With the rapid development of electric vehicles, the charging pile and car charger solutions have become a hot spot in the market. In this type of application, the input voltage is mostly a three-phase AC input. After three-phase PFC, the DC bus voltage will be as high as 7,800V. Such a high DC bus voltage brings the design of the DC/DC converter of the latter stage. Great challenge.
The first is the choice of device, 800V bus voltage, the DC / DC Mosfet rated voltage needs at least 1000V, and the MOS tube selection at this voltage level is very limited. Therefore, most of the current schemes use a three-level circuit that uses two 600V Mosfets in series to solve the MOS tube stress problem caused by the high bus voltage. Secondly, the switching losses under high voltage are so large that we have to choose the circuit topology of the soft switch. The LLC converter can achieve ZVS over the full load range, enabling high switching frequencies and high switching frequencies. The figure below shows the circuit of a typical three-level full-bridge resonant converter.
Three-level full-bridge LLC converter
Three-level converters have their own advantages. For example, each Mosfet only needs to withstand half of the input voltage. Of course, there are also disadvantages. For example, each bridge arm requires 4 MOSFETs and their respective drivers, which increases the system complexity. For example, each bridge arm requires its own clamping diode, which increases system cost.
In this article, we will introduce the design of our 8KW LLC converter. Using Cree's 1200V silicon carbide Mos tube instead of the two series-connected MOS in the above figure, the three-level converter is simplified into a conventional two-level full-bridge converter, as shown below. At the same time, we set the switching frequency to 160KHz, which reduces the volume of the magnetic device and the entire converter.
8KW Silicon Carbide Full Bridge LLC Solution
Here is a picture of our prototype.
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