Transistor Selection Considerations to Improve Performance and Lower Cost in Power Electronics

In modern power converter and wireless power transfer systems, the choice of transistor is critical to deliver the optimal balance of performance, energy efficiency and price.

In this blog we will look at how to select the MOSFETs for switching power electronics, starting with an overview on the transistor before examining the types of losses in power MOSFETs, a comparison of silicon vs wide bandgap semiconductors – GaN, SiC and GaAs – outlining when each technology should be applied and finally, we will show how the soft-switching ZVS and ZCS techniques used in EPIC controllers and Eggtronic patented architectures improve performance for both on and off transitions to improve performance and/or lower the cost of the power electronics.

Transistor 101 (trans resistor)

The transistor is a fundamental component of electronics that acts as a variable resistor controlled by a control terminal. They can be used in linear mode, as a continuous dial to linearly and continuously regulate the resistance of the transistor, or switching mode, where the transistor is either fully on (with minimum resistance) or off (with infinite resistance).

Transistor size varies according to the application’s power requirements, with small devices used to handle small amounts of power - i.e. signal transistors in integrated circuits, and large devices for audio amplifiers, power converters and similar applications.

Finally, to complete the overview, transistors can be BJTs (bipolar junction transistors), or MOSFETs (metal oxide semiconductor field effect transistors), in which a channel between the drain and the source of the transistor can be controlled (opened and closed) to apply an electric field to the gate. Today, MOSFETs are the most commonly used type for power electronics.

Losses in power MOSFETs

In these switching converters, the FET can have two kinds of losses, static and dynamic.

Static losses come from the resistance of the transistor and can be improved by increasing the width of the transistor and decreasing length – to use the classic water analogy, the transistor is like a pipe, with larger, shorter pipes creating a lower resistance against the flow of water.

Dynamic losses are created as the FET transitions from on to off, and vice versa. If the voltage during a transition from off to on is not zero, then energy stored in the parasitic capacitance (when turning on) or parasitic inductance (when turning off) will be discharged and wasted. Analogously, if the current is not zero during the transition from on to off, then the integral over the transition time of the resistance (which is increasing from almost zero to infinite) of the transistor multiplied by the square of the current (which is decreasing down to zero), lead to a resistive dynamic loss. These dynamic losses can be mitigated by making transistors smaller and faster, and improving the package.

For a given working condition (e.g. voltage and current stress in the FET), and for a given power semiconductor technology (e.g. silicon FETs, which are the most commonly used today), the reduction of static losses through a larger channel usually increases the parasitics of the FET, worsening dynamic losses, with slower FETs, and higher parasitic capacitance.

Wide bandgap semiconductors

The bandgap is the energy needed to free an electron from its orbit around the nucleus and semiconductors such as gallium nitride (GaN) or silicon carbide (SiC) have a bandgap that is wider than that of standard silicon – 3.39 eV, 2.26 eV and 1.12 eV respectively. Thus they are known as wide bandgap (WBG) semiconductors.

If we look at the Fermi Energy for a metal, there is overlap between the valence band and conduction band, allowing electrons to flow. For a semiconductor, this overlap is removed, with a small gap that still allows some electrons to jump to the conduction band under normal temperature conditions.

As we move to WBG semiconductors, the required energy to jump increases. And in insulators, this gap is significantly higher still, making it impossible at normal temperatures to have electrons in the conduction band.

Wide bandgap semiconductors bring several advantages for power electronics. Even under high temperatures, these semiconductors function properly, with commercial SiC MOSFETs typically being guaranteed to temperatures over 150^oC and demonstration models have been produced to tolerate over 800^oC, whereas silicon becomes conductive as the temperature rises, with even silicon IGBTs (insulated-gate bipolar transistors – used in very high-power applications) typically restricted to 125^oC.

These WBG materials can also sustain higher electric fields, which means higher voltage peaks for a given length of the FET channel and reductions in conduction losses for the same maximum voltage.

Other fundamental parameters to be considered in the choice of the semiconductor for power applications are electron mobility, thermal conductivity and melting point, all of which are plotted in the chart below for each of Si, GaN, and SiC, as well as for gallium arsenide (GaAs).

There are trade-offs for each semiconductor material, however silicon is typically best compromise when cost is critical; whereas SiC is ideally suited to high voltage applications, especially when temperature is critical; and GaN is ideally suited to medium-voltage applications (hundreds of volts) and for higher frequency switching. Finally, GaAs is best suited to extremely high-frequency applications such as microwaves and RF.

Alternative approaches: ZVS and ZCS techniques to lower dynamic losses

There is another way to reduce dynamic losses and improve the performance of power converters and wireless power transfer systems, and this is through the soft-switching Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS) techniques.

ZVS dramatically reduces dynamic losses caused by the parasitic capacitance of the MOSFET in turn-on transitions, with ZCS reducing losses in turn-off transitions.

All Eggtronic EPIC controllers are all capable of these soft switching capabilities and by implementing the ZVS / ZCS techniques in standard Si FETs it’s possible to reduce the performance gap between silicon and wide bandgap semiconductors. What’s more, unique features of Eggtronic EPIC controllers are the fact that ZVS / ZCS is ensured in every working point (for every input voltage, and for every load condition), and the Bill of Materials of the converters based on Eggtronic reference designs is extremely reduced in comparison to traditional ZVS architectures.

Employing GaN and SiC transistors working in a ZVS / ZCS converter or wireless charger shows even better performance, with lower losses, a higher maximum frequency, and lower heat dissipation than the same wide bandgap semiconductor working in a hard-switching architecture.

Ultimately, deploying Eggtronic EPIC controllers boosts the performance of every transistor - be it silicon-based, GaN, or SiC. And the combined use of EPIC + WBG leads to unmatched levels of performance, ensuring both cost-effectiveness and reduced converter size.

Conclusions

Choice of transistor is one of the fundamental steps in the development of power electronics, and wide bandgap semiconductors can help to reduce power consumption, and size.

However, the proper choice of the power architecture and the IC controller is even more important as it can enable lower-cost transistors or, for the same budget, boost transistor performance.