10. September 2020, 15:00 Uhr | Ralf Higgelke
Another topic TDK has addressed is the frequency dependence of the ESR. Why is the equivalent series resistant so important?
Fernando Rodríguez: The ESR is the predominant cause of losses in a capacitor. And if you take a standard power capacitor, the ESR is rising sharply over frequency. To illustrate this, we measured the losses of a capacitor with a 20 A ripple current at a frequency of 50 kHz and at a frequency of 200 kHz. The losses were 1.6 W with a peak temperature of +42.7 °C and 3.6 W with +74.2 °C, respectively (see Fig. 4). And in the higher-frequency case the temperature distribution was much more inhomogeneous.
Fig. 4: Thermography of a capacitance element which was subjected to a ripple current of 20 A at different frequencies. This illustrates the influence of the non-linearity of the ESR.
There are at least for root causes for this ESR rise over frequency. First, the impedance is inhomogeneous and there are internal resonances with the other elements of a real-world capacitor like the equivalent series inductance ESL. Second is the negative electromagnetic interaction we have seen in the measurement I mentioned before. Third is the winding geometry and metal profile of the leads which contribute the frequency dependency. Forth and last is the well-known skin effect.
The most potential for improvement have the first two ones: the inhomogeneous impedance and the internal resonances as well as the negative electromagnetic interaction.
Fig. 5: In a capacitor with two windings the current will be shared inhomogenously between the windings due to the different ESL.
Can you please get into more detail?
Fernando Rodríguez: A standard film capacitor consists of two parallel stacked windings. If you look at the current sharing over frequency, you see that it is very inhomogeneous. At high frequencies, virtually no current flows into the winding which is much more away from the terminals (see Fig. 5). In consequence, also the internal temperatures of the windings are different. The root cause for this is the difference in ESL of the winding.
In order to flatten the frequency curve of the ESR of the capacitor we had to apply a new set of design rules to the inner construction of the capacitor.
Which are these design rules?
Fernando Rodríguez: First of all, the impedances of all capacitive elements within the capacitor have to be the same above, below and close to capacitor resonance frequency. This is the only way to make sure that the current will be distributed homogenously inside the capacitor and its capacitive elements. Second, it is mandatory to use of a finite-element analysis. Only by using this technique you can be sure you have taken into account all the parasitic elements in the structure of the component. So you are able to avoid negative impacts of these parasitic elements like resonances and inhomogeneous distributions of the current. And third, you have to implement an overlapped busbar structure from terminals to the winding connection point. By this you can minimize the parasitic inductance. To give you one example, we were able to halve the stray inductance of an automotive capacitor with busbar structure from 12 nH to 6 nH.
Fig. 6: With the new high-frequency capacitor the AC losses for a 100 A RMS current are reduced by 40% at 45 kHz and by 80% at 100 kHz.
TDK is going to release a film capacitor with this new blended high-temperature film dielectric and based on these design rules. Can you give me a number how far you might be able to reduce losses?
Fernando Rodríguez: The AC losses for a 100 A RMS current are reduced by 40% at 45 kHz and by 80% at 100 kHz (see Fig. 6). Also we were able to make the frequency characteristic of the ESR linear. Standard capacitors have an exponential ESR characteristic.
When we will see first products?
Fernando Rodríguez: We are already delivering samples to key customers like power semiconductor suppliers. We want to get field gather field experience with this new dielectric. But we are expecting to be ready to release in 2021.
Doctor Cabo and Mister Rodríguez, many thanks for your time.
The interview was conducted by Ralf Higgelke.