Design of 7.5kVA Single-Phase Aviation Static Converter

introduction

Aerospace static converters require high reliability, small size, light weight and good electrical performance, especially high reliability. To this end, we analyze and apply a novel dual-tube forward converter as the front stage of the inverter DC input. It has the advantages of high reliability such as no shoot-through phenomenon in the internal bridge arm of the dual-tube forward converter and low voltage stress on the primary switch tube. At the same time, it overcomes the weakness of the dual-tube forward converter that the output rectifier and freewheeling diode voltage stress is too high and cannot be reliably applied to high-voltage output occasions [1][2]. The inverter part of the static converter adopts a three-state hysteresis current control inverter, which has the advantages of fast response speed and good stability compared to the voltage control inverter, and has the advantages of low output voltage THD and high efficiency compared to the two-state hysteresis current control inverter [4].

1 Principle Analysis

The structural block diagram of the 7.5kV·A single-phase aviation static converter system is shown in Figure 1. It consists of three parts: a three-phase uncontrolled rectifier bridge, a dual-tube forward DC/DC converter, and a three-state hysteresis current controlled single-phase inverter. The DC converter increases the pulsating DC voltage of 240~300V output by the uncontrolled rectifier bridge to 360V to meet the minimum input voltage amplitude requirement of the inverter.

1.1 Analysis of DC/DC Converter Principle

In a dual-switch forward converter and its isolated DC converter, since the voltage stress of the secondary output rectifier and freewheeling diode is proportional to the output voltage, the higher the voltage stress of the fast recovery diode, the more serious the voltage spike and thermal stress problems generated by the reverse recovery process when it is turned off, which endangers the safe operation of the diode. Therefore, the secondary output diode is the weakest link when the isolated converter outputs high voltage. The higher the voltage rating of the transformer secondary output freewheeling fast recovery diode, the longer its reverse recovery time is[5], and the greater the transformer leakage inductance energy storage during the reverse recovery process; when the reverse recovery process ends, the higher the voltage spike generated on the freewheeling diode. In addition, the longer the reverse recovery time of the secondary output freewheeling diode, the more serious the loss of the converter output duty cycle, and the number of turns of the transformer secondary side must be increased, which further increases the voltage stress of the output rectifier and freewheeling diodes. Therefore, the key to improving the reliability of the converter when it outputs high voltage is to reduce the voltage stress of the secondary output diode. Although the diode voltage stress can be reduced by connecting diodes in series and adding an absorption circuit, the voltage-equalizing capacitor and absorption capacitor in series will increase the current stress of the switch tube on the primary side of the transformer and may cause oscillation of the transformer leakage inductance and the voltage-equalizing capacitor. In addition, the power consumption of the RC absorption circuit also reduces the efficiency of the entire converter.

In order to effectively reduce the voltage stress of the output diode on the secondary side of the transformer, we adopted a novel dual-transistor forward converter combination [2][3]. As shown in Figure 2, the four dual-transistor forward converters on the primary input side of the transformer are connected in parallel, and the secondary output side is connected in parallel first and then in series.

Figure 2

The input and output voltage relationship of the combined converter is:

Vo=4N(2L)DVin (1)

Where: Vin and Vo are input and output voltages respectively;

D is the conduction duty cycle of the power switch tube (Q1~Q8);

N (2L) is the ratio of the number of turns of the secondary side to the primary side of the high-frequency transformer (T1~T4).

The voltage stress VF (2L) of the secondary output freewheeling diode (D10 and D14) is

VF(2L)=N(2L)Vin （2）

The voltage stress VR(2L) of the secondary output rectifier diode (D9, D11~D13) is

VR(2L)=2N(2L)Vin (3)

For the two-switch forward converter, its output voltage Vo, secondary rectifier diode voltage stress VR(1L) and secondary freewheeling diode voltage stress VF(1L) are respectively shown in equations (4) to (6) [1].

Vo=N(1L)DVin （4）

VR(1L)=N(1L)Vin （5）

VF(1L)=N(1L)Vin （6）

Where: N(1L) is the ratio of the number of turns of the secondary side to the primary side of the high-frequency transformer of the dual-switch forward converter.

When the input and output voltages and on-duty ratios of the two converters are the same, the relationship can be obtained from equations (1) to (6):

Equations (7) and (8) show that the voltage stress of the secondary output freewheeling diode of the novel combined converter is only 1/4 of the corresponding secondary diode of the dual-tube forward converter, which greatly reduces the diode withstand voltage level. To this end, we use a fast recovery diode DSEI30-06A with a withstand voltage of 600V as the secondary freewheeling diode. Its reverse recovery time is 35ns, and no additional RC absorption circuit is required. Its voltage peak does not exceed 350V. Therefore, the novel combined converter can be reliably applied to high-voltage output occasions.

1.2 DC/AC inverter principle and main parameter analysis

The block diagram of the three-state hysteresis control current source inverter system is shown in Figure 3. It consists of a voltage outer loop and a current inner loop. The current inner loop regulates and controls the current to make it close to the given signal, thereby improving the dynamic performance of the system; the error signal generated by the voltage outer loop is used as the given signal of the current inner loop, thereby achieving the purpose of voltage stabilization and making the system have excellent electrical performance. Its main circuit topology is shown in Figure 4, and the output voltage is across the filter capacitor Cf. Assuming Ig is the current given signal and Δ is the hysteresis loop width, the three-state hysteresis control process is as follows:

Ig>If＋Δ, Q1 and Q4 are turned on at the same time, and If rises (i.e., +1 state);

Ig

If－Δ

The main function of the inverter filter inductor Lf in hysteresis control is to filter out the output high-order harmonics together with the filter capacitor Cf; as an integral link, it provides a slope integral function for the current closed-loop control and participates in the control. Its value not only affects the output waveform, but also affects the dynamic performance of the system. If Lf is too large, the inductor current cannot track the change of the given current, resulting in system imbalance. If Lf is too small, although the dynamic performance of the system can be improved, the current pulsation increases, affecting the quality of the output waveform. From the input and output characteristics and working principle of the inverter, it can be obtained that the filter inductor must satisfy equations (9) and (10) [3][4].

Where: vOmax is the output voltage peak;

ω is the output voltage angular frequency;

Igmax is the maximum value of the current given signal;

ton is the conduction time;

ΔiLfmax is the maximum ripple of the inductor current.

The function of the inverter filter capacitor Cf is to filter out the high-frequency components of the output. A larger value can reduce the THD of the output voltage, but it will increase the current capacity of the switch tube and other devices; if the value is too small, the THD of the output waveform will increase. Taking all factors into consideration, Cf should satisfy formula (11).

Cf<(αIomax/ωVo) （11）

Where: The coefficient α is preferably around 0.5 in engineering.

If the hysteresis width Δ is too small, the switching frequency of the power tube will increase and the switching loss will increase. If it is too large, the inductor current pulsation will increase and the output waveform will deteriorate. Generally, about 10% of the inductor current is taken as the corresponding Δ value [3][4]. The final selection must be determined through actual debugging.