Pre Switch - APEC 2022

Inverter Benefits

The Pre-Switch soft-switching solution effectively eliminates switching losses for wide bandgap (WBG silicon carbide + gallium nitride) transistors, and reduces IGBT switching losses by approximately 65-80%, resulting in increased efficiency. Specifically, efficiency is improved in the low operating points for inverters where switching losses dominate total losses. With Pre-Switch, the result is an inverter with higher peak efficiency, higher average efficiency and higher low load efficiency.

The elimination of switching losses also reduces cost by allowing designers to pass more current through a transistor with the same transistor loss budget. This is possible because the transistor is no longer dissipating switching losses as wella as conduction losses. There is also a second order improvement in transistor conduction losses because the transistors are operating at lower temperatures. Both factors combine to reduce cost and size of an inverter.

The elimination of switching losses means that inverter switching speeds – which today are limited to slow (10-15 kHz) frequencies in order to maintain efficiency – can be run 5-10X faster (50-100kHz) with the same or higher efficiency. Pre-Switch’s higher switching frequencies have profound inverter and motor benefits. They include: a high quality sine wave sent to the motor which improves motor efficiency; a reduction in the DC link capacitor size and cost by up to 90%; the elimination of audible noise that can be heard by human ears; and reduced common mode noise in the motor between the rotor and stator which is known to ruin motor bearings.

A Pre-Switch inverter with a 10X faster switching frequency produces a low distortion sine wave output – a ‘clean wave’. This improves motor efficiency and adds range – especially where EVs are driven at low rpm and low torque conditions. Hence, we have adopted the name ‘CleanWave™’ for our inverter reference design.

The Pre-Switch technology enables designers to build a lighter and lower cost inverter, as discrete transistors can be used instead of heavy and expensive power modules. Until Pre-Switch, discrete transistors have been of only limited use in high current applications due to the challenge of current sharing, EMI mitigation and assembly costs. With Pre-Switch, transistors have no current or voltage across them when they turn on and turn off. The result is that simple transistor paralleling can be used and a dramatic reduction in EMI achieved. Both these factors help reduce inverter cost, size and weight.

Lastly, Pre-Switch technology virtually eliminates EMI because each transistor is turned on and off with no voltage or current present.

Drivetrain Benefits

Pre-Switch’s efficient, economic and reliable elimination of switching losses opens up a new level of drivetrain performance and cost optimization. Pre-Switch inverters are higher efficiency at peak and low load outputs, and can be made significantly smaller and at lower cost. This enables the inverter to be embedded into the motor housing. Pre-Switch’s 10X higher switching frequencies have been shown to increase the motor’s efficiency allowing higher RPM motors to be used. Inverter dV/dt is configured using a free lossless dV/dt filter which is part of the architecture.

Motor Benefits

A Pre-Switch-enabled inverter reduces sine wave output distortion by 10X, enabling motors to run more efficiently. In a conventional hard switching design, the output ripple current of the half-bridge circuit switching back and forth at 10-15 switching events per fundamental frequency causes a significant level of distortion. The distortion is effectively an induction heater in the motor coils and does no useful work. Pre-Switch technology minimizes this ripple by switching 10x faster. The lower distortion fundamental sine wave to the motor is what we call a ‘clean wave’ and improves motor efficiency predominantly at lower RPM and lower torques which is where EV’s are driven and increases EV range.

The second benefit of the Pre-Switch soft switching architecture is that inverter dV/dt is configurable with a free lossless dV/dt filter that is part of the architecture. Reducing dV/dt improves motor reliability and reduces motor winding insulation allowing higher power density motors. Due to the fast edge speeds of WBG (SiC; GaN) transistors, high dV/dt is traded off for reduced switching losses. But high dV/dt speeds of above 15-20V/ns can cause insulation damage. Inverter designers in the past accommodate these excessive dV/dt speeds by adding extra insulation in the motor. This approach has the adverse affect of reducing motor power density and increasing motor costs. In contrast, the Pre-Switch architecture slows edge speeds but allows increased switching frequencies, eliminating the problem of high dV/dt speeds and reducing the insulation required.

The faster switching speeds enabled with Pre-Switch can be used to spin motors faster. In some applications a lower cost, lighter and higher RPM motor can be used.

The final benefit for motor design is that because Pre-Switch-enabled systems switch so fast, low inductance motors can be used which have the benefit of being smaller and lighter and lower cost. This is particularly suitable for applications such as electric aircraft, where designers are trying to reduce the amount of iron in the motors to keep weight to a minimum.

Pre Switch – APEC 2022

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