DC/DC converter for locomotive air conditioning

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DC/DC converter for locomotive air conditioning [Copy link]

Abstract: In order to improve the working environment of locomotive drivers and save energy, the railway department is currently vigorously promoting locomotive variable frequency air conditioning. Since the locomotive power supply voltage is DC110V, it cannot meet the requirements of locomotive air conditioning, so first of all, the voltage must be increased to 300V through DC/DC conversion, and then converted into an AC voltage that meets the requirements of air conditioning through inversion. This paper mainly discusses the design and implementation of the DC/DC converter used for locomotive air conditioning, and gives the experimental results.

Keywords: locomotive air conditioner; boost converter; inverter

　

　

1 Overview

Locomotives run on railway lines all year round. In order to improve the working environment of locomotive drivers, the railway department is gradually equipping locomotives with air conditioning systems. The 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 capacity limitation and the impact of frequent start and stop of the air conditioner, the normal operation of other loads of the generator is seriously affected. For this reason, the railway department stipulates that the impact problem must be solved when installing the air conditioner to achieve soft start. At present, most manufacturers use general-purpose inverters for soft start. Although the impact problem is solved, it is obviously very "wasteful" to use general-purpose inverters only to achieve soft start of air conditioners. General-purpose inverters cannot meet the special requirements of variable-frequency air conditioners. Therefore, it is very meaningful to develop a special variable-frequency speed regulation system for locomotive air conditioners, which can not only achieve soft start, but also achieve temperature regulation through variable-frequency air conditioners to achieve energy saving.

At present, variable frequency compressors are generally driven by three-phase 200V asynchronous motors, and the operating frequency range is 0-120Hz. The inverter suitable for this is usually a DC300V voltage level. A DC generator on a diesel locomotive can provide a DC110V power supply, so a boost device must be used to convert the DC110V voltage into DC300V through boosting, and then convert it into an AC voltage that meets the requirements through an inverter. The basic structure of the locomotive variable frequency air conditioning controller is shown in Figure 1.

Figure 1 Basic structure diagram of locomotive variable frequency air conditioning controller

This paper mainly discusses the design and implementation of DC/DC converter for locomotive air conditioner. Firstly, the converter structure which is easy to implement is selected, then the circuit is designed, and finally the experimental results which meet the design requirements are given.

2 DC/DC converter main circuit structure selection and design

2.1 Main circuit structure selection

There are many possible structures for DC/DC boost converters. Usually, a full-bridge DC/DC converter circuit with transformer isolation is used for converters above 1kW. However, this converter circuit requires four power switching devices, which makes the system structure complex. At the same time, the DC bias magnetic problem of the isolation transformer must be considered in the circuit design, which undoubtedly increases the difficulty of control. Due to the harsh working environment of the locomotive variable frequency air conditioning controller, it is hoped that the circuit structure will be as simple as possible. Through analysis and experiments, it is believed that the Boost topology is a better implementation solution. This structure only requires one switch device, one boost diode and a boost inductor, and its control circuit is also relatively simple. Of course, this structure requires a larger capacity of the switch tube when the power is large [1]. This is why this topology is not selected for general high-power DC/DC converters. Considering the actual situation of this system and the current level of devices, it is still feasible to use the Boost topology. Its principle is shown in Figure 2.

Figure 2 DC/DC converter principle

The power of the locomotive air conditioner is 5kW. According to the requirements of the locomotive air conditioner, the DC/DC conversion circuit needs to convert DC110V into DC300V. The main circuit of the converter is a typical Boost structure, and the control circuit is implemented by the general PWM control chip SG3524. The PWM signal output by the control circuit is isolated and amplified by HCPL316J to drive the IGBT. HCPL316J is a dedicated IGBT drive circuit that implements overcurrent protection by detecting the saturation voltage drop of the IGBT. Compared with the general IGBT dedicated drive circuit with overcurrent protection, it has the advantages of simple circuit structure and low price. The current waveforms in the inductor and the voltage waveforms across the IGBT under continuous and intermittent current conditions of the Boost circuit are shown in Figure 3.

(a) When the current is continuous

(b) When the current is intermittent

Figure 3 Waveforms of inductor voltage _v L , _current i L _and voltage vs across IGBT

2.2 Calculation of main circuit parameters

2.2.1 Selection of operating frequency

Usually, the operating frequency of a small power switching power supply is as high as tens of kHz or even hundreds of kHz. However, in this circuit, due to the high power, the current flowing through the switch tube when it is turned on is very large, and the switching loss is very large, so the switch tube is not suitable for working at a very high frequency. Considering the actual situation, the switching frequency is selected as 15kHz.

2.2.2 Calculation of inductance

It is known that the compressor load power is 5kW, and the output voltage of the Boost circuit V _o =300V, so the equivalent load resistance R _L of the Boost converter is 18Ω, and the equivalent output load current I _o =17A.

In high-power applications, it is generally desirable to operate in a continuous inductor current state. From Figure 3 (a), based on the principle of volt-second balance of the voltage across the inductor in one cycle, we can get

V _i t _on－( V _o－V _i )( T－t _on )=0（1）

From formula (1), we can get

（2）

The current ripple in the inductor is

ΔI=t_on=DT（3）

Ignoring the converter loss, the converter input power is equal to the output power, that is,

V _i I _{L (AV)} = V _o I _o（4）

Where: I _{L (AV)} is the average value of the inductor current.

From formula (4), we get

I _{L (AV)} = I _o = I _o（5）

To ensure current continuity, the inductor current should satisfy equation (6).

I _{L (OFF)} ≥Δ I /2（6）

Considering equations (3) and (6), the inductance value that satisfies the current continuity condition is

L≥R_LTD(1－D)2（7）

Equation (7) should be satisfied under all duty cycle conditions, and the current should be continuous above 10% rated load. 10% load is equivalent to R _L = 180Ω. When D =

L ≥ ×180× × =0.89mH, in the actual circuit, L =1.1mH.

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 formula (8) [1].

C≥(8)

According to the design requirements, when the input voltage is 55V, the output voltage should still be 300V. In this way, the maximum duty cycle D _max = = =0.82, considering the maximum duty cycle and full load, and taking the voltage ripple factor as 2%, the switching frequency as 15kHz, and the load resistance as 18Ω, C = 160μF can be obtained. In the actual circuit, C = 220μF.

2.2.4 Selection of IGBT power switching device

The peak current flowing through the IGBT is the peak current flowing through the inductor, that is,

I _S(M) = I _{L (M)} = I _{L (AV)}＋Δ I _L（9）

Where: IL _(M) and IS _{(M) are the peak value of the inductor current and} the peak value of the current flowing through the IGBT respectively.

Substituting equation (3) into equation (9), under full load conditions, we get IS _(M) = 150A, and then consider a double safety margin. When the switch is turned off, the voltage across it is the input voltage, i.e. 300V, and also consider a double safety margin, so a 600V/300A IGBT is selected.

3 PWM control and IGBT drive circuit

3.1 PWM control circuit[2]

The PWM control uses the SG3524 controller, and its functional block diagram is shown in Figure 4.

Figure 4 3524 principle block diagram

The DC power supply _Vs is sent from pin 15 to the input of the reference voltage _regulator to generate a stable +5V reference voltage, which is then sent to other internal and external circuits as a power supply. Pin 7 must be connected to an external capacitor _{CT , and pin 6 must be connected to an external resistor RT} , so that a sawtooth wave is generated at pin _7. Different oscillation frequencies can be generated by selecting different CT and RT . The output of the oscillator is divided into two paths: one is sent to the bistable trigger and two NOR gates in the form of a clock pulse; the other is sent to the non-inverting end of the comparator in the form of a sawtooth wave (pin 7). The reverse end of the comparator is connected to the error amplifier. The error amplifier is actually a differential amplifier, one of whose inputs is connected to the output voltage after voltage division, which plays a feedback role. V _REF is connected to the other end of the amplifier as a given signal through resistor voltage division, and pin 9 is the compensation end. The output of the error amplifier is compared with the sawtooth wave. The output of the comparator is a pulse signal whose width changes with the output voltage of the error amplifier. The pulse signal is then sent to the input of the NOR gate. The other two inputs of the NOR gate are the output signals of the trigger and the oscillator respectively. Finally, two pulse waves with a difference of 180° are sent out. SG3524 has an external shutdown function. When an external fault occurs, the PWM output of SG3524 is blocked through pin 10 to play a protective role.

In this scheme, pin 12 and pin 11 are connected in parallel with pin 13 and pin 14 respectively, and the total output pulse is widened, so that the original two pulses with a duty cycle of 0-50% are widened to a pulse with a duty cycle of 0-100%. In actual use, in order to prevent the main circuit from overcurrent caused by excessive pulse width, a limiting circuit is added to pin 9.

3.2 IGBT drive circuit [3]

Since the selected IGBT has a large power, the pulse signal output by SG3524 must pass through the isolation amplifier circuit to drive the IGBT. Considering reliability and economy, HCPL316J was selected as the driving circuit. In addition to isolation and driving functions, HCPL316J also has overcurrent protection function. Overcurrent protection is achieved by measuring the saturation voltage drop across the IGBT. When overcurrent occurs, HCPL316J blocks the IGBT drive signal and sends a fault signal at the same time. In this solution, the fault signal output by HCPL316J is connected to the SHUTDOWN terminal of SG3524 to achieve more effective protection. The principle block diagram of HCPL316J is shown in Figure 5.

Figure 5 HCPL316J principle block diagram

4 Experimental Results

According to the above design, a DC/DC converter for locomotive was built in the laboratory, and a series of experiments were carried out. Figure 6 shows the experimental waveform.

(a) Waveform when the load is light

(b) Waveform when the load is heavy

Figure 6 Inductor current and voltage waveforms across the switch under resistive load

When the load is light, the voltage across the switch tube will oscillate due to the influence of distributed capacitance. Under full load, the DC input voltage changes from 55V to 165V, and the output voltage of the DC/DC converter can be stabilized at 300V, with good regulation capability. However, due to the structure of the circuit itself, the lower the input voltage, the greater the current flowing through the switch tube and the Boost inductor, so the heat dissipation of the switch tube and the inductor must be considered.

5 Conclusion

The DC/DC converter for locomotive air conditioning proposed in this paper has the advantages of simple structure and convenient debugging. The laboratory experimental results show that the scheme is feasible and needs to be tested in operation and continuously improved.