AU J.T. 6(4):193-198 (Apr. 2003)
Faculty of Engineering, Assumption University
Bangkok, Thailand
This paper deals with the steady state performance of a novel controlled rectifier, using an IGBT A.C controller on the primary side of the input transformer, while a center tapped secondary and normal diodes are used for rectification. Such a rectifier is suitable for low voltage high current applications. The IGBTs are controlled such that this type of controlled rectifier has a better displacement power factor because the line current drawn is symmetrical in relation to the peak of the a.c input voltage. Thus the fundamental component of the line current drawn will be in phase with the line voltage.
Theoretically expected and experimentally obtained waveforms of currents and voltages are presented.
Keywords: IGBT a.c controller, symmetrical triggering, controlled rectifier displacement power factor, harmonic distortion.
Many industrial applications need low- voltage, high-current d.c for their operation.
Examples of such industrial applications are electroplating, extraction of metals by electrolysis, etc. Most often voltage/current is to be varied and controlled. Since the standard electrical supply available is a.c, controlled rectifiers are used to obtain the variable d.c from an a.c source. Normally a transformer is
(a) use silicon controlled rectifiers on the secondary side to rectify a.c and produce d.c, or
(b) use a.c controller (two back-to-back connected SCRs) on the primary side of the transformer and then rectify the secondary voltage with power diodes. Such schemes are shown in Fig.1 using a single-phase supply. In practice, the scheme shown in Fig.1(b) is used as the current commutation takes place on low current side. The principles of operation of such a scheme are well understood and used to step down the a.c to the required level and also provide isolation. There are two options to obtaining the controlled d.c voltage:
.
discussed widely in textbooks in power electronics (Sen 1988).
L
N
L
L
In this paper, a novel controlled rectifier using Insulated Gate Bipolar L
Transistor (IGBT) a.c controller (IR 2002)
R
L
N instead of SCR-based a.c controller to achieve
(a)
Fig.1. Conventional low-voltage controlled rectification
(b)
L
193

In this paper a novel controlled rectifier using insulator gate bipolar transistor (IGBT) a.c controller instead of SCR based a.c controller to achieve the same function is described. The scheme is shown in Fig. 2. The IGBT a.c controller is on the primary side of the step down transformer, while the secondary side is a full wave rectifier with a center tapped secondary. It will be shown that this arrangement will have better performance from a power factor point of view, compared to the scheme in Fig 1(b).
L
G1 G2
L
R
L
N
Description of IGBT a.c Controller based
Controlled Rectifier
Fig.2. Controlled rectification with IGBT a.c controller
The basic schematic circuit is shown in
Fig. 2. Two IGBTs (MOSFETs can also be used) are connected in series opposition and the combination itself is in series with the incoming a.c line. The IGBTs must have in built diodes otherwise external diodes are to be connected across each IGBT. The IGBT1 is switched on and off during the positive half cycle while the IGBT2 is switched on and half a.c wave using appropriate gate control strategy. The secondary side consists of an ordinary full wave rectifier with a large smoothing inductor to maintain load current ripple free.
ν g2
ν g1 during the negative half cycle. The conduction period is made symmetric about the peak of the
The signal waveforms at different points in the circuit are shown in Fig. 3 under ideal conditions of operation. In order to maintain high displacement power factor, the gate drive signals have to be displaced by 180 o and symmetric with respect to the peak of a.c wave.
Out put current control is obtained by varying the pulse width of the gate drive signal. No special commutation circuits are required even though an R-C snubber is required across the primary winding in practice to absorb the
ν
S energy in the magnetizing inductance when both IGBTs go into off state.
π
π
2π
2π
3π
3π
ω t
ω t
ν p
π 2π 3π ω t
ν o i
L
π
π
2π
2π
3π
3π
ω t
ω t i
S
π
π
2π
2π
3π
3π
ω t
ω t
Fig. 3. Ideal waveforms of voltages and currents at different points
(v s
=a.c supply voltage, v p
= voltage applied to the primary of the transformer, v g1,
vg
2
=gate drive signals, vo = rectified secondary output , I current and I s
= a.c supply current drawn )
L
= load
194

From the voltage waveforms, the average output voltage equation is given by:
V od.c
=
1 p
π
α
− α
V om sin ?
d ?
=
2 cos α p
V om for α varying from 0 - 90 o
Assuming a large smoothing inductance in series with the load, the load current can be regarded as constant d.c. In such a case the a.c supply current I
S
will be square pulses as shown in the Fig. 3. The Fourier series for this type of current is given by:
I s
=
+
4 p
I s
4
5 p cos
I s
α sin cos5 a
?
t sin5 ?
3 p t
4
+
I s
...
cos 3 α sin 3 ?
t
The fundamental component of the supply current drawn from the line is in phase with the supply voltage. Hence, the displacement power factor (also called fundamental power factor) is unity and there is no fundamental reactive power transport to and from the load. This feature is in contrast with the controlled rectifier scheme shown in Fig.
1(b). However, there is some reactive power transport due to harmonic components of current.
It is interesting to analyze what happens during the blanking period when both IGBTs are in off condition, thus making primary current zero. This means that the primary can not have equivalent balancing ampere-turns corresponding to secondary ampere-turns. Also the load current on secondary side continues to flow due to the presence of large inductance in series with the load. In order to produce zero secondary ampere-turns and keep the load current constant, the load current splits equally into the two halves of the secondary as shown in Fig. 4 during this period. Both diodes conduct simultaneously and equally. This phenomenon takes places naturally twice in each cycle of the input a.c wave when IGBTs go off. This fact has been verified during experimentation. Again when an IGBT is given gate drive, corresponding diode takes over the full current and the other diode commutates off.
L
N i s
= 0 i
L
/2 i
L
/2
L i
L
R
L
Fig. 4. Load current flow path during blanking period
Description of Experimental Set Up
In order to verify the working principles and waveforms, the schematic circuit shown in
Fig. 5 is constructed and tested. The microcontroller AT 89c51 is programmed to develop the required gate drive signals and two
Darlington photo isolators are used to isolate high power circuits from micro-controller and other electronic circuits. Briefly the operation of the circuit is as follows: Sharp pulses from zero crossing detector (not shown) interrupt the microcontroller at 10 millisecond intervals at port pin 3.2 which is the interrupt INT0. Upon receipt of this interrupt, the controller will start
Timer0 after loading it with a number derived from the output of the ADC connected to port1.
This number can be changed by changing the analog control voltage V
C
. The run time of timer0 is the initial delay α . When timer0 overflows it generates an interrupt when it will be stopped and timer1 is started after loading it with a number corresponding to ( π - 2 α ) and simultaneously either P2.3 or P2.4 is made high depending on whether the a.c wave is going through the positive or negative half cycle. This positive or negative half cycle information is made available to the controller
195

89C51 at port pin P2.1 in the form of a square wave derived from the a.c line using a zero crossing detector (not shown). Corresponding IGBT will be made on by driving its gate high using the transistor 2N2222 and photo Darlington 4N33.
When timer1 overflows, it is stopped and also the IGBT, which was on, is made off by making its gate drive zero. Now, the controller is waiting for the next interruption at INT0 after receipt of which the procedure described above is repeated.
The R-C circuit across the primary of the transformer is required in order to provide a path for the magnetizing current of the
5V 15V
P2.4
P2.3
P2.1
P3.2
P1
D0-D7
2N2222
2N2222
ADC
4N33
5V
V
C
15V
4N33
5V transformer when IGBT a.c controller goes into off state. The design of these components is difficult as the waveform of the voltage applied to the primary varies depending on the pulse width of the gate drive signals applied to the
IGBTs. However, the reactance of the capacitor
X
C
is made equal to the magnetizing reactance
Xm of the transformer referred to as primary.
This is only a guideline. The series resistance R is there to limit the initial capacitor charging the current at the instant of application of the voltage to the primary by the a.c controller.
The initial capacitor current is to be limited to be well within the rating of the IGBT used.
R
G
R
G
L
R
C
N
Fig. 5. Circuit diagram of the experimental set up
R
L i
L
L
196

Experimental Waveforms
The set up is started and load current is adjusted to have a value of about 5 amp. The waveforms of signals at different points in the set up are recorded and shown in Fig. 6. It is interesting to compare these practical wave- forms with the expected ideal waveforms shown in Fig. 3.
Fig. 6. Experimentally obtained waveforms of currents and voltages
197

A novel IGBT a.c controller based, controlled rectifier, has been constructed and tested in the laboratory. The experimentally obtained voltage and current waveforms at different points in the circuit agree closely with the theoretically expected waveforms. At medium voltage and power levels this type of controlled rectifier can replace the conventional
SCR based controlled rectifier with an added advantage of having unity fundamental power factor. The controlled rectifier has been tested in a steady state and under open loop (manual) control. Control circuits are being developed in the department of electrical engineering to maintain output d.c voltage/current constant under varying load conditions by incorporating closed loop control on the rectifier. These will be reported in a future paper.
The author acknowledges the help rendered by his colleagues, Mr. Wuttikorn
Threevithayanon and Mr. Chairat Kumrueng, of the Faculty of Engineering, AU, in recording the test waveforms and in the preparation of this manuscript.
Sen, P.C. 1988 Power Electronics. Tata
McGraw-Hill, New Delhi, India.
IR. 2002. Application Note AN-1017a.
International Rectifier, El Segundo, CA,
USA.
198