An isolated PoE-PSE converter for eGaN FETs and silicon power devices

Isolated brick converters are widely used in telecommunications systems to power network equipment. These converters are available in a variety of standard sizes and input/output voltage ranges. Their modularity, power density, reliability and versatility simplify isolated power applications and, to some extent, commercialize the isolated power market. A common feature of these converters is that the input/output power devices are rated at 100V or less. However, isolated converter applications on the market require higher device voltages, such as PoE-PSE (Power over Ethernet powered devices). These converters benefit from the advantages of increased voltage ratings of GaN field-effect transistors (eGaN FETs). This article will build an eGaN FET-based half-brick converter and compare it to a similar state-of-the-art silicon MOSFET brick converter.

Introduction to Isolated PoE-PSE Converter

Over the past few years, Power over Ethernet (PoE) standards have evolved. The main focus is on the systematic increase in power in new and new types of equipment. According to the IEEE 802.3at standard for Power over Ethernet, the Power Supply Equipment (PSE) requires PoE Type 1 output voltage between 44V and 57V, and PoE Type 2 (PoE+) output voltage between 50V and 57V. Each port of the Ethernet switch must be capable of outputting 15.4W (Type 1) or 25.5W (Type 2) power. For power supplies, the output requires some form of regulation, but no strict regulation is required. Interestingly, the minimum voltage increase is due to the increased power level and the maximum linear voltage drop is increased, and future power supply equipment may require a smaller voltage range close to a maximum of 57V. For a typical Ethernet switch with 24, 36 or 48 ports, the total power supply required may be as high as 1.2 kW. This has driven the need for converters with higher efficiency and higher power density.

Due to the limited size constraints of these brick converters, engineers are continually trying to use innovative methods to increase their output power and power density. Although these ideas are numerous and ever-changing, they are all related to improving system efficiency. This is a physical limitation due to the fixed volume of the converter and the method of heat dissipation. For half-brick converters, it is difficult to remove losses beyond 35W, even with powerful airflow and/or substrates. Figure 1 shows the relationship between the minimum full load efficiency and the achievable output power required for a half brick converter. Since most commercial half-brick power-supply converters are already 95% efficient, even a half-percent efficiency improvement is important and an additional 100W of output power can be added. However, the cost per watt ($/W) is the most important consideration. Increasing brick converter efficiency and output power can reduce the total cost per watt of the module.

Figure 1: The minimum efficiency required for a half-brick converter to achieve a specified output power (assuming a maximum power consumption of 35W).

Compare different isolated PoE-PSE converters

When trying to compare half-brick PoE-PSE converters, a simple one-to-one comparison is not possible because different commercial converters have a very diverse design. The output power of each generation of power supplies has increased because the manufacturer's "optimal" design has been improved in terms of structure, layout and topology. Determining the "best" solution is an iterative process, and the definition of the "best" solution is different, further increasing the complexity of the problem. The design of half-brick applications is diverse, and an excellent example is the choice of building two interleaved converters or building a single converter. In addition, current commercial products have methods that use single-stage conversion or two-stage conversion.

For larger brick sizes (such as half-brick sizes), the output power and the total power consumption of the converter are sufficiently high that each switch typically requires the use of multiple power devices - from the perspective of required thermal management, and The same is true for the minimum on-resistance (maximum wafer size). If the converter is divided into two (each responsible for half of the power), then the total number of power devices will not be affected. The cost and volume increase of using more inductors and transformers is also problematic because these devices are smaller and the interleaving of the converters allows the output capacitance to be reduced. Furthermore, the size of the bricks (especially the height limit) means that the height of a single high power transformer is limited, and the length of the core channel may not be optimal compared to the core of two smaller transformers. The remaining differences (gate drive and control) will likely be the decisive factor, that is, can we accept the increased cost to achieve higher efficiency and output power?

Like an eighth-brick converter, developing an eGaN-based FET converter is not necessarily the general best solution. Compared to current commercial systems, our design goal is to increase the operating frequency a lot, demonstrating that eGaN devices can help engineers who specialize in power supply designs to develop state-of-the-art next-generation products with higher efficiency and higher output power.

Prototype eGaN FET based PSE converter

For 48V to 53V eGaN FET-based half-brick power supply converters, a phase-shifted full-bridge (PSFB) converter with a full-bridge synchronous rectifier (FBSR) topology can be selected (see Figure 2). Due to the higher power, two interleaved converters are built in half-brick volume instead of a single converter with parallel devices. This not only avoids the complexity of parallel devices, but the use of two independent converters theoretically allows phase-cutting to improve efficiency at light loads. Figure 3 shows the efficiency results for one-phase and two-phase operation, where the light load efficiency with a simple phase cut is increased by at least 2%.

Each converter operates at 250kHz and has an output ripple frequency of 1MHz. Figure 4 shows a more complete schematic. The purpose is to show that two such converters can be built in a limited volume due to the increased switching frequency and the relatively small size of the gallium nitride device. Choosing a 4:7 transformer turns ratio means that when VIN is 60V, the secondary winding voltage (excluding the switching spike) is about 105V. Therefore, the secondary side can use a 200V device and the primary side can use a 100V device.

The actual prototype based on eGaN FET is shown in Figure 5. As can be seen from the figure, unlike traditional brick designs, the magnetic components are not integrated on the main printed circuit board, but are placed on several separate printed circuit boards. This not only reduces the number of layers required for the main printed circuit board, but also allows the output filter to use conventional surface mount inductors. The converter uses eight layers of two ounces of copper printed circuit board per layer. Transformer windings are created by stacking two eight-layer boards (parallel) in the winding window.

Figure 2: 350W fully regulated phase-shifted full-bridge (PSFB) topology with full-bridge synchronous rectification (FBSR) (two half-brick, interleaved 250kHz converter) using eGaN FETs. (electronic system design)

Figure 2: 350W fully regulated phase-shifted full-bridge (PSFB) topology with full-bridge synchronous rectification (FBSR) (two half-brick, interleaved 250kHz converter) using eGaN FETs.

Figure 3: Efficiency data for a half-brick PSE converter based on an eGaN FET prototype design with single-phase (half converter power down) and normal two-phase operation. (electronic system design)

Figure 3: Efficiency data for a half-brick PSE converter based on an eGaN FET prototype design with single-phase (half converter power down) and normal two-phase operation.

Figure 4: Schematic of an eighth-brick, 38 V~60 V to 53 V/70 W converter operating at 250 kHz switching frequency with an eGaN FET design. (electronic system design)

Figure 4: Schematic of an eighth-brick, 38 V~60 V to 53 V/70 W converter operating at 250 kHz switching frequency with an eGaN FET design.

Figure 5: Top and bottom views (in inches) of a 48V to 53V half-brick PSE converter designed with eGaN FETs. (electronic system design)

Figure 5: Top and bottom views (in inches) of a 48V to 53V half-brick PSE converter designed with eGaN FETs.

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