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.

7 comments on “Plug-In Chargers: Implications for Capacitors

  1. Parser
    December 3, 2010

    Electric vehicles use lithium-ion rechargeable batteries. Very sophisticated charging systems count the charge going into and coming out of the batteries. Supervision circuits measure currents to several digits of accuracy and time to milliseconds (1/1000 seconds) to know what is the state of charge. Voltage monitoring and temperature gives information on aging capacity of the battery. To keep the charger high efficiency a switching power supply is used with its own active power factor correction circuit. To match a traditional 60 hp engine it requires 44.7 kW electric motor. This is a small power plant inside each electric vehicle, which typically has over 100 kW electric motors. 

  2. t.alex
    December 4, 2010

    In terms of super caps, I am interested to know more about the market itself. How big is the market and who are the players? Making super caps for HEV is definitely not easy when there are lots of safety regulations to go through. 

  3. Eldredge
    December 4, 2010

    Ron, do you anticipate supply issues for capacitors, either in general or by type? Seems like there could be a significant increase indemand.

  4. Mydesign
    December 6, 2010

       “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?”
       Can you suggest some of the issue with Emi and power quality?  Any way the Emi varies with the size and the time taken for charging. This can happens, while discharging also. The main two equation connected with capacitor are T=RC and f = 1/2πRC

  5. tioluwa
    December 6, 2010

    This means a whole new sets of inivations for capacitors just to meet the demands of the HEV market.


    It's not just an issue of super capacitors, which are a type of capacitors, but a greater demand on efficiency and tolereance and all other know parameters of the capacitors in general, which have to be improved upon to meet the demands of HEV.

    It's a whole new market opening up and i'm sure both the supply chain and OEMs are up to the task.

  6. t.alex
    December 7, 2010

    I guess the cost for producing these capacitors are still high, and so this might be one of the hurdle for mass producing HEV?

  7. prabhakar_deosthali
    December 15, 2010

    If these chargers are put on-board then aprt from the cost the size and weight will also play an important role for the capacitor selection.

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