Designing and building an isolated step-down DC/DC converter with a wide input voltage range and high output current using silicon MOSFETs at the standard 5 V output can be achieved, but with limited performance, especially at low loads and high input voltages. What's more, as silicon matures, it can be challenging to squeeze more juice out of a wide-input step-down DC/DC converter at light loads at high input voltages. Unlike silicon MOSFETs, enhancement-mode GaN-based field-effect transistors (Egans) promise better performance over a set of input and output parameters of the same wide load. In fact, because these wide-bandgap devices operate at higher speeds, have higher breakdown voltages, and lower resistances, they can provide higher efficiency over a wider range of load variations while minimizing space and cost.
Because Egan FETs are built on silicon substrates, the cost differential is narrowing and business is accelerating. Efficient Power Conversion (EPC), for example, has been offering GaN on silicon-based Egan FETs for the past four years and continues to expand its product portfolio. In addition, to help designers from silicon Egan FETs, the company has built a number of evaluation boards to compare the performance of silicon MOSFETs with Egan FETs in specific buck converter designs (see the Hi-Tech article. "Development Boards Make Evaluating Egan FETs Simple"). In addition, EPC has built a number of demo boards that provide a complete reference design using these broad bandgap devices in specific DC/DC converter circuits.
For example, in this article we will explore the design of a wide input, 20 non-isolated buck DC/DC converter using EPC's eGaN FETs, such as the epc2001 and epc2021, over a wide load variation. This design uses the ltc3891 of the linear technology of the buck controller, which integrates the drive and adopts a constant frequency current mode architecture. This DC/DC buck converter is designed for distributed power solutions in telecom, industrial and medical applications.
Wide Input, High Current Buck
Before investigating a wide input, high current isolated buck converter design using GaN transistors, let's first look at the performance characteristics and the Egan FETs epc2001 and epc2021 incorporated into this design. The company's data shows that the epc2001 is a 100 V device with an RDS(ON) of 7 mΩ and a leakage current (ID) capability of 25. The epc2021 is an 80 V Egan transistor with an RDS(ON) of 2.5mΩ and a leakage current (ID) capability of 60. Pulse ID is rated at 420 A. Gallium nitride transistors offer much lower conduction losses due to their low on-resistance. At the same time, because they are designed for higher switching frequencies, these GaN FETs also have lower switching losses. Additionally, to reduce package inductance, Egan FETs are used to passivate the mold to form solder sticks. These Egan FETs also overcome silicon MOSFETs enabling very efficient and compact high step-down ratio synchronous buck converters with minimum on-time over a wide input voltage range.
Taking advantage of the properties of Egan FETs, EPC has prepared a development board marked EPC9118 to simplify the construction of non-task isolated 20 step-down DC/DC 5 V fixed DC output converters with an input voltage range of 30 to 60 volts. It includes a complete power stage that includes Egan FETs epc2001 and epc2021, drivers, inductors and input/output capacitors (Figure 1). As shown, the controller ltc3891 adopts GaN FET driver. Since GaN transistors are capable of switching at high frequencies, the buck converter in this design is switched to 400 kHz.
EPC's Egan FET epc2001 and epc2021 images
Figure 1: The power stage includes the Egan FET driver, inductor, and input/output capacitors.
According to the demo board's quick start guide, the epc9118 is a 2.5" square board that contains a fully closed loop optimized control Buck converter. Based on the power stage shown in Figure 1, a schematic of a complete input 20, a non-isolated buck converter with a 5 VDC output is shown in Figure 2. This complete circuit is assembled on this demo board with proper layout to minimize losses and EMI. Since the demo board includes a closed-loop controller, the efficiency measurement must include losses due to the controller. The demo board's guide provides a procedure to measure the efficiency of this step-down DC/DC converter.
Image of EPC's epc9118 step-down DC/DC converter (click for size)
Figure 2: Complete schematic of a wide-input non-isolated step-down DC/DC converter with an output voltage of 5 volts at 20 A.
This guide presents the measured efficiency performance of a wide-input buck converter operating at a switching frequency of 400 kHz, as shown in Figure 3. It has been shown that this compact board can deliver over 93% full load power efficiency while delivering 20 36 VDC input at 5 VDC output. As the output load current varies from 5 A to 20 A, the conversion efficiency is still over 93% with a 36 VDC input. When the load current drops below 2.5 A, it starts to drop sharply. Also, when the input voltage is higher, say 48 volts DC, its output voltage is 5 volts, and the efficiency drops a little or so. For example, with 48 VDC input and 5 VDC output, the measured efficiency shown in Figure 3 exceeds 92%. With a load current of 5 A and the same input and output voltage parameters as before, this efficiency drops below 92%. When the load current begins to drop below 2.5 A, the efficiency begins to drop sharply. However, it still provides about 80% efficiency performance even with a load current of only 1. Similar performance for a 56 VDC input is also illustrated in Figure 3.
Typical Measurement Efficiency Graph
Figure 3: Typical measured efficiency curve for a wide-input 20-buck DC/DC converter utilizing Egan FETs.
In conclusion, the development board epc9118 demonstrates that high efficiency, wide input 20 buck converters can be easily designed and built using high frequency switching Egan FETs. Power circuits constructed with these wideband bandgap transistors can achieve over 93% conversion efficiency over a 20 VDC output range at 5 VDC output, with a wide input voltage range of 30 V to 60 V, even with loads down to 5 A Below, the efficiency is still high.