Migrating from Silicon to Silicon Carbide
Introduction
Migrating from silicon (Si) metal oxide semiconductor field-effect transistors (MOSFETs) to silicon carbide (SiC) MOSFETs involves several important considerations. Simply replacing a Si device with a SiC device in an existing design will not automatically yield the benefits of faster switching, improved efficiency, and lower noise.
Key considerations for this migration include:
- Body diode characteristics: SiC MOSFETs have different body diode properties compared to Si MOSFETs, which can affect circuit performance.
- Voltage drop and RDSon: The on-resistance (RDSon) and voltage drop characteristics of SiC devices differ from those of Si devices, impacting efficiency and thermal performance.
- Short Circuit Withstand Time (SCWT): SiC MOSFETs typically have a shorter SCWT, necessitating careful circuit protection design.
- Gate drive requirements: SiC devices often require different gate drive voltages and currents, which may necessitate changes to the gate driver circuitry.
- Circuit protection: Enhanced protection mechanisms may be needed to safeguard SiC devices due to their different electrical characteristics.
- Bus structure: The layout and design of the bus structure are crucial for minimizing parasitic inductances and capacitances, which can affect switching performance.
- Thermal management: Improved efficiency of SiC devices can lead to reduced thermal requirements, allowing for smaller heat sinks, smaller fans, or even the elimination of fans. This can result in an overall lower system cost.
By carefully addressing these considerations, you can fully leverage the advantages of SiC MOSFETs, such as higher efficiency, faster switching speeds and reduced thermal management needs, ultimately leading to a more cost-effective and reliable system.
Body Diode/Reverse Recovery
Please refer to the points in Figure 1. In MOSFETs, SiC devices exhibit significantly lower reverse recovery compared to their Si counterparts. This lower reverse recovery translates to faster switching speeds, reduced losses, and higher overall efficiency.
Figure 1: Body Diode and Reverse Recovery
However, it is important to note that SiC devices generally have a higher forward voltage drop across the body diode compared to Si devices. This is due to the higher energy bandgap of SiC. Despite this, SiC Schottky barrier diodes can be used to mitigate this issue.
SiC Schottky diodes have a positive temperature coefficient, making them easier to parallel compared to Si PN junction diodes, which have a negative temperature coefficient. This positive temperature coefficient ensures that current sharing between parallel diodes is more balanced, enhancing reliability and performance.
In summary, the lower reverse recovery of SiC MOSFETs leads to faster switching, lower losses and higher efficiency. While the forward voltage drop of the body diode in SiC is higher, the use of SiC Schottky diodes can effectively address this, making SiC devices a superior choice for high-performance applications.
DC Bus and Decoupling
In high-power systems, a direct current (DC) voltage is maintained with the help of capacitance to store energy for when MOSFETs turn on and off. However, parasitic elements such as inductance in the bus bar, wiring, and lead inductance can negatively impact performance. Minimizing these parasitic inductances is crucial to achieving the benefits of faster switching. Refer to Figure 2.
Figure 2: DC Bus and Decoupling
To address this, laminated bus bars should be used to reduce inductance. Additionally, ceramic capacitors, which have very low impedance, should be placed as close as possible between the drain of the high-side MOSFET and the source of the low-side MOSFET. This proximity helps to minimize the inductive loop and improve switching performance.
Film and electrolytic capacitors, which also contribute to lower impedance, can be positioned slightly further away from the actual circuit. These capacitors help to smooth out voltage fluctuations and provide additional energy storage.
Finally, selecting packages that minimize parasitic inductance is essential. Whether using discrete packages like TO-247 or high-power system modules, choosing designs that reduce internal inductance will further enhance performance and reliability.
By carefully managing the DC bus and decoupling strategies, you can significantly improve the efficiency and performance of high-power systems.
Review and Additional Considerations
Refer to Figure 3 showing considerations for migrating from Si to SiC.
Figure 3: Review and Additional Considerations
VCE vs VDS: IGBT vs SiC
When comparing Insulated Gate Bipolar Transistors (IGBTs) to SiC MOSFETs, the voltage drop across the collector-emitter (Vce) in IGBTs is generally higher than the drain-source voltage (Vds) in SiC MOSFETs. SiC MOSFETs offer lower conduction losses and can operate at higher voltages, making them more efficient for high-power applications.
Switching Loss
SiC MOSFETs exhibit significantly lower switching losses compared to Si IGBTs and MOSFETs. This is due to their faster switching speeds and lower reverse recovery charge. Lower switching losses translate to higher efficiency and reduced heat generation.
Body Diode/Reverse Recovery/Forward Voltage Drop
SiC MOSFETs have a lower reverse recovery charge and faster recovery times than Si devices. However, the forward voltage drop of the body diode in SiC is higher. Using SiC Schottky diodes can mitigate this issue, as they have a positive temperature coefficient, making them easier to parallel and more efficient.
Gate Drive
SiC MOSFETs require different gate drive voltages and currents compared to Si devices. They often need higher gate drive voltages and faster switching capabilities, necessitating specialized gate driver circuits to fully leverage their performance benefits.
Switching Speed and Its Effect on EMI, Ringing, and Overshoot
The faster-switching speeds of SiC MOSFETs can lead to increased Electromagnetic Interference (EMI), ringing and voltage overshoot. Proper circuit design, including the use of snubber circuits and careful layout, is essential to mitigate these effects and ensure reliable operation.
Circuit Protection
Enhanced circuit protection mechanisms are required for SiC devices due to their different electrical characteristics. This includes over-voltage, over-current, and thermal protection to safeguard the devices and ensure reliable operation.
Bus Structure: Minimizing Parasitic Inductance/Capacitance
Minimizing parasitic inductance and capacitance in the bus structure is crucial for optimizing the performance of SiC MOSFETs. Laminated bus bars and strategically placed ceramic capacitors can help reduce these parasitics, improving switching performance and reducing losses.
Short Circuit Withstand Time (SCWT)
SiC MOSFETs typically have a shorter short-circuit withstand time compared to Si devices. This necessitates careful design of protection circuits to ensure that the devices can handle short-circuit events without damage.
Thermal Management
Improved efficiency of SiC devices can lead to reduced thermal requirements, allowing for smaller heat sinks, smaller fans, or even the elimination of fans. This can result in an overall lower system cost.