An inverter power supply design based on AN8026

　　1. Introduction

　　Frequency converters are of great significance in energy conservation and power environmental protection. Electric motor drives are major consumers of electricity, consuming about 65% of the country's electricity. In the past thirty years, frequency conversion speed regulation has been widely used in steel, metallurgy, petroleum, chemical industry, electricity and other industries. Other household appliances such as frequency conversion refrigerators, frequency conversion washing machines, frequency conversion microwave ovens, etc. have also appeared one after another. Therefore, it is particularly important to design a reliable and high-performance frequency converter power supply .

　　Frequency conversion technology has been widely used at present, and the reliable and stable operation of the frequency converter determines the performance index of the frequency converter. As the basic hardware, the efficient and reliable operation of the frequency converter power supply is crucial. As shown in Figure 1, the topology of the frequency converter is mainly composed of a rectifier unit, a pre-charging circuit, a brake unit and an inverter unit. As can be seen from the figure, the frequency converter power supply provides a DC power supply for the drive circuit and the control circuit, and the drive circuit provides the inverter unit with a strong driving capability and a fast response speed. Therefore, it is particularly important to design efficient and reliable frequency converter power supply hardware.

　　2. Power soft switching technology and circuit principle

　　AN8026 is a flyback single-ended output RCC type quasi-resonant soft switch driver controller developed by Panasonic. It is packaged in a SIP 9-pin package. The definition of each pin number is shown in Table 1. The internal block diagram is shown in Figure 2. Its features are as follows [4]:

　　The supply voltage is from 8.6V at the lower limit to 34V at the upper limit;

　　The output pulse is a single-ended totem pole drive pulse;

　　The output driving current is +1A, directly driving the MOSFET tube;

　　The starting current is 8uA, which reduces the power consumption of the starting resistor;

　　Built-in cycle-by-cycle overcurrent protection circuit;

　　Built-in hysteresis characteristic input undervoltage 8.6V protection circuit;

　　·Overvoltage protection circuit of external voltage regulator.

　　The limit parameters of AN8026 are shown in Table 2, and Figure 3 is the circuit diagram of AN8026. The soft switching power supply is mainly composed of the control chip AN8026, MOSFET K2225, TL431, pulse transformer TR1, etc. VCC is powered by the DC bus through the current limiting and voltage dividing of the starting resistors R1, R3, R5, and R6. The resistance value of the starting resistor can be adjusted according to the different input DC bus voltage levels. When the starting voltage is reached, the converter starts to work, and then the control chip is powered by the secondary +18V auxiliary winding.

　　R8 limits the inductive pulse current of the auxiliary winding of TR1, and the diode D6 is negatively clamped, and a 2.8V positive pulse is sent to the first pin for the flux reset detection of TR1, which can avoid the impact current caused by the switch tube turning on when the magnetic energy of TR1 is not completely released, and at the same time ensure the stability of the output voltage; the second pin is connected to C10 and R14 to set the minimum off time, and C10 sets the minimum on time; the fourth pin is connected to R15 as the source current sensor of the switch tube, which is proportional to the peak voltage of the switch current; the overvoltage protection of the eighth pin is sampled from the Vcc end by the voltage regulator ZD4, so that the starting working voltage does not exceed the upper limit of 34V, and DZ uses a 22V voltage regulator tube, and at the same time, it can realize the output overvoltage protection when the driving pulse is out of control; in order to prevent the startup voltage of AN8026 from exceeding 28V before starting, causing malfunction and making the circuit unable to start, C13 in the circuit absorbs the instantaneous overvoltage spike.

　　The soft switching of the switching power supply is mainly achieved through the quasi-resonant circuit composed of C5, C6, C7 and the primary winding inductance Lp. The specific device parameters need to be calculated according to the switching frequency of the switching power supply. At the same time, the current detection of the fourth foot realizes over-current protection. The eighth foot uses R16 and C18 to perform over-voltage locking at startup to achieve soft start of the switching power supply. The feedback loop composed of TL431 realizes the stability of the output voltage.

　　3. Experimental Verification

　　According to the circuit schematic design, the PCB was drawn using the software AltiumDesigner Summer 09, and the prototype was debugged. Figure 4 shows the soft switching power supply experimental waveform.

　　From the experimental waveform of the switching power supply , it can be seen that the voltage stress of the MOSFET switch tube of the switching power supply is almost zero when it is turned on and off, and the switch tube loss is approximately zero. This greatly improves the efficiency of the switching power supply, while increasing the life of the switch tube, improving the reliability of the power supply, and thus improving the reliability and stability of the inverter.

　　4. Conclusion

　　The inverter power supply board in this design scheme has efficient and stable working performance. The soft switching power supply designed based on the AN8026 control chip is efficient, stable and reliable, and can meet the power supply needs of the drive board and the control board. From the experimental results, it can be seen that the voltage stress of the MOSFET switch tube is almost zero when it is turned on and off, and the switch tube loss is approximately zero. This greatly improves the efficiency of the switching power supply, while increasing the life of the switch tube and improving the reliability and stability of the inverter, thereby confirming the feasibility of the scheme.