A RECHARGEABLE AND PORTABLE AC POWER SUPPLY WITH THE MINIMUM OF ELECTRONIC COMPONENTS Dominic E. Trotman and Charles Pollock Department of Engineering University of Warwick Coventry, U.K.,CV4 7AL
Abstract - This paper presents the development of a rechargeable portable power supply and includes many novel features. A new topology is proposed which combines the functions of rectifier, inverter, step-up and stepdown converter into a single circuit reducing components used resulting in a unit that is compact, lightweight, reliable and cheap. The concept of a variable voltage DC link between the power conversion stages and a new inverter switching scheme has been proposed and realised. The concepts presented have been amply verified and are also applicable to unintermptible power supplies and stand-by power systems.
INTRODUCTION The aim of this research was to develop a rechargeable and portable power supply with minimum cost, weight, size and circuit complexity while also reducing electrical stresses on the semiconductor devices. This has been achieved by incorporating the novel circuit design described in this paper. A rechargeable portable supply could have many uses. Supplying
mains power to remote sites such as building sites, radio masts, lock-up garages and garden sheds and running appliances such as electric drills, soldering irons and radio equipment can often be a problem. This is because a long extension cord from a mains supply can be dangerous or inconvenient or a chemically fuelled generator might be too noisy, expensive and unreliable. Therefore for remote power operation, an electrical unit which is as cheap, light, compact and reliable as possible, is highly desirable.
more common to use a power transformer operating at the supply frequency [2,3]. In the design proposed in this paper the transformer is eliminated completely by utilising power electronics. It should be noted that an energy storage element in the form of an inductance is still required when incorporating the power electronics although its size is small compared to a power transformer. Another important aim of this research was the minimisation of switching stresses in the devices. The electrical and thermal stress that a switch experiences during switching can be severe, especially during turn-off with an inductive load [4]. Often the voltage of the d.c ‘link’ of an inverter is fixed and a large capacitor is used to smooth d.c voltage prior to inversion. Therefore, the switching devices switch on and off at high frequency against the maximum supply voltage and electrical stresses can be considerable. The paper describes a control methodology which reduces these electrical stresses while still producing a sinusoidal voltage at the output of the inverter. This novel technique is called a variable voltage DC link. The variable voltage DC link concept can also be applied to the charging system in which mains voltage is rectified and converted to a lower voltage to charge the battery. Using the variable voltage DC link would mean that the large and costly smoothing capacitor required for a conventional constant voltage link could be eliminated.
The results described in this paper are also applicable to uninterruptible power supplies (UPS) and stand-by power supply systems. The portable power supply, UPS system and standby power supply can be represented by the same general block diagram as shown in Fig. 1. Power supplies for these applications have three main components: the rectifier, the battery charger and the inverter. Additional components and modifications are required depending on the power output, quality of output waveform and load characteristics. The reduction in circuit complexity described in this paper has been achieved by combining the separate units i.e. rectifier, charger and inverter into a single converter. Fig. 1 Block diagram of UPS. stand-by or rechargeable, portable power supply
A method of matching the supply voltage to the battery voltage is required. Although high voltage batteries are available 111 it is
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PROPOSED CIRCUIT TOPOLOGY There are two very distinct routes of power conversion required for the rechargeable portable power supply. Firstly, there is the charging route and this is where the mains voltage is converted into a DC voltage to charge the battery; this will be termed the charging mode. Secondly. there is the opposite route in which a low DC voltage from the battery is converted into an AC supply of the correct voltage and frequency; this will be termed the portable power mode. The understanding of these two separate modes is well known and both units can very readily be purchased. However, the concept of combining both systems into a single unit is relatively unknown. The proposed topology for a rechargeable portable power supply is shown in Fig. 2 and offers the following features: - transformerless design, - same inductor for step-up and stepdown operation, - combined inverter and rectifier units, - variable voltage DC link.
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The variable voltage link concept can also be applied to the stepdown mode of operation in which the mains voltage is rectified but not smoothed before being steppeddown to charge the battery.
OPERATION OF PROPOSED CIRCUIT TOPOLOGY Charging Mode Incoming AC mains is applied to the input of the rectifier. Diodes D1, D2, D3 and D4 perform rectification and unsmoothed rectified mains appears across the 'link' between the rectifier and the DC to DC converter. Capacitor C1 provides some filtering but is not sufficient to produce a smooth DC voltage. Switch TR5, diode D6. inductor L and capacitor C2 form the stepdown converter with the battery acting as its load. The control circuitry shown in Fig. 3 controls the switching action of TR5 which is pulse width modulated. As the input to the stepdown converter is unsmoothed rectified mains, then the control circuit functions to maintain a fixed charging current into the battery by adjusting the PWM accordingly.
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Fig. 2. Proposed topology for a rechargeable and portable power supply DESIRED CHARGING 1
The inverter circuit used in the portable power mode requires four freewheeling diodes in parallel with the switching devices. The proposed topology utilises these freewheeling diodes for use in the rectifier reducing the circuit complexity. The d.c voltage at the input to an inverter is usually constant and therefore can be termed a constant DC link. The control scheme chosen in the inverter then switches the constant DC link voltage using pulse width modulation to obtain the required AC output voltage. In this application capacitor C1 in the proposed topology would need to be a very large polarised component to smooth the output of the step-up convertor prior to inversion. This is a relatively expensive component of the system. Also the devices switch on and off at high frequency against the maximum supply voltage and electrical stresses can be considerable. If, instead, the waveform applied to the inverter consists of half wave rectified sinusoidal pulses, then with a very simple inverter switching scheme, it is possible to obtain the required output voltage as before. Using this new technique switching stresses will be reduced and inverter design simplified. This novel technique is therefore a variable voltage DC link.
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Fig. 3. Block diagram of the control circuit for the charging mode
The desired charging current is set by the user and this is compared with the actual current. An error signal which constitutes the difference between these two signals is fed to the PWM generator. The output of the PWM generator is adjusted to reduce the error and thus maintain a constant charging current to the battery, even though the input to the step-down circuit is unsmoothed rectified mains. The current sensor is a Hall effect device and provides electrical isolation between the current being measured and the control electronics. The control scheme could be easily modified to ensure that a pure sinusoidal waveform is drawn from the a.c supply minimising the need for any filtering.
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It is necessary to introduce a blanking time or ‘dead time’ between
Portable Power Mode
DC power is fed from the battery
DC converter configured in step-up mode with components L. D5 and TR6 to the
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constituting the step-up converter. The system block diagram is shown in Fig. 4.
switching one device in the leg off and switching the other device on. preventing a ‘shoot through’ condition. This is achieved using delay circuitry on the tum on time of the pulses on both channels 1 and2. RESULTS
Charging Mode
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The complete circuit proposed in the previous section has been constructed and tested in both charging mode and portable power mode. In charging mode a variable DC voltage of up to 250 V was applied to the rectifier and the charging current measured. The control circuit shown in Fig. 3 maintained the charging current to the preset level despite the changes in the input voltage. The value of energy storage inductance chosen was sufficient to maintain continuous current in the inductor and load. Figure 5 shows the switch voltage waveform together with the inductor current for an input voltage of 250 V.
li Fig. 4. Block diagram of control circuit for the portable power mode Voltage sense circuitry is provided to monitor the stepped-up voltage. The output of the voltage sense circuitry is fed to an operational amplifier along with a low voltage 50 Hz rectified sine wave. Control circuitry is used to pulse width modulate the DC to DC converter so that a rectified sinusoidal voltage appears on its output. This is achieved by modulating an 18 kHz triangular wave carrier with the output of the feedback amplifier which will consist of rectified sinusoidal pulses of the desired frequency with its DC level shifted depending on the output of the voltage sense circuit. The PWM switching instants are determined by the points of coincidence of the carrier and modulating waveform. The output of the DC to DC converter is connected to the input of the inverter via the variable voltage d.c link. The next stage is to invert the ‘link’ voltage with a simple switching scheme. The inverter stage consists of switches TR1, TR2, TR3 and TR4. The same sinewave used in the step-up stage control circuitry is fed to a zero crossing detector circuit to generate square wave pulses. The square wave is then separated into two channels, channel 1 and channel 2 respectively. Channel 2 is then inverted. The switching scheme is required to alternatively switch the ‘variable voltage DC link’ pulses to obtain the required mains sinusoid. This is achieved by switching TR1 and TR2 together and then TR3 and TR4 together etc (at a frequency of 50 Hz). Therefore, if channel 1 is used to switch TRI, TR2 and channel 2 is used to switch TR3, TR4, the desired output is obtained.
Fig. 5 Circuit waveforms in charging mode (Vin = 250 V) (a) switch voltage, (b) inductor current
The control circuit shown in Fig. 3 and illustrated by the waveforms in Fig.5 ensures that the current supplied to the battery is constant despite the sinusoidal variation in the voltage applied to the rectifier. This does however result in a non sinusoidal supply current waveform containing considerable harmonic components. Although these can be filtered it would be preferable to modify the control circuit so that the supply current contained less harmonics.
Portable Power Mode The circuit has also been tested in portable power mode. A 500 R resistive load was placed across the inverter output terminals and the circuit performance was monitored for different values of DC link capacitance. Figure 6 shows traces of inverter output current and voltage with a capacitance of 0.47 pF in the variable voltage DC link of the circuit. The waveforms taken without a link capacitor show a high harmonic content in the inverter output voltage and current which has a detrimental effect on efficiency. A capacitor of 0.47 pF
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was required to produce high quality output. This capacitance is significantly less than would have been required for a fixed voltage d.c link inverter.
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example medical and computer applications. The waveforms in the portable power mode have been obtained from a system that incorporated the same inductor used for the charging system and thus a further reduction in weight, size and cost will be obtained in any unit produced for commercial use. It should be noted however, that the output voltage from the inverter was restricted to 50 V. This was because of the very simple design of the control circuit which was built for evaluation purposes only. Now that the concept of the variable voltage DC link has been proven a full experimental prototype needs to be designed. The experimental prototype would also incorporate a high frequency isolated transformer to improve the voltage matching between the supply and the battery.
CONCLUSIONS 1
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A topology for a rechargeable portable power supply has been presented, in which the concept of a variable voltage DC link and new inverter switching scheme has been proposed and realised.
Fig. 6 Waveforms in portable power mode with C1 = 0.47 FF (a) inverter output current, (b) inverter output voltage
There are a minimum number of power devices with only six transistors and six diodes required for a complete battery charger/inverter system. 0 01 A I I U
The same inductor is used for both step-down and step-up DC to DC conversion removing the need for a low frequency transformer, resulting in less weight, size and cost. If output isolation is required the circuit can be easily modified, replacing the inductor with a lightweight transformer operating at the switching frequency.
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The variable voltage DC link between the DC to DC converter stage and inverterhectifier stages reduces the need for expensive smoothing capacitors and also considerably reduces the electrical stresses on the switching devices in the inverter. The inverter can now operate a simple switching scheme at the supply frequency with the switches only changing state when the variable voltage DC link is at a minimum.
Fig. 7 Circuit waveforms in portable power mode with C1 = 0 (a) inverter output current, (b) inverter output voltage
The waveforms presented in the previous figures clcarly show successful operation of the conversion of 12 volts 1 X into an alternating voltage of higher magnitude and at a frequency of 50 Hz. Comparing Fig. 6 and Fig. 7 shows that a small amount of capacitance is required across the variable voltage I X link to ensure that the harmonic content of the output waveform is reduced and that system efficiency is not drastically reduced. Furthermore, the control strategy could be modified to reduce the harmonic distortion in the output waveform [5]. The capacitor used in the variable voltage DC link was a polypropylene unpolarised capacitor and is substantially cheaper than the polarised electrolytic capacitor which would normally be employed to provide smoothing of a constant volrage DC link. The success of the variable voltage DC link will also mean reduced electrical stresses on semiconductor devices due to lower voltage and currents being switched simultaneously and thus ultimately a more reliable unit. This would be of prime importance for applications where reliability was esscntial for
REFERENCES 1. Divan, D.M. (1989) "A new topology for single phase UPS Systems", IEEE Industry Applications Society Annual Meeting, San Diego. pp 931-936. 2. Harada. K., Sakamota, H. and Shoyama, M. (1988). "Phase controlled DC - AC Converter with high frequency switching", IEEE Transactions on Power Electronics, vo1.3 No.4, October, pp.406. 3. Manias, S.. Ziogas, P.D. and Oliver, G. (1987). "Bilateral DC to AC convertor using a high frequency link", IEE h o c . Pt.B, Vol. 134,. No.1, pp. 15-18. 4. Blackbum, D.L. (1987) " Tum-Off Failure of Power MOSFET's IEEE Transactions on Industrial Electronics ",N0.2. April 1987, pp 136 - 142. 5. Evans, P.D., Close, P.R. (1987). "Harmonic distortion in pwm inverter output waveforms". IEE Proc. Pt. B, Vol. 134, July 1987, pp.224-231.
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