Selecting a ripple generator

The 10 critical selection criteria for choosing a ripple generator

Choosing the correct ripple generator might be a difficult task. Several factors come into play, the most important being these:

1. Maximum DC voltage is defined by the DC bus voltage of the HV components. It defines the DC source as well as the maximum insulation voltage of the coupling unit (transformer). Typical voltages are in the range of 400 to 1000V.

2. Maximum DC current depends heavily on the power consumption and HV DC voltage of the component. Batteries and inverter are high power component where currents in the range of 500 – 1000 A are common. Auxiliary devices required less current, typically below 100A. The DC current defines the required power of DC source. It also gives an indication about the possible impedance of the component

3. Ripple voltage test levels in V_pp are defined by the standard and/or the component test specifications. Typical values are between 10Vpp to 80Vpp. A big variety in required test levels exist and the ripple generator must cover them. The voltage levels must be reached at the terminals of the EUT.

4. Impedance of the component is a very important parameter to determine the required ripple current and power. The impedance can be estimated (DC resistance), simulated or measured. It is critical to know the impedance not only at DC but up to the maximum ripple frequency (i.e. 150 kHz for ISO 21498). Typical values are around 50 mΩ for HV batteries, 100 mΩ for inverters and 1 Ω for auxiliary systems (DC-DC converters, chargers, etc.).

5. Ripple current in A_pp is given by the ripple voltage test levels and the impedance. Important is to calculate the ripple current over the complete frequency range. Typical values are in the range of  300 - 500 A_pp for an motor inverter and less for auxiliary equipment. The ripple generator must be able to generate these ripple amplitudes. To prevent that the tested component is destroyed, a fast current limiter is required.

6. Accessibility of the EUT some components might be required to be placed in a protected environment or climate chamber where the accessibility is difficult. Longer cables and remote voltage measurement are required that ensure the ripple test is performed according the specifications of the standard.

7. Integration into test stand ripple tests are time lengthy tests that can take several days or weeks. HV batteries need to be tested at different SoC (state of charge), which makes it necessary to charge and discharge them during ripple testing. An automated test is helpful and the ripple generator must provide the necessary remote control interfaces.

8. Safety for personnel ripple is a high voltage test and a potential danger to test engineers. Therefore, the ripple test system must be safe and have the possibility to be integrated into existing safety systems. A galvanic insulation between DC and AC, as offered by the transformer coupling method, increases safety.

9. Upgradeability EV is a very dynamic market where technologies and also testing evolve quickly. DC voltages go to higher levels to reduce losses and (cable-) weight. Faster charging is the key to customer acceptance which means increased DC current (and lower impedance). The ripple generator must cover the testing requirements of today and tomorrow – so upgradability is the key. This is true for the hardware and – even more importantly – for the software to include changes in standards.

10. Ease of use Building and operating a ripple test system for HV components used to be a task for test specialists. Today compact and fully integrated ripple test systems are available that offer features and functionalities that allows everyone to perform the test.