Circuit (hardware) implementation of DSP's intermediate frequency power supply

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Circuit (hardware) implementation of DSP's intermediate frequency power supply [Copy link]

1 Overview Medium frequency power supply generally refers to a single-phase or three-phase variable frequency power supply with an output frequency of medium frequency (commonly used to be 400 Hz), which is widely used in aerospace, ships, locomotives, induction heating, radar, communication switches and other equipment. As a power conversion device, it must convert the industrial frequency input voltage into a frequency and voltage suitable for industrial applications. Early medium frequency special power supplies were mostly generated by motor units, usually composed of asynchronous motors and synchronous generators. The asynchronous motor is powered by three-phase industrial frequency AC. On ships, the three-phase industrial frequency AC is generated by a diesel generator set, and the asynchronous motor then drives the synchronous generator to rotate to generate the required amplitude and frequency of medium frequency AC. This power supply has low harmonic content, good sinusoidality, and the stability of output voltage and frequency can meet the requirements. However, since the power supply is composed of a rotating motor, it is used in aerospace, ships, and locomotives, and its mechanical noise and large body are very obvious. With the development of power electronic devices, the structure of static medium frequency power supply has been proposed. However, the power pollution and harmonic distortion caused by power electronic devices are urgent problems to be solved. At present, most of the static medium frequency power supplies adopt the analog control method, that is, the triangle wave and sine wave generating circuits composed of analog devices are used to send the generated triangle carrier signal Ut and the sine modulation wave signal Ur to the voltage comparator respectively, thereby generating the SPWM sequence. The advantage of this modulation method using analog circuits is that the time used to complete the comparison of Ut and Ur signals and determine the pulse is very short, almost instantaneous, and the intersection of Ut and Ur is very accurate without any approximate processing. However, the disadvantages of this method are poor flexibility, more hardware required, and more troublesome debugging; and it is impossible to achieve voltage regulation while frequency modulation, that is, frequency modulation and voltage regulation can only be performed separately; in addition, due to the drift phenomenon of analog devices, the power supply stability is poor. Applying computer digital technology to the control link of medium frequency power supply has the following obvious advantages: 1) In terms of design, it is easy to adopt advanced control methods and intelligent control strategies, which makes the inverter power supply more intelligent, with more perfect performance and convenient system upgrade. Even the control algorithm can be modified online without changing the hardware circuit; 2) The stability of the output voltage The error of the output voltage of the power supply using DSP technology can be controlled within 1%; 3) The output voltage waveform distortion The output fluctuation of the power supply caused by the load form or load change is more obvious in the traditional medium frequency power supply. The DSP chip can detect the harmonics of the output voltage and compensate for the reactive power due to its powerful computing power, which greatly reduces the harmonic content and reduces the waveform distortion. In addition, computer digital technology can realize intelligent monitoring and other functions, which is the development trend of medium frequency power supply nowadays. This paper mainly introduces the development of digital medium frequency power supply using TMS320LF2407 DSP as the core control. 2 Hardware Implementation of Medium Frequency Power Supply 2.1 System Overview This system studies a 1kVA intelligent medium frequency power supply controlled by DSP. The technical parameter requirements are: Input AC power 220(1±10%)V Frequency 50(1±5%)Hz Power factor>0.8 Rated output voltage AC 220 V Rated frequency 400 Hz Voltage stability accuracy ±2% Frequency stability accuracy ±0.01% Power 1 kV·A A rectifier bridge is used to rectify the single-phase industrial frequency power supply into DC, and then IGBT is used to form the main power bridge inverter circuit. The generation of SPWM signals and the implementation of digital control algorithms are realized by programming on the basis of DSP as the main controller chip. At the same time, the DSP chip also performs real-time monitoring, fault processing and status display on the input and output of the power supply and the power circuit. The principle block diagram is shown in Figure 1. As shown in Figure 1, the power supply system is mainly composed of the main circuit and the control circuit. The main circuit is divided into two parts: the rectifier circuit and the inverter circuit. The rectifier circuit converts the AC power into DC power and then provides it to the inverter circuit to form 400 Hz constant frequency and constant voltage AC power. This article focuses on the inverter part. The control circuit includes feedback detection and SPWM generation. DSP completes all detection control and calculation. It detects the voltage set value and the output voltage feedback value, and generates the output voltage control command through digital PID operation to make the output voltage value consistent with the set value. At the same time, DSP generates SPWM signal through calculation to realize the opening and closing of the inverter bridge, so that the amplitude and frequency of the output voltage are consistent with the set value, and the output value is displayed to the user. 2.2 Main power circuit 2.2.1 Circuit topology The main circuit of the medium frequency power supply system is shown in Figure 2, which adopts AC/DC/AC composite conversion circuit. It includes three parts: full-bridge rectifier circuit, DC/AC inverter bridge and output filter. The AC/DC rectifier circuit converts the input 220 V AC into a DC voltage as the input voltage of the DC/AC inverter bridge. The DC/AC inverter bridge is composed of power MOSFET devices S1, S2, S3 and S4, and its function is to convert the DC voltage output by the full-bridge rectifier circuit into a modulated voltage wave with a fundamental frequency of 400 Hz. The output low-pass filter circuit is composed of a filter inductor Lf and a filter capacitor Cf. Its function is to filter the modulated voltage wave Uab output by the DC/AC inverter bridge, which contains high-order harmonic components and has a fundamental frequency of 400 Hz, into an AC sine wave Ua with a low harmonic content (THD) of 220 V/400 Hz for use by the AC load. 2.2.2 Power circuit parameter design The power circuit includes the selection of full-wave rectifier bridge and inverter bridge power devices and the determination of low-frequency filter parameters, which are explained one by one below. 2.2.2.1 Calculation of rectifier device parameters The input is an AC voltage of 220 V, and its peak voltage is 220 aunt 2 = 311 V. Due to the effect of the filter capacitor, its maximum output DC voltage is about 310 V. The rated output power of this power supply is 1 kW. Considering the loss of the filter and the switching loss of the power switch tube, assuming that the overall efficiency is 80%, the input power is 1 000/0.8=1 250 W. When the system is working, considering that the voltage will drop, assuming that the voltage value is (1.1~1.2)220 V, the average input current Iun is 1 250/(1.1×220)=5.17 A. Considering the fluctuation of the input voltage and the derating, due to the surge current caused by the fluctuation of the power grid, a certain safety margin must be reserved. Therefore, the rectifier circuit device selects the rectifier module as a standard rectifier module of 20 A and 600 V. 2.2.2.2 Selection of inverter devices Power MOSFET has excellent switching characteristics, short switching time, low switching loss, and can work under ultrasonic frequency pulse conditions. As a voltage-controlled majority carrier device, it has a very high gate circuit impedance, so it is suitable for multi-tube parallel operation. Because its drive circuit is simple, it can be directly driven by CMOS logic without adding a buffer circuit, easy to protect, and basically has no secondary breakdown phenomenon. It is one of the preferred power devices in medium and low power circuits, and generally uses products from Mitsubishi, Fuji, APT, and IR. The operating frequency of the inverter in this article is 20 kHz. According to the above calculation, the system selects a 600 V MOSFET device. The current level of the device is determined according to the maximum peak current it passes. Considering that the system overload factor is KA=1.5 and the safety factor KR=1.2, the peak value of the MOSFET device is about 27.7 A when the system output power is 1 kV·A. Considering the current ripple and the reverse recovery peak current of the anti-parallel diode, the current rating of the device can be taken as 40 A. Finally, we choose the MOSFET model 47N60S5.