Improve power density in charger and adapter designs with high-efficiency GaN converters

Today, the most commonly used power converter topology for charger and adapter applications is the quasi-resonant (QR) flyback topology because of its simple structure, easy control, low bill of materials (BOM) cost, and high energy efficiency through valley switching. However, the switching loss and transformer leakage inductance energy loss, which are closely related to the operating frequency, limit the maximum switching frequency of the QR flyback converter, thereby limiting the power density.

The use of GaN HEMTs and planar transformers in QR flyback converters helps to increase switching frequency and power density. However, to achieve higher power density in ultra-thin charger and adapter designs, soft switching and transformer leakage inductance energy recovery become indispensable. This inevitably leads to the selection of converter topologies that are inherently more efficient.

This article explains how to apply Infineon’s CoolGaN™ Integrated Power Stage (IPS) technology to active clamp flyback (ACF), hybrid flyback (HFB) and LLC converter topologies. This approach makes it faster and easier to design charger and adapter solutions to create smaller and lighter products, or higher power products of the same size for fast charging of devices or charging multiple devices with one adapter.

Converter topologies enabling higher power density

It has been proven that half-bridge (HB) topologies such as active clamp flyback (ACF), hybrid flyback (HFB) and LLC converter can achieve high efficiency even at very high switching frequencies thanks to zero voltage switching (ZVS) and no snubber losses.

Active Clamp Flyback (ACF) Topology

Figure 1 shows a typical application example of CoolGaN™ IPS for an active clamp flyback (ACF) converter. In the ACF topology, when the main switch is off and the clamp switch is on, the energy stored in the transformer leakage inductance (Llk) can be recovered through the clamp switch. Cclamp and Llk resonate together with the transformer through the clamp switch to transfer energy to the load. Compared to the passive clamp flyback topology, the energy stored in the traditional RCD clamp circuit Llk gradually decays, and this energy recovery improves system efficiency. A well-designed ACF topology can operate under soft-switching ZVS conditions, so its operating switching frequency is much higher than that of the quasi-resonant (QR) flyback topology operating under hard switching conditions. This helps to reduce the size of magnetic components, including transformers and EMI filters.

Figure 1: ACF converter application circuit diagram

The components of the ACF converter include: high-side switch and low-side switch, transformer, clamp capacitor Cclamp, rectifier output stage and capacitor. The typical operating waveform shown in Figure 2 briefly explains the working principle of the ACF converter.

Figure 2: ACF converter operation

When the low-side power switch is turned on, the ACF converter stores energy in the primary inductor and the leakage inductor (Llk). Thereafter, when the low-side power switch is turned off, the energy is transferred to the output. During the off-state of the low-side switch, when the high-side switch is turned on, the energy stored in the leakage inductor is transferred to the output. In addition, the switching ZVS operation further improves the efficiency. This operation ensures that the ACF converter achieves high-efficiency performance.

Hybrid Flyback (HFB) Topology

Figure 3 shows a typical application example of CoolGaN™ IPS in a hybrid flyback (HFB) converter topology.

Figure 3: HFB converter application circuit diagram

The components of the hybrid flyback converter include: high-side switch and low-side switch, transformer, resonant tank (Llk and Cr), and rectifier output stage and capacitors. This topology also benefits from the soft switching operation of the power switch, which can achieve high power density and high energy efficiency. Using the same technology as the LLC converter, in this topology, the transformer leakage inductance and magnetizing inductance can resonate with the capacitor. In addition, the advanced control scheme based on non-complementary switching mode can support a wide range of AC input voltage and DC output voltage, which provides the necessary conditions for realizing universal USB-C PD operation.

HFB can achieve full ZVS operation on the primary side and full ZCS operation on the secondary side. The leakage inductance energy is then recovered to achieve high energy efficiency. The hybrid flyback topology can easily achieve a wide output range through variable duty cycle . This overcomes the limitations of the LLC topology in wide output range applications. For more information on the hybrid flyback converter, refer to [1].

Figure 4 shows the typical operating waveforms, which briefly illustrate the working principle of the hybrid flyback converter. When the high-side switch is turned on, the hybrid flyback converter stores energy in the primary-side inductor. When the low-side switch is turned on, this energy is transferred to the output. By properly timing the transitions of the two MOSFET switches, the HFB operates under ZVS conditions for both switches, which ensures high system efficiency without the need for additional components. Thanks to the high efficiency achieved by ZVS operation and the additional efficiency improvement brought by ZCS operation on the secondary side, the hybrid flyback converter provides a cost-competitive solution for ultra-high power density converters such as USB-PD fast chargers.

Figure 4: HFB converter operation

LLC Converter

Figure 5 shows a typical application example of CoolGaN™ IPS for a half-bridge LLC topology. The LLC converter is a member of the resonant converter family, which means that the voltage regulation is not done using conventional pulse width modulation (PWM). The LLC converter operates with a 50% duty cycle and a fixed 180° phase shift, and regulates the voltage by frequency modulation. The components of a half-bridge LLC converter include: high-side and low-side switches, transformer, resonant tank (Lr and Cr), and rectifier output stage and capacitors.

Figure 5: Half-bridge LLC converter application circuit diagram

Figure 6 shows the typical operating waveforms, which briefly illustrate the operating principle of the half-bridge LLC converter. When the high-side switch is turned on, the half-bridge LLC converter operates in power supply (PD) mode. In this switching cycle, the resonant tank is excited by a positive voltage, so the current resonates forward. When the low-side switch is turned on, the resonant tank is excited by a negative voltage, so the current resonates negatively. In the PD operation mode, the current difference between the resonant current and the magnetizing current is transferred to the secondary side via the transformer and rectifier to power the load.

Figure 6: Half-bridge LLC converter operation

In addition to this, all primary-side MOSFETs are turned on with ZVS resonance, fully recovering the energy stored in the MOSFET parasitic output capacitance. At the same time, all secondary-side switches are turned off with ZVS resonance, minimizing the switching losses typically associated with hard switching. All switching devices in the LLC converter operate resonantly, which minimizes dynamic losses and improves overall efficiency, especially at higher operating frequencies ranging from hundreds of kHz to MHz.

To achieve zero voltage switching (ZVS) operation of the high voltage switches, all three topologies utilize circulating current in the transformer to discharge the switch QOSS. Obviously, the larger the QOSS, the larger the circulating current and the longer the discharge time required. The circulating current increases transformer losses (core and winding losses), while the discharge time significantly increases the dead time. Dead time reduces the effective duty cycle and causes higher RMS currents in the circuit, which increases conduction losses. Therefore, minimizing dead time is critical for very high switching frequency operation. GaN HEMTs have an excellent FOM (RDS(on)×QOSS), which helps to reduce dead time and reduce circulating currents in the circuit. Thanks to this advantage, as well as low drive losses and zero reverse recovery, GaN HEMTs are perfect for ACF, HFB and half-bridge LLC converters.

CoolGaN™ IPS and 65 W ACF Converter Evaluation Board

To further optimize system size, Infineon Technologies recently launched the CoolGaN™ Integrated Power Stage (IPS), which combines a 600 V enhancement-mode CoolGaN™ switch with a dedicated gate driver in a small, thermally enhanced QFN package.

To demonstrate the performance of CoolGaN™ IPS, a 65 W active clamp flyback converter based on the CoolGaN™ IPS IGI60F1414A1L was developed (Figure 7). [2]

Figure 7: Front view of the 65 W ACF evaluation board with CoolGaN™ IPS half-bridge

The measured energy efficiency curve (Figure 8) shows that its four-point average efficiency and 10% load condition efficiency meet the CoC Tier2 and DoE Level VI efficiency requirements.

Figure 8: ACF evaluation board efficiency curves at different input voltages and load conditions

Summarize

Today’s high power density charger and adapter applications often use GaN HEMTs because they have a much improved figure of merit (FOM) compared to silicon MOSFETs, which allows for high frequency switching. CoolGaN™ IPS technology integrates gate drivers in a compact package and supports high operating frequencies, making it particularly suitable for active clamp flyback (ACF), hybrid flyback (HFB) and LLC converters, thus helping to further increase the power density of charger and adapter designs.