The Inverter Stage: Unlocking the Power of Power Electronics

The inverter stage is the “muscle” of the drive – a power electronics block that provides the regulated, conditioned power directly to the motor, driving it in the manner required by the end application, providing the amperes needed for torque production, the voltage needed for speed and magnetic flux regulation, and the frequency and phase relationships required for control of the speed and torque in the most efficient manner.

In a previous blog, we finished our consideration of the power supply functions within the variable speed drive (VSD). In this blog, we move into the heart of the drive – the inverter stage. This is highlighted in Figure 1.

Figure 1: Detailed Variable Speed Drive Architecture

The inverter stage fundamentally has two sets of inputs and one set of outputs. The main power input is the DC bus (discussed in the previous blog on the input stage). The main power outputs are the three-phase lines to the motor. The main control inputs are the gate signals to each of the switching power transistors in each leg of the inverter. There are various flavors of an inverter with different numbers of phases, and different power electronics topologies (multi-level, matrix, etc) but the vast majority of inverters on the market are three-phase, two-level inverters as shown in Figure 2.

Figure 2: Inverter Stage Driving Three-Phase Motor

Power Transistors

The power transistors in each leg of the inverter are power-switching devices that turn fully on or fully off at a high frequency (usually in the range of 5-20kHz) and a controlled duty cycle or modulation index. They act as quasi-ideal switches that modulate the voltage applied at each motor phase winding and re-create a waveform with low-frequency components (typically sinusoidal) related to the motor velocity (typically in the range DC-1kHz) and instantaneous position and an averaged voltage amplitude related to the motor velocity and rated magnetic flux. This is illustrated in Figure 3. The output inverter phase-to-negative voltage is a pulse width modulated square wave switching between the DC bus voltage and zero. The inherent inductance of the motor windings will filter this signal to result in a motor current at the required low-frequency phase, frequency, and amplitude, with some undesirable switching frequency ripple present.

Figure 3: Inverter phase output voltage and current

The power transistors and associated thermal management (heat sinks, fans) are usually amongst the most expensive components in a VSD and also tend to take up the most space, especially at higher power levels. Power losses in the transistors come from the fact that they are not ideal switches, have a small voltage drop across them when conducting current, and also have non-zero turn-on and turn-off switching times when high voltage and high current within the switch overlap for short periods resulting in switching losses.  Historically MOSFETs and IGBTs have dominated the power transistor market, but in recent years, there has been a trend to look to wider band-gap SiC and GaN-based power transistors. These can offer lower switching losses, which can allow for higher switching frequencies or more efficient inverters at the same switching frequency. The incentive for VSD designers to move to SiC and GaN is not quite as compelling as in other application spaces, where moving to higher switching frequencies allows for smaller filters. In VSDs, the filter is the motor winding, so in many applications, a higher switching frequency does not give a huge advantage.

Gate Drivers

Gate drivers are responsible for converting the logic level signals from the motor controller to signals that have the voltage amplitude and current drive capable of fully controlling the power transistors, and for ensuring that those drive signals are correctly referenced, as illustrated in Figure 4.

Figure 4: Switching function of a gate driver

Other functions provided by the gate drivers can be:

• Power transistor protection (e.g. desaturation detection in inverter/motor short-circuit conditions)
• Managed switching of the power transistor (e.g. application of different slew rates to the gate drive signal to manage EMI in certain conditions)
• Galvanic and safety isolation of the motor controller from the inverter stage
• Reporting of fault conditions in the inverter and power transistors

So gate drivers can be very complex devices with full communication interfaces (e.g. ADuM4177), or quite simple devices with only driving functionality (e.g. MAX22700). Whatever features are included in the gate driver, robustness to transients (expressed in specifications such as Common Mode Transient Immunity – CMTI) is an important feature as these devices live in a very noisy domain in the inverter, and need to be able to sit between the quiet, low voltage environment of the motor controller, and the high voltage, high current, noisy domain of the power transistors.

Summary

The power inverter is the heart of the VSD and manages the currents and voltages applied to the motor. Safe, robust, efficient switching of the power transistors within the power inverter is an important function of the gate drivers within a VSD. The next blog will consider some of the signals that are measured within the inverter stage to accurately control its operation.