Basic Fundamentals of Inverter

Power Inverters for Electric Vehicles

Technological advancements in the electric vehicle power inverter sector have played a crucial role in the growth of the market. Several technological advancements in electric vehicle power inverters have been made in recent years.

Power inverters used in electric vehicles are now capable of handling higher voltage and power ratings than ever before. This allows electric vehicles to travel longer distances on a single charge and to have faster acceleration and top speeds. Table 1 below show the summary of the rated power and power densities of the reviewed traction inverter implementations:

Current EV Market Designs

 

 Nissan LEAF inverter. 1) Control Board 2) DC-link capacitor 3) DC-link busbar 4) Phase busbars 5) Current sensor 6) Gate driver board 7) IGBT power modules 8) Serpentine heat sink

 Tesla Model S inverter components. 1) Control board 2) Aluminium shield 3) Phase current sensor ferrite ring 4) Gate driver board 5) Phase busbars 6) Heat sinks 7) TO-247 package IGBTs 8) IGBT clips 9) DC-link capacitors.

 

Second generation Chevrolet Volt inverter components: 1) Control board 2) Gate driver board 3) Top heat sink 4) Power board 5) DSC IGBT modules 6) Bottom heat sink 7) Phase busbars 8) DC-link capacitor

Toyota “power card” DSC IGBT module stack with heat exchanger

BorgWarner Viper Inverter Model

 

Borgwarner High Voltage Inverters

Technical Features:

1. Using the Viper power switch patent. The switch has the dual-sided cooling capabilities. This unique cooling structure provides a more compact, efficient power module. When compared with previous generations of inverters, this dual-cooled power switch enables weight reductions of 40%, overall size reductions of 30% and higher power densities of 25%.
   
   
2. The switch’s design also eliminates wire bonds – the complex web of wiring that provides the path for the current to flow through the switch via the inverter’s connections to and from the battery and motor. Eliminating the wire bonds and moving to soldered interconnectors, translates into better durability.
   
   
3. The use of silicon carbide (SiC) MOSFET. SiC devices offer 2–3 times lower on-state voltage drop than Si, 10x higher breakdown field than Si. It has better thermal conductivity and better high-temperature stability. The higher voltage means that the cables are lighter than the low-voltage cables. It also has a smaller power size than silicon and a faster switching speed. Faster switching speeds mean lower switching losses, with SiC reducing switching losses by 70%. Therefore, it is especially suitable for high switching speed or power occasions. The advantages of SiC power electronics are particularly evident in 800 V battery systems.

 

 

Power Inverter: Power Module Thermal Design

Power modules are the major, key elements of the power conversion emobility systems. Being able to reduce the size of existing arrangements or packing double the power in current inverter sizes is a global trend that drives the e-mobility crowd. These fast switching systems need to have controlled waveforms with high frequency responses that allow for minimal losses and as a result, better performance and reliability.

Power modules consist of:

1. IGBT (integrated gate bipolar transistor) modules

2. MOSFET (metal oxide semiconductor field effect transistor) modules 

3. IPMs (intelligent power modules)  

4. SIP (system in package) modules.

Power modules are used extensively in high power inverter applications like alternative energy conversion, battery backup systems, motor drives for industrial equipment, electric vehicles, traction and shipping.
 

Tesla Model 3 Main Inverter 

 

DC Link Capacitors

1. Thermal Interface Materials

2. Encapsulation material

MOSFET modules in the inverters

1. Thermal Interface Materials

2. Liquid Encapsulants

Application Trends

• Smaller size and lower weight
• Higher Frequency and high power rating
• Built-in. Integration capability

Size and weight are highly driven by the thermal capabilities or a design. MOSFETs, IGBTs etc are reducing the overall design size but they are also much more thermally intensive. With SiC MOSFETs you can pretty much cut in half the design size but you also have much more heat to dissipate. That's why choosing the correct thermal interface materials is a crucial enabler of this technological trend.

On the same note SiC switches can offer higher switching speeds with minor losses but the cost is, once again, the higher thermal output, requiring Epoxy molding compounds specifically made for SiC MOSFETs.

Finally, integration cannot be achieved without getting rid of cables and useless interconnections. This is only possible with busbars, requiring busbar coating powders with great electrical insulation to be able to make compact and integrated designs.

Power Module: Subcomponents

Power modules can be anything from IGBT modules to MOSFET , IPMs and SIP modules. They are used extensively in high power inverter applications. These modules are mainly lead by next gen dies such as Silicon Carbide, Gallium Arsenide and Gallium nitride, with the later being reserved mainly for smaller, consumer-grade applications. Gate driver controllers can use all of those technologies depending on the output and performance requirements.

SiC Switches in Viper Inverter (Borgwarner)

IGBT Package

 

Chart. Power Module Component properties for different applications

Phase change materials (PCM) are designed to minimize thermal resistance at interfaces and maintain stable performance through the rigorous reliability testing required for long product life applications. 
Their main benefits are:

• Min BLT 20~30um
• High thermal conductive
• Non silicone based material
• No pump out, bleed out, dry out issue
• Superior long-term reliability
• Stencil printable and dispensable

Thermal Gap Pads (TGPs) provide high thermal performance with ease of use for many applications. Ultra-high compressibility enables low stress and excellent conformity to mating surfaces. Honeywell Thermal gap pad models are naturally tacky, and require no additional adhesive which could inhibit thermal performance.

Thermal gap pads provide thermal performance with ease of use across a multitude of applications. They have been designed to minimize thermal resistance at interfaces, exhibit minimal bleeding and maintain effective performance through reliability testing. TGP models are silicone based, therefore they offer a certain anti-shock effect, with electrical isolation and non-flammability. While all interfaces bleed (with zero exemptions), our pads exhibit very low bleeding even under pressure due to the low molecular weight of the remaining unlinked chains.

Power Inverters: Functions & Application