GaN + Digital Control + High-Performance Magnetics
GaN Talk – Renee Yawger
Apr 07, 2021
Designing an Ultra-thin, Highly Efficient (>97%), Multilevel DC-to-DC Converter
GaN-based solutions coupled with digital control and high-performance magnetics can increase efficiency, shrink the size, and reduce system costs for high density computing applications like ultra-thin laptops and high-end gaming systems.
As computers, displays, smart phones and other consumer electronics systems have become thinner and more powerful over the past decade, there is increasing demand for addressing the challenge of thinner solutions while extracting more power out of limited space.
The multilevel converter is an exceptional candidate to shrink the size of the magnetic components and achieve high efficiency in a compact solution. Leveraging the advantages of eGaN® FETs, such as small size and low loss, further enhances the performance of a multilevel solution. This blog will evaluate the EPC9148, a 48 V to 20 V, 250 W three-level converter using eGaN FETs and digital control which achieves a peak total system efficiency of 97.8% and only 4.1 mm component height.
Why Digital Control?
Owing to its flexibility, the ultra-thin converter design employs digital control, which is especially effective in the development of complex control schemes. Regulating control based on compensator controllers for current and voltage loops, together with various circuit protection functions, can be easily managed digitally. And the high time resolution available from these controllers allows for optimal dead-time management for GaN, whose dead time can be within 10 ns and is much lower than that of the Si MOSFET and its controller/driver.
Digital control also enables the use of GaN-compatible gate drivers that have high drive strength for fast switching and high-side gate voltage clamping for gate overvoltage protection.
The EPC9148 features a Microchip Technology dsPIC33CK32MP102 Digital Signal Controller (DSC). This 100 MHz single core device is equipped with dedicated peripheral modules for Switched-Mode Power Supply (SMPS) applications, such as a feature-rich 4-channel (8x output), 250 ps resolution pulse-width modulation (PWM) logic, three 3.5 Msps Analog-To-Digital Converters (ADC), three 15 ns propagation delay analog comparators with integrated Digital-To-Analog Converters (DAC) supporting ramp signal generation, three operational amplifiers as well as Digital Signal Processing (DSP) core with tightly coupled data paths for high-performance real-time control applications. The dsPIC33CK device is used to drive and control the converter in a fully digital fashion where the feedback loops are implemented and executed in software.
Design of an eGaN-FET-based three-level converter
The simplified schematic of the EPC9148 eGaN-FET-based three-level buck converter with synchronous bootstrap circuit is shown to the right. The circuit has three operating modes at duty cycles lower than 50%: 1) input voltage charges up the flying capacitor and load inductor through Q1 and Q3; 2) flying capacitor discharges while the load inductor charges through Q2 and Q4; 3) inductor current discharges through Q3 and Q4 (either via the equivalent body diode of one and the channel of the other during deadtime, or via the channel of both FETs). The steady-state operation follows the cycle of 1→3→2→3. The effective frequency seen at the output inductor is thus twice the switching frequency of the FETs, allowing the use of a lower inductance value than required in a conventional synchronous buck converter. The switching frequency of the converter is optimized at 400 kHz so that the effective frequency seen at the inductor is 800 kHz, high enough to allow the use of the Wurth 7443762504022, a 4.1 mm tall, 2.2 μH inductor, while maintaining low switching loss and thus high overall efficiency and good thermal performance. The cascaded synchronous bootstrap circuit that ensures adequate gate voltage (>4.5 V) for the upper FETs has been adopted. Three control loops are implemented using a digital controller to regulate the output voltage, output current, and flying capacitor voltage respectively. Flying capacitor voltage should be kept at half of the input voltage at any time to avoid overstressing any of the FETs and ensure correct circuit operation.
High performance eGaN FETs for the three-level buck converter
In the EPC9148 design, Q1 blocks the 48 V input voltage before the flying capacitor voltage is established. As Q2-Q4 only need to block half of the input voltage, they only need to be rated for 24 V. Therefore the 100 V rated EPC2053 with RDS(on) of 3.8 mΩ and the 40 V rated EPC2055 with R DS(on) of 3.5 mΩ shown in figure 2 are selected for Q1 and Q2-Q4 respectively. Both eGaN FETs are of tiny size and can operate at up to 150°C junction temperature.
The EPC9148 three-level buck converter was built to verify the mulitlevel design. The overall thickness of the circuit including the circuit board is only 5 mm. The circuit was tested with no forced air up to 12.5 A output current with a maximum temperature rise of 40 °C in a 25 °C ambient. The capacitor voltage was well-balanced during charge and discharge phases. The overall power efficiency of the three-level converter operating at 20 V output and with 800 LFM forced air reached a peak efficiency of 97.8%. It maintains efficiency above 97% above 4 A load current. The overall power efficiency at 12 V output and with 800 LFM forced air reached a peak efficiency of 97%. This is all achieved within the 5 mm height limit.
The GaN-FET-based multilevel buck topology with digital control and high-performance magnetics can be used for designing an ultra-thin and highly efficient DC-to-DC converter. A 48 V to 20 V, 250 W three-level buck converter built using eGaN FETs achieved a peak efficiency of 97.8% and an overall thickness of only 5 mm. The multilevel topology allows the use of a thin inductor with low inductance value. The eGaN FETs not only reduce the area occupied with their tiny footprints, but also improve the overall power efficiency with their fast-switching capability.
How to Design a Synchronous Buck Converter Using eGaN FETs