By David Woodcock
Power Systems Manager, Future Design Centre, Future Electronics
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Power switches made with wide bandgap semiconductor material, either silicon carbide (SiC) or gallium nitride (GaN), are now widely used in power converters. The high-speed switching characteristics, alongside the low reverse-recovery charge of SiC transistors, or the zero reverse-recovery charge of GaN high electron-mobility transistors (HEMTs), enable designers to make power systems which are smaller than silicon-based alternatives, and which are also highly efficient.
For all the advantages of GaN and SiC, however, these switch types do still have something of an exotic character compared to the tried and trusted silicon MOSFET or IGBT. Reference and demonstration designs such as the GaNdalf II 2 kW bridgeless totem pole PFC stage, and GaNSTar 500 W LLC converter boards, developed by Future Electronics, can provide a useful blueprint for designers approaching GaN and SiC power switch-based projects for the first time.
Now the Future Electronics power system design laboratory, based in London, UK, has implemented a SiC MOSFET-based design for a 3 kW LLC power supply which steps a nominal 390 V dc input down to a 49 V dc nominal output.
With the SiC MOSFETs switching at a fast 250 kHz, this new SoniC demonstration design board can achieve a total space saving of around 30% compared to a 3 kW silicon MOSFET-based design, in which the maximum switching frequency would typically not exceed 100 kHz. The faster switching performance of the SoniC board enables the use of smaller magnetics and capacitors.
The principle underpinning the design approach was to use standard off-the-shelf components or, in the case of the transformers, a conventional design which can readily be produced at low cost by any small or medium-sized OEM. The design also avoids less conventional methods for magnetic component assembly, such as copper tape on the secondary, or planar magnetic design and construction.
With the SoniC board, Future Electronics has developed a successful architecture which achieves peak efficiency at full load of 97.4%.
Fig. 1: The SoniC LLC converter board using SiC MOSFETs is around 30% smaller than an equivalent silicon MOSFET-based design
A 3 kW LLC converter design for the mass market
The board’s bill-of-materials (BoM) consists of components that are readily available to OEMs via the distribution channel:
The SiC MOSFETs switching at 250 kHz allowed the use of a much smaller resonant inductor and transformer than are required in silicon MOSFET-based 3 kW converters, contributing to an estimated space saving of 30% compared to all-silicon designs, as shown in Figure 2.
Fig. 2: A block diagram of the SoniC board shows the NCP4390 PFM controller on the secondary side
In implementing wide bandgap semiconductor technology to produce space and weight savings, designers must start by choosing between the SiC and GaN materials. In the SoniC design, the Future Electronics engineers opted for SiC on the primary side because of the broad availability of high-voltage SiC products from multiple device manufacturers.
In a sign of the maturity and quality of SiC production technology today, testing of the primary side of the converter showed faultless performance. In a soft-switching topology, the onsemi SiC MOSFETs and the drivers maintain stable and robust operation at 250 kHz, which is shown in Figure 3.
Fig. 3: Switching waveforms of the SoniC board at a 3 kW load with a 390 V dc input voltage.
Green – Input current; Yellow – Primary SiC MOSFET drain-source voltage; Orange – Secondary MOSFET drain-source voltage; Purple – Output current
Full-bridge configuration achieves stable operation on secondary side
The introduction of SiC power switches, then, did not generate any difficulties for the implementation of a small, high-power LLC dc-dc converter. But to achieve the aim of a 30% size reduction, the system needs to operate at high frequency 250 kHz at full load, and potentially up to 690 kHz during start-up and up to 500 kHz in light-load operation.
The onsemi NCP4390 LLC power controller, which supports switching frequencies up to 690 kHz, was an ideal choice for this application. It is a PFM controller for LLC resonant converters with synchronous rectification. The power controller employs a current-mode control technique which provides a better control-to-output transfer function in the power stage than voltage-mode control, simplifying the design of the feedback loop.
In early iterations of the SoniC board design, however, the high switching frequency did give rise to problems in the transformer.
The architecture of the SoniC board adopts a standard LLC converter implementation, making it straightforward for OEMs to produce in large quantities. Such an architecture would normally use a half-bridge circuit or centre-tapped transformer on the secondary side, to optimize dc-dc stage efficiency.
But the attempt to use a centre-tapped transformer configuration on the secondary side resulted in too much ringing on the drain-source voltage switching waveforms: the peak voltage exceeded the 150 V rating of the original silicon MOSFETs. Cost, availability and efficiency concerns militate against the use of MOSFETs with a voltage rating higher than 150 V.
When the design was modified to use a full-bridge configuration on the secondary side, however, the ringing problems disappeared. This then allowed the use of 80 V silicon MOSFETs, which offer lower on-resistance than the 150 V MOSFETs, and therefore the potential to cut conduction losses.
In testing, in fact, the SoniC board achieves minimum efficiency at full load of 97%, and 93% across the load range from 10% to 100%, which is shown in Figure 4.
Fig. 4: System efficiency plot of the SoniC board
Due to the modular and flexible design of the SoniC board, the Future Electronics design team was able to change its approach to the full-bridge implementation on the secondary side without changing the design of the transformer. With the secondary side operating in full-bridge mode, it is hard to achieve synchronous operation at 3 kW due to the low on-resistance of the silicon MOSFETs, a feature necessary to achieving the target for conversion efficiency. Nevertheless, effective operation is achieved at below the resonant frequency of the LLC converter in discontinuous mode (DCM). This is suitable for a wide range of high-power applications.
But if required, the design could be modified to enable the LLC converter to run in continuous conduction mode (CCM) above the resonant frequency, to support input voltages above 390 V. Two options are possible:
New components offer potential to enhance SoniC converter design
The SoniC board, which is available on request from Future Electronics, demonstrates the stable, efficient operation of onsemi SiC MOSFETs when operating at a high switching frequency in a high-power LLC converter. The emergence of new and more advanced components offers the potential to improve on the already high performance of the SoniC board.
One option is to upgrade the silicon MOSFETs used on the secondary side. The original SoniC board used the best onsemi silicon MOSFETs available at the time of development, but they have since been superseded by the T10 family, which is based on new process technology developed by onsemi. The 80 V NTMFWS1D5N08X MOSFET is suitable for use in SoniC full-bridge configuration. The advantage of using this T10 MOSFET is that it offers improved switching performance, thanks to its lower reverse-recovery charge. On-resistance is also lower than in the previous generations of onsemi MOSFETs, promising an increase in system efficiency.
Looking further ahead, a second option is to investigate the scope to use 150 V GaN FETs in a half-bridge configuration on the secondary side. The most notable feature of the GaN switches is the absence of reverse-recovery charge; a half-bridge circuit based on these GaN switches would be free of the ringing observed in the half-bridge configuration using 150 V silicon MOSFETs.
Today, there is only limited availability of such low-voltage GaN switches, while both unit cost and on-resistance are higher than for the silicon equivalent. The choice of package styles and footprints is also much smaller in GaN FETs than in silicon MOSFETs.
But production roadmaps from GaN device manufacturers suggest that supply is set to ease considerably, and the portfolio from which design engineers can choose is expected to widen over the coming months.
These options for modification of the Future Electronics design blueprint hold some promise for efficiency improvements and BoM cost reduction. Even in its current form, however, the SoniC board demonstrates conclusively that the use of SiC MOSFETs in a high-power LLC dc-dc converter stage enables a substantial 30% space saving while achieving stable operation and high efficiency, without recourse to any exotic components or non-standard topologies.
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