Design of AC drive inverter composed of IGBT

Electric energy is divided into AC and DC. The process of converting AC into DC is called rectification, and the process of converting DC into AC is called inversion. An inverter is a device that converts DC into AC.

AC motors are generally powered by AC-DC-AC inverter power supply, that is, the AC power provided by the power grid is converted into DC power through rectification and filtering, and then the DC power is converted into the required AC power supply through the inverter to power the motor. Therefore, the inverter is one of the more critical technical devices. With the development of semiconductor devices, IGBT is increasingly being applied to AC transmission technology. This article mainly analyzes the main circuit structure of the AC transmission inverter composed of IGBT, including the main circuit form, the realization of the drive circuit and the buffer absorption protection circuit.

1 Main circuit structure schematic diagram

Figure 1 is a typical schematic diagram of the inverter structure. It consists of three parts: inverter circuit, drive protection circuit, control and signal acquisition circuit.

The inverter circuit is mainly responsible for the conversion of electric energy, that is, converting the input DC electric energy into three-phase AC electric energy that can be used by the motor load to provide energy for the motor. Figure 2 shows the schematic diagram of the inverter circuit. This inverter circuit consists of 6 insulated gate bipolar transistors T1~T6 and freewheeling diodes D1~D6. By controlling the on and off of IGBT tubes T1~T6, the input DC power supply is inverted into a frequency-adjustable rectangular wave AC output to the three-phase motor. The function of the freewheeling diodes D1~D6 is to provide a release channel for the electric energy stored in the motor coil when T1~T6 changes from on to off; when the motor brakes, it provides a channel for the regenerative current to flow back to the DC power supply.

2 Protection absorption circuit structure

Due to the existence of distributed inductance in the circuit and the high switching speed of IGBT, when the IGBT is turned off and the reverse recovery diode connected in parallel recovers in reverse phase, a large surge voltage Ldi/dt will be generated, thus threatening the safety of the IGBT. Therefore, measures must be taken to suppress the surge voltage and protect the IGBT from being damaged. The surge voltage can be suppressed by adding a protection absorption circuit. Its schematic diagram is shown in Figure 3. The protection absorption circuit has a good suppression effect and has the advantage of small loss in the protection absorption.

2.1 Analysis of protection circuit principle

Taking the turn-off moment of the switch tube T1 as the starting point to analyze the working principle of the absorption circuit, its working process can be divided into three stages: linear commutation, bus parasitic inductance Lp resonant energy and absorption capacitor Cs discharge.

The linear commutation phase starts from when the switch tube T1 receives the shutdown signal and ends when the switch tube T1 is completely turned off. The bus current flowing through the bus parasitic inductance Lp is divided into two branches, T1 and the absorption circuit.

After the linear commutation phase is over, the switch tube T1 is completely turned off. At this time, the parasitic inductance Lp of the main circuit resonates with the absorption capacitor Cs, and the energy stored in Lp is transferred to Cs. When the voltage on the absorption capacitor reaches the maximum value, that is, the resonance peak, the resonant current i is zero, the absorption circuit diode D2 is turned off, and the clamping voltage prevents oscillation.

After the second stage, the overshoot energy on the absorption capacitor Cs is discharged through the absorption resistor R, the power supply and the load. During the discharge process, the load is approximately considered to be a constant current source.

2.2 Component parameter selection

a. Selection of buffer capacitor Cs

The capacitance value of the buffer capacitor Cs in the buffer circuit is:

Where, L is the parasitic inductance of the main circuit, Io is the collector current when the IGBT is turned off, VCEP is the final value of the buffer capacitor voltage, and Ed is the DC power supply voltage.

b. The value of the buffer resistor Rs

The function of the snubber resistor is to release the charge of the snubber capacitor before the next IGBT shutdown. Therefore, before the next IGBT action, under the condition of 90% discharge of the stored charge, the snubber resistor value formula should satisfy the following formula:

Where f is the switching frequency.

3. Driving circuit structure

To ensure the reliable operation of IGBT, its driving circuit plays a vital role.

3.1 IGBT drive circuit requirements

The basic requirements for IGBT drive circuits are as follows:

(1) The drive circuit must be very reliable and provide a low-impedance charging and discharging circuit for the gate capacitance of the IGBT;
(2) Under the condition that the switching characteristics and power consumption are met, the gate resistance can be appropriately increased to limit the instantaneous voltage drop peak;
(3) The drive circuit can transmit high-frequency pulse signals at the kHz level;
(4) The maximum voltage drop between the IGBT gate and emitter is ±20V. The forward drive voltage is usually +15V and the reverse drive voltage is -8V.

3.2 Driving circuit composed of M57959L

According to the above drive circuit design principles, various forms of drive circuits can be designed according to different requirements. Common drive circuits include drive circuits composed of discrete components and dedicated integrated drive circuits. Compared with drive circuits composed of discrete components, dedicated integrated drive circuits have strong anti-interference ability, high degree of integration, fast speed, and perfect protection functions, which can achieve the best drive of IGBT.

M57959L is a hybrid integrated IGBT driver produced by Mitsubishi Corporation of Japan. Its internal principle structure is shown in Figure 4. It has high-speed photoelectric isolation input, high insulation strength, and is compatible with TTL level. It has a built-in timing logic short-circuit protection circuit and a protection delay characteristic. The chip is powered by positive and negative power supplies, which overcomes the disadvantage of unstable negative voltage when powered by a single power supply. It has high driving power and can drive 200A/600V or 100A/1200V IGBT modules. The driving circuit composed of M57959L is shown in Figure 5.

When using, you should pay attention to the value of the gate resistor. The value of the gate resistor Rext can affect the oscillation suppression effect, slow down the di/dt when the switch is turned on, improve the current overshoot waveform, and reduce the surge voltage. From a safety perspective, Rext should be a larger value, but a larger Rext affects the switching speed and increases the switching loss; from the perspective of increasing the operating frequency, a smaller value should be taken. When the switching frequency is met, a larger Rext should be taken.

4 Main circuit installation and layout

Since the IGBT switching frequency is very fast and the power is also very high, the inverter composed of IGBT will produce strong interference to other components. These interferences not only affect the normal operation of the circuit, but may even damage the inverter due to instantaneous short circuit. Therefore, sufficient attention should be paid to electromagnetic interference, and reasonable installation and layout can reduce electromagnetic interference.

Common interferences and corresponding measures include:

(1) Isolation of power supply to suppress IGBT switching interference Due to the influence of the distributed capacitance and coupling inductance of the power supply transformer, when one IGBT is turned on or off, the strong spike pulse will interfere with the normal operation of other IGBTs through the distributed capacitance (inductance). Therefore, each trigger circuit of the full-bridge inverter must be isolated to suppress this interference.

(2) Since the average operating current and instantaneous peak current of the inverter are very large, the leakage inductance in the inverter circuit and even the small lead inductance cannot be ignored. If the PCB layout is not carefully designed, these magnetic fluxes will pass through the closed PCB wires and form current. To this end, the following measures can be taken to suppress interference:

a. The trigger circuit components of each IGBT should be concentrated in a narrow area to avoid crossing each other;

b. The trigger circuits of the same phase should be adjacent, and the distance between the two groups should be relatively far;

c. The leads between PCB and IGBT should be as short as possible and twisted together.

5 IGBT voltage and current parameter selection

In the protection absorption circuit, when T1 is turned on and T2 is turned off, the voltage Uce2 that T2 bears is:

Considering the grid fluctuation of +/-10%, the mature voltage of T2 is Uce2:

Considering the instantaneous voltage when the circuit is turned on and off, and the voltage that the IGBT module can withstand should have a margin of 50% to 80%, the selected module voltage BVce should be:

Considering the fluctuation of the power grid and the influence of the current spike at startup, the Icm of the selected IGBT module is:

Where Pn is the inverter output power, δ is the pulse duty cycle, and η is the inverter efficiency.

6 Conclusion

This article mainly introduces the main circuit composition and IGBT parameter selection of the inverter for motor transmission composed of IGBT, the composition and parameter selection of the drive circuit and buffer absorption circuit, and the issues that should be paid attention to in the installation and layout of the main circuit. It has certain value for the inverter design in practical applications.