Design and Implementation of IGBT based Constant Voltage Battery Charger for Railway Coaches

Design and Implementation of IGBT based Constant Voltage Battery Charger for Railway Coaches

Sushma S. Shetty

M. Tech Power Electronics R.V.C.E.

Bangalore, India

Dr. V. Chayapathy

Associate Professor, Dept. of EEE R.V.C.E.

Bangalore, India

Abstract This paper presents a 6.5kW IGBT based DSP controlled constant voltage battery charger designed and implemented for Railway Coaches. The proposed battery charger comprises of a three phase uncontrolled bridge rectifier at the front end and an IGBT based full bridge DC-DC converter at the output end. The rectifier circuit converts the three phase 415V AC supply to DC. The rectified DC output is given as input to the full bridge DC-DC converter. The converter then generates a regulated and isolated DC voltage which is used to charge the battery. A complete analysis of full bridge DC-DC converter, design considerations and validation of simulation results are discussed in this paper.

Keywords DSP, constant voltage battery charger, three phase uncontrolled bridge rectifier, full bridge DC-DC converter, isolated DC output.

voltage input to the battery throughout the charging process.

Digital controllers are more beneficial than analog controllers as they have the following advantages: Reduced noise levels, programmable compensator, high reliability and high speed. TMS320F28027 has advanced on-chip control peripherals like PWM modules, analog comparators with digital analog converter (DAC), slope compensation hardware and 12- bit high speed ADCs coupled with an efficient 32-bit CPU.

1. PROPOSED CONVERTER TOPOLOGY

   The basic block diagram representation of the proposed battery charger circuit is as shown in Fig. 1.

   1. INTRODUCTION

      Modern locomotives have various electrical & electronic equipment, such as lighting, heating and communication systems. They must continue to work even in case of power failures. Hence, the battery packs are a critical part of the on-board safety equipment. Valve regulated lead acid (VRLA) batteries are widely being used in passenger coaches of Indian Railways. These batteries are capable of operating under extreme temperatures, highly humid and dusty atmosphere and can withstand vibrations. Nine 12V batteries are connected in series to form an 110V battery of 70Ah capacity. It is essential to charge these batteries efficiently in order to prolong the battery life and so a battery charger plays a very important role.

      In recent times, several high efficiency, high-power density AC-DC switched mode power supply topologies have been proposed for charging the battery [1]. Amongst various topologies, a diode based bridge rectifier coupled with a full bridge DC-DC converter is presented in this paper. A full-wave rectier produces a DC voltage from a three phase 415V±15%, 50 Hz AC source, and a full bridge DC-DC converter reduces the rectified DC voltage to an appropriate level suitable for the application. A full bridge DC-DC converter is chosen because of its high power handling capability and stability [3, 4].

      The proposed battery charger is operated in Constant Voltage Mode with the help of a DSP controller (TMS320F28027). In constant voltage mode, irrespective of the battery's state of charge, the charger maintains nearly the same

      Fig 1. Block Diagram Representation of Battery Charger

      The core part of the proposed battery charger circuit is the full bridge DC-DC converter which consists of 4 IGBTs connected in a bridge formation feeding power to a high frequency transformer which provides galvanic isolation between input and output. The secondary side of the high frequency transformer is connected to an output rectifier and LC filter circuit.

      1. Analysis of full bridge DC-DC converter

         The circuit diagram of a full bridge DC-DC converter is as shown in Fig 2:

         Fig 2. Full Bridge DC-DC Converter

         The switching topology used for the full-bridge converter is the bipolar voltage switching, where the transistors are switched in pairs. Transistors T1 and T4 are considered as one switch pair and transistors T2 and T3 are considered as the other switch pair. The switching sequence is as shown in Fig 3:

         Fig 3. Switching sequence

         When T1 and T4 are closed, the voltage across the transformer primary is Vs. When T2 and T3 are closed, the transformer primary voltage is Vs. For an ideal transformer, having all switches open will make = 0. Diodes D1 and D2 on the transformer secondary, rectify this waveform to produce the voltage Vx as shown in Fig 4:

         ()

         = [ ] (1)

         Mode 2: Transistors T2 and T3 are closed

         • Transformer primary voltage, =

         • Diode D2 is forward biased and diode D1 is reverse biased.

         • Therefore, = 2 = ()

           Voltage across the filter inductor = = ()

           Transistors T2 and T3 are also switched ON for a period of DT. Therefore,

           ()

           = [ ] (2)

           Mode 3: When all the transistors are switched open

         • Transformer primary voltage, = 0

         • Transformer secondary voltages, 1 = 2 = 0

         • Therefore, = 0

           With both switches open, the current in each of the primary windings is zero. The current in the lter inductor L must maintain continuity, resulting in both D1 and D2 becoming forward-biased. Inductor current divides evenly between the transformer secondary windings.

           Voltage across the filter inductor = .

           The change in inductor current is given by,

           2

           = ( ) (3)

           Since, average voltage across inductor is assumed to be zero, net change in inductor current over one period is zero. Therefore,

           ( )

           [ ] +

           T

           (2

           ) = 0

           ( )

           [ ] =

           T

           (2

           )

           Fig 4. Voltage Vx

           Mode 1: Transistors T1 and T4 are closed

         • Transformer primary voltage, =

         • Diode D1 is forward biased and diode D2 is reverse biased.

         • Therefore, = 1 = ()

           Voltage across the filter inductor = = ()

           Assuming a constant output voltage , the voltage across L is a constant, resulting in a linearly increasing current in L. In the interval when T1 and T4 are closed, the change in current in L is,

           () =

           2

           = ( ) (4)

           The waveform of current through the filter inductor is as show in Fig 5:

           Fig 5. Inductor current waveform

           The waveforms of current through the filter capacitor and output voltage ripple are as shown in Fig 6:

           Fig 6. Capacitor current and Output voltage ripple waveforms

           The output voltage ripple of the full bridge converter is given by,

           Fig 7. Three Phase Uncontrolled Bridge Rectifier

           Output voltage of three phase uncontrolled bridge rectifier =

           33

           = 472.66V

           This rectified voltage is given as input to the full bridge DC-DC converter. Therefore, the supply voltage to full bridge DC-DC converter is Vs = 472.66V

           Calculation of Duty Ratio (D) Required Output Voltage, = 110V

           Ideally, when a switch is ON, the voltage drop across the switch

           is zero. However it is not true in practical cases. Considering the forward voltage drops of the switch and diode, the output voltage equation of the full bridge DC-DC converter is,

           = 12

           (5)

           322

      2. System Requirements

         Transformation ratio of the high frequency transformer,

         = 138

         415

         IGBT (IGW40T120) forward voltage drop () = 1.7V Diode (DSI 30-16A) forward voltage drop () = 1.6V Therefore, D (max) = 35.4 %

         The switching frequency of the IGBT () is chosen to be 8 kHz, considering the associated losses and efficiency of the converter.

         Calculation of Filter Inductor Value (L)

         Output Current of battery charger circuit = 50A

         According to the system requirements, current ripple should be less than 3% of output current

         IL < 3% of 50 A < 1.5A

         Value of Filter Inductor is given by,

      3. Design Considerations

         The circuit is designed for critical value of input supply voltage. Three phase input supply line voltage = 415 15% = 350V Peak Line Voltage = 350 x 2 = 494.97V

         Phase Voltage (Star Connected Source) = 350 / 3 = 202.07V Peak Phase Voltage (Vm) = 202.07 x 2 = 285.77V

         Taking IL =1.5A, L = 1.33mH Therefore, choose L = 3mH

         Calculation of Filter Capacitor Value (C)

         According to system requirements, output voltage ripple should be less than 2% of output voltage

         < 2% < 0.02

         12

         In a Full bridge DC-DC converter,

         Value of filter capacitor,

         1 (2 0.354)

         = 322

         =

         3

         32 3 10

         3 2 = 2.376

         0.02 (8 10 )

         Therefore, choose C = 4700

      4. Feedback System

      Voltage Sensing Element

      The output voltage of the battery charger is sensed and fed to the ADC of the DSP Processor (TMS320F28027) so that the output voltage is maintained constant even when there is a variation in the input supply voltage by changing the duty ratio of PWM pulses correspondingly. In order to provide a convenient analog input range to the embedded analog-to- digital converter, a voltage sensing element (AMC1100) from TI is chosen. Fig 8 shows how the voltage sensing element is connected to the battery charger circuit.

      Fig 8. Output Voltage Sensing

      A voltage divider circuit (R1 & R2) is connected across the battery charger output. Values of R1 and R2 are chosen such that the differential analog input range, (VINP) (VINN), is ±250 mV with a maximum of ±320 mV before clipping occurs. Once the VDD1 and VDD2 power is applied to the AMC1100, the analog output is available with a fixed gain of 8. Therefore, with an input voltage of ±250 mV, the nominal output is ±2.0 V.

      IGBT Gate Driver

      The PWM pulses generated by the DSP processor do not have the brute drive capability required for switching the IGBT. Therefore, an IGBT gate driver IC (UCC27324) from TI is chosen. UCC27324 is capable of delivering 4 A of current to an IGBT gate. It also helps in reducing the effect of high- frequency switching noise. Fig 9 shows the typical connection of IGBT Gate Driver IC.

      Fig 9. IGBT Gate Driver

2. SIMULATION AND RESULTS

   The proposed battery charger circuit is simulated using MATLAB SIMULINK to validate the design. It has been simulated in two parts. Fig 10 shows the SIMULINK model of three phase uncontrolled bridge rectifier and Fig 12 shows the SIMULINK model of full bridge DC-DC converter. Corresponding waveforms are as shown in Fig 11, Fig 13, Fig 14, Fig 15 and Fig 16. The simulation results are in agreement with the theoretical calculations.

   Fig 10. Simulink model of Three Phase Uncontrolled Bridge Rectifier

   Fig 11. Phase and line voltages of three phase input and output voltage waveforms

   Fig 12. Simulink Model of Full Bridge DC-DC Converter

   Fig 13. PWM pulses to (T1&T4) and (T2&T4)

   Fig 14. Transformer Primary and Secondary Voltages

   Fig 15. Voltage Vx and Inductor Current Waveforms

   Fig 16. Output Voltage Waveform

   The results are tabulated as shown in Table 1.

   TABLE 1: SIMULATION RESULTS

   PARAMETER	VALUE
   Output Voltage Ripple	0.001%
   Output Current Ripple	1.33%
   Efficiency	95%
   Regulation	Less than 2%

   The hardware implementation of the battery charger circuit is as shown in Fig 17.

   Fig 17. Hardware Setup

3. CONCLUSION

An IGBT based constant voltage battery charger circuit was designed and validated. The DSP controller enabled the circuit to be free of human intervention. The desired efficiency and output regulation was achieved.

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