DC-DC Reference Design Board Targets e-Mobility

A bidirectional converter developed by gallium nitride specialist EPC in collaboration with MPS operates at an advertised peak efficiency of 97 percent.

The EPC9165 buck converter is a 2-kW, two-phase 48V–14V bidirectional device aimed at 48-V battery packs providing high density and power–typically used in e-mobility applications. For 4kW operation, EPC said two converters can be run in parallel; for 6 kW, three converters can be combined; for 1kW, only one phase can be employed. Output voltage is 14 volts.

The architecture also can be changed to support inputs ranging from 12 to 36 volts since the converter's hard-switching topology provides regulated output. A compatible controller module (EPC9528) includes a Microchip dsPIC33CK256MP503 16-bit digital controller. Most analog controls are incompatible with GaN FETs, requiring extra circuitry to match gate driver behavior. Digital solutions offer an effective means of implementing advanced power and temperature protection features.

In addition, digital control, such as the dSPIC33CK, enables dynamic adjustment of downtime along with single- to multi-phase designs.

Cecilia Contenti, EPC's vice president of strategic marketing, said 48V – 12V bidirectional converters are widely used in e-mobility, micro EV, and light mobility applications running at between 2-2.5kW as well as small hybrids with power up to 4kW. "One example is the city car, which has 2kW of power. Compared to a Si MOSFET solution, limited to 100kHz, a GaN solution can operate at 500kHz while allowing more power per phase", Contenti said. That's due to smaller inductor value and higher current in the same form factor.

EPC claims its converter is 58 percent smaller and lighter than competing devices with peak efficiency of greater than 97 percent compared to 95 percent for Si MOSFETs using a similar inductor. Given GaN's thermal properties and efficiency, Contenti said was cooling is not required.

Board features

The EPC9165 reference design consists of the EPC2302 GaN FET in a QFN package, using EPC's latest 100-V GaN technology. The device delivers approximately 101A of continuous current and 408A, pulsed. The package offers 0.2 degC/W thermal resistance at the heatsink, and a typical R_DS(on) of 1.4mOhm and a typical Q_G of 23nC.

Thermal management is critical for ensuring consistent operation. Even with the heatsink fitted, sufficient cooling is necessary for operating across a specified output current range. According to EPC, a mounted heatsink dissipates heat. As indicated, the Wakefield 567-94AB heatsink fits the 1-mm-tall SMD threaded (M2) board spacers, leaving a 0.3-mm gap between FETs and heatsink. Strong heat conductivity across the 0.3-mm gap is provided by T-Global A1780 TIM with a thickness of 0.5mm. The pre-installed heatsink and TIM materials (Figure 1) offers appropriate cooling for testing (Figure 2).

Figure 1.

The EPC9165 reference design also includes a new MPS MPQ1918 100V half-bridge driver developed specifically for use with GaN FETs. The MPQ1918 is available in a 3x3mm FCQFN package with a peak current of 1.6A, and a pull-down/pull-up resistor of 0.2Ω / 1.2Ω to enable high-power FETs.

Figure 2: PCB 3D model showing TIM and heat sink installation

Figure 3: Functional block diagram overview of the EPC9165 board.

Many companies offer GaN-optimized drivers to boost performance. Optimization includes 5V gate-drive voltage and a high-side bootstrap voltage "clamp" to avoid overcharging the bootstrap capacitor beyond 6V. Also included is a phase-node PIN capable of negative voltage of about -3V, under-voltage lockout at 3.6V (for disable) and 4.0V (for enable), high dv/dt capability for switch node, typically greater than 100V/ns, and up to 200V/ns. Dead-time management is ideally less than 10ns, said Contenti.

Moreover, according to EPC, the MPS 100V gate driver offers increased power density while simplifying e-mobility designs.

The EPC9165 includes logic power supplies for 5V and 3.3V. It also supplies power to the controller card through an edge connector. The output inductor current and input current are all measured using a 0.2mΩ sensing resistor and a 50-V/V amplifier, resulting in a current sense gain is 0.01V/A. Input and output voltages are measured using a resistor divider network (100k and 5.36k), with gain of 0.05087.

"A shunt is placed between the main filter inductor and the low voltage terminal," Contenti said. "Due to the high current, a very low resistance of just 200µΩ is used, which keeps loss as low as possible without compromising on signal-to-noise ratio."

"The shunt voltage is amplified using a high common-mode voltage (65V) shunt amplifier, which keeps the component count low, occupies the smallest area and has sufficient bandwidth for the converter," she added.

Due to high switching frequencies, "the inductance of the shunt becomes an important factor in the design, most notably the current return path. Extra attention was given to minimize this inductance in the layout and it was further compensated for by using passive integration techniques to cancel that inductance in a similar manner to DCR current sensing techniques. The completed combination yields a high-performance current sense system that when combined with the digital controller can easily exceed 8kHz control bandwidth," said Contenti.