Application of Four-way Two-Tube Forward Series-parallel Output Method in Maglev Train DC-DC Converter

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Application of Four-way Two-Tube Forward Series-parallel Output Method in Maglev Train DC-DC Converter [Copy link]

As China's maglev trains develop from experiments to industrialization, the power supply voltage of its main circuit has moved from the previous DC: 750V to a higher voltage DC: 1500V. Although high-voltage power supply solves the copper loss problem of the transmission line, it also brings the problem of the power devices of the subsequent DC-DC converter to withstand high voltage. The input voltage of this experiment is: 1050V~1800V output ≤300V. Herein, a control method of four-way double-tube forward in series and parallel is proposed, which is suitable for high-voltage input working occasions. The series four-way double-tube forward high-power DC-DC converter has the following advantages:
① No output superposition transformer is required, but direct superposition.
② IGBT (SKM300GAR or L-128D) with a withstand voltage of 1200V can be used as a power switch. The voltage equalization problem when IGBT is used in series is solved.
③ Compared with other high-power three-level DC-DC conversion circuits, there is no voltage and current imbalance problem of the power tube rectifier tube.
④ Compared with half-bridge and full-bridge, there is no problem of power tube direct connection and resulting in tube explosion.
⑤ Use peak current control to improve loop response speed.
⑥ There is no primary loop current caused by wide voltage range input in phase-shifted full-bridge or LLC resonant control mode.
⑦ Use three-stage isolated feedback. That is, the input part, output part, and feedback part are completely isolated. Reduce the interference to the control part when the high-power switch is working.
1. Circuit working principle
In the past, when the staggered parallel double-tube forward combination converter was used in high-voltage output occasions, the secondary voltage of the converter was high and the voltage stress of the high-frequency rectifier diode was large. Usually, multiple fast recovery diodes are connected in series to solve the voltage withstand problem, but the voltage balancing design is difficult. The series-parallel double-tube forward combination converter changes the series-parallel connection of the device into the series-parallel connection of the circuit, which can realize the dynamic voltage and current balancing of the high-frequency diode, and is suitable for high input voltage and high power working occasions. Compared with the full-bridge or half-bridge converter, the double-tube forward converter does not have the risk of bridge arm direct pass and has the advantage of high reliability. Disadvantages: Due to the need for core reset, the working duty cycle of the two-tube forward converter must be less than 50%, resulting in low transformer utilization, high transformer secondary voltage, and high voltage stress on the secondary high-frequency rectifier diode. Especially in high output voltage and high power applications, the high voltage of the transformer secondary makes it difficult to select the high-frequency rectifier diode, which often becomes a key factor restricting the converter design and ultimately affects the converter efficiency. In order to reduce the converter secondary voltage and increase the converter capacity, four two-tube forward converters can be combined. The circuit shown in Figure (1) is a new type of staggered series-parallel two-tube forward combination converter. During operation, the control pulses of the two series-connected two-tube forward converters are 180° phase-shifted relative to the other two. In some electric locomotives with high voltage input (1800V) applications, the output voltage of the converter is about 300V. If a conventional two-tube forward circuit is used, such a high voltage stress will bring certain difficulties to the selection of the primary switch tube and the secondary high-frequency rectifier diode. Usually, multiple IGBTs and diodes are connected in series to solve the voltage balancing problem, but dynamic voltage balancing design is more difficult. In response to this problem, this new type of parallel-series dual-tube forward combination converter is used under the same input and output voltage conditions. By replacing the series-parallel connection of devices with the series-parallel connection of the circuit, the voltage and current stress of the devices is reduced, and the dynamic balance problem of IGBT and high-frequency diode is solved well. The forced balanced three-level method is adopted to make the maximum voltage of the switch tube to be half of the input voltage. The two-way series connection and the dual-tube forward work alternately make the circuit under high power output; the midpoint potential of the voltage-dividing capacitor can be guaranteed to be constant.
The new combination converter is obtained by staggered parallel connection of four identical dual-tube forward converters at the secondary freewheeling diode, and then connected in series, and the output shares a set of filter circuits. In Figure 1), Ud is the branch bus voltage (1050~1800V), U0 is the output voltage, and the transformer ratio N=Ns/Np. In order to simplify the circuit state analysis, the transition process is ignored and only the steady-state process is analyzed, because it is assumed that all devices in the parallel-series dual-tube forward combination converter are ideal, and the circuit has a total of 6 working stages when it is in a stable working state.
S1, S2, S3, and S4 are turned on at the same time, D1 and D2 are turned on, D5 and D6 are turned off, and the voltage applied to the filter inductor L is NUd-U0, and the inductor current rises linearly. The excitation current of T3 and T4 is still being reset, CR5, CR6, CR7, and CR8 are still turned on, the voltage of S1, S2, S3, and S4 tubes is clamped at the DC bus voltage 1/2Ud, and the reverse voltage applied to D3 and D4 is NUd. S1, S2, S3, S4, D1, and D2 continue to be turned on, and the inductor current continues to rise linearly. t2 reset ends, the voltage on S1, S2, S3, and S4 is 1/Ud, and the reverse voltage on D3 and D4 is 1/2Ud.
S1, S2, S3, and S4 are turned off at the same time, D1 and D2 are turned off, and the freewheeling diodes D5 and D6 are turned on. The voltage applied to the inductor is -U0, and the inductor current decreases linearly. CR1, CR2, CR3, and CR4 are turned on, and the excitation current of T1 begins to reset. The reverse voltage applied to D1 and D2 is 1/2Ud.
S5, S6, S7, and S8 are turned on, and D3 and D4 are turned on. The voltage applied to the inductor is NUd-U0, and the inductor current rises linearly. The excitation current of T1 and T2 is still being reset, and CR1, CR2, CR3, and CR4 are still turned on. The voltage of S1, S2, S3, and S4 tubes is clamped at the DC bus voltage 1/2Ud, and the reverse voltage applied to D1 and D2 is NUd.
S5, S6, S7, and S8 continue to remain turned on, and the inductor current continues to rise linearly. The reset of T1 and T2 is completed, and the voltage on S1, S2, S3, and S4 is Ud/4, and the reverse voltage on D1 and D2 is NUd/2.
S5, S6, S7, and S8 are turned off at the same time, D3 and D4 are turned off, freewheeling diodes D5 and D6 are turned on, the voltage on the inductor is -U0, the inductor current decreases linearly, CR5, CR6, CR7, and CR8 are turned on, and the excitation current of T3 and T4 begins to reset.
In Figures 2 to 7, let the switching period be T, the time from t0 to t2 is DT, and D is the duty cycle. When the circuit is working stably, according to the magnetic balance of the inductor, it can be obtained that: DT(NUd-U0)=(0.5-D)TU0
Where: Ud——input DC bus voltage;
U0——output DC bus voltage;
L——output filter inductor;
So: U0=2NDUd
From Figure (4), the pulsation of the inductor current is;
ΔIL=Nd(1-2D)TUd/
The main characteristic relationship of the parallel-series double-tube forward converter (circuit 1) and the staggered parallel double-tube forward converter is compared below. Under the same input voltage, output voltage and duty cycle working conditions, the transformer ratio N of circuit 1 is only half of the high-frequency transformer ratio of the conventional circuit. The voltage stress of the secondary rectifier and freewheeling diode is reduced by half, which greatly improves the working conditions of the secondary high-frequency diode.
In high input, output voltage, wide range, and high power applications, the traditional double-tube forward converter and its combination converter often use multiple IGBTs in series to solve the problem of high voltage stress of the primary switch tube and the secondary high-frequency diode to solve the voltage withstand problem. If the series-connected IGBTs and high-frequency diodes cannot balance the voltage well during operation, the circuit may be damaged. The new four-way parallel-series dual-tube forward combination converter achieves better dynamic voltage balancing of IGBTs and high-frequency rectifier diodes by changing the series connection of the primary IGBT and the secondary rectifier diodes into a series connection of the circuit. And from the experimental data, it can be seen that a 1000V tube is enough for the secondary high-frequency diode, which improves the reliability of the circuit and reduces the conduction loss of the diode.
2. Drive feedback protection circuit
During the operation of high-power switch tubes, many factors that affect the reliability of the power supply will be generated, such as: the anti-interference ability of the drive circuit, the stability of the closed-loop feedback part, etc. Since the power of this DC-DC converter is large and the peak value (≤180 seconds can reach 80kW), the volume of the whole machine is relatively large, which increases the distance between the feedback and control parts, making the signal transmission susceptible to interference and affecting the stability of the system. In order to eliminate interference and improve reliability, the following measures are taken:
① Use peak current control mode. The chip uses (UC3825)
② The driver uses M57962AC, the drive signal line is directly laid on the PCB board, and an amorphous alloy magnetic ring is set between the gates of the IGBT. To eliminate interference.
③ The output voltage sampling uses a linear isolation sensor. The isolation is ≥DC2500V.
④ The transmission signal with the microcomputer communication port uses A/D, D/A and wide-body high-isolation optocoupler (HP4503).
⑤ The output current sampling uses a magnetic balance Hall device from the Swiss LEM company. The purpose is to reduce the current sampling loss and improve the signal ratio of signal transmission.
⑥ The conventional DC contactor is cancelled in the battery reverse connection protection part, because the life and operation reliability of DC contactors are generally poor, especially when working at high voltage, it is easy to arc. This machine uses IGBT as a battery backflow protection tube and a reverse connection protection tube, which makes the battery protection circuit a contactless working mode.
⑦ The fan power supply adopts temperature control PWM type. To reduce the running time of the fan and extend the service life.
⑧ The transformer, inductor, absorption circuit and other heating devices are directly installed and fixed on the radiator. The fan only blows the heat dissipation air, not the devices, to reduce the dust accumulation caused by the fan heat dissipation.
III. Prototype experimental method
① The experimental power supply uses a 100kW high-power three-phase voltage regulator. Due to the high experimental voltage, two 50kW three-phase power frequency transformers are used, and the output is supplied to the next stage DC-DC after 12-pulse rectification.
② The load uses a self-made 30kW small resistance box to share 6 devices for switching.
③ Temperature probes are installed on the switch tube, transformer, and inductor of the machine to observe temperature changes
④ Isolate high-voltage probes and current clamp probes to observe the working waveforms of the switch tube and the main circuit.
⑤ Collect data and make files to compare with mathematical modeling simulation data to determine component parameters.
IV. Device selection:
Switching tube: 4 SKM300GAR128D of Ximenkang,
4 300GAL128D ,
Fast recovery: 16 APT 2X100D100J
. V. Conclusion:
Through the experiment of four-way series-parallel combination double-double tube circuit, it reflects the reasonable application of this circuit in high-voltage input and output occasions. Solve the reliability problem of high-power switching power supply.