Maximizing power handling in HEVs and EVs with power magnetics
The LLC topology converter includes a resonant cell (resonant capacitor + resonant choke) and a power transformer. Figure 1 shows a Premo kit box that includes all the magnetics components needed to operate the two stages of a 22 kW on-board charger:
2 x (½ resonant choke + 11kW transformer + ½ resonant choke) = 22kW
This kit box includes optimized molded integrated magnetics that can be implemented directly in the converter of our customers developing 22kW OBCs. The AECQ200-qualified unit optimized for an on-board charger applications integrates split resonant chokes and deliver optimum power density, efficiency and performance. With a footprint of just 213x73mm and only 67mm high, this magnetic component kit has a power density of 19.5kW/dm3. Its thermal transfer is maximized by a heat conductive aluminium casting.
Typically, the electrical system of EV/Plug-in HEV involves three types of converters: an On-Board Charger taking the mains to the HV battery, a DC/DC converter connecting the HV battery to a low voltage 12V battery / 12V network and a traction converter providing power from the HV battery to the engine.
The most common electronic topologies for these converters include an LLC topology for the On-Board Charger, an ZVS topology for the DC/DC converter and a power inverter for the traction converter.
DC/DC converter: ZVS topology
DC/DC converters in automotive applications typically deliver 12VDC to supply the 12V network. The ZVS topology (Zero Voltage Switching) is required to reduce, losses, heating and reliability issues in these converters.A ZVS topology is more suitable for a DC-DC application (400V to 12V) because the output voltage must be adjusted only slightly in the range 10.5V to 15.5V. This enables the global effective turn ratio not to fluctuate significantly.
In these topologies, an input “SHIM inductor” is often needed to achieve the phase-shift function. Premo suggests smart magnetic solutions without any additional SHIM inductor component. The leakage inductance of the transformer can be used to implement the necessary input serial inductance. However, that must be managed carefully because the leakage inductance is a “parasitic” parameter naturally intrinsic in the transformer. This value depends mainly on the winding structure.
Physically, we can say that this is the result of a poor coupling between primary and secondary windings. The method consists of considering different winding structures.
Thanks to a finite element analysis software – like Maxwell – electromagnetic simulations can be performed to investigate different winding structures for a specific leakage inductance. This will provide the required input inductance value without adding a choke.
OBC: LLC topology
An LLC topology is more suitable for OBC applications (400V to 400V) since the output voltage should be adapted according to the battery charging voltage range (270V to 470V). The resonant cell allows the level of voltage to be adjusted with the switching frequency. Efficiencies up to 98% can be achieved with LLC resonant converters when integrated magnetics are used. Resonant converters have been around for a long time. However, power electronics for automotive HEV/EV is boosting demand as there is an absolute need to improve efficiency. The typical structure of a resonant converter LLC topology is shown in figure 6.
The resonant tank consists of Lr, Cr and Lp. The magnetizing inductance of the transformer itself can be used to create the Lp function of the resonant cell. Thanks to the resonant tank filter, the voltage is naturally adapted by adjusting the switching frequency, so the total losses of the converter are limited compared to a pure PWM structure.
Premo offers combined magnetics solutions which integrate the resonant choke Lr, the parallel inductor Lp and the transformer. For example, an integrated magnetics solution can include two transformers and the resonant series inductor Lr split into two identical symmetrical parts at the beginning and the end of each primary winding. Figure 7 shows a real example with 2 x 11kW transformers with 2×7.5µH Lr chokes.
Integrated magnetics use the common magnetic circuit of a transformer, a series resonant choke and the flux cancellation to increase power density and save cores.
As the power demand increases and EMI issues become more critical, the advantages of resonant converters with integrated magnetics are their soft switching of transistors that provide a higher operating frequency and open the door to GaN-transistors based sinusoidal power processing.
About the author:
Abdelkader Birch is R&D engineer at Premo – www.grupopremo.com