Basic knowledge of inverter and IGBT

The frequency converter is a static frequency converter that can convert the 50Hz frequency AC power of the power grid into frequency-adjustable AC power. It is widely used as a power supply device for electric motors at home and abroad. The use of frequency converters can save energy, improve product quality and labor productivity, etc.

1. Inverter composition principle

1.1 Basic structure of inverter

The speed control inverter consists of: main circuit, control circuit, protection circuit

Working principle of inverter main circuit

Structural diagram of voltage and frequency conversion device

According to different control methods, AC-DC-AC inverters can be divided into the following three types:

The control method adopts controlled rectifier voltage regulation and inverter frequency regulation, and its structural block diagram.

The characteristics of the control method of controlled rectifier voltage regulation and inverter frequency regulation:

In this device, voltage regulation and frequency regulation are performed separately in two links, coordinated in the control circuit, with a simple structure and convenient control. However, since the input link uses a thyristor controlled rectifier, when the voltage is adjusted to a low level, the power factor at the grid end is low. The output link mostly uses a multi-beat inverter composed of thyristors, which changes phase six times a week, and the output harmonics are large, so this type of control method is rarely used now.

The control method adopts uncontrolled rectifier rectification, chopper voltage regulation, and inverter frequency modulation, and its structural block diagram.

The characteristics of the control method of uncontrolled rectifier rectification, chopper voltage regulation, and inverter frequency regulation are as follows:

The rectifier uses a diode uncontrolled rectifier, which only rectifies but does not regulate the voltage. A chopper is then set up separately to regulate the voltage using pulse width. This method overcomes the disadvantage of low power factor; however, the output inverter link remains unchanged and still has the disadvantage of large harmonics.

The control method of using uncontrolled rectifier rectification and pulse width modulation (PWM) inverter to simultaneously regulate voltage and frequency is shown in its structural block diagram.

The characteristics of the control method of not controlling the rectifier rectification and the pulse width modulation (PWM) inverter to simultaneously adjust the voltage and frequency:

In this type of device, if uncontrolled rectification is used, the input power factor remains unchanged; if (PWM) inversion is used, the output harmonics can be reduced. PWM inverters require fully controlled power semiconductor devices, and the degree of reduction of output harmonics depends on the switching frequency of PWM, which is limited by the switching time of the device. When using insulated bipolar transistors IGBT, the switching frequency can reach more than 10kHz, and the output waveform is very close to a sine wave, so it is also called SPWM inverter, which has become the most promising device form at present.

Voltage type inverter structure diagram:

Voltage type inverter:

In an AC-DC-AC inverter, when a large capacitor is used to filter the intermediate DC link, the DC voltage waveform is relatively flat. Ideally, it is a constant voltage source with zero internal impedance, and the output AC voltage is a rectangular wave or a step wave. This type of inverter is called a voltage-type inverter.

Current source inverter structure diagram:

Current type inverter:

When the intermediate DC link of the AC-DC-AC inverter uses large inductance filtering, the DC current waveform is relatively flat, so the internal impedance of the power supply is large. It is basically a current source for the load, and the output AC current is a rectangular wave or a step wave. This type of inverter is called a current type inverter.

2 Main circuits of several typical AC-DC-AC inverters

①AC-DC-AC voltage frequency conversion circuit

Commonly used AC-DC-AC voltage type PWM frequency conversion circuit.

The AC-DC-AC voltage type PWM frequency conversion circuit uses diodes to form a rectifier to complete the conversion from AC to DC, and its output DC voltage Ud is uncontrollable; the intermediate DC link is filtered by a large capacitor C; power transistors V1~V6 form a PWM inverter to complete the conversion from DC to AC, and can achieve simultaneous adjustment of output frequency and voltage. VD1~VD6 are the feedback diodes required for the voltage type inverter.

②AC-DC-AC current type frequency conversion circuit

Commonly used AC-DC-AC current type frequency conversion circuit.

AC-DC-AC current-type frequency conversion circuit: The rectifier uses a controllable rectifier circuit composed of thyristors to complete the conversion from AC to DC, output a controllable DC voltage U, and realize the voltage regulation function; the intermediate DC link is filtered by a large inductor L; the inverter uses a series diode current-type inverter circuit composed of thyristors to complete the conversion from DC to AC and realize the adjustment of the output frequency.

③Performance comparison between AC-DC-AC voltage inverter and current inverter

3Insulated Gate Transistor (IGBT)

1. Structure and basic working principle of IGBT

Insulated gate transistor IGBT, also known as insulated gate bipolar transistor, is a newly developed composite power electronic device.

Because it combines the characteristics of MOSFET and GTR, it has the advantages of high input impedance, fast speed, good thermal stability and simple driving circuit, as well as low input on-state voltage, high withstand voltage and large current. All these make IGBT more attractive than GTR.

IGBT plays a dominant role in inverter driven motors, medium frequency and switching power supplies, as well as areas requiring fast speed and low loss.

1. Basic structure and working principle of IGBT

1) Basic structure

IGBT is also a three-terminal device, the three poles are drain (D), gate (G) and source (S).

(a) Internal structure (b) Simplified equivalent circuit (c) Electrical graphic symbols

2) Working principle

The driving principle of IGBT is basically the same as that of power MOSFET, and it is a voltage-controlled device.

The turn-on and turn-off are determined by the voltage UGE between the gate and the emitter. When UGE is positive and greater than the turn-on voltage UGE (th), a channel is formed in the MOSFET and the base current is provided to the transistor to turn it on.

When a reverse voltage or no voltage is applied between the gate and the emitter, the channel in the MOSFET disappears, the transistor has no base current, and the IGBT is turned off.

2. Basic characteristics and main parameters of IGBT

Transfer characteristics and output characteristics of IGBT

(a) Transfer characteristics (b) Output characteristics

1) Basic characteristics of IGBT

① Static characteristics

The transfer characteristic of the IGBT, which describes the relationship between the collector current IC and the gate-emitter voltage UGE, is similar to the transfer characteristic of the power MOSFET.

The turn-on voltage UGE(th) is the minimum gate-emitter voltage at which the IGBT can achieve conductivity modulation and turn on.

UGE(th) decreases slightly with increasing temperature. When the temperature increases by 1°C, its value decreases by about 5mV. At +25°C, the value of UGE(th) is generally 2~6V.

The output characteristics of IGBT, also known as the volt-ampere characteristics, describe the relationship between the collector current IC and the collector-emitter voltage UCE when the gate-emitter voltage is used as the reference variable.

IGBT switching process

2) Main parameters

① Collector-emitter rated voltage UCES

②Gate-emitter rated voltage UGES

③Rated collector current IC

3. IGBT holding effect and safe operating area

From the structure of IGBT, it can be found that the IGBT current may be out of control, just like the mechanism that after an ordinary thyristor is triggered, even if the trigger signal is removed, the thyristor still maintains conduction due to entering the positive feedback process. Therefore, it is called the holding effect or self-locking effect.

The cause of the holding effect may be excessive collector current (static holding effect) or excessive maximum allowable voltage rise rate duCE/dt (dynamic holding effect). Increased temperature will also increase the risk of holding effect.

The dynamic holding effect allows a smaller collector current than the static holding effect, so the maximum collector current allowed is actually determined based on the dynamic holding effect.

The maximum collector current, maximum collector-emitter voltage and maximum collector power consumption can determine the parameter limit range of the IGBT in the on-state; the maximum collector current, maximum collector-emitter voltage and maximum allowable voltage rise rate can determine the parameter limit range of the IGBT in the blocking state, that is, the reverse bias safe operating voltage (RBSOA).

4IGBT drive circuit

(1) Requirements for drive circuit

① IGBT is voltage driven, has a threshold voltage of 2.5~5.0V, and has a capacitive input impedance. Therefore, IGBT is very sensitive to gate charge. Therefore, the drive circuit must be very reliable to ensure a low-impedance discharge loop, that is, the connection between the drive circuit and the IGBT should be as short as possible.

② Use a driving source with low internal resistance to charge and discharge the gate capacitance to ensure that the gate control voltage UCE has sufficiently steep leading and trailing edges to minimize the switching loss of the IGBT. In addition, after the IGBT is turned on, the gate driving source should be able to provide sufficient power to prevent the IGBT from exiting saturation and being damaged.