Designing an uninterruptible power supply based on the high efficiency push – pull converter

This paper presents an implementation of the DC/DC push–pull converter for an uninterruptible power supply (UPS). Some classical DC/DC converters are presented and analyzed for pointing out their advantages and drawbacks. Besides, an original system based on a push-pull converter associated with a dynamic modulation control is chosen. The main advantage is the possibility to control the delivered electric power in a wide range from very low level to high level of voltage within the same basic architecture. It can reduce the switching power losses and increase the power conversion efficiency. This paper proposed a new control scheme of the DC/DC converter and DC/AC inverter. The suggested system consists of a high efficiency DC/DC converter and a singlephase DC/AC inverter has been simulated using Matlab/Simulink and designed basing on the DSP TMS320F28027. Both results show high performances of the DC link and AC load voltages, when load changes from zero to rated. The performance of the proposed system has been verified through a 1kW prototype of the system for a 50 Hz/220-230 VAC load sourcing by two series connected batteries of 12V. The proposed DC/DC converter achieves a high efficiency of 93.0%. The system including the DC/DC converter and DC/AC inverter achieves an efficiency of 91.2% and Total Harmonic Distortion (THD) of AC load voltage reached 1.9%.


ABSTRACT:
This paper presents an implementation of the DC/DC push-pull converter for an uninterruptible power supply (UPS).Some classical DC/DC converters are presented and analyzed for pointing out their advantages and drawbacks.Besides, an original system based on a push-pull converter associated with a dynamic modulation control is chosen.The main advantage is the possibility to control the delivered electric power in a wide range from very low level to high level of voltage within the same basic architecture.It can reduce the switching power losses and increase the power conversion efficiency.This paper proposed a new control scheme of the DC/DC converter and DC/AC inverter.The suggested system consists of a high efficiency DC/DC converter and a singlephase DC/AC inverter has been simulated using Matlab/Simulink and designed basing on the DSP TMS320F28027.Both results show high performances of the DC link and AC load voltages, when load changes from zero to rated.The performance of the proposed system has been verified through a 1kW prototype of the system for a 50 Hz/220-230 VAC load sourcing by two series connected batteries of 12V.The proposed DC/DC converter achieves a high efficiency of 93.0%.The system including the DC/DC converter and DC/AC inverter achieves an efficiency of 91.2% and Total Harmonic Distortion (THD) of AC load voltage reached 1.9%.

INTRODUCTION
An uninterruptible power supply is an electronic device that provides an alternative electric power supply to connect electronic equipment when the primary power source is not available.The UPS systems provide for a large number of applications in a variety of industries.

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Conventional UPSs, which are shown in Figure 1, have very low efficiency, about 70-80%.Because of the topology consists of AC transformer, its power loss is very high.Moreover, semiconductors of the DC/AC inverter work with high current, leading power loss decreases.Some new UPSs have efficiency of 87% -[1].
To correct disadvantages of conventional UPS and increase their efficiency, the paper proposed a new topology of UPS, a block scheme is shown in Figure 2.
In the new proposed topology, in UPS applications, they need dischargers to draw power from batteries.The voltage level of batteries is much lower than that of DC-Link bus; and in order to generate 50 Hz, 220-230Vac.Thus, a converter with a high step-up voltage ratio is required for the dischargers.Furthermore, to effectively utilize the energy stored in batteries, the dischargers should be designed with high efficiency and the output voltage of the DC/DC converter should be high enough to generate the DC-link voltage [1], [2], [3].Up to now, various DC/DC converters have been investigated for high step-up applications.These DC/DC converters are divided into five basic topologies: Fly-back Converter, Forward Converter, Half-Bridge Converter, Full-Bridge Converter and Push-Pull Converter [2][3].After analysis advances and drawbacks of these converters, a push-pull DC/DC converter has been chosen.
In this paper, a push-pull converter for UPS applications is proposed.The proposed converter is depicted ina DC-link voltage controller is described for a constant DC-link voltage regulation.A digital model has been designed on DSP TMS320F28027, and prototype 1kW with a 50 Hz/220 Vac has been implemented.Experimental results show that ripple of Dc link is less than 1.5%, total efficiency is 91.2%.

ANALYSIS OF DC/DC TOPOLOGIES
Because the output voltage of the DC/DC converter should be high enough to generate the DC-link voltage around 350 V, and battery has a low voltage ranges from 20 V to 24 V, so a DC/DC converter has been investigated for high step-up applications.For this application, below DC/DC converters are chosen.These DC/DC converters provide an electrical isolation between the input and output of the converter.Selection of a topology depends on careful analysis of the design specifications, cost and size requirements of the converter.Operation of each of the above topologies is described in the following sections of this application note.Details of the topology selection and hardware design are provided in below sections.[4], [5]

Forward Converter
A forward converter, which can be a step-up or step-down converter, is shown in Figure 3(a).
When the transistor Q is ON, VIN appears across the primary, and then generates output voltage determined by equation 1.The diode D1 on the secondary ensures that only positive voltages are applied to the output circuit while D2 provides a circulating path for inductor current if the transformer voltage is zero or negative.A third winding is added to the transformer of a forward converter, also known as a "reset winding".This winding ensures that the magnetization of the transformer core is reset to zero at the start of the switch conduction.This winding prevents saturation of the transformer.One problem with the Forward topology is that the primary switch voltage can rise essentially unconstrained.When the switch turns off, energy stored in the transformer primary wants to cause current to continue to flow toward the FET drain.And resetting of a transformer is a cause to limit the maximum duty cycle that a Forward converter can operate.These issues tend to limit the power level, that a Forward converter is best used for power levels between 30W and 150W [1], [2].Moreover, Forward Converter hashigh input ripple current.So it tends to decrease a quality of output voltage and current

Fly-back Converter
The fly-back converter suffers from high power losses due to its hard-switching operation.Especially, for the high step-up applications like battery, the hard-switching operation causes high power losses.Considering its operating conditions for the harshest environment, the life time of the UPS system is reduced [1], [2],[3].In this converter, the reversal of the magnetic field is achieved by reversing the direction of the primary winding current flow.In this case, two capacitors C1 and C2 are required to form the DC input mid-point.Transistors Q1 and Q2 are turned ON alternately to avoid a supply short circuit, in which case the duty cycle, d, must be less than 0.5.For the half-bridge converter, the output voltage VOUT equals that of equation 3.  The half-bridge topology has some disadvantages: it features a split capacitor bus, twice the device current and twice transformer turn ratio, leading to increased transformer loss and size.The Half-Bridge DC/DC Converters have efficiency lower than the other one during a light load.They are working at half the supply voltage the switching transistors are working at twice the collector current compared with the basic push pull circuit.[1], [2], [3].

Half-Bridge Converter
The asymmetrical half-bridge fly-back converter has been utilized to reduce the switching power losses for low voltage DC source.It reduces the switching power losses by the soft-switching operation for the switching power devices [8], [9].The half-bridge fly-back converter in [8], however, requires a large turns ratio of the transformer for generating a high DClink voltage from the low voltage batteries.The large transformer turns ratio causes high voltage stresses on the switching power devices.

Full-Bridge Converter
The full-bridge converter topology is shown in Figure 4 (b), is basically the same as the halfbridge converter, where four transistors are used.Diagonal pairs of transistors (Q1-Q4 or Q2-Q3) conduct alternately, thus achieving current reversal in the transformer primary.Output voltage equals that of equation 4.
The full-bridge topology is well suited for applications that require a wide input voltage range.The full-bridge converter with a phaseshift control has been used for high step-up applications.However, the phase shifted fullbridge converters [10,11] require lots of power switching devices and the associated control circuits.So, the manufacturing cost of the system increases, which limit the practical utilization of the UPS system.
To optimize the global efficiency of the boost converters based on classical inverters described above, we designed a converter with a symmetric architecture using push-pull structure.The Push-Pull converters have several advantage characteristics in comparison of other topologies.A Push-Pull Converter is converter with a bidirectionally driven isolation transformer.Push-Pull transformers and filters are much smaller than others.A Push-Pull Converter has a low output ripple current, a lower input ripple current, and a simple gate drive.Moreover, the better core utilization and the lack of a realistic duty cycle limit in the Push-Pull architecture allows them to operate at significantly higher power levels.For moderate input voltages Push-Pull converters are useful to 500W and beyond [1], [2],[3].

THE OPERATION OF THE PROPOSED PUSH-PULL CONVERTER
The Figure 5 represents the electronic schema of the proposed boost voltage converter using the push pull structure.We can clearly observe in this schema the symmetrical structure of the output stage.Within this configuration, the transformer works in a forward mode at high frequency 20 kHz, and, thus, it allows the possibilities to minimize the coupled coils element even in case of conversion of high power.Moreover, the symmetrical structure of the push-pull stage allows the use of the two quadrants of the magnetic cycle of the transformer, the main advantage being the size optimization of the coils.
The operation of push-pull converter: When Q1 switches ON, current flows through the upper half of the T1 transformer primary and the magnetic field in T1 expands.The expanding magnetic field in T1 induces a voltage across the T1 secondary; the polarity is such that D2 is forward-biased and D1 is reverse-biased.D2 conducts and charges the output capacitor C2 via L1.L1 and C2 form an LC filter network.When Q1 turns OFF, the magnetic field in T1 collapses and after a period of dead time (dependent on the duty cycle of the PWM drive signal), Q2 conducts, current flows through the lower half of T1's primary, and the magnetic field in T1expands.At this point, the direction of the magnetic flux is opposite to that produced when Q1 conducted.The expanding magnetic field induces a voltage across the T1 secondary; the polarity is such that D1 is forward-biased and D2 is reverse-biased.D1 conducts and charges the output capacitor C2 via L1.After a period of dead time, Q1 conducts and the cycle repeats.There are two important considerations with the push-pull converter: • Both transistors must not conduct together, as this would effectively short circuit the supply.This means that the conduction time of each transistor must not exceed half of the total period (D < 0.5) for one complete cycle, otherwise conduction will overlap.
• The magnetic behavior of the circuit must be uniform; otherwise, the transformer may saturate, and this would cause destruction of Q1 and Q2.This behavior requires that the individual conduction times of Q1 and Q2 must be exactly equal and the two halves of the center-tapped transformer primary must be magnetically identical.These criteria must be satisfied by the control and drive circuit and the transformer.The output voltage equals that of Equation 5. Where: D is the duty cycle of the transistor Q1 and Q2 0<D<0.5

Push-Pull Switching Waveforms
Figure 6 shows oscilloscope waveforms for the drain voltages of the two primary switches and the output inductor current.When neither switch is active then both drain voltages are at the input voltage potential.It can be seen in the figure that there can be significant overshoots beyond the expected 2•Vin.

PROPOSED CONTROL SCHEMES
To achieve a high quality of load voltage and current (a rate amplitude and low THD), two control schemes have been developed.The first one allows keeping DC-link constant and the second one allows keeping AC voltage, current under rate parameters.

Push-Pull Control Loop
The push-pull converter is controlled with a voltage mode control scheme.The PWM module is configured for Push-Pull mode with an independent time-base.The DC Link voltage is measured by the voltage sensors and sent to DSP control.This value is subtracted from the voltage reference in software to obtain the voltage error.The voltage error is then fed into a control algorithm that produces a duty cycle value based on the voltage error, previous error, and control history.The output of the control algorithm is also clamped to minimum and maximum duty cycle values for hardware protection.The voltage mode control algorithm must be executed at a fast rate in order to achieve the best transient response.Therefore, the control algorithm is executed in the ADC interrupt service routine, which is also assigned the highest priority in the UPS code.A block diagram of the push-pull converter control scheme is shown in Figure 7.

Inverter Control Loop
The inverter output is generated by varying the voltage reference using a sinusoidal PWM.The measured output voltage is subtracted from the present reference value and the voltage error is obtained.The voltage error is fed into the voltage error compensation algorithm.The output of the voltage error compensator produces the current reference value.The measured output current is subtracted from the current reference to obtain the current error.The current error is used as the input to the current error compensation algorithm to produce the command signal for the PWM module.The result of the control loop is added to the nominal duty cycle for one leg of the full-bridge inverter and subtracted from the nominal duty cycle for the second leg.A block diagram of the full-bridge inverter control system is shown in Figure 8.

SIMULATION OF THE PROPOSED CONTROL SCHEME
The proposed control scheme has been simulated by Matlab/Simulink software and presented in Figure 9.In the scheme, the battery presented as an ideal DC power source voltage is 24V.Semiconductor components in the Push-Pull are MOSFETs.The transformer is isolated and has a transformer ratio 1:18.Semiconductor components of the DC/AC inverter are IGBT, and AC load is static load under rated power 1000W.The coefficient Kp, Ki in the control block is defined experimentally during a simulation process.It should be noted, the load current peak value has not been exceed rated, so saturation block must be in the current control block circuit DC/AC.Moreover, LC filter with the following L=2mHC=3.3uFis used to achieve true sin AC voltage output of the inverter.

THE DESIGN PROTOTYPE AND EXPERIMENTAL RESULTS
To illustrate the analysis and discussion, a 1kW prototype with proposed topology has been built.The schematic diagram of the proposed converter is depicted in Figure .2.The new control algorithm is programmed in the control board DSP TMS320F28027 to generate the command pulses for DC/DC push-pull converter and DC/AC full bridge inverter.The central processor unit of proposed converter is TMS320F28027.This is a 32 bit DSP controller of Piccolo family of Texas Instrument.F28027 has 2 Enhanced Pulse Width Modulator (ePWM) which have 12 PWM output compatible for 1 phase and 3 phase inverter.

CONCLUSIONS
This paper has proposed a novel converter based on push-pull DC/DC converter for battery sourcing applications in transformer-less single phase inverter.This specific architecture provides high efficiency and high step-up DCDC conversion with the possibility of an independent adaptation single link to the converter ratio of the transformer.In the paper, analysis of the converter has been presented in detail, from which design equations and circuit parameters were derived.The proposed converter can be operated with PWM control and constant switching frequency 20 kHz.Experimental results have verified that the proposed converter can achieve high efficiency over a wide load range.The proposed converter achieves a high efficiency of 91.2 % for its rated power.This paper proposed a new control schemes for pushpull DC/DC converter anDC/AC inverter, which allows to keep DC link and output voltage stable under input voltage and load variations, that ripple of Dc link is less than 1.5%.Moreover, the control scheme reduced transience time.The suggested converter is expected to be a good candidate for a standalone UPS, and PV application.
Designing an uninterruptible power supply based on the high efficiency push-pull converter  Le Minh Phuong  Phan Quoc Dzung  Nguyen Minh Huy  Nguyen Hoai Phong University of Technology,VNU-HCM (Manuscript Received on July 08 th , 2013, Manuscript Revised September 03 rd , 2013)

Figure 3 .
Figure 3.A DC/DC Forward converter (a); And DC/DC Fly-back converter (b)

Figure 3
Figure 3(b) shows a fly-back converter circuit.When transistor Q1 is ON, due to the winding polarities, the diode D1 becomes reverse-biased.Therefore, transformer core flux increases linearly.When transistor Q1 is turned OFF, energy stored in the core causes the current to flow in the secondary winding through the diode D1 and flux decreases linearly.Output voltage is given by 2 1

Figure 4 (
Figure 4(a) shows the half-bridge converter.In this converter, the reversal of the magnetic field is achieved by reversing the direction of the primary winding current flow.In this case, two capacitors C1 and C2 are required to form the DC input mid-point.Transistors Q1 and Q2 are turned ON alternately to avoid a supply short circuit, in which case the duty cycle, d, must be less than 0.5.For the half-bridge converter, the output voltage VOUT equals that of equation 3.

Figure 4 .
Figure 4.A DC/DC Half-Bridge converter (a) and DC/DC Full-Bridge converter (b)

Figure 7 .
Figure 7.A Block diagram of Push-pull voltage control loop

Figure 8 .
Figure 8.A Block diagram of the full-bridge inverter control system

Figure 9 .
Figure 9. Simulation model of proposed push-pull topology

Figure 10 .
Figure 10.The wavefrom of current load, DC link and load voltage: for case study 1(a), case study 2(b)

Figure 12 .
Figure 12.Experimental waveforms :DC link voltage, output voltage and output current

Figure 13 .
Figure 13.Experimental waveforms: DC link voltage, output voltage and output current and a spectrum of AC output voltage

Table 1 .
Power efficiency of UPS