Inverter switching power supply common sense analysis

Variable-frequency drive (VFD) is a power control device that controls the AC motor by changing the working frequency of the motor by applying variable frequency technology and microelectronic technology. The switching power supply circuit provides the control power of the inverter, which is a prerequisite for the normal operation of the inverter. To repair the inverter, you must first understand the switching power supply circuit.

The switching power supply circuit used in the frequency converter is a straight-through type inverter circuit. It is a voltage and power converter that converts the DC voltage and power into a pulse voltage and rectifies it into another DC voltage. The input and output voltages are isolated by the switching transformer, and the switching transformer functions as a power transfer and voltage/current conversion. The switching transformer is a step-down transformer. The characteristics of the switching power supply are as follows:

1) The oscillation and voltage regulation mode of the switching power supply is to adjust the output voltage by changing the pulse width or period. It is called time proportional control, and is divided into PWM (width adjustment) and PFM (frequency modulation) control modes.

2) From the energy conversion characteristics of the circuit, it can be divided into two working modes: forward and reverse. When the switch tube is saturated and turned on, the rectifier connected to the secondary winding is turned off by the reverse bias, and the primary winding of the switching transformer flows into the current to store energy (electromagnetic conversion). When the switch tube is turned off, the secondary winding discharges electric energy (magnetoelectric conversion) through the load circuit. The positive mode is the opposite, and there are not many practical applications.

3) From the primary circuit structure of the switching transformer, there are two circuit forms composed of discrete components and integrated oscillating chips. Therefore, from the source of the oscillating signal, it is divided into self-excited (discrete parts) and other exciter (IC circuit) switching power supply. Both circuit configurations have applications.

4) The switch tube has a bipolar device and a field effect transistor.

5) Small-power inverters use single-ended forward-excited circuits, and large- and medium-power inverters often use double-ended forward circuits. Generally, the switching power supply of the inverter often provides the following voltage outputs: CPU and auxiliary circuits, control circuit, +5V power supply for operation display panel; ±15V power supply for fault detection circuit and control circuit such as current, voltage and temperature; control terminal 24V power supply for working relay coils. The four-way isolated power supply is about 22V. The four-way power supply is often processed by the voltage regulator circuit into +15V, -7.5V positive and negative power supplies for the drive circuit to provide excitation current for the IGBT inverter output circuit. .

In any electronic device, the failure rate of the power circuit is always quite high because it has to provide the power supply of the whole machine, and the burden is the heaviest. The switching power supply circuit of the frequency converter is simple in form and simple in structure. But simple circuits can also cause troubles. The maintenance of the switching power supply is not as intuitive as the linear power supply. Any abnormality in the oscillation, voltage regulation, protection, load, etc. of the circuit will cause various faults in the circuit.

No response after power-on, no display on the operation display panel, the inverter does not seem to be powered. The control voltage of the measurement control terminal and the 10V frequency adjustment voltage are both 0. It is normal to measure the resistance of the main terminal of the inverter, so it can be concluded that the problem is in the switching power supply circuit.

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