New Industry Products

6.6kW Bidirectional CLLLC Resonant Dual Active Bridge Reference Design

July 18, 2019 by Paul Shepard

The TIDM-02002 CLLLC resonant dual active bridge (DAB) with bidirectional power flow capability and soft switching characteristics is a reference design from Texas Instruments designed for hybrid electric vehicle/electric vehicle on-board chargers and energy storage applications.

This design illustrates control of this power topology using a C2000™ MCU in closed voltage and closed current-loop mode. The hardware and software available with this design can help accelerate time to market.

System Description

On-board chargers (OBCs) are an essential part of EVs and HEVs. An OBC typically consists of an ac-dc PFC stage and an isolated dc-dc converter. C2000 MCUs are designed to implement advanced digital power control that automotive applications demand.

The ability to charge the battery fully overnight is highly desired for most EV Level 1 and Level 2 chargers. With battery capacity increasing, the OBCs need to be designed for even higher power. With the increasing power capacity of the OBC, specifications such as power density and efficiency are even more important, due to limited space and cooling capacity in the car.

CLLLC DAB topology

The CLLLC (Capacitor-Inductor-Inductor-Inductor-Capacitor)—with its symmetric tank, soft switching characteristics, and ability to switch at higher frequencies—is a good choice for these applications. In this design, control and implementation of a CLLLC topology, as shown in figure below, is illustrated.

TIDM-02002 Bidirectional CLLLC resonant dual active bridge (DAB) reference design for HEV/EV onboard charger simplified schematic (click on schematic to enlarge)

A full-bridge LLC converter, as shown in Figure 5, falls under the broad category of DAB converters. Under DAB converters, the converter can be classified on the basis of model or operation:

  1. A phase-shifted DAB converter is one of the most popular converters historically.
  2. Resonant DAB converters have different variants on resonant tanks (LC, LLC, CLLC, CLLLC, and so forth).

Resonant DAB converters are of interest because high efficiency, high power, and high density are achievable with such converters. CLLLC, with its symmetric tank, is capable of bidirectional operation. The problem with using an LLC structure for bidirectional use is that the switching frequency, when operating in the reverse power flow mode, is governed by the transformer winding capacitance and the leakage inductance. This offers little or no control on the gain of the power stage and the switching frequency.

Therefore, the CLLLC type of structure is preferred as it offers much better control on the switching frequency and an additional degree of freedom on the gain. One of the biggest challenges with resonant converters is the operation across a wide input/output voltage range. This is particularly challenging with battery-charging type applications where the battery voltage varies widely from 280V to 450V.

Techniques in literature have been presented to mitigate this with a variable dc link voltage from the PFC. This design relies on the ability to regulate the PFC voltage in a 380V to 600V range; a resonant tank is designed to cover that range of voltages.

Summary of features:

  • Vprim: 380- to 600-Vdc; Vsec: 280- to 450-Vdc
  • Power Max: 6.6kW, 98% peak efficiency
  • CLLLC resonant tank with 500-kHz nominal pwm switching (300kHz to 700kHz range) enables higher power density
  • Soft switching with Zero Voltage Switching (ZVS) on the primary; Zero Current Switching (ZCS) and ZVS on the secondary enable higher efficiency
  • Active synchronous rectification scheme implementation using Rogowski coil sensor enables higher efficiency
  • Software Frequency Response Analyzer (SFRA) and Compensation Designer for ease of tuning of control loops

Software support for TMS320F28004x device with the Control Law Accelerator (CLA), which enables integrated OBC design with ac-dc and dc-dc controlled using a single C2000 MCU