Author:
Matthias Kasper and Jon Azurza, Infineon Technologies
Date
10/31/2022
PDF
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Figure 1: Design specifications for a next generation USB charger
As part of an initiative to reduce the amount of electronic waste, the European Union has created a requirement for the development of a compact, universal charging device based on the USB-C standard, suitable for all types of portable devices like E-bikes, mobile devices and ever more powerful portable computers, all of which require periodic and rapid recharging. Such a charging device will be expected to cover a wide range of output voltages, be able to operate from all types of mains voltages around the world, and have a power rating sufficient to charge multiple devices simultaneously. This requirement will drive the demand for power levels in USB-C chargers upwards from 65 W towards 240 W. At these levels, power factor correction (PFC) and achieving high power density while maintaining voltage regulation across a wide voltage range all become increasingly challenging. Gallium-Nitride high electron mobility (GaN HEMT) devices have been proposed as a possible key enabler for highest power density. Therefore, Infineon undertook to investigate if they could deliver on the promise to provide the level of power density required for next generation charging applications. In this article we discuss the approach Infineon used and the findings.
Addressing the challenge
This charger design presented multiple challenges, including wide input (90-265 VRMS) and output voltage ranges (5-48 V) in addition to PFC, and the goal of providing two independent USB-C output ports in a high power-density form factor. Attaining a high level of power density was particularly challenging because heat could only be removed from the charger by means of natural convection and radiation. A graph of the required operating efficiency versus power density, alongside the design specifications for this 240 W charger are shown in Figure 1. To maintain a maximum surface temperature of 70°C, with maximum power density, required the design to be at least 96% efficient.
Topology Selection
Different options for the control and isolation functions were proposed, in order to help identify the optimum design topology. For the PFC (rectifier) stage, buck, boost, and buck-boost type PFC topologies were considered. Boost PFCs provide a DC-link voltage which is higher than the peak grid voltage, but the lower voltages provided by a buck PFC stage simplify the design of the DC/DC stage which follows. However, a disadvantage of using a buck converter in single-phase system is that it has discontinuous input current which creates harmonics which are unacceptable on the grid side, especially at lower voltages and for this reason, a buck converter could not be used. Requirements for the DC/DC converter included galvanic isolation (to provide safety) and to allow each of the two USB-C ports to be controlled independently. This could be achieved in a variety of ways. One option was to use a Hybrid Flyback (HFB) DC/DC converter followed by two buck converters, to provide simultaneous isolation and regulation. Alternatively, the first DC/DC conversion stage could be used to provide isolation only (no regulation), by using the "DC-transformer" (DCX) converter with a fixed conversion ratio. A third method would be to use two isolating and regulating converters (like the HFB) - one for each output port. However, this approach could not provide the required level of power density as it would require two transformers, each rated for the full power of the converter.
USB Charging Solution and Performance
Having considered the relative advantages and disadvantages of these alternative approaches using a detailed multi-objective (efficiency vs. power density) Pareto optimization, a 2x interleaved Totem-Pole PFC with boost-follower modulation, in combination with a DCX followed by two buck stages was the chosen solution (Figure 2).
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Figure 2: Final USB charger design
By operating with continuous conduction mode (CCM) with a fixed switching frequency (400 kHz) in each bridge-leg (i.e., 800 kHz effectively) in combination with small boost inductance values, the PFC stage was able to achieve zero voltage switching (ZVS) over the full line cycle for all load conditions and input rms-voltages. This mode causes large ripple currents in the boost inductors and the switches, so two interleaved high-frequency bridge-legs were used and this approach brought several advantages. Firstly, the mean current in each leg of the bridge is 50% of the overall PFC current, which also reduced by half the current ripple required in each inductor, in order to achieve ZVS. Secondly, phase-shifting the bridge-legs by 180° resulted in a doubling of the effective switching frequency for the EMI filter. This lowered the level of attenuation which the filter was required to provide, and as a result, the EMI filter could be made smaller. Finally, because power losses were spread across more components, hot spots did not occur.
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Figure 3: View of PCB for USB charger
The DCX converter operated at a resonant frequency of 425 kHz and was designed to achieve ZVS (independent of the load) by only using the magnetizing current for the ZVS transition. The turns ratio of the transformer (5.6:1) was selected such that the DC-link voltage range of 300 – 400 V mapped to the input voltage range of the buck stage (52 – 71 V). This enabled the use of 100 V rated Schottky Gate (SG) GaN HEMT devices as synchronous rectifiers in the DCX stage, and also in the two output buck stages.
Conclusion
HEMT GaN devices have the ability to perform soft and hard switching at high switching frequencies, properties which allow the use of advanced topology, modulation, and control schemes. As a result, this 240 W USB-C charger design was able to achieve an overall system efficiency of 95.3 % at full load operation for 90 Vrms input and 48V output voltages. The power density of the system was 42 W/in3 (uncased), a level which surpasses that achieved by available silicon-based chargers by approximately a factor of 2. These results show that HEMT GaN devices are well placed to meet the requirement of next generation wide input/output voltage range, high power density USB-C based chargers.
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