Power Electronics: Correct Switching Frequency Decisive

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In terms of efficiency, comfort, and acoustics the power electronics of the electric drive are decisive. Its software controls so variably that it meets all requirements.

Andreas Neemann,

Andreas Neemann has already challenged some schedules for interviews with his curious questions. The automotive author with a passion for complex topics has been writing for ZF media internally and externally for over two decades.

Firmly press the "gas" pedal and electric vehicles unleash impressive driving dynamics from the off. Unlike combustion engines, electric drives put down their high torque onto the road instantly. And with understated acoustics compared to the engine roar accompanying this power delivery from their gasoline-powered counterparts.

Power electronics: The control center in the electric drive

However, these dynamics also sometimes give engineers a headache – especially when it comes to combining this dynamic handling with efficient energy use. A key concern for ZF developers. Here, the power electronics come into play as the central switching point for the electric drive: Functionally arranged in the driveline between the battery and the electric motor, this aluminum housing measures around 30 by 30 centimeters. One task of the power electronics is to convert the direct current from the battery into alternating current for use in the electric motor. Hence, this part of the power electronics is called the inverter. Put simply, here you have several transistors that switch on and off at extremely short, regular intervals. When switched on, they transfer the battery current to the motor. Together, these high-frequency switching operations produce a sinusoidal voltage curve which provides the ideal traction energy for the electric motor.

The power electronics transfer the traction energy from the battery to the electric motor, converting that energy from direct to alternating current. ZF software varies the frequency without compromising efficiency, comfort, NVH behavior or the battery's service life.

Transistor Software

"In the early days of electromobility, the transistors in the power electronics switched at a fixed clock rate of between 8 and 10 kHz," explains Olaf Moseler, head of software function development for electric drives at ZF. Today, a fixed clock rate cannot meet the wide-ranging requirements for power electronics. This ultimately involves more than just supplying the electric motor with the alternating current it needs. It also entails a whole list of other tasks: The power electronics conversely rectify the electrical energy produced in generator mode during braking recuperation, feeding the energy back into the battery. Drivers who relish this sort of driving dynamics and play with the power pedal pose additional challenges through a combination of initial highpower demand followed by relaxed cruising. Plus of course the high expectations on comfort and good NVH behavior (NVH = Noise, Vibration, Harshness). And particularly the consistent demand for maximum efficiency, or a long range per kilowatt-hour to put it another way. "We now have a conflict of objectives – and there's no longer one 'right' switching frequency," says Moseler. If the transistors switch at high clock frequencies of around 14 kHz, that's ideal for dynamics. The current has a low ripple, i.e., undesirable rapid oscillations overlaid on a desired direct current, resulting in low losses in the electric motor. However, significant electrical losses occur with each switching operation on a transistor. High clock frequencies therefore lead to high energy losses in the power electronics. By contrast, low switching frequencies of around 2 kHz prove highly efficient but result in higher ripple currents and prove challenging in terms of acoustics.

"Simply by using software that varies the switching frequency, we can increase the range of electric drives by up to 1.5 percent."

Olaf Moseler, Head of Software Function Development, Electric Drive

Variable switching frequency resolves this conflict of objectives

The solution provides a control software that varies the switching frequency of the transistors in the inverter. A microcontroller in the low-voltage portion of every ZF power electronics module houses this software. "Thanks to the variable switching frequency, we can get the best out of the system for each operating point," says Moseler. System developers at ZF have the necessary experience, expertise and, especially, measurement data to know which operating points are ideal for adjusting the shift frequency. The software engineers translate this know-how into the corresponding switching commands for the transistors. In this way, ZF combats the specific current losses caused by the switching operations themselves: "We can reduce the system's own losses by up to 900 watts," says Moseler. "The upshot is an increased range of between 0.5 and 1.5 percent."

Variable switching frequency does what fixed switching frequency cannot: It provides the optimum solution for the specific requirements.