Technical field

The present invention relates to a switching power supply, specifically a high-voltage DC power supply using a multi-level inverter, a high-frequency, and a resonant soft switch-based high-voltage DC power supply. [Read]

Background technology

High-voltage DC power supplies are widely used in electrostatic dust removal, high-voltage capacitor charging and medical imaging equipment. Traditional high-voltage DC power supplies usually use a power supply scheme of thyristor phase-controlled rectification and power supply with an industrial frequency transformer to boost the power supply. However, this low-frequency power supply method makes the volume and weight of transformers and filter devices relatively large, and the input and output of the power supply contain a large number of low-order harmonics that are difficult to filter out. In recent years, with the widespread application of new generation power devices (such as igbt, color t, etc.), the speed of microprocessors has been further improved, and high-frequency inverter technology has become more and more mature, creating conditions for the development of a high-performance high-power high-voltage DC power supply.

High frequency can make the high-voltage power supply device smaller and lighter, but at the same time, the switching loss will also increase accordingly, the power efficiency will seriously decrease, and the electromagnetic interference will also increase, so simply increasing the switching frequency is not possible. In the application of high-power high-voltage DC power supply, due to the conventional PM (pul color idth touch dilation, pulse width modulation), the switching tube works in the state of a hard switch, with large electromagnetic interference, and the switching tube loss and damage probability is high, which is not conducive to further improving the switching frequency and also affecting the stability and efficiency of the power supply. In response to these problems, soft switching technology is proposed, which uses resonance-based auxiliary converter means to solve the switching loss and switching noise problems in the circuit, so that the switching frequency can be greatly increased.

After searching the existing technical literature, it was found that "High-Power High-Voltage DC Power Supply Based on Resonant Soft Switches" uses the leakage inductance and external capacitors of high-frequency transformers in the power main circuit to form a series resonant circuit, which can improve the switching environment of the switch tube, and adopts a modulation method combining pam (pulse amplitude modulation) and pm (pulse frequency modulation). Pam control uses a thyristor phase control rectifier circuit to adjust the DC bus voltage to adjust the output power, and the pm control adjusts the output power by changing the working frequency of the inverter circuit. Pam controls the thyristor phase, which will generate switching losses, and the switching frequency of the thyristor is low, which determines that Pam cannot respond quickly; pm can only eliminate the single loss when the switching tube is turned on or when it is turned off, and when the switching frequency is high, the switching loss is still high, which still has certain restrictions on the switching frequency.

Contents of invention

The object of the present invention is to overcome the shortcomings in the prior art. A high-voltage DC power supply based on resonant soft switching technology can completely eliminate the switching loss of the inverter and the rectification loss of the high-frequency uncontrollable rectifier circuit. The entire power supply system control strategy is simple, efficient, small output voltage fluctuations and fast response.

The present invention is implemented through the following technical solutions, which include: an industrial frequency uncontrolled rectifier, which is configured to give the inverter a stable input voltage; the inverter converts the input stable DC voltage into a variety of pulse level outputs to adjust the amplitude of series resonance; the series resonant circuit consists of an external capacitor and the leakage inductance of the transformer. If the leakage inductance of the transformer is insufficient, an inductor can be applied to convert the pulse level output by the inverter into a sine waveform to facilitate the voltage boost of the transformer; the high-frequency uncontrolled rectifier rectifies the high-frequency high-voltage sinusoidal voltage, and the series effect of the n-level rectifier can increase the output DC voltage by n times.

The described industrial frequency uncontrolled rectifier is to rectify the voltage of the power grid, and the number of rectifiers included is determined by the number of output levels of the inverter. The rectifier is connected in series. The secondary dual windings of the low-frequency transformer ensure that the current and voltage phases in each rectifier are the same, and the corresponding diodes are turned on at the same time, so that the series capacitor group is voltage-charged.

The switching frequency of the inverter is high, and soft switch control is used to eliminate high-frequency switching losses. The inverter adds a switching tube. There are two input DC voltages. According to the different conduction methods of the switching tube, the output of the inverter has 5 states, namely 2 forward resonance, 1 forward resonance, free resonance, 1 reverse resonance and 2 reverse resonance. The output state of the inverter is summarized as forward resonance, free resonance and reverse resonance. Forward resonance means that the pulse voltage direction of the inverter output is the same as the resonant current direction, which strengthens the resonant current; free resonance means that the pulse voltage output of the inverter is zero, which is for resonance.

The current has no effect; reverse resonance means that the pulse voltage direction output by the inverter is opposite to the resonant current direction, which weakens the resonant current. In the same state, different directions of the resonant current correspond to different switching conduction methods. The state of the switch tube is switched at the zero crossing point of the resonant current so that the switching loss is zero, and the switching frequency and the series resonant frequency are always the same. According to the detected capacitance voltage, resonant current and output voltage, the five states of the inverter determine the output state at the next moment according to the decision curve obtained by simulation. The action period of each state is set to an integer multiple of half of the series resonant period.

The series resonant circuit is composed of the leakage inductance of the external capacitor and the transformer in series. If the leakage inductance of the transformer is insufficient, an inductor can be applied. The capacity of the capacitor and the inductor is determined, and the series resonant frequency and the switching frequency of the inverter are also determined. The capacity of the capacitor and the inductor is determined by the withstand voltage and current withstand current conditions of the inverter's switching tube and the capacitor charging speed required by the uncontrolled rectifier. The inductor value is inversely proportional to the peak of the resonant current and is inversely proportional to the capacitor charging speed of the rectifier. The capacitor voltage is only related to the resonant frequency.

The high-frequency uncontrolled rectifier rectifies the high-voltage AC current output from the high-frequency transformer and outputs the high-voltage DC voltage. The multiple of the output voltage increase is determined by the initial, secondary turns ratio, number of secondary windings and the number of rectifiers connected to each secondary winding. Each secondary winding of the transformer is connected to a multi-stage rectifier, and the rectifiers connected to different secondary windings are connected in series. The multi-stage rectifier connected to the secondary winding increases capacitors, and the capacitor capacity connected to each stage rectifier is the same. When the current flows through is zero, the corresponding diodes of each rectifier are turned on at the same time to ensure that each series capacitor is charged equally and without rectification loss.

When the boosting factor of the high-frequency transformer remains unchanged, the turns of the two secondary windings remain unchanged, that is, the high-frequency transformer will not increase the capacity and volume. The high-frequency transformer outputs high-voltage high-frequency alternating current, and the diodes in the high-frequency uncontrolled rectifier must use fast diodes. The output voltage is connected in series by multiple capacitors, and the withstand voltage value of each capacitor is reduced by multiple times. However, the selection of capacitors still follows the principle of small capacity and high withstand voltage. Small capacity can make the output voltage boost faster.

A boost method without overshoot and does not affect rapidity. In a series resonant circuit, the capacitance voltage and resonant current need to be limited to protect the switching tubes and diodes in the inverter and high-frequency uncontrollable rectifier. During the boost phase, the output voltage given value is not directly the target value, but gradually increases and converges to the target value. Before the output voltage given value rises to 95% of the target value, the output voltage given value increases in a forward resonant state so that the amplitude of the increase of the output voltage increases, so that it can be

At this time, if the table looks up, it is forced to be a free resonant state at the next moment. The capacitance voltage and resonant current exceed the limit value, and the next state is also forced to be a free resonant state. After the output voltage set value reaches 95% of the target value, the output voltage set value rises with a small amplitude and quickly converges to the target value. The case where it is determined to be a free resonant state is forced to be a reverse resonant to ensure that there is no overshoot of the output voltage during the entire voltage rise process.

Compared with the prior art, the present invention has the following beneficial effects: the structure of the inverter is simple and the control strategy is easy to implement. Based on the resonant soft switch control technology, switching losses can be completely eliminated and the switching frequency can be further improved. Due to the increase of the output level of the inverter, the output voltage is adjusted more finely, making the output voltage fluctuation smaller and the response is faster. In order to adapt to the designed inverter input voltage mode, the series structure of the power frequency rectifier is adopted to equally charge the series capacitor group, ensuring the stability of the inverter input voltage. Moreover, the power frequency rectifier does not need to adjust its output voltage, and an uncontrolled rectifier is used to simplify the control complexity of the entire system; the high-frequency uncontrolled rectifier adopts a multi-stage rectifier series method to increase the same capacity of capacitors between the rectifiers at all levels, eliminating the loss of the high-frequency uncontrolled rectifier and improving the efficiency of the entire system.

Attached description of the drawings

The features and advantages of the invention will be better understood when referenced to the following detailed description, in which, throughout the drawings, similar characters represent similar parts. Where:

Figure 1 is a high voltage power supply topology known in the art;

Figure 2 is a high voltage power topology of five-level inverter 40 according to an embodiment of the present invention. The industrial frequency uncontrolled rectifier 50 adopts the secondary two windings of the power frequency transformer 42, and the high frequency uncontrolled rectifier 60 adopts the secondary two windings of the high frequency transformer 44, respectively, and is connected in series together;

Figure 3 is a high voltage power topology using a five-level inverter 40 according to an embodiment of the present invention. The industrial frequency uncontrolled rectifier 70 adopts a 2-stage rectifier, and the high frequency uncontrolled rectifier 80 adopts a 4-stage rectifier;

Figure 4 shows the five working states of the inverter 40, 1-the output voltage of the inverter 40, and 2-the resonant current of the series resonant circuit. Among them, i-2 forward resonance, ii-2 reverse resonance, iii-free resonance, iv-1 forward resonance, and v-1 reverse resonance;

Figure 5 is the ideal rise curve for the given value of the output voltage, 1-ideal rise curve for the given value, and 2-the high-voltage DC voltage output curve obtained by simulation;

Specific implementation methods

As shown in Figure 1, the topology of high-frequency high-voltage DC power supply 100 known in the art. The high-voltage DC power supply 100 uses a three-stage power circuit to convert the three-phase AC voltage 11 in the power grid into a adjustable stable high-voltage DC voltage 17. The three-phase AC voltage 11 of the power grid is obtained through the controllable rectifier circuit 30 and a larger capacity electrolytic capacitor 52 to obtain the DC bus voltage 13 of the inverter 10. The controllable rectifier circuit 30 adopts a pam control strategy to continuously adjust the DC bus voltage 13 according to the output high-voltage DC voltage 17. Here, the controllable rectifier has switching losses, but the switching frequency is low and the loss is very small. It is precisely because of the low switching frequency that the output response of the controllable rectifier circuit 30 is very slow, and it is not easy to adjust the output DC bus voltage 13 frequently.

The DC bus voltage 13 to high-frequency AC high voltage 15 is realized through the inverter 10, series resonance circuit and high-frequency boost transformer 26. The inverter 10 is composed of four fully controlled switch tubes in reverse parallel each, and an external capacitor 22 and the leakage inductance of the transformer 26 form a series resonance circuit. If the leakage inductance is not enough, an inductance 24 can be added. The high-frequency pulse voltage output by the inverter 10 is input to the transformer 26 through the series resonance circuit. The high-frequency AC voltage and current are obtained through the boosting effect of the transformer 26. The high-frequency AC voltage 15 is obtained. The inverter 10 often adopts a pm and pm control strategy, which can continuously track the output.

Although the change in the output voltage 17 is adopted, a primary switching loss will still occur when the switch tube is turned on or off, and the loss of the harder switch will be reduced by more than half. The rectifier circuit in high-voltage DC power supply generally uses a multi-stage rectifier 20, which can reduce the withstand voltage value of the rectifier diode and capacitor and reduce the volume. Since the high-frequency AC voltage 15 is rectified, the multi-stage rectifier 20 uses a fast rectifier diode. The fast rectifier diode here does not conduct at the zero point of the current crossing. The rectifier circuits at each stage are turned on in turn, and the diodes will generate large switching losses, which reduces the overall efficiency of the high-voltage DC power supply 100.

As shown in Figure 2, according to one embodiment of the present invention, the topology of the high-voltage DC power supply 200. Inverter 40 adds a fully controlled switch tube 28. If the switch tube 28 is disconnected, the structure of the inverter 40 is the same as that of the inverter 10. A capacitor group is added at the DC bus voltage, and two capacitor groups are connected in series. Considering the voltage equalization of the capacitor groups 36 and 38, the front end can be implemented by transformer 42 and uncontrolled rectifiers 46 and 48. In the initial, the secondary winding turns ratio of the secondary winding is 1:1, and the secondary winding two windings are generated. The same voltage is charged by charging the two capacitor groups 36 and 38 through the uncontrolled rectifiers 46 and 48, which can ensure the equalization of the capacitor group in series. After the charging is completed, the inverter 40 starts to work, and the DC bus voltage cannot be adjusted.

As shown in Figure 3, the inverter 40 has added a switching tube 28, which can output 5 pulse levels. The values ​​of the 5 pulse levels are fixed and only 5 discrete values. The switching tubes 2, 4, 6, 8, 28 only switch when the resonant current crosses the zero point, so the switching frequency is fixed and is the resonant frequency. There are 5 working states of the inverter 40. They are called 2 forward resonance, 1 forward resonance, free resonance, 1 reverse resonance and 2 reverse resonance. The working periods of the 5 states are also fixed, which are an integer multiple of half of the resonance period. Different values ​​can be used in the boosting stage and the stabilization stage of the five states, but they are all integer multiples of half of the resonance period.

The switching conduction methods in five states are: (1) When the resonant current is positive, the 2 forward resonance is on the switch tubes 2 and 8; when the resonance current is negative, the 2 forward resonance is on the switch tubes 4 and 6. (2) When the resonance current is positive, the 1 forward resonance is on the switch tubes 28 and 8; when the resonance current is negative, the 1 forward resonance is on the switch tubes 28 and 6. (3) When the resonance current is positive, the free resonance conducts the switch tubes 2 or 8, and the switch tubes 2 and diode 16 are turned on to form a loop. When the switch tubes 8 and diode 14 are turned on to form a loop; when the resonance current is negative, the free resonance conducts the switch tubes 4 or 6, the switch tubes 4 and diode 18 are turned on to form a loop, and the switch tubes 6 and diode 12 are turned on to form a loop. (4)

Regardless of whether the resonant current is positive or negative. 1 reverse resonance is to turn on the switch tube 28. When the resonant current is positive, the switch tube 28 and diode 16 cause the series resonant circuit to feed back the capacitor group 36; when the resonant current is negative, the switch tube 28 and diode 8 cause the series resonant circuit to feed back the capacitor group 38. (5) Regardless of whether the resonant current is positive or negative, the 2 reverse resonance is to turn off the switch tubes 2, 4, 6, 8 and 28. When the resonant current is positive, the diodes 14 and 16 are to turn on the series resonant circuit to feed back the DC bus; when the resonant current is negative, the diodes 12 and 18 are to turn on the series resonant circuit to feed back the DC bus.

The output state of the inverter is summarized as forward resonance, free resonance and reverse resonance. Forward resonance, the DC bus gives the series resonance circuit and load electrical energy, and the load voltage 17 will increase. The higher the DC bus voltage, the greater the output power, the more electrical energy stored in the series circuit, and the greater the amplitude of the increase of the load voltage 17; free resonance, the electrical energy stored in the series resonance circuit supplies power to the load, due to the consumption of the load, the load voltage 17 will inevitably drop, but the amplitude of the decrease is small; reverse resonance, the electrical energy stored in the series resonance circuit not only supplies power to the load, but also feeds the electric energy back to the DC bus, and the load voltage 17

It will inevitably decrease, and the amplitude is large. Therefore, if the power of the DC bus voltage is exactly equal to the load consumption, the load voltage will not fluctuate and remain unchanged. Then the DC bus voltage is not easy to change frequently, which will cause instability of the entire high-voltage DC power supply, greatly increase the harmonics, and bring more harm. Therefore, the more pulse levels output by the inverter 40, the smaller the fluctuation of the load voltage 17 will inevitably be. When using a 9-level inverter, the fluctuation of the output voltage 17 can be extremely small, which can meet the equipment with extremely high demand for power quality. If the level of the inverter is continued to increase the level, the effect will no longer be obvious, and instead increase the complexity of the hardware circuit.

There is a certain correspondence between the DC bus voltage, the electrical energy stored in the series resonant circuit and the output voltage 17, which determines the choice of 5 states. A simulation model can be established. The difference between the given voltage value and the measured value 17 and the curve of the 5 states under different capacitance voltage 32 is drawn. When implementing, the state output method can be determined by using a comparative method. The hardware circuit of the inverter 40 is simple and can output 5 levels. It is just that the capacitance voltage 32, the output voltage 17 and the zero crossing point of the resolution resonant current 34 is required. The signal acquisition circuit is high, and the speed of the control processor must be fast enough. However, due to the simple algorithm and control, it can be realized by using the low- and medium-side pld/pga.

The conduction of the rectifiers of the multi-stage rectifier 20 in Figure 1 is inconsistent. Since it is high-frequency and high-voltage rectifier, the conduction and disconnection of the fast rectifier diode will cause large power loss, affecting the service life of the fast rectifier diode, and also affecting the equalization of the capacitor group charging, which reduces the quality and stability of the output voltage 17. The secondary of the high-frequency transformer 44 uses two windings, and the turn ratio of the secondary winding to the primary winding is reduced to half of the transformer 26, while the transformer

The boosting multiple of 44 remains unchanged. The number of turns of the overall winding remains unchanged, so the volume occupied is the same. The multi-stage rectifier 60 is based on an embodiment of the present invention. It adopts two two-stage rectifiers in series. The output current waveforms of each stage rectifier are exactly the same, which realizes the equalization charging of the capacitor well. Moreover, the fast rectifier diode is turned on or off when the current is zero, so no rectification switching loss is generated, which further improves the efficiency of the high-voltage DC power supply 200.

As shown in Figure 4, the topology of the high-voltage DC power supply 300 according to another embodiment of the present invention is changed. The DC input voltage circuit of the inverter 40 is changed. The transformer is not required, and the fast uncontrollable rectifier circuit in the topology of the high-voltage DC power supply 200 is directly used. The frequency of the power grid is low, so a general rectifier diode can be selected in the uncontrollable rectifier circuit 70. In order to improve the output DC voltage quality, the capacitance of the capacitor groups 36 and 38 must be large enough, and the rectification circuit 70 also has no switching loss. The high-frequency transformer 26 has not been changed. A single four-stage rectifier 80 is used, and the boost multiple has not changed. The structure of the four-stage rectifier 80 has no rectification loss. The capacitor capacity relationship between the connected rectifiers is relatively complicated. It is not easy to choose. The inverter structure and its control method are the same, and the high-voltage DC power supply 300 can achieve the same performance of the high-voltage DC power supply 200.

As shown in Figure 5, according to the boosting process of the high-voltage DC power supply 200, the five states output by the inverter 40 are fixed, and the output voltage 17 is changed through the switching of the five states. If the output voltage set value is directly set to the target value, this discrete control method will inevitably lead to overshoot in the boost stage. Therefore, the output voltage given value must gradually increase in the boost stage until the target value is reached. Under the conditions of limiting the capacitor voltage 32 and the resonant current 34, a curve of continuous increase in the output voltage given value is designed. Forward resonance causes the output voltage to rise, free resonance causes the output voltage to decrease slightly, and reverse resonance causes the output voltage to decrease significantly. The given voltage plan curve is based on this point. When the output voltage does not reach 95% of the target value, the given voltage is given. The given voltage is not reached 95% of the target value, the given voltage is given.

The voltage rises at the fastest speed, that is, the amplitude of the output voltage increase by 2 forward resonance. If the capacitor voltage 32 and resonance current 34 exceed the limit value, the next state is set to free resonance, and the reverse resonance state is avoided as much as possible. After the output voltage reaches 95% of the target value, if the capacitor voltage 32 and resonance current 34 exceed the limit value, the next state is set to reverse resonance, and try to avoid 2 forward resonance, and use 1 forward resonance to make the output voltage rise slowly to the target value. Curve 2 in Figure 5 is the ideal curve for the increase of the output voltage. The actual rise curve of the output voltage does not track the ideal curve well because of the limitation of the capacitor voltage 32 and resonance current 34 to avoid excessive voltage or current causing the switching tube loss of the inverter 40.
Chapter completed!