These are the requirements of the input stage of the power supply.
•
This is basically an incoming mains rectification unit, which can provide a low ripple d.c. link voltage and should draw low distortion input line currents from the supply utility.•
The overall circuitry and components should be robust and reliable in use, and be produced at an economic cost.Figure 8.3 Generic component arrangement of an SMPS system
1In effect, this is the application of a short duration wave (e.g. typical pulse width 50–100 μs)
supplied by the inverter feeding the HV transformer.
•
Ideally it should have a power factor approaching unity such as to minimise electromagnetic interference (EMI) problems and harmonic distortion of the supply.•
To minimise the size and weight of the high voltage high frequency trans- former, it may be advantageous to have an input stage topology which is capable of boosting the d.c. link voltage in excess of the nominal 560 V d.c. as derived from a standard bridge arrangement. If this approach were adopted, an ideal target link voltage would be 800 V d.c., which, while remaining well within the limits of current semiconductor switch technology, would add cost to the system and introduce further power losses.•
The overall circuit efficiency must be high and, if possible, but not critical, should be capable of energy recovery to minimise power losses, particularly when 50–100μs pulse widths are being used to mitigate high resistivity dust reverse ionisation difficulties.From the above requirements of an electrostatic precipitator power supply, it was concluded that there were three possible circuit topologies for the input stage of an SMPS power supply:
(1) three phase six switch unity power factor (UPF) converter; (2) three phase boost type unity power factor rectifier;
(3) three phase full wave rectifier, with a.c. side filtering.
8.3.1 Three phase six switch mode UPF converter
This circuitry is illustrated in Figure 8.4. This is a well proven approach and is essentially a four quadrant switch mode inverter. By pulse width mode (PWM) switching of the semiconductor devices, usually IGTBs, the converter is capable of drawing sinusoidal input currents at near unity power factor, which will reduce the EMI and distortion problems [2]. Figure 8.5 indicates a PsPice
Figure 8.4 Three phase six switch mode UPF converter circuitry
waveform simulation of the topology, indicating minimum distortion of the line current. The power flow through this topology is a direct function of switching control, therefore reversal of power flow is possible.
The cost of this topology is high because of the larger number of semi- conductor switching devices (6) and the heat sink requirements of these switches. The number of switching devices in this topology also means that a complex switching and control system will be required further inflating the cost.
8.3.2 Three phase boost type UPF rectifier
The circuitry for the three phase boost type of unity power factor (UPF) rectifier is illustrated in Figure 8.6. Again it is fairly widely used and hence is a reliable option for an input device. By control of the switching frequency and duty ratio of the semiconductor switch, the rectifier can be made to draw nearly sinusoidal input current from the utility with a power factor approaching unity, hence EMI and distortion is minimised [3]. Figure 8.7 shows a PsPice waveform simulation of the line current and voltage indicating minimal distortion of these waveforms. As the name suggests this topology is also capable of boosting the output d.c. voltage to a pre-determined level.
This topology has the advantage that it achieves all that the switched mode converter can offer (except for bi-directional energy flow) with just one switching device and one fast recovery power diode instead of six used for the previous system.
Figure 8.5 PsPice simulation of a three phase six switch mode UPF converter circuit
8.3.3 Three phase full wave rectifier with a.c. side filtering
The basic circuitry for this topology is illustrated in Figure 8.8. Although the approach is widely used in some current SMPS designs being relatively low cost, it can create harmonic distortion within the power distribution network and give Figure 8.6 Three phase boost type UPF rectifier topology
Figure 8.7 PsPice simulation of a three phase boost type UPF rectifier circuit
rise to higher peak currents. Figure 8.9 presents a PsPice simulation indicating the large distortion of the line current with this approach. Unlike the initial topology this system is not capable of bi-directional energy flow; however, this is not considered a serious drawback to its usage as the system is very robust and reliable, because only diodes and not switching semiconductors are employed in the circuitry.
8.3.4 Comments on the various input stage topologies
Based on the criteria upon which these particular topologies have been con- sidered and analysed, the ‘three phase boost type UPF rectifier’, although the most expensive, has several potential operational advantages over the ‘three phase six switch mode UPF inverter’ and the ‘three phase full wave rectifier with a.c. side filtering’ approaches, as follows.
(a) It is capable of unity power factor (UPF) operation and boosting the volt- age with the minimum number of switching devices and hence should be more reliable at a lower cost than the three phase six switch approach. Figure 8.8 Three phase full wave rectifier with a.c. side filtering circuit
(b) The ability to boost the d.c. link output voltage can be advantageous because it reduces the turns ratio requirement of the high voltage trans- former and hence reduces harmonic distortion, but at an additional cost over the other topologies.
(c) Although bi-directional energy flow is only possible with the three phase boost topology, this is not envisaged to be a major problem with other systems, since the pulse energy could be fed back to the d.c. link rather than to the utility.