As the demand for smaller, more powerful electronic devices grows, so does the need for faster and more efficient transistors. Infineon’s Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are an ideal choice for applications which require high switching frequencies, such as USB-C adapter and charger designs.
Today, I will explore other approaches to implementing topologies usually found in USB Type-C power delivery (PD) charger designs, using discrete or fully integrated GaN HEMTs. I will also discuss the benefits of integrated switching controllers, and hopefully this will answer some questions you may have.
Let’s start with the benefits. The active clamp flyback (ACF) converter, seen in Figure 1, is a popular topology that facilitates zero voltage switching (ZVS). This type of transformer is better for higher frequency operation than a quasi-resonant flyback. Its ZVS operation and complete recovery of the energy in transformer leakage inductance makes it more efficient overall. This makes the ACF well-suited for high-frequency operation compared to the quasi-resonant flyback topology.
Figure 1. Discrete implementation of the active flyback converter topology
There are two main types of control method: the complementary (CP) and the non-complementary (NCP). The CP control is more effective at EMI shielding. In CP mode, both switches turn on and off at the same time during each cycle, which creates a current in the transformer that has a sinusoidal shape. However, this can impact light-load and standby efficiency, as well as cause EMI issues. NCP ACF can be used to overcome both of these problems. In this mode, the active clamp switch only turns on when there is enough magnetic energy to keep the main switch ZVS. This way, circulating clamp capacitor currents are reduced.
The active clamp flyback topology – Integrated GaN switch
As you can see in Figure 2, there is an alternative implementation of the active clamp flyback (ACF) using a CoolGaN™ integrated power stage (IPS). In this design, the clamp switch provides a path to recover the energy stored in the transformer’s leakage inductance (Llk) when the main switch turns off. Then, as the clamp switch turns on, Cclmp and Llk resonate together through the clamp switch and the transformer, resulting in energy transfer to the load. This energy recovery increases the system efficiency compared to the passive clamp flyback, where the energy is stored in Llk damps in a traditional RCD clamp circuit.
Figure 2. Active flyback converter topology implemented using a GaN-based Integrated Power Stage (IPS)
The hybrid flyback (HFB) converter, shown in Figure 3, is another resonant topology that not only employs ZVS, but zero current switching (ZCS) too. The primary-side converter has resonant-type current waveforms, which means high-frequency, high efficiency is possible due to ZVS operation with lower RMS currents.
Figure 3. Discrete implementation of the hybrid flyback topology with XDPS2201 controller
The capacity of the resonant capacitor helps with energy storage, which means the transformer size can be smaller than for other flyback-type topologies. Thanks to the half-bridge structure with a self-voltage clamp to Vbus on the primary side, the voltage stress on the switching device is better than for the ACF topology. HFB has an additional switch on the primary side compared to QR flyback, which requires special care for light load efficiency due to circulating currents of resonance type operation. Designing for universal input specifications is a careful process, and the control complexity required for HFB is much higher than that needed for QR flyback topology. However, Infineon’s XDP™ digital power XDPS2201 controller uses a dedicated control algorithm, making this task much simple.
Figure 4 shows a hybrid flyback converter (HFB) topology with the CoolGaN™ Integrated Power Stage (IPS). The converter consists of a high-side and a low-side switch, the transformer, the resonant tank (Llk and Cr), the output stage rectifier and capacitors.
Figure 4. Implementing a hybrid flyback topology using a GaN-based Integrated Power Stage (IPS)
The advanced control scheme with its non-complimentary switching pattern provides a solution that supports a wide range of AC input and DC output voltages - which is exactly what's necessary for universal USB-C PD operation. When the high-side switch is turned on, the hybrid flyback converter stores energy in the primary-side inductor. This energy is then transferred to the output when the low-side switch is turned on. With proper timing control during the switch transition of both switches, HFB runs under ZVS for both devices, ensuring high system efficiency without requiring additional components. Both devices benefit from improved efficiency and lower costs (from ZVS and ZCS operation on the secondary side), making hybrid flyback a great choice for high power density converters, like USB-PD fast chargers.
PFC and hybrid flyback topology
The new USB-PD with extended power range (EPR) standard allows for higher power levels, which is great news for many different applications. This presents a new challenge for the currently used converter topologies, though. The size and power density have become increasingly essential requirements in this application. The wider output voltage range (from 5 V to 48 V) presents new challenges for existing converter topologies, so combining an AC-DC power factor correction (PFC) boost converter with a DC-DC hybrid flyback (HFB) stage provided the most suitable combination for USB-PD chargers and adapters with a wide input and output voltage range. Take a look on Figure 5 below.
Figure 5. Power architecture for USB-PD extended power range
The XDP™ XDPS2221 integrates two power controllers in a single package - an AC-DC power factor correction (PFC) controller and a DC-DC hybrid flyback controller. These two stages work together to help meet regulatory requirements and reduce the cost of materials for external components.
This architecture uses a modern controller to provide a powerful combination of efficiency and power density that meets international standards. Plus, it allows for more effective control of the output voltage in USB-PD standards, and it requires a much smaller transformer than conventional flyback topology.
The XDP™ XDPS2221 enables high-power densities because of the zero voltage switching, resonant energy transfer and other topology advantages. The PFC stage is enhanced with automatic enable/disable functionality and adaptive bus voltage control to maximize efficiency. Optionally, the integrated PFC function can also be disabled to support any external PFC Controller. The flyback stage uses peak current control operation for robust regulation and fast dynamic load response.
For more insights about Infineon’s solutions and products for USB-PD chargers and adapters – click here.
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