Technical field

The present invention relates to a switching power supply, specifically a high-voltage DC power supply based on resonant soft switching using a multi-level inverter, high frequency. Pen × fun × Pavilion www. ｂｉｑｕｇｅ。 ｉｎｆｏ

Background technology

High-voltage DC power supplies have a wide range of applications in equipment such as electrostatic precipitation, high-voltage capacitor charging, and medical imaging. Traditional high-voltage DC power supplies usually use a power supply scheme of thyristor phased rectification followed by step-up of industrial frequency transformer. However, this low-frequency power supply mode makes the volume and weight of transformers and filter devices relatively large, and the input and output terminals of the power supply contain a large number of low-order harmonics that are difficult to filter. In recent years, with the wide application of a new generation of power devices (such as IGBTs, MOSFETs, 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 miniaturized and lightweight, but at the same time, the switching loss will also increase, the power efficiency will be seriously reduced, and the electromagnetic interference will also increase, so it is not possible to simply increase the switching frequency. In the application of high-power high-voltage DC power supply, due to the conventional PWM (Pulse Width Modulation), the switch works in the state of hard switching, the electromagnetic interference is large, and the probability of loss and damage of the switch tube is large, which is not conducive to further improving the switching frequency, and also affects the stability and efficiency of the power supply. In order to solve these problems, the soft switching technology is proposed, which uses the auxiliary commutation method based on resonance to solve the problems of switching loss and switching noise in the circuit, so that the switching frequency can be greatly increased.

After searching the existing technical literature, it is found that the "high-power high-voltage DC power supply based on resonant soft switch" uses the leakage inductance and external capacitance of the medium and high frequency transformer in the power main circuit to form a series resonant circuit, which can improve the switching environment of the switch, and adopts the modulation mode combining PAM (pulse amplitude modulation) and PFM (pulse frequency modulation). PAM control regulates the output power by adjusting the DC bus voltage using a thyristor phased rectifier circuit, and PFM control regulates the output power by changing the operating frequency of the inverter circuit. PAM controls the thyristor phase, which will produce switching losses, and the low switching frequency of the thyristor determines that the PAM cannot respond quickly, and PFM can only eliminate the single loss when the switch is turned on or off, and the switching frequency is high. Switching losses are still high, and there are still certain limits on switching frequencies.

Contents of the invention

The object of the present invention is to overcome the deficiencies in the prior art, to provide a high-voltage DC power supply based on resonant soft switching technology, which can completely eliminate the switching loss of the inverter and the rectification loss of the high-frequency uncontrollable rectifier circuit. The control strategy of the whole power supply system is simple, the efficiency is high, and the output voltage fluctuation is small and the response is fast.

The present invention is realized through the following technical scheme, the present invention comprises: power frequency uncontrollable rectifier, the rectifier is configured to give a stable input voltage to the inverter; the inverter converts the input stable DC voltage into a variety of pulse level outputs, and is used to adjust the amplitude of series resonance; the series resonant circuit is made up of an external capacitor and the leakage inductance of the transformer. If the leakage inductance of the transformer is insufficient, an external inductor can be added to convert the pulse level of the inverter into a sinusoidal waveform to facilitate the transformer to boost the voltage; the high-frequency uncontrollable rectifier rectifier to the high-frequency and high-voltage sinusoidal voltage, the series effect of the n-stage rectifier can increase the output DC voltage by n times.

The power frequency uncontrollable rectifier is a grid voltage rectification, and the number of rectifiers included is determined by the output level quantity of inverter. The rectifier is connected in series, and the secondary double winding of the low-frequency transformer ensures 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 charged at equal voltage.

The switching frequency of the inverter is high, and the soft switching control is adopted to eliminate the high-frequency switching loss. The inverter adds a switch tube. There are two types of input DC voltages. According to the different conduction modes of the switch, the output of the inverter has 5 states, which are 2 forward resonance, 1 forward resonance, free resonance, 1 reverse resonance and 2 reverse resonance. The output states of the inverter are summarized as forward resonance, free resonance, and reverse resonance. Forward resonance means that the direction of the pulse voltage output by the inverter is the same as the direction of the resonant current, which strengthens the resonant current, free resonance means that the pulse voltage output by the inverter is zero, which has no effect on the resonant current, and reverse resonance means that the direction of the pulse voltage output by the inverter is opposite to the direction of the resonant current, which weakens the resonant current. In the same state, different directions of resonant current correspond to different switching conduction methods. The state of the switch is switched at the zero crossing of the resonant current so that the switching loss is zero, and the switching frequency is always the same as the series resonant frequency. 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 from the simulation. The period of action for each state is set as an integer multiple of half of the series resonant period.

The series resonant circuit is made up of an external capacitor and a leakage inductance of a transformer in series. If the leakage inductance of the transformer is insufficient, an external inductor can be applied. The capacitance of the capacitor and inductor is determined, as is the series resonance frequency and the switching frequency of the inverter. The capacitance of capacitors and inductors is determined by the withstand voltage and current of the inverter's switch and the capacitor charging speed required by the uncontrollable rectifier. The inductance value is inversely proportional to the peak resonant current and inversely proportional to the capacitor charging speed of the rectifier. Capacitor voltage is only related to the resonant frequency.

The high-voltage alternating current of the high-frequency uncontrollable rectifier is rectified to the high-voltage transformer, and the high-voltage direct current voltage is output. The increase in output voltage is determined by the ratio of the primary and secondary turns of the high-frequency transformer, the number of secondary windings, and the number of rectifier stages connected to each secondary winding. Each secondary winding of the transformer is connected to a multi-stage rectifier, and the rectifiers connected by different secondary windings are connected in series. When a capacitor is added to a multi-stage rectifier connected to the secondary winding, and the capacitor capacity is the same as that connected to the rectifier at each stage, the current flowing through is zero. The corresponding diodes of each rectifier are turned on at the same time to ensure that each series capacitor is charged at a uniform voltage without rectification loss.

Under the condition that the boost factor of the high-frequency transformer remains unchanged, the sum of the turns of the two windings of the secondary stage does not change, that is, the high-frequency transformer will not increase the capacity and volume. The output of the high-frequency transformer is high-voltage high-frequency alternating current, and the diode in the high-frequency uncontrollable rectifier must use a fast diode. The output voltage is provided by multiple capacitors in series, and the withstand voltage value of each capacitor is reduced by many times, but the selection of capacitors should still follow the principle of small capacity and high withstand voltage, and small capacity can make the output voltage rise faster.

A boost method that does not overshoot and does not affect rapidity. In series resonant circuits, capacitor voltages and resonant currents need to be limited to protect switches and diodes in inverters and high-frequency uncontrollable rectifiers. During the boost phase, the output voltage is not directly targeted, but is gradually increased. Convergence to the target value. Before the given output voltage rises to 95% of the target value, the output voltage rises in a positive resonant state so that it rises at the fastest rate. At this time, if you look up the table and judge that the next moment is a reverse resonant state, it is forced to be a free resonant state. If the capacitor voltage and resonant current exceed the limit value, the next state is forced to be free resonant. When the given value of the output voltage reaches 95% of the target value, the given value of the output voltage rises by a small amplitude and quickly converges to the target value, and the case judged to be free resonance is forced to reverse resonance. In order to ensure that there is no overshoot of the output voltage in the whole voltage rise process.

Compared with the prior art, the present invention has the following beneficial effects: the structure of inverter is simple, the control strategy is easy to realize, based on the resonant soft switching control technology, the switching loss can be completely eliminated, the switching frequency is further improved, because the output level of the inverter increases, the output voltage is regulated more finely, the output voltage fluctuation is smaller, the response is faster; Moreover, the power frequency rectifier does not need to adjust its output voltage, and the uncontrollable rectifier is adopted, which simplifies the control complexity of the whole system; the high-frequency uncontrollable rectifier adopts the multi-stage rectifier series mode, which increases the capacitance of the same capacity between the rectifiers at all levels, eliminates the loss of the high-frequency uncontrollable rectifier, and improves the efficiency of the whole system.

Description of the drawings

The features and advantages of the present invention will be better understood when reading the detailed description below, wherein, within all drawings, similar characters denote similar parts. Thereinto:

Figure 1 shows the known high-voltage power supply topology in this art.

Fig. 2 is an embodiment of the present invention, the high-voltage power supply topology of five-level inverter 40 is adopted, the power frequency uncontrollable rectifier 50 adopts the power frequency transformer 42 secondary two windings to rectify respectively, and the high-frequency uncontrollable rectifier 60 adopts the high-frequency transformer 44 secondary two windings to connect the 2-stage rectifier respectively. and in series;

Fig. 3 is an embodiment of the present invention, the high-voltage power supply topology of adopting five-level inverter 40, the power frequency uncontrollable rectifier 70 adopts a 2-stage rectifier, and the high-frequency uncontrollable rectifier 80 adopts a 4-stage rectifier;

Figure 4 shows the five working states of the inverter 40, the output voltage of the 1-inverter 40, and the resonant current of the series resonant circuit 2-series. Thereinto. I-2 forward resonance, II-2 reverse resonance, III-free resonance, IV-1 forward resonance, V-1 reverse resonance;

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

The specific embodiment

As shown in Figure 1, the topology of the well-known high-frequency and high-voltage DC power supply 100 in this art. The high-voltage DC power supply 100 uses a three-stage power circuit to convert the three-phase AC voltage 11 in the grid into an adjustable and stable high-voltage DC voltage 17. The three-phase alternating current voltage 11 of the power grid obtains the DC bus voltage 13 of the inverter 10 through the controllable rectifier circuit 30 and the larger capacity electrolytic capacitor 52. The controllable rectifier circuit 30 adopts a PAM control strategy, and the DC bus voltage 13 can be continuously adjusted according to the output high-voltage DC voltage 17. Here, the controllable rectifier thyristor has switching loss, but the switching frequency is low and the loss is very small. It is also because the switching frequency is low, 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.

DC bus voltage 13 to high-frequency AC high-voltage 15 is realized by inverter 10, series resonant circuit and high-frequency step-up transformer 26. The inverter 10 is made up of four full-control switch tubes and is connected with a diode in reverse and parallel respectively, and the leakage inductance of capacitor 22 and transformer 26 forms a series resonant circuit, and if the leakage inductance is not enough, an inductance 24 can be added. The high-frequency impulse voltage output by the inverter 10 is a sinusoidal voltage and a current into the transformer 26 through a series resonance circuit, and the high-frequency alternating current 15 is obtained through the step-up effect of the transformer 26. The inverter 10 often adopts the control strategy of PWM and PFM, can continuously track the change of output voltage 17, although adopts resonant soft switching technology, when the switch is turned on or when it is turned off, a switching loss is still generated, and the loss of the hard switch is reduced by more than half. The rectifier circuit in the high-voltage DC power supply generally adopts a multi-stage rectifier 20, which can reduce the withstand voltage value of the rectifier diode and capacitor and reduce the volume. Due to the rectification of the high-frequency AC voltage 15, the multistage rectifier 20 adopts a fast rectification diode. The fast rectification diode here is not turned on at the zero crossing of the current. The rectifier circuits at all levels are turned on sequentially, and the diodes will produce large switching losses, which reduces the overall efficiency of the high-voltage DC power supply 100.

As shown in Figure 2, a high-voltage DC power supply 200 topology according to an embodiment of the present invention. The inverter 40 adds a full-control switch 28. If the switch pipe 28 is disconnected, the structure of the inverter 40 is the same as that of the inverter 10. A capacitor bank is added at 23 DC bus voltages, and two capacitor banks are connected in series. Considering the equal-voltage charging of capacitor groups 36 and 38, the front end can be realized by using transformers 42, uncontrollable rectifiers 46 and 48. The ratio of turns of primary and secondary windings of transformer 42 is 1:1, and two windings of the secondary stage. The same voltage is generated and charged by uncontrollable rectifiers 46 and 48 to two capacitor banks 36 and 38, which can ensure the voltage equalization charging of the series capacitor banks. To be charged completely, inverter 40 starts to work, and DC bus voltage 23 can not be adjusted.

As shown in Figure 3, the inverter 40 adds a switch tube 28, which can output 5 kinds of pulse levels, and the values of the 5 kinds of pulse levels are fixed, but only 5 discrete values. Switches 2, 4, 6, 8, and 28 are switched only 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 action period of the five states is also fixed, which is an integer multiple of half of the resonance period, and the working cycle of the five states can also be selected with different values in the boost stage and the stabilization stage, but they are all integer multiples of half of the resonance period.

The switching conduction modes of the five states are as follows: (1) when the resonant current is positive, 2 the positive resonance is the on-switch 2 and 8, and when the resonant current is negative, 2 the positive resonance is the on-switch 4 and 6. (2) When the resonant current is positive, the positive resonance of 1 is the on-switch 28 and 8, and when the resonant current is negative, the 1-positive resonance is the on-switch 28 and 6. (3) When the resonant current is positive, the free resonant conduction switch 2 or 8, and the conduction switch 2 and the diode 16 make the series resonant circuit form a loop. When the resonant current is negative, the free resonant switch tube 4 or 6 is turned on, the conduction switch tube 4 and the diode 18 make the series resonant circuit form a loop, and the conduction switch tube 6 and the diode 12 make the series resonant circuit form a loop. (4) Regardless of whether the resonant current is positive or negative. 1. The reverse resonance is that the switch tube 28 is turned on, and when the resonant current is positive, the switch tube 28 and the diode 16 make the series resonant circuit feed back electric energy to the capacitor group 36; when the resonant current is negative, the switch tube 28 and the diode 8 make the series resonant circuit feed back electric energy to the capacitor group 38. (5) Regardless of whether the resonant current is positive or negative, 2 reverse resonance is to turn off switches 2, 4, 6, 8 and 28. When the resonant current is positive, diodes 14 and 16 turn on so that the series resonant circuit feeds back power to the DC bus, and when the resonant current is negative. Diodes 12 and 18 are turned on so that the series resonant circuit feeds back power to the DC bus.

The output states of the inverter are summarized as forward resonance, free resonance, and reverse resonance. Forward resonance, the DC bus provides electrical energy to the series resonant circuit and the load, and the load voltage 17 will rise. The higher the DC bus voltage, the greater the power output, the more electric energy stored in the series circuit, the greater the amplitude of the rise of the load voltage 17; free resonance, the electric energy stored in the series resonant circuit supplies power to the load, due to the consumption of the load, the load voltage 17 will inevitably decrease, but the decrease is smaller; reverse resonance, the electric energy stored in the series resonant circuit not only supplies power to the load, also feeds back the electric energy to the DC bus, and the load voltage 17 is bound to decrease, and the amplitude is larger. Therefore, if the power provided by the DC bus voltage is exactly equal to the consumption of the load, then the load voltage will remain unchanged without fluctuation. Then the DC bus voltage is not easy to change frequently, which will cause the instability of the entire high-voltage DC power supply, and the harmonics will be greatly increased, bringing more harm. Therefore, the more pulse level that inverter 40 outputs, the fluctuation of load voltage 17 must be smaller, and when using 9-level inverter, the fluctuation of output voltage 17 is extremely small, and can meet the equipment that has extremely high demand for power quality. If you continue to increase the level, the effect will no longer be obvious, but will increase the complexity of the hardware circuit.

There is a certain correspondence between the DC bus voltage 23, the electric energy stored in the series resonant circuit and the output voltage 17, which determines the choice of five states. Simulation models can be built. The difference between the given voltage value and the measured value 17 and the curve of the five states under different capacitance voltages 32 are plotted, and the state output can be determined by the comparison method during implementation. The hardware circuit of the inverter 40 is simple, can output 5 levels, but needs to collect the zero crossing point of the capacitance voltage 32, the output voltage 17 and the resolving resonant current 34. The requirements for the signal acquisition circuit are high, and the speed of the control processor must be fast enough. However, due to the simple algorithm and control, it can be achieved with low-end CPLD/FPGA.

The conduction of the rectifier of each level of multistage rectifier 20 in Figure 1 is inconsistent, because it is high-frequency and high-voltage rectification, the conduction and disconnection of fast rectifier diode can cause large electric energy loss, the service life of fast rectifier diode is affected, and also affects the equalization of capacitance group charging, and the quality and stability of output voltage 17 are reduced. The secondary stage of high-frequency transformer 44 adopts two windings, and the turn ratio of secondary winding and primary winding is reduced to half of transformer 26, and the step-up factor of transformer 44 remains unchanged. The number of turns of the overall winding is the same, so it occupies the same volume. The multistage rectifier 60 is an embodiment according to the present invention, adopts the form of two two-stage rectifiers in series, the output current waveform of the rectifier of each stage therein is exactly the same, the equalizing voltage charge of the capacitance is well realized, and the fast rectifier diode is turned on or turned off when the current is zero, so the rectifier switching loss is not generated, and the efficiency of the high-voltage DC power supply 200 is further improved.

As shown in Figure 4, the topology of the high-voltage DC power supply 300 according to another embodiment of the present invention. Among them, the DC input voltage circuit of the inverter 40 is changed. No transformer is required, and the fast uncontrollable rectifier circuit in the high-voltage DC power supply 200 topology is directly adopted. 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 quality of the output DC voltage. The capacitance of capacitor groups 36 and 38 should be large enough, and the rectifier circuit 70 also has no switching loss. The high-frequency transformer 26 has not been changed, a single four-stage rectifier 80 is adopted, the step-up multiple has not changed, the structure of the four-stage rectifier 80 has no rectification loss, and the capacitor capacity relationship connected between each rectifier is more complicated. Not easy to choose. The inverter structure and its control mode are the same, and the HVDC power supply 300 can achieve the same performance of the HVDC power supply 200.

As shown in Figure 5, the step-up process of the high-voltage DC power supply 200 is used. The action period of the 5 states of the inverter 40 output is fixed, and the output voltage 17 is changed by the switching of the 5 states, and if the given value of the output voltage is directly set to the target value, this discrete control mode will inevitably lead to the overshoot of the boost stage. Therefore, a given value of the output voltage must be gradually increased during the boost phase until the target value is reached. Under the condition of limiting the capacitance voltage 32 and resonant current 34, a curve with a given value of the output voltage increasing is designed. Forward resonance increases the output voltage, free resonance decreases the output voltage, and reverse resonance decreases the output voltage, based on which the given voltage planning curve is based. When the output voltage does not reach 95% of the target value, the given voltage rises at the fastest rate, i.e., 2 positive resonances increase the output voltage. If the capacitance voltage 32 and resonant 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, try to avoid 2 forward resonance, and use 1 forward resonance to make the output voltage rise slowly to the target value. The curve 2 in Fig. 5 is the ideal curve of the output voltage rise, and the actual rise curve of the output voltage does not follow the ideal curve well because the capacitance voltage 32 and resonant current 34 are limited to avoid the switch loss of the inverter 40 caused by the excessive voltage or current.

Although the specific features of the present invention have been illustrated and illustrated here, many modifications and alterations can be made by those skilled in the art. Therefore, it should be understood that the attached claims are intended to cover all such modifications and alterations that fall into the true spirit of the invention. (To be continued.) )