IGBT, MOSFET and GaN: An Overview of Efficiency, Power and System Cost for Inverter Design

There are numerous applications in which IGBTs have found their potential use (High Voltage and High Current applications) since the designed Peak Power Requirements are exceeding 250kW. For example, High Power Inverter for Centrifuge in Industrial Sector, Motor Control of a Wind Tunnel for Aerodynamic Applications, and Motor Drive Control System in Automotive Sector. In applications where Peak Power requirement is higher, IGBTs would be the perfect choice.

The next design requirement which defines the selection of a device is Switching Frequency. To make a better comparison between the selection of IGBTs, MOSFETs and GaN in an inverter, let’s assume a design requirement of peak power up to 10kW-20kW (Mid-Range Inverter). This region of Peak Power is an overlapping area between IGBTs, MOSFETs and GaN, since all of these devices are capable to handle this power.

After defining the power requirements, the designer then selects the switching frequency of the application which would then determine the selection of the component. The question now arises which Switching Frequency should be opted? To answer this question better, let’s have a look at some of the key design parameters that are impacted by Switching Frequency.

a. Power Losses- Switching Losses
One of the key concerns that a designer has to deal with is to reduce the power losses and switching losses in high-efficiency applications. Switching losses are directly proportional to switching frequency i.e. Higher the frequency, higher would be the losses associated with it.

The factor that contributes to overall power dissipation is the extra “Internal (On) Resistance” the circuit has to face. This Internal (On) Resistance of the switching component is known as On Resistance (RdsOn). The higher the RdsOn, the higher would be the power dissipated. Usually, the devices having higher RdsOn have lower current ratings (IdMax) if the Drain to Source Voltage is kept constant (devices having similar Voltage Ratings in the same package)

b. Harmonics
Losses that occur due to harmonics have also a major impact on the efficiency of the system. Harmonics are usually the multiple of the fundamental switching frequency that are generated due to non-linear switching components. The effect of these distortions can cause major damage to the components hence decreasing components’ life (premature aging). However, the amplitude of these harmonics is inversely associated with the switching frequency. The higher the switching frequency, the lower would be the effect of Harmonics.

The applications like Solar Inverters and Traction Inverters where it is vital to have lower losses and increased efficiency it is better to use the component that has lower Internal (On) Resistance (RdsOn) and supports higher switching frequency. In this case, GaN would be a better fit. MOSFET or Silicon Carbide MOSFET would take the second place since MOSFETs can handle higher switching frequencies as compared to IGBTs.

Let’s have a look at some of the other key parameters that can influence the selection of a switching component based on the design requirements.

Power Density and Thermal Stability
GaN devices have better numbers of power density as compared to IGBTs and MOSFETs. They are capable of delivering more power and fewer losses while being in a compact package which enables designers to make more compact Printed Circuit Boards (PCBs) for Inverters.

The key factors that contribute to thermal performance are Internal (On) Resistance (RdsOn), Gate Charge (Qg) and Junction-Gate Capacitance. The overall power dissipation caused by these parameters is directly linked to the thermal behaviour (Heat Dissipation) of the device. The higher the values, more would be the dissipated power which would contribute to higher temperatures. From the experiments and available literature, it has now been established that GaN devices offer approximately 80% less power losses as compared to IGBTs and MOSFETs while providing approximately two times the power density in the same form factor.

Efficiency and Figure of Merit:
As discussed above, there are some inverter applications in which Efficiency is considered the “most important” parameter. Since it is the ratio of the delivered power to the input power, all the effort is put in to maximizing the output to achieve better efficiency in such inverter applications.

While designing an efficient inverter, some internal parameters like Internal (On) Resistance (RdsOn) and Gate Charge should also be kept in mind. These two parameters are used for the calculation of FOM (Figure of Merit), which directly influences the overall efficiency of the design. The Better the FOM, better would be the efficiency.

FOM = On Resistance (RdsOn) x Gate Charge (Qg)

In the same Voltage range, GaN and MOSFETs provide better FOM as compared to IGBTs.

BOM Cost:
There are various technical features that add to the BOM cost of the application.

• Complex Cooling
  Due to high Internal (On) Resistance, heating can be a major issue in Inverters which contributes to thermal instability. To counter this problem heat sinks additional cooling is employed in mid-range to high power Inverters to ventilate extra heat, contributing to an additional BOM cost. Generally, Cooling requirements are quite low in GaN devices since they do not tend to overheat because of high electron mobility, thermal stability and efficiency.
• Output Filters and Magnetic Components
  Filters are used to stabilize the output and block certain frequencies of the inverter. They are designed using magnetic components (Capacitors and Inductors) and employed to reconstruct the PWM and SPWM signals at the Inverter Output. For higher frequencies small output filters (Small Values of Inductances and Capacitances) are required as compared to low frequencies. This would also affect the size of the PCB and add to the BOM Cost.

  With GaN devices small output filters are required to stabilize the output (since they are used in high-frequency applications). In the case of IGBTs (lower frequencies), powerful output filters with higher specifications are required which add to the BOM cost.

• Efficiency of the other Components
  In high-power systems, the distortions caused by harmonics and overheating generated in the circuit can cause pre-mature aging of the components. The overall efficiency of the component will decrease over time making it unsuitable for application use and hence needs replacement in the long run. Replacing a failed component takes time, effort and additional BOM Cost.

The qualitative overview of IGBTs, MOSFETs or GaN in inverter applications is shown in the matrix below. This comprehensive overview of strengths and weaknesses can help to identify application-specific performance parameters when considering them in a design.