Weighing Automotive WBG Power Options

Automobile power electronic designs continue to diverge in electric vehicles (EV), hybrid, and gasoline autos where silicon power metal-oxide-semiconductor field-effect transistors (MOSFETs) and wide-bandgap (WBG) semiconductor devices, such as gallium nitride (GaN) and silicon carbide (SiC) devices, create effective design alternatives.

The range of electronic vehicle options spans between the full hybrid electric vehicle (FHEV), plug-in hybrid electric vehicle (PHEV), and mild hybrid electric vehicle (MHEV). The voltage requirements for the hybrid-electric-vehicle (HEV) power systems range from 12V to 800V with hundreds of ampere currents. To settle power issues, wide-bandgap SiC or GaN devices—compared to traditional silicon MOSFETS —at a higher price bring higher efficiency, switching frequency, operating temperature, and operating voltage to bridge this power conversion gap.

Previous solutions had lower voltage supply rails, making the complete MOSFET solutions appropriate. While these older solutions work, they are unable to survive higher supply voltages and faster transients demands. GaN, SiC, and alternative MOSFET designs deliver higher efficiency, switching frequency, operating temperature, and operating voltage to the automotive power system.

Finding a suitable device technology to build your automotive power system doesn't have to be complicated. Understandably, the plethora of specifications can be overwhelming. Also, selecting a less-than-ideal device may result in compromises, potentially increasing system cost or affecting performance. This article simplifies device selection by exploring key considerations when assessing both your automotive system's power supply needs and available device performance specifications.

The primary MOSFET devices must conduct high currents from the battery to the system. If the configuration of these MOSFETs is in parallel, the system challenge keeps current and temperature imbalances under control (Figure 1).

Figure 1 shows three MOSFETs in a ring configuration. In this arrangement, the MOSFET sources connect to a star point. The symmetrical connections to the drain loop connect the electrical and thermal paths between the MOSFETs. MOSFETs must dissipate as much power as possible to optimize MOSFET performance and keep the junction temperature of the hottest MOSFET below the maximum safe temperature of 175°C.

To realize this goal, matching and minimizing the thermal resistance between each MOSFET's mounting base and the mounting bases of all the other MOSFETs is critical. The mounting of each MOSFET is symmetric and as close together on a thermally conductive surface as possible.

MOSFET breakdown voltage (BVDSS) occurs when the reverse-biased body-drift diode breaks down and significant current starts to flow between the source and drain by the avalanche multiplication process. This voltage can be greater than 200V.

A low thermal resistance path allows the easy flow of heat between MOSFETs. Heat flow can be analogous to electric current flow—so, the MOSFETs' thermal bonding points or drain tabs should be on a thermal ring main. The MOSFETs' mounting base temperatures closely track when heat flows easily between all the MOSFETs in the group.

The wide-bandgap semiconductor devices must provide high electric breakdown fields, thermal conductivity, and saturated electron drift velocity. Regarding these specifications, the GaN and SiC versus the IGBT Si device characteristics provide significant design advantages.

GaN transistors switch much faster than silicon MOSFETs, which can achieve lower switching losses and low gate capacitance to enable faster turn on and turn off while reducing gate drive losses. As an example, GaN offers a gate charge of less than 1nC-Ω versus 4nC-Ω for silicon. Also, these devices provide significantly lower output capacitance, enabling designers to achieve higher switching frequencies without increasing associated switching losses and to shrink the size and weight of magnetics in the system. The output charge of a typical GaN device is 5nC-Ω versus comparable silicon at 25nC-Ω.

GaN and SiC each have different power needs. The voltage levels of SiC devices are as high as 1,200V with high current-carrying capabilities, making them a good fit for automotive traction inverters applications. GaN FETs, on the other hand, are typically 600V devices and can enable high-density converters in the range of 10kW and higher. GaN applications include electric vehicle onboard chargers and DC/DC converters. Despite these differences, the two technologies do overlap in some applications below 10kW.

Silicon carbide's carbon and silicon constituents are respectively the fourth and eighth most abundant elements on earth. SiC as a wide bandgap semiconductor has revolutionized power conversion performance, yielding efficiency figures previously unattainable at high frequencies, with further knock-on benefits of smaller associated passive components, particularly magnetics.

SiC FETs, as a cascade arrangement of a Si-MOSFET and SiC JFET, achieve less than 7mΩ on-resistance for 650V devices and less than 10mΩ at 1200V rating (United SiC).

A braking action causes energy to flow from the combustion engine torque to the Belt-Driven Starter Generator (BSG) and to a 48V battery, which acts as a generator. A three-phase inverter via a silicon MOSFET intrinsic diodes rectifies the BSG electric waveforms to charge the 48V battery with direct current (Figure 2).

Click image to enlarge

 Figure 2: An auxiliary 48V system (Source: Author)

Energy flows to the BSG from the 48V battery during start-stop activities to act as a motor. During this time, the 48V battery provides power to the BSG and draws power by using three-phase silicon power transistor inverters. A DC-DC buck converter lowers the 48V to 16V, powering 3-phase inverter gate drivers. The DC-DC buck converter provides the BSG with the proper motion sequence.

The BSG accomplishes three tasks: Starting the engine during start-stop, improving acceleration performance by boosting the torque, and causing braking actions to charge the battery. The 48V battery also powers pumps, fans, compressors, electric power steering racks, and aids start-stop systems. A 48V battery can deliver the same 12V battery power with a quarter of the current.

A lithium-ion MHEV battery specification can be 1kWh, 48V, 21Ah. The "VDA320: Electric and Electronic Components in Motor Vehicles 48V On-Board Power Supply" document recommends that the battery voltage operating range be between 36V and 52V. This specification allows limited operating modes between 20V and 60V and a dynamic overvoltage up to 70V. The 60V maximum operating voltage is the maximum permissible safe contact voltage for human operators.

DC-DC Buck Converter Robustness

The 48V buck converter in Figure 2 can be subject to voltage spikes as high as 70V and electrical stress up to 40ms. Operation above this limit can cause permanent device damage. Accordingly, the absolute maximum rating of the buck converter's input voltage needs a margin above 70V.

Auto Power Electronics Require Low EMI

Electromagnetic interference (EMI) is narrow or broadband electrical “noise” that originates from an external source. EMI interference can affect an electrical circuit by electromagnetic induction, electrostatic coupling, or conduction. For this reason, automotive power management electronics must have EMI protection.

In automotive environments, the 48V buck converter must meet the EMI's CISPR25 Class 5 specifications. Fixed-frequency converters usually attenuate spikes during both conducted and radiated testing. Adjustable DC-DC frequency allows engineers to filter specific frequencies when passing EMI tests. By distinction, constant on-time architectures exhibiting those variable frequencies seldom have good EMI performance.

The 48V Front-End Buck Converter

Automobiles have numerous electronic control units (ECUs) with good EMI performance. The robust front-end 48V buck converter interface withstands the battery's static and dynamic voltage conditions. This interface also supports various 16V to 20V output voltage and motor-control gate drivers while providing MCU backup power if a 12V battery is disconnected.

The 48V buck converter versus the 12V buck tends to have higher switching loss (Equation 1).

Eq. 1

PSW = ½ x C x V² x f              

Where C is the parasitic capacitances

V is the buck converter output

f is the operating frequency

By reducing the frequency of operation (f), the switching loss diminishes. Additionally, the adaptation of an advanced process with smaller minimum features lowers parasitic capacitances (C). The control technique leads to low-duty cycle operation. For example, a 16V output and a 48V input leads to the use of Equation 2.

Eq. 2

D = BUCK₁/ BUCK₂                 

D = 16 / 48

D = 0.33

Where D is duty cycle

BUCK₁ and BUCK₂ rated output voltages

From this calculation, the high-side transistor of the buck converter conducts 33 percent of the time while the low-side transistor conducts 67 percent. This calculation can guide the sizing of the power transistors for optimum performance.

Choosing which device technology is most suitable for an EV's electric power can be difficult. When there are choices, there will be tradeoffs. Using silicon MOSFET power devices can solve the overvoltage imbalance problems, providing an effective power conversion solution. In contrast, WBG devices can better bridge the power supply gap among the various EV options. Also, compared to traditional silicon devices, WBG—SiC and GaN—devices provide higher efficiencies, switching frequencies, operating temperatures, and operating voltages, but at a higher cost. Although both technologies offer competing solutions, with some clever engineering design, it may still be possible to extend the life of power semiconductor MOSFET devices. These inexpensive low-voltage rating devices can still provide an efficient, low-cost power conversion alternative to remain competitive in the automotive environment. 

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