Plug-In Chargers: Implications for Capacitors

Electromagnetic interference (EMI) filters are critical for proper operation of hybrid electric vehicles (HEVs), and their use will no doubt spread throughout critical controls in traditional vehicles. Regardless of the exact application, HEVs will drive all capacitors to a higher level of performance in terms of reliability, current and voltage handling -- whether RMS (Root Means Square) or transient -- and temperature rating.

There is no doubt we can expect a new concern over capacitor inductance and frequency response to emerge.

It is appropriate to briefly discuss the charge systems that a Plug-In HEV (PHEV) is connected to for recharge. At first glance, PHEV chargers might be thought of as a gigantic 12 volt battery charger. This over-simplification is far from true. PHEV chargers are much more complex.

First, safety is paramount. The charger has to be intelligent and foolproof with safety interlocks. For instance, it must be able to provide high-quality power for the PHEV, sense when charge is being completed, and turn off when instructed to during normal operation as well as emergency safety disconnect scenarios. Also, if the PHEV is to be used to back-feed the grid, there is a need to be able to isolate and turn off the back-feed in the event of grid shutdown and repair.

The amount of data these chargers will process is equally impressive. Time-of-use metering will be very beneficial for end users charging their PHEVs. Likewise the charger will need to be intelligent enough to sense load types and state-of-charge scenarios to avert overcharge and potential litigation.

Then there is the issue of EMI and power quality. Filters and conservative design will be the rule. What all this points to is conservative design rules, the use of high-quality components, and failsafe/redundant architecture. What does it mean for capacitors?

The requirements for stable capacitors across time and temperature are easy to predict, as is the need for fail-safe devices. EMI filters will be needed to isolate the charger from the grid. High-quality power factor correction capacitors are required -- currently these are mostly aluminum electrolytic, due to the capacitance values needed, but might quickly trend to films as thinner dielectrics help films catch up to aluminum electrolytic values.

Also the trend to higher voltage systems and failsafe operation should bolster the film capacitor possibility.

There will be a need for high-frequency and high-voltage capacitors in the charge circuit, most likely addressed by ceramics and films as well. The output filters will probably go with aluminum electrolytic with some safety circuit failure event isolation circuits.

Of course the data extraction and logic circuits will use traditional ceramic, tantalum, or niobium oxide capacitors, low-voltage electrolytics, and super capacitors.

Bottom line: It's not a gigantic 12v battery charger.