Development of DC/DC converter for emergency ventilation power supply of light rail vehicle-EEWORLD

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1 Introduction
When a power failure occurs in a light rail vehicle, in order to ensure normal ventilation and passenger safety in the vehicle, the main circuit in the front-stage DC/DC converter must increase the voltage from 24 V to 600 V, which is a large step-up factor. The literature briefly introduces several high-gain DC/DC converters. Since the operating time of the emergency ventilation power supply depends on the battery, the design standard is 45 minutes. Therefore, in the case of short-term operation, if the symmetry of the pulse can be guaranteed, the transformer bias problem of the full-bridge converter can be ignored.
Combined with the specific working conditions of the emergency ventilation power supply, the full-bridge converter is used as the main circuit. Since the full-bridge converter has parasitic oscillation of the rectifier bridge, as the transformer turns ratio increases, the oscillation caused by the leakage inductance and the output diode junction capacitance of the full-bridge converter secondary becomes more serious, so an absorption circuit must be used to suppress the oscillation. This article introduces the structure and parameters of the front-stage DC/DC converter of the light rail emergency ventilation power supply, and compares and analyzes three RCD absorption circuits in combination with actual working conditions, and gives specific parameter designs.

2 System introduction and absorption circuit selection
2.1 System main circuit structure and principle
Figure 1 shows the system main circuit structure.

Its working principle is: 24V DC is converted to AC through square wave inverter, and then stepped up by a high-ratio transformer and uncontrolled rectified by a diode rectifier bridge. After rectification, it is converted into the required 600 V DC through LC filtering. The output voltage can be controlled by controlling the duty cycle of the switch tube. Since the actual application requires a very small transformer, the switching frequency is set to 50 kHz. C1 and C2 are the support capacitors on the power supply side and the output side, respectively.
For the full-bridge converter topology, the voltage across the secondary rectifier diode will have large oscillations, which is mainly caused by the parasitic oscillation between the leakage inductance of the high-frequency transformer and the junction capacitance of the rectifier diode (or the distributed capacitance of the winding). In this design, due to the high step-up multiple, the oscillation caused by the primary switching process is transmitted to the secondary through the transformer, which will also cause a great impact. Experiments show that the oscillation peak is close to 1.2 kV, so it is necessary to add absorption circuits to both the primary and secondary to suppress the spikes at both ends of the rectifier diode.
2.2 Comparison of several RCD topologies
Figure 2a. b shows two RCD absorption topologies. Topology I is a traditional RCD absorption circuit, and its operating waveform is shown in Figure 2c.

[page] For topology I, in the steady state before charging, the voltage uCs on Cs is the same as the output voltage Udc. When reaching time t0, the transformer secondary voltage becomes Uin/n, the rectifier bridge starts forward conduction, the other set of diodes is turned off, and the secondary winding leakage inductance begins to oscillate with the junction capacitance of the rectifier bridge diode. The rectifier bridge output voltage u0 rises and reaches Udc at t1, then VDs is turned on, and Cs joins the resonance. uCs and ur0 rise to the clamping voltage Uc, and then drop to Uin/n at the end of resonance at t2. When VDs is turned off at time t3, uCs discharges through Rs and Lf until uCs and Udc reach equilibrium at t4. During this off time, ur0=0.
In this snubber circuit model, the maximum voltage stress of the rectifier bridge is controlled to be less than 2 times 2Uin/n Uc. From the working process, since the voltage drop on Rs is almost zero, iRs=0 in [t1, t3]. There is only one current at [t3, t4], at which time the loss Edis on Rs and Lf can be calculated linearly as:

The disadvantage of this snubber circuit is that when the additional voltage on Cs is discharged, it must pass through the filter inductor, which slows down the discharge process and causes a relatively high Uc.
In topology II, Rs and Lf are connected in parallel. Its working process is similar to that of topology I, but when VDs is turned off at t3, the discharge current only passes through Rs and not Lr, which speeds up the discharge speed.
In this circuit mode, part of the resonant energy is transmitted to the terminal output through Rs. As shown in Figure 2c, when uCs is higher than Udc, there is a small current iRs' on Rs to maintain the charge and discharge balance of Cs. Its size depends on the value of Rs, and this current is not enough to release the voltage on Cs to balance with Udc during the turn-on time. Therefore, the amplitude of uCs can be stabilized at Uin/n before the rectifier bridge is turned off at t3.
This discharge loop is a first-order model and is relatively simple to calculate. Since energy will be consumed on Rs as long as uCs is higher than Udc, the circuit loss Edis is larger than that of topology I, which can be approximately calculated as follows:

Figure 3a is another commonly used RCD absorption circuit, in which the absorption resistor and capacitor are connected in parallel. Figure 3b is its working waveform. When the rectifier bridge is turned off, uCs is completely discharged through Rs. As long as there is enough time, all the energy on the capacitor will be consumed through Rs, and uCs will be zero at time t4. This discharge process is faster, but the energy loss is also greatly increased compared with the previous circuit.

The calculation formula for Edin is:
The calculation formula for Edis is:

By comparison, it can be seen that the clamping absorption effect of topology III is the best, and the clamping absorption effect of topology II is slightly better than that of topology I.
2.3 Absorption circuit structure and parameter selection
The basic working principle of RCD absorption circuit: During the reverse recovery process of the rectifier diode, the transformer leakage inductance and the circuit parasitic inductance resonate with the diode junction capacitance, causing the diode to withstand a reverse spike voltage; when the voltage spike is higher than uCs in the RCD circuit, due to charge conservation, Cs and the diode junction capacitance distribute the charge according to the capacity ratio; Cs is often much larger than the junction capacitance, so it is equivalent to clamping the peak voltage to:

Where: Ucm is the starting voltage of Cs half a cycle; Qrr is the reverse recovery charge.
For the RCD absorption circuit, when the RsCs product is large, the discharge speed of Cs is slow and cannot drop to Uin within half a cycle, so Uc is high, but because its Rs is large, the loss will be relatively small, which is called weak absorption, and vice versa. It is called strong absorption.
When the RCD absorption circuit works stably, its voltage and energy are balanced, that is, the voltage raised by the Cs clamp must be released through Rs within half a cycle, and the energy of the voltage spike must be
absorbed by Rs. If Rs increases, the discharge will slow down. According to the balance condition, Us will be raised, and the loss on Rs will decrease.
Since the switching frequency of this design is very high, 50 kHz, it requires a very short Cs discharge time, otherwise it is very easy to enter a weak absorption state and raise Uc. Therefore, it is necessary to determine the working state of the absorption circuit in combination with the working conditions of the switch tube, and comprehensively consider the values of Rs and Cs in combination with the heating conditions.
[page] Adding an RCD absorption circuit at both ends of the primary MOSFET, because the input voltage is only 24 V, the peak value is small, and the loss generated on Rs is relatively small. Therefore, topology III is selected, and Rs with a smaller resistance value and Cs with a smaller capacitance value are selected to make the RCD absorption circuit work in a strong absorption state, thereby limiting the voltage spike to the greatest extent. According to the experimental results, taking into account the absorption effect and temperature rise, Rs is selected as a power resistor of 300Ω/8 W, and Cs is selected as a CBB capacitor of 1 000 pF/100 V.
When the absorption circuit is added after the secondary rectifier bridge, the voltage spike across the rectifier diode is very large. At the same time, since the output voltage is very high, if topology III is used, huge losses will be generated, so it is excluded.
For topologies I and II, if Rs is very small, the loss will also be very serious, so Rs must take a larger resistance value to make the system work in a weak absorption state. In this state, the smaller RsCs is, the smaller Uc is, so the Cs capacitance should be as small as possible, but it must be ensured that the capacity of Cs is sufficient to absorb the peak voltage. Through experiments, considering the absorption effect and heating conditions, Rs selects a 10 kΩ/50 W aluminum shell resistor, and Cs selects a 4700 pF/1 kV high-frequency absorption capacitor. To make up for the lack of absorption effect, an RC absorption circuit is connected in parallel next to the RCD absorption. R is selected as a 10kΩ/50W aluminum shell resistor, and C is selected as a 4700pF high-frequency absorption capacitor.
Under the selected parameters, the secondary RCD absorption circuit adopts topologies I, II, and III. The Rs10 min temperature rise is 15℃, 18℃, and 55℃ respectively; Uc is 960 V, 938 V, and 930 V respectively. Considering the absorption effect and the heating of the resistor, the topology II absorption circuit is selected.

3 Experimental verification
Figure 4 is the optimized main circuit. The output power of the DC/DC converter is 1 kW, and the output voltage is 500~650 V. According to P=UI, Io can be obtained as 1.54~2 A. The input voltage is 24 V, the fluctuation range is 18~30V, and the effective value of the primary current is 33.3~55.6A. It can be seen that the primary works in a low-voltage and high-current state, and the secondary works in a high-voltage and low-current state. The selection of the device is based on this calculation result.

Select 150 A/100 V MOSFET as the switch tube, select FFPFF10F150S as the diode, its main parameters are 10 A/1.5 kV, the transformer turns ratio is 3:100, Lf=6 mH, G is 1μF/1 kV CBB capacitor, C1 is 1 000μF/100V electrolytic capacitor, C2 is 200μF/1 kV electrolytic capacitor.

Figure 5a shows the waveforms of the input voltage Uin before optimization and the voltage ur0 across the diode measured experimentally. As can be seen from the figure, ur0 oscillates very severely, with a peak value of nearly 1.2 kV. This peak poses a great threat to the normal operation of the system and causes severe heating of the rectifier diode.

Figure 5b is the waveform of ur0 after adding the absorption circuit. It can be seen from the figure that the voltage spike at both ends of the diode has been greatly improved, and the peak value has been reduced by about 300 V. Figure 5c is the waveform of the output current Io, input bus voltage Uin, transformer primary voltage u1 and output voltage Uo of the converter when it is fully loaded, and the output power is greater than 1 kW. It can be seen that through good absorption and filtering, the output voltage and current waveforms are good.

4 Conclusions
This paper introduces the structure and parameter design of the front-stage DC/DC converter of the emergency ventilation power supply of light rail vehicles. Aiming at the parasitic oscillation problem of the rectifier bridge, the working principles and advantages and disadvantages of three RCD absorption topologies are analyzed in detail, and the structure and parameters of the absorption circuit are selected according to the actual working conditions. By building a prototype, the design scheme and the effect of the absorption circuit are verified.

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