Design of variable frequency power supply based on PWM controller and IPM module

Abstract: A low-power variable frequency power supply is designed by using the intelligent, high-precision PWM controller SA866 and the intelligent power module PS21255 . The system has a simple hardware circuit, fewer components, a compact structure, a high cost-effectiveness, flexible adaptability, and is safe and reliable.

1 Introduction

With the development of power electronics technology, microelectronics technology and control theory, the stability and reliability of inverters have been greatly improved. Various inverters are widely used in various industries, and variable frequency speed regulation technology is developing rapidly. At present, the most commonly used is the variable frequency power supply using sinusoidal pulse width modulation technology S PWM . The electric traction system driven by this variable frequency power supply has the advantages of high efficiency, small torque fluctuation, low noise, fast response, good speed regulation characteristics, reliable operation, and excellent control characteristics. S PWM technology and its control performance are becoming more and more perfect, and dedicated PWM integrated circuits have been introduced one after another. They are increasingly widely used in variable frequency power supply and speed regulation control, making the system circuit simple, convenient for control and adjustment, and highly intelligent.

(2) Serial interfaces SDA, SCL and CS are used to obtain data from the EEPROM, and are data, clock and chip select signals respectively.

(3) Control and output SETPOINT is the frequency setting terminal. The input voltage of this pin will determine the operating frequency of the system; RACC and RDEC determine the acceleration and deceleration time respectively; RPHT, YPHT, BPHT and RPHB, YPHB, BPHB are the bridge arm pulse signal outputs, among which RPHT, YPHT and BPHT correspond to the upper bridge arm of the three-phase output respectively; RPHB, YPHB and BPHB correspond to the lower bridge arm of the three-phase output respectively; DIR controls the three-phase sequence. This pin corresponds to high and low levels and has two directions of PWM waves for users to choose. When the level is high, the output phase sequence is R-B-Y, and when the level is low, the output phase sequence is R-Y-B.

The layout of the external terminals of the IPM (PS21255) module separates strong and weak electricity. P and N are DC input terminals, P is the positive terminal and N is the negative terminal; U, V, W are the three-phase output terminals of the inverter; UP, VP, WP are the pulse signal input terminals of the upper bridge arm U, V, W phases; UN, VN, WN are the pulse signal input terminals of the lower bridge arm U, V, W phases; FO is the fault output terminal (low level is valid).

5. Control of power supply system

The control circuit mainly includes control power supply, SPWM wave generator with SA866 as the core, isolation drive and protection circuit, as shown in Figure 3.

The DC voltage obtained by rectifying and filtering the 220V AC is used as the inverter DC voltage input of PS21255. SA866 generates three pairs of SPWM signals with a phase difference of 120°, which are applied to the control input end of PS21255 through photoelectric isolation, and output three SPWM waves with a phase difference of 120° at the output end of PS21255 to drive the asynchronous motor. By changing the output frequency, the asynchronous motor variable frequency speed regulation is realized. PS21255 has overheating, over (under) voltage, overcurrent and overheating detection and protection circuits. When any fault occurs, it will block the internal 6 IGBT tubes and send out the fault signal FO at the same time.

The control power supply uses 7805 and 7815 to provide DC regulated power supply.

SA866AE can realize continuous speed adjustment and forward and reverse switching through a 10-bit digital-to-analog converter and an external forward and reverse direction pin. SA866 works in mode N3, and the frequency is given by connecting SETPOINT through an external circuit, and RACC and RDEC are connected to the corresponding circuit to realize frequency acceleration and deceleration, and the SERIAL terminal is suspended. All operating parameters, including carrier frequency, waveform, minimum pulse width, dead zone pulse width and V/f curve, are programmed through an external EEPROM.

The system uses an external EEPROM. The EEPROM uses AT93LC46 produced by Atmel. It only needs +5V voltage to work and can be repeatedly erased and written 106 times. The chip is packaged as DIP-8, where Vcc and Vss are the positive and negative terminals of the 5V power input respectively, CLK is the clock signal input terminal, DI is the data input terminal, DO is the data output terminal, and ORG is the storage structure of the internal data. It can be selected as 8 or 16 bits. Its corresponding pins are connected to the SDA, CS and SCL pins of SA866AE respectively. All programmable parameters are stored in the EEPROM. PAGE0 and PAGE1 are used to select the 4 pages of data in the memory 93LC46. After the system is powered on or reset, it automatically downloads through the serial port, reads the parameter word from the EEPROM to the SA866, and generates the corresponding pulse waveform according to the set parameter word to control the opening or closing of the module in the main circuit.

In order to ensure the safe and reliable operation of the inverter system, a protection and isolation drive circuit is set between the IPM main circuit and the control circuit. When the IPM has undervoltage, overcurrent, overtemperature, or short-circuit protection, that is, any of the UV, OC, OT, and SC faults, the fault output signal duration tfo is 1.8ms (SC duration will be longer). During this time, the IPM will block the gate drive and shut down the IPM. After the fault output signal duration ends, the IPM automatically resets internally and the gate drive channel opens. Therefore, the fault signal generated by the device itself is non-retentive. If the source of the fault is still not eliminated after tfo ends, the IPM will repeat the automatic protection process and act repeatedly. Overcurrent, short circuit, and overheating protection actions are all very bad operating conditions, and their repeated actions should be avoided. Therefore, the device's self-protection cannot be fully realized by relying solely on the IPM internal protection circuit. In order to make the system truly safe and reliable, auxiliary peripheral protection circuits are required.

Since the input voltage and feedback energy will be directly reflected in the DC link, the voltage and current detection and protection sampling of the entire system are concentrated in the DC link. In this design, a high-speed optocoupler 6N137 with a control end is placed in front of the SPWM interface circuit SA866, so that the SPWM signal generated by SA866 drives the three-phase 6-way pulse signal input end of the IPM after isolation and amplification. The fault output signal FO of the IPM is sent to the controllable optocoupler 6N137, and is connected to the SETTRIP end of the SA866 after isolation. When the IPM fault alarm occurs, the signal FO quickly sends a protection high level to the SA866, quickly cuts off the control signal channel of the SPWM, shuts down the IPM, and realizes the protection function.