What is ripple and where it occurs
Ripple is a disturbance signal that is superimposed on a supply line. Typically, it originates from switching of power electronics i.e. MOSFETs or IGBTs. On an electric vehicle the biggest source for ripple is the electric propulsion system, namely the inverter part.
Why an inverter
The 3 phase AC induction motor is the preferred choice of the mainstream automotive industry as the primary drive for the latest generation of EVs. This type of electric motor provides the constant torque designers seek, is relatively simple in its construction so requires little maintenance, and is robust and highly reliable. Using such a motor, however, presents some very specific challenges. The obvious one is the electrical power stored in the battery to drive the motor is DC whilst the motor requires an AC 3 phase signal.
To overcome this an inverter is used to create an artificial 3-phase AC source to drive the motor. An important characteristic of the inverter is that it has the capability to create the AC source at varying voltages and frequencies. It is this feature that controls the speed of the motor, and ultimately the speed of the vehicle. By maintaining a linear relationship between voltage and frequency the AC motor can provide constant torque whilst speed increases and decreases.
The problem
The switching of the inverter’s power electronics generates large amplitudes of ripple current (up to several hundred App). Through the impedance of the high voltage bus cabling, the ripple current generates large voltage amplitudes (up to 100 Vpp). These are present at the supply input of other high voltage components that are connected to the same HV bus, i.e. DC-DC converters, chargers or HV batteries.
Ripple might lead to negative effects on the HV component:
- Malfunction of the equipment i.e. unexpected opening of the relay on an HV battery because the BMS (battery management system) is disturbed by the ripple voltage.
- Destruction of the component, i.e. when the input filter of a DC-DC converter is stressed at its resonance frequency
It is essential therefore that the high voltage components in the EV must be immune to the effects of ripple generated by the inverter. The different levels of ripple immunity and how to conduct the test are detailed in various standards and are discussed further in a separate post.
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