May 2012

Solutions for on-board chargers

All charged up

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EPCOS and TDK components perform key functions in the Finepower’s on-board chargers for electric and hybrid vehicles. They include inductors, transformers, capacitors and protective components.

Finepower - an engineering specialist for power electronics - has developed a highly compact prototype of an on-board charger (OBC). This device, based on specifications and technical conditions of the German automotive industry, delivers high efficiency at the same time as high power density. Both are important factors for incorporating OBCs into electric vehicles. The space-saving design of the charging system was made possible by the development of a range of application-specific chokes and transformers qualified to the AEC-Q200 specifications for reliability tests.

Well-protected power input
An EPCOS B72220F0271K101 type varistor protects the power input of the converter from overvoltages. A special coating significantly improved the mechanical stability of the varistor for use in electric vehicles. The EPCOS B57364S1509M ICL is used to limit the high inrush currents. To ensure EMC and provide for EMI filtering, Finepower’s developers opted for EPCOS standard chokes of the kind also used in industrial power supplies. The B32933C3155M types from the EPCOS heavy-duty series of X2 capacitors were selected, which are characterized by high reliability and a long service life. The DC link circuit is stabilized by the B43508B5337M and B43504B5337M types of EPCOS aluminum electrolytic capacitors. They provide a capacitance of 330 µF at a rated voltage of 450 V DC. TDK Mega Caps of the CKG57NX7R2J474MT series are connected in parallel to them to reduce the ESR. This is the only way of keeping the DC link circuit highly compact despite the high power density.

Special PFC concept boosts efficiency
In the development process, special attention was paid to achieving maximum charger efficiency. In order to maximize the power drawn from the line, the charger must include active power factor correction (PFC). This rectifies the AC voltage from the power line and simultaneously generates an internal DC link circuit of 400 V DC that supplies the charging circuit. PFC circuits based on the principle of the boost converter represent the state-of-the-art and are widespread.

Sources of losses sustained in converting the power voltage in the 400 V DC link circuit are:

  • EMI filters (copper losses)
  • Bridge rectifiers
  • PFC chokes
  • Power switches (MOSFET)
  • PFC diodes
  • Other losses are incurred by components such as the aluminum electrolytic capacitors in the DC link circuit, the shunt etc.

The conventional solution produces an efficiency of between 96 and 97 percent at an input voltage of 230 V AC, depending on design and optimization. At an input power of 3.65 kW, this means that between 110 and 146 W of this figure is converted to heat losses before it is available at the DC link circuit.

Figure 1: PFC stage in the interleaved process

The parallel connection of two PFC branches with corresponding EPCOS PFC chokes splits up the high ripple current load. This boosts efficiency and improves EMC.

Finepower uses the synchronous interleaved PFC process (Figure 1) in order to eliminate these drawbacks as much as possible. This topology operates with two PFC stages connected in parallel to the joint output, thus already reducing by 50 percent the adverse effect of only a single PFC choke carrying the whole load and thus also the ripple current. Although the interleaved process requires two chokes, each of them can be dimensioned for half the current (8 ARMS or 22 APK), leading to considerable simplification. A highly compact design from the newly developed e-mobility platform series of EPCOS PFC chokes is used for this purpose (Figure 2).

Figure 2: EPCOS PFC choke from the new e-mobility platform
Despite their compactness, these EPCOS chokes are rated for peak currents of up to 22 A.

The EPCOS PFC chokes are very well suited for this application, as their low volume, special core material and use of an RF stranded wire for the winding produce low losses. Figure 3 shows the principle of the superposition of both choke currents.

Figure 3: Superposed choke currents

The superposed choke currents produce an approximately sinusoidal current input from the power line.

Measurements on the 230 V power line show that at 40 to 85 percent of the rated load (1.3 to 2.8 kW), the efficiency of the PFC stage exceeds 98 percent and reaches as much as 97.5 percent at full load (3.3 kW). Compared with a conventional PFC stage, therefore, power losses of between 35 and 70 W are saved in this first stage of the OBC alone.

DC/DC converters with high efficiency
Fed with a regulated and stabilized intermediate circuit voltage of 400 V DC, the DC/DC converter must supply the charging voltage for the battery with reliable electrical separation from the power line. Depending on the design and charging state of the battery, the range of possible output voltages of the OBC is set between 200 and 420 V DC. Both the charging current and the final charging voltage have to be programmable and controllable via the communications interface of the device (CAN bus).

In order to minimize the total losses of the OBC, the DC/DC converter must also maximize its efficiency. Due to the need for reliable electrical separation in this switching stage, however, its efficiency cannot be quite as high as in the PFC stage. At the current state of the art, the highest efficiencies are attained in insulated DC/DC converters with resonant bridge topologies. Either phase shifting converters or LLC circuits may be used for this purpose. In both of these, the transistors are always connected in when the drain-source voltage is zero and the losses in the semiconductors are consequently very low.

LLC converters (Figure 4) benefit from the fact that this fully resonant topology always permits sufficient energy to be stored in the load circuit to assure autonomous switching of the bridge nodes.

Figure 4: Full-bridge LLC converter

Finepower uses inductors from the EPCOS e-mobility platform in the full-bridge
converter. This applies both for the resonance inductor (LR) and the transformer (TF).

Inductors from the EPCOS e-mobility platform series are also used in the full-bridge LLC converter. The maximum output voltage is 420 V DC, i.e. in the same range as the input voltage of 400 V DC. The transfer function of an LLC converter permits the voltage to be increased and reduced. A useful option is to operate with a transfer ratio of 1:1 in the transformer, thus minimizing its losses. The transformer on the primary side is operated by a full bridge at ±400 V, which is converted back to 400 V DC after bridge rectification on the secondary side.

Apart from the load current, a significant part of the resonant current must also be transformed in the LLC process, which leads to additional power losses not only in semiconductor components but also in copper components (circuit board, resonance inductor, transformer). If a full-bridge circuit doubles the voltage on the primary side of the transformer compared to a half bridge, the current is halved. The increased transfer ratio leads to a higher copper resistance (linear) and the semiconductor sections exhibit double the RDS(on). Because the line losses are a quadratic function of the current (PD=I2R), unnecessary power losses are avoided.

The full-bridge LLC concept in rated operation (1.2 to 3.3 kW) permits an efficiency exceeding 97 percent to be invariably reached despite reliable separation from the power line. At the same time, the quasi-resonant full-bridge principle improves the EMC properties.

Table: EPCOS and TDK components in the on-board charger from Finepower

TypeOrdering code
Aluminum electrolytic capacitorsEPCOSB43504B5337M
Gate Drive TransformerEPCOSB82801C2245A200
PFC chokeEPCOSB78547P2176A005
Resonance inductorEPCOSB78547P2175A005
Power transformerEPCOSB78547P2177A005

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