Abstract: In order to improve the working environment and energy saving of locomotive drivers, the railway department is currently promoting locomotive inverter air conditioners. Since the locomotive power supply voltage is DC110V, it can not meet the requirements of locomotive air conditioning. Therefore, the DC/DC conversion must first be used to raise the voltage to 300V, and then through the inverter to become the AC voltage that meets the air conditioning requirements. The design and implementation of DC/DC converter for locomotive air conditioning are mainly discussed, and the experimental results are given. Key words: locomotive air conditioning; Boost converter; inverter DC/DC 1 Overview The locomotive runs on the railway line all the year round. In order to improve the working environment of the locomotive driver, the railway department is gradually equipped with an air conditioning system on the locomotive. Early installations were generally three-phase fixed-frequency air conditioning systems. The power supply on the diesel locomotive is generated by a three-phase 380V generator. Due to the limitation of capacity and the frequent start and stop of the air conditioner, the normal operation of other loads of the generator is seriously affected. To this end, the railway department stipulates that the installation of the air conditioner must solve the impact problem and achieve a soft start. At present, most manufacturers use general-purpose inverters for soft start. Although the problem of impact is solved, it is obviously "wasted" to use the general-purpose inverter only to realize the soft start of the air conditioner, and the general-purpose inverter can not meet the special requirements of the inverter air conditioner. Therefore, it is very meaningful to develop a special frequency conversion speed control system for locomotive air conditioners, which can realize soft start and temperature adjustment through inverter air conditioner to achieve energy saving. This paper mainly discusses the design and implementation of DC/DC converter for locomotive air conditioning. First, the converter structure that is easy to implement is selected, and then the circuit is designed. Finally, the experimental results that meet the design requirements are given. 2 DC/DC converter main circuit structure selection and design 2.1 main circuit structure selection For DC/DC boost converters, many forms of construction are possible. A full-bridge DC/DC converter circuit with transformer isolation is usually used above 1 kW. However, this conversion circuit requires four power switching devices, which makes the system structure complicated, and must overcome the DC bias problem of the isolation transformer in circuit design. This undoubtedly increases the difficulty of control. Due to the harsh working environment of the locomotive inverter air conditioner controller, it is hoped that the circuit structure is as simple as possible. Through analysis and experimentation, it is considered that the Boost topology is a better implementation. The structure requires only one switching device and one boosting diode and boosting inductor, and the control circuit is relatively simple. Of course, the structure requires a large capacity of the switch tube when the power is large [1], which is why the general high-power DC/DC converter does not select such a topology. Considering the actual situation of the system and the current level of the device, it is still feasible to use the Boost topology. The principle is shown in Figure 2. 2.2 Calculation of main circuit parameters 2.2.1 Selection of working frequency Usually low-power switching power supplies operate at frequencies up to tens of kHz or even hundreds of kHz. However, in this circuit, due to the large power, the current flowing in the switching tube is large when the conduction is turned on, and the switching loss is very large, so the switching tube should not work at a very high frequency. Consider the actual situation, choose the switching frequency is 15kHz. 2.2.2 Calculation of inductance It is known that the compressor load power is 5 kW, and the output voltage of the Boost circuit is Vo=300 V, so that the equivalent load resistance of the Boost converter is RL=18 Ω, and the equivalent output load current Io=17A. Viton-(Vo-Vi)(T-ton)=0 (1) Available from formula (1) Vo/Vi=1/(1-D) (2) The current ripple in the inductor is ΔI=(Vi/L)ton=(Vi/L)DT (3) Ignoring converter losses, the converter input power is equal to the output power, ie ViIL(AV)=VoIo (4) Where: IL(AV) is the average of the inductor current. Obtained by formula (4) IL(AV)=(Vo/Vi)Io=(1/ID)Io (5) In order to ensure continuous current, the inductor current should satisfy equation (6). IL(AV)≥ΔI/2 (6) Considering equations (3) and (6), the inductance value that satisfies the continuous current can be obtained. Equation (7) should be satisfied for all duty cycles, taking into account the continuous current above 10% of rated load. The 10% load is equivalent to RL=180Ω, and when D=, the inductance value when the current is continuous is obtained. =0.89mH, L=1.1mH in the actual circuit. 2.2.3 Calculation of Output Filter Capacitor Capacity In order to meet the requirements of the relative value of the output ripple voltage, the filter capacitor is determined by equation (8) [1]. C≥(VoDT/Δ VoRL ) (8) According to the design requirements, the output voltage should still be 300V when the input voltage is 55V. Thus, the maximum duty ratio Dmax===0.82, considering the maximum duty cycle and full load, and taking the voltage ripple coefficient of 2%, the switching frequency of 15kHz, and the load resistance of 18Ω, can be obtained C=160μF, the actual circuit Take C=220μF. 2.2.4 Selection of IGBT for power switching devices The peak value of the current flowing in the IGBT is the peak value of the inductor current flowing, that is, IS(M)=IL(M)=IL(AV)+1/2ΔIL (9) Where: IL(M) and IS(M) are the peak current of the inductor current and flow through IGBT current peak. Substituting equation (3) into equation (9), at full load, available IS(M)=150A, consider twice the safety margin; when the switch is turned off, the voltage across it is the input voltage, ie 300V, and also considers the safety margin twice, so choose the 600V/300A IGBT. . 3.1 PWM control circuit [2] The PWM control uses the SG3524 controller, and its block diagram is shown in Figure 4. The DC power supply Vs is sent from the pin 15 to the input of the reference voltage regulator to generate a stable +5V reference voltage, which is then sent to the internal and external circuits as a power supply. Pin 7 must be connected to external capacitor CT, and pin 6 must be connected to resistor RT to generate sawtooth wave on pin 7. Different CT and RT can be selected to generate different oscillation frequencies. The output of the oscillator is divided into two ways: one is sent as a clock pulse to the flip-flop and two NOR gates; the other is sent to the non-inverting terminal of the comparator in the form of a sawtooth wave (pin 7). The opposite end of the comparator is connected to the error amplifier. The error amplifier is actually a differential amplifier with one input connected to the divided output voltage for feedback. VREF is connected to the other end of the amplifier by a resistor divider as a given signal, and pin 9 is the compensation terminal. The output of the error amplifier is compared with the sawtooth wave. The output of the comparator is a pulse signal that changes in width according to the output voltage of the error amplifier, and then the pulse signal is sent to the input of the NOR gate, and the other two inputs of the NOT gate. The terminals are the output signals of the flip-flop and the oscillator, and finally send two pulse waves with 180° difference. The SG3524 has an external shutdown function that protects the PWM output of the SG3524 via pin 10 when an external fault occurs. 3.2 IGBT drive circuit [3] Due to the large power of the selected IGBT, the pulse signal output by the SG3524 must pass through an isolation amplifier circuit to drive the IGBT. In view of reliability and economy, HCPL316J was selected as the drive circuit. In addition to isolation and drive functions, the HCPL316J also features overcurrent protection. The overcurrent protection is realized by measuring the saturation voltage drop across the IGBT. On the one hand, the HCPL316J blocks the IGBT drive signal and sends out the fault signal. In this scenario, the fault signal output by the HCPL316J is connected to the SHUTDOWN terminal of the SG3524 for more effective protection. The block diagram of HCPL316J is shown in Figure 5. According to the above design, a DC/DC converter for a locomotive was formed in the laboratory, and a series of experiments were carried out. Figure 6 is an experimental waveform. When the load is light, the voltage across the switch will oscillate due to the influence of the distributed capacitance. At full load, the DC input voltage is varied from 55V to 165V, and the output voltage of the DC/DC converter can be stabilized at 300V, which has good regulation capability. However, due to the structure of the circuit itself, the lower the input voltage, the larger the current flowing through the switching transistor and the Boost inductor, so the heat dissipation problem of the switching transistor and the inductor should be considered. 5 Conclusion The DC/DC converter for locomotive air conditioner given in this paper has the advantages of simple structure and convenient debugging. The results of laboratory experiments show that the program is feasible and needs to be tested and improved.
At present, the inverter compressor is generally driven by a three-phase asynchronous motor of about 200V, and the working frequency range is 0 to 120 Hz. The inverter to which this applies is typically a voltage level of DC300V. A DC generator on a diesel locomotive can supply DC110V power. Therefore, a boosting device must be used to convert the DC110V voltage into DC300V, and then convert it into an AC voltage that meets the requirements. The basic structure of the locomotive inverter air conditioner controller is shown in Figure 1.
The power of the locomotive air conditioner is 5kW. According to the requirements of locomotive air conditioning, the DC/DC converter circuit needs to convert DC110V into DC300V. The main circuit of the converter is a typical Boost structure, and the control circuit is realized by a general-purpose PWM control chip SG3524. The PWM signal output by the control circuit is isolated and amplified by the HCPL316J to drive the IGBT. HCPL316J is an IGBT-specific drive circuit that provides overcurrent protection by detecting the saturation voltage drop of the IGBT. Compared with the IGBT dedicated drive circuit with overcurrent protection, it has the advantages of simple circuit structure and low price. The current in the inductor and the voltage waveform across the IGBT in the continuous and intermittent current of the Boost circuit are shown in Figure 3.
In high power applications, it is generally desirable to operate in a continuous state of the inductor current. According to Figure 3 (a), according to the principle that the voltage across the inductor is balanced in volt-seconds in one cycle,
3 PWM control and IGBT drive circuit
Figure 5
In this solution, the legs 12 and 11 are respectively connected in parallel with the legs 13 and 14 to widen the total output pulse, so that the original two-way duty ratio is 0-50% pulse broadening to a duty ratio of 0-100%. All the way to the pulse. In actual use, in order to prevent overcurrent of the main circuit due to excessive pulse width, a limiter circuit is added to the pin 9.
Figure 6
4 Experimental results
September 23, 2018