harmonics at the out-
put using a digital technique.
3. Concept of Impedance Control and
Implementation
3.1. Basic Concept of Series Power
Semiconductor Array
A conceptual approach for changing the effective overall
AC resistance of a power BJT array over a wide range is
shown in Figure 2. Figure 2(a) shows the simplified
concept of control of the transistor. Figure 2(b) indicates
how an opto isolator can be used to control current di-
version in the power semiconductor. Figure 2(c) indi-
cates how series connected infrared emitter diodes in an
opto isolator can be used to control a series connected
bipolar power transistor array with AC operational capa-
bility. When the transistor array is used in 230 V AC
applications such as in an AC voltage regulator, the in-
stantaneous values could often vary up to a maximum of
approximately 3302 V for a range of input AC RMS
voltages from 160V to 260V [25]. This high voltage re-
quirement at high power loads suggests the use of multi-
ple power semiconductors to share the loading.
For an n-element BJT array similar to the case of Fig-
ure 2(c), it can be shown that,
1
x
B
Array
b
i nR
Ri






(1)
when
 
12 3
;; ;
12
BB B
B
BB Bn
RR R
RR RR
nn n
 

B R
(2)
where RArray is the approximate effective instantaneous
resistance at the AC input of the circuit in Figure 2(c), ib
is the instantaneous base current, ix is the amount of
base current diverted by the opto transistors and RB is the
resistance between collector and base of the nth transistor
[22]. For the case of the 4 element array in Figure 2(c),
41
x
B
Array
b
i R
Ri






(3)
and,
1234
;;;
432
BBB
B
RRR
RR RRR
(4)
Note that the base emitter voltage drops are neglected
in these approximations.
Based on the simplified relationship in Equation (3),
the resistance between the collector and the emitter can
be easily controlled either by varying RB or suitably
changing ix. This in effect indicates that we need to con-
trol the ratio
x
b
ii, which is defined as the base current
diversion ratio (BCDR). This technique in addition pro-
vides the necessary electrical isolation between the low
voltage control circuits and the power stage. In a practi-
cal application with Darlington pairs, the compound base
emitter voltage will be between 1 to 2 Volts. This practi-
cally permits the concept of controlling the BCDR using
opto isolators and similar low voltage control circuits.
For details [23] is suggested. A similar design approach
could be used with other power semiconductors such as
MOSFETs and IGBTs, by modifying the above tech-
nique, a discussion of which is beyond the scope of the
paper.

252 N. KULARATNA ET AL.
3.2. Limits and Boundaries of the Achievable AC
Resistance
Due to Darlington pairs in Figure 2(c) a cutoff condition
is reached when the compound VBE value for the Dar-
lington pair is approximately less than about 1.0 Volt.
This occurs at a higher value of the opto transistor cur-
rent and at that point the controllability of the array im-
pedance diminishes. This is the case beyond the maxi-
mum BCDR, where the base current of each of the tran-
sistors is totally removed by the action of the optoisola-
tors. Under this condition, the effective resistance of the
array is not controlled by the transistors, except for the
leakage effects. If the transistor leakage effects are ne-
glected and the conditions in Equation (2) are maintained,
the effective maximum resistance of the array reaches
the value given by the series combination of the resistors
RB1 to RBn,
max 234
B
BB B
CE B
RRR R
RR n  (5)
At the other extreme, when the current through the
input diodes of the optoisolators is zero (the case of
minimum BCDR), the effective resistance of the array
reduces to B nR

. In between these two limits the
overall resistance of the array, RCE, can be controlled by
varying the current through the series connected diodes.
(b) (c)
(d)
Figure 2. Concept of AC impedance control with a BJT array. (a) Simplified concept; (b) Control of the base current using
opto transistor; (c) Implementation of a 4 element system; (d) Effective impedance (Rarray) of a 4 element array versus opto
diode current for RB values of 180 kΩ and 270 kΩ.

N. KULARATNA ET AL.
253
From Equation (5) the maximum value of effective re-
sistance for an array of 4 elements is approximately 2.1
RB, neglecting the effects of leakage currents in transis-
tors. Equation (3) indicates that the minimum resistance
for the array is approximately 4RB/β. This clearly indi-
cates a wide range of ideal performance possible within
the boundaries. If the array is to work as a high voltage
capable switching element, the value of RB can be set to a
suitable value for the designer to get the overall result of
4RB/β to reach the lowest necessary, based on the circuit
components. For a 4-element array the ratio of off-im-
pedance/on-impedance is around β/1.9 and for a well
configured Darlington pair this can be in the range of 3
orders.
Figure 2(d) depicts a typical example of the variation
of the effective resistance versus control input IF (opto-
diode current) for RB values of 270 kΩ and 180 kΩ for a
four element array (as in Figure 2(c)) capable of 100W
dissipation. It is clear that the lowest value reaches the
theoretical value expected from 4RB/β. As indicated in
Figure 2(d) for the case of 50 V AC input with RB = 270
kΩ, the array reaches a maximum at higher values of IF
as per theoretical predictions and SPICE simulations.
However the maximum value is significantly lower than
the expected due to leakage effects.
Also the graphs indicate the dependence of the effec-
tive resistance on the AC line voltage, due to device
nonlinearities and the dependence of β on the instanta-
neous collector current over the AC cycle. Another prac-
tical situation is that the transistors could have non iden-
tical β values. However it is easy to compensate for this
variation by slightly adjusting the RB values deviating
from the relationship in Equation (2).
4. Application Examples
4.1. Design of an AC Regulator Based on the
Technique
Figure 3(a) indicates the basic approach where the trans-
former T1 allows the boost or buck operation. If you con-
sider that the transformer is an ideal one, where the series
winding has N times the turns as in the primary winding
which is in series with the power semiconductor array
placed across the bridge points of the full wave rectifier.
Under this arrangement, the following approximate
relationship holds true, for any general load connected at
the output.

2
1
out inarrayL
VV NRNI (6)
where Out
V and in V are the output and input voltages
and
L
I
is the load current in vectors respectively.
Figure 3(b) indicates the phasor diagrams for the case
where the input voltage (in V) is less than the required
regulated output (out V). In this example we consider the
load current is lagging the input voltage by an angle Φ.
Based on the relationship in Equation (6), and assuming
that the impedance of the transistor array is purely resis-
tive, and the transformer is ideal with no leakage induct-
ances or resistances, the control circuits could regulate
the output voltage, by maintaining the regulated Vout
within the arc of the circle with a radius of out V in the
region where the tangential points of the worst case
phase angles of the load falls within ±Φmax. Beyond these
limits of the tangential points T and T’ of the phasor dia-
gram, regulation is not possible, for a given turns ratio N.
Few interesting and practically useful observation are
that,
1) If the load is purely resistive, the regulated output
voltage will be in phase with the input voltage;
2) For a given input voltage if the transformer is con-
figured to have the case of

1
out in
VV N

, dissipation
in the array is minimum;
3) Condition 2) above suggests having multiple taps in
the transformer, to have the best efficiency under wide
range of input voltage fluctuations;
4) When the transformer turns ratio increases, the al-
lowable phase angle of the load decreases.
Figure 3(c) indicates the case of phasor diagram,
where the input voltage is higher than the required regu-
lated output voltage. In this situation, by creating a
higher voltage across the array, and practically reversing
the voltage at the primary winding, the regulation is
achieved. In case the load is purely resistive, as expected
the regulated output will be in phase with the input volt-
age. Also in this situation, when the input voltage rises
above the required regulated output value, it is possible
to reverse the connections of the primary winding, so
that the dissipation across the array is reduced. In Fig-
ures 3(b) and 3(c) the arc included within T and T’ indi-
cates the limits of the reactive component of the load to
achieve regulation, assuming that the transformer is con-
sidered ideal.
As shown in the phasor diagrams the technique is
useful in situations with non resistive loads as well, while
regulating the output for both voltage sags and surges
with out any transformer configuration changes.
Figure 4(a) indicates the implementation block dia-
gram of a 230 V/50 Hz capable 1 KVA regulator based
on the technique[17,18] developed to overcome the fre-
quency sensitivity, waveform distortion and the lower
overall efficiency of the commonly used ferro-resonant
regulators and the slow response of motor driven variacs
[26,27]. To cater for the worst case line voltage situations
such as in [8], this AVR prototype was developed to op-
erate within a wide range such as from 160 V to 260 V.

254 N. KULARATNA ET AL.
(a)
(b) (c)
Figure 3. BJT array based AC regulator and phasor diagrams. (a) Basic concept; (b)Phasor diagram when input voltage is
less than the regulated output; (c) Phasor diagram when input voltage is higher than the regulated output.
L1
N1
T1
L
N
(a) (b1)
(b2) (b3)
Figure 4. AVR implementation of a 1 kVA prototype and measured performance. (a) Overall block diagram; (b1) Load regu-
lation; (b2) Line regulation; (b3) Efficiency.

N. KULARATNA ET AL.
255
For RMS output voltage control, the effective resis-
tance of the array is varied depending on the load and the
input voltage. RMS control circuit compares the actual
AC output sample, converted to a DC value using an
RMS-DC converter IC, with a reference DC voltage.
Current injection circuit adjusts the current injected into
the series connected input diodes of the opto isolator,
based on the output of the RMS control circuits. This
effectively controls the value of Rarray in such a way that
the loop keeps the output RMS value regulated at a pre-
set value such as 230 V, for a wide range of input volt-
ages.
Figures 4(b1) to 4(b3) indicate the measured per-
formance of an AVR prototype of output capacity of
1kVA based on Darlington pairs of 2N3773 and 2N4923
[17]. Load regulation graph in Figure 4(b1) indicates
that the prototype regulates around 230 ± 6% V AC
within an input voltage range of 170 V to over 250 V.
The line regulation graph in Figure 4(b2) indicates that
the technique is usable almost up to 160 V, however
needs adjusting the lowest possible resistance of the ar-
ray.
As per graph of Figure 4(b3) we see that the effi-
ciency keeps dropping as the input voltage keeps in-
creasing towards the nominal regulated value of 230 V.
In the particular prototype tested [17], we have used a
transformer with a turns ratio (N) of 70/160, where the
array is expected to have the lowest resistance, at a worst
case of input voltage of 160 V rms. If the input voltage
reaches 230 V rms, the primary winding should have
zero value so that the correction applied at the secondary
side is zero. This indicates that the quantity
should be equal to
2
Array L
NRI

23070 160

V, for the case of a
pure resistive load. Similarly if the input voltage goes
towards a surge situation with over 230 V rms value, this
quantity should be further increased, to generate a nega-
tive correction at the secondary winding.
In this prototype where the efficiency is optimized
around 160 V, as the input voltage increases the effi-
ciency drops. However the overall performance is com-
parable with a similar capacity ferro-resonant version. As
discussed in a previous paragraph, by using multiple taps
and reversing taps under surge voltage conditions at the
input overall efficiency could be improved.
This gapless transformer based technique can be en-
hanced with a digital control subsystem to minimize the
harmonics at the output. By suitable control circuitry,
transformer tap changes can also be incorporated to fur-
ther enhance the overall efficiency, recognizing the fact
that the technique has a higher efficiency at the worst
case sags, where only a minimum resistance value of the
array is required as per Equation (6).
4.2. Electronic AC Load
Another useful application of the technique is in an elec-
tronic AC load. The design approach for an AC elec-
tronic load with processor control is indicated in [23-25].
A discussion on this is beyond the scope of this paper.
5. Processor Based Approach for
Linearizing of the Array Resistance
Given the non-linear behaviour of the transistor array as
per Figure 2(d), the instantaneous current in the array
will be nonlinear under general conditions, and will de-
pend on the instantaneous AC line voltage as well as the
dependency of the gain of transistor on its collector cur-
rent. In order to control the non linearity of the value of
RArray which is equivalent to ()
()
Array rms
Array rms
V
I, collector current
in the opto transistors based on the following relationship
could be controlled [27].
'
p
F
C
F
I
IK
I



(7)
K, '
F
I
and p are the parameters for opto isolator pairs
[27].
From the basic transistor parameter relationships and
assuming that all transistors are identical,
eBE
qV kT
Array S
II (8)
where q is the electron charge, k is the Boltzmann con-
stant and T is the absolute temperature of the transistor
junction. Is is the saturation current for the identical tran-
sistors Q1 to Q4 in Figure 2(c).
By substituting the relationships in Equation (7) and (8)
in Equation (3),
'
41
e




















BE
p
F
F
B
CE qV
kT
S
I
KI
R
R
I
(9)
With suitable mathematical manipulations [23], we
can also arrive at the following relationship for voltage
across the array (VCE) and the opto-diode forward current
(IF) for a given RCE value, neglecting the vBE compared to
the instantaneous values of vCE.

'
ln lnBCE
CE F
p
CEB F
KnRR
VpI RnRI



 (10)
Based on the relationship of Equation (10) and using
experimental data similar to Figure 2(d) for the circuit

256 N. KULARATNA ET AL.
arrangement in Figure 2(c), curve fitting techniques can
be used to obtain the logarithmic relationships for volt-
age across the array (VCE) and the current fed through the
photo diodes (IF) of the opto isolator for each value of
expected array resistance. Implementation of this is
shown in Figure 5 depicting the hardware block diagram
and the flow chart applicable. By plotting these curves
from the experimental results (which tallies with the
SPICE simulation results) together with a straight line fit,
it could be easily seen that a reasonably accurate values
for m and c values for a straight line approximation can
be obtained. Two selected examples from [25] are shown
for array resistances of 500 Ω and 50 Ω in Figure 6.
From these graphs, one can see that the relationship is
very close to a straight line fit, with matching R2 values
close to 1.
(a)
(b)
Figure 5. Approach to digital control of the harmonics at
the output. (a) Processor and the control; (b) Flow chart.
(a)
(b)
Figure 6. Graphs of Logn (VCE) versus Logn (IF) for two dif-
ferent RCE values together with straight line fit in each case
(a) For RCE = 500 Ω; (b)For RCE = 50 Ω.
Given these relationships for a particular case of a
transistor array, the values of m and c can be fed into the
digital control algorithm, in an overall arrangement as in
Figure 5(a). A look up table can store the experimental
values for the particular array, and then derive the ap-
proximate values m and c suitable for each case of a
straight line fit.
The above discussion leads towards the digital control
approach to solve the linearity issue of the array, by con-
trolling the two parameters m and c indicated above. Us-
ing a digital control algorithm, injected opto diode cur-
rent can be controlled to adjust the instantaneous current
through the array, by taking relatively larger number of
samples of the AC voltage waveform via sampling.
Overall effect of the processor based system is to control
the opto diode current to achieve the expected impedance,
based on the behaviour of the array as per Figure 2(c)
during each sampling period. 1 kHz sampling rate was
used in the proof of concept system. With the sampling
of the instantaneous AC line voltage, microprocessor

N. KULARATNA ET AL.
257
program calculates the required opto diode current (out-
put from the DAC) over each sampling period within the
50 or 60 Hz AC cycle. More details are available in [25]
and a related US patent application.
Figure 7 compares the array performance with and
without digital control at different resistance settings. It
is clear from these oscillographs forms that the digital
control technique reduces the harmonics in the waveform.
These results were obtained in an electronic AC load
capable of 150 VA capacity, based on an 8 bit Zilog Z8
Encore processor [25]. Figure 8 indicates the fast Fou-
rier transforms (FFT) of the current waveform at 1.3 A
with and without the digital controller. This FFT plots
indicates that the technique reduces the 3rd, 5th , 7th and
9th harmonic etc by significant amounts, proving that the
algorithmic approaches used is clearly suitable. More
information is available in [23].
Overall achievements in this work are:
1) A versatile AC impedance control technique which
can be used in applications such as AVRs and power
conditioners;
2) A new digital control technique where additional
digital waveform control can be added to the system to
minimize harmonics in the current waveforms.
(a)
(b)
Figure 7. Current drawn by the array for a different setting of the array resistance with and without digital waveform cor-
rection (a)75 Ω setting; (b) 200 Ω setting control.

258 N. KULARATNA ET AL.
(a)
(b)
Figure 8. Comparison of the FFT of the current waveform at 1.3 A (a) with digital control; (b) without digital control.
7. Conclusions
The concept of using a series transistor array with opto
isolator based isolation can be used in several AC power
control applications suitable for low power and single
phase requirements. Paper provides a general analysis of
the array, and the design details of a 1 KVA capacity
AVR prototype, which can be extended into other power
levels. One secondary advantage of the technique is its
ability to incorporate digital control algorithms to mini-
mize harmonics due to the non linear nature of the power
semiconductor array. Given these details, the technique
could be used to develop AVRs with performance far
superior to ferro-resonant versions and other slow re-
sponding transformer tap changers etc.
In addition, this technique has the potential to combine
with common AVR techniques such as transformer tap
changers etc for higher overall efficiency. Another pos-
sible application is in electronic AC loads with harmonic
control.
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