GaN Systems GS-EVM-CHG-250WPFCLLC-GS1 Evaluation Board User Manual

250W PFC-LLC Adapter Reference Design
Abridged version
Technical Manual

Scope and Purpose

This document provides a comprehensive functional description and design guide with the 250W AC/DC charger reference design (part number: GS-EVM-CHG- 50WPFCLLC-GS1). This adapter reference design uses 650V Gallium Nitride (GaN) transistors from GaN Systems (GS-065-011-2L). This manual describes the system operation and covers technical aspects essential to the design process. Test results and waveforms are also included.

Introduction

The 250W GaN-based charger reference design provides a cased turn-key solution with the following key features:

• High-frequency PFC and LLC controller with GaN FET
• High power density 16W/in3 (256cc)
• High efficiency above 96%
• Cost-effective topologies with Boost PFC+ Half-Bridge LLC
• Low Total Harmonic Distortion (THD)
• Passes EN55023 Class B for conduction and radiation electromagnetic interference (EMI)
• Meets IEC 62368-1 touch temperature requirement
• Comprehensive system protections such as TSD, OLP, OVP, OCP, and OPP

2.1 System Block Diagram
The block diagram of the 250W adapter is shown in Figure 1. This adapter has a two-stage power conditioning system, including a boost AC/DC PFC stage and an isolated DC/DC half-bridge LLC stage.

1. 1Boost PFC stage: A boost architecture regulates the AC grid voltage to a 400V DC bus voltage. Also, this boost converter is responsible for PFC. The PFC controller of this converter provides hybrid continuous/discontinuous current mode (CCM/DCM) with a digital average current control scheme. In particular, CCM is provided for heavy loads to benefit from the reduced peak current, and DCM is provided for light loads to reduce the switching frequency (i.e., improve the efficiency). Two parallel GaN Systems’ GS-065-0112-L transistors (650V, 150mΩ) are utilized in this converter. Having parallel GaN transistors improves the thermal and electrical performance of the converter.
2. Half-bridge LLC stage: The second stage isolated LLC converter steps down the 400V DC bus voltage to 19V. The LLC converter is capable of providing soft switching (ZVS). Besides, the LLC converter features quasi-sinusoidal currents, which leads to reduced EMI. This converter utilizes GaN Systems’ GS-065-011-2-L transistors (650V, 150mΩ) for the half-bridge devices and SRs in the transformer’s secondary. The controller of the LLC converter employs current mode control. The duty cycle of this converter is set to 0.5, and the variable switching frequency provides the output voltage regulation.

2.2 System Specifications
Table 1 summarizes the key specifications for this 250W GaN-based charger reference design.
Table 1. Key parameters and performance of GS-EVM-CHG-250WPFCLLC-GS1 reference design

(6.7inchX2.7inchX0.9inch)
Surface temperature rise| <53 °C
Touch Temperature @19V/13.2A| IEC 62368-1
EMI Standard| EN55032 CE & RE Class B
System Protections| TSD, OLP, OVP, OCP, SCP, Open Loop

2.3 Reference Design Board
The PCBA photos of the 250W adapter reference design are shown in Figure 2. According to this figure, the surface mount devices (SMD) are placed on the bottom layer, and through-hole components are placed on the top layer. The PCB of this adapter is based on a 4-layer 2Oz FR4 design. The Key components placements are highlighted in Figure 2. As a general rule, in the PCB design of adapters, it is essential to keep a clearance distance between the significant heat sources (e.g., magnetic components and bridge diodes on the top layer) and the GaN devices on the bottom layer. This provides minimum heat effect applied from other sources to GaN transistor junction temperature. The key magnetic components (e.g., PFC inductor and LLC transformer) are implemented using low-profile magnetic cores suitable for high frequency and high-power density.

GaN Value Proposition

Table 2 summarizes key Figures of Merit (FOM) between the GaN transistor and Si Super Junction (SJ) MOSFET. As can be seen, a GaN transistor has much lower FOMs compared to Si SJ MOSFETs. The combined advantages of GaN transistors: low gate charge, low parasitic capacitor, and low on-state resistance in the converter lead to a more efficient system. In particular, lower switching energy and parasitic capacitance of GaN transistors provide higher efficiency with hard-switching of the boost PFC and hard-switching turn-off of the LLC stage. Moreover, the zero-reverse recovery and optimized packaging of the GaN power stage enable less EMI noise. In addition to significantly improved efficiencies, the lower switching energy of GaN transistors provides fewer thermal challenges, which is a critical advantage, especially for adapter applications.

Table 2. GaN vs Si MOSFET parameters for both PFC and LLC stage of 250W adapter reference design

On resistance
Rayon (mOhm) Typ.
Tj=25C| 150| 159| 300| 300| Almost same or 2x smaller
typical Rayon
Total gate charge Qg (NC)| 2.| 24| 13| 16.| Lower gate driver loss
Gate to drain charge
Qgd (NC)| 0.7| 8| 4| 6| Lower switching loss
Reverse recovery charge
Qrr (nC)| 0| 2900| 740| 1000| Lower switching turn-on loss
for PFC stage; no hard
commutation failure for LLC

Moreover, there are several values for GaN at the LLC stage in the 250W adapter. The LLC converters should not continuously operate at the resonant frequency due to the regulatory requirements. Therefore, it is essential to highlight GaN advantages for various situations (e.g., when the switching frequency is below/above the resonant frequency). Figure 3 demonstrates multiple operating points of the LLC converter. GaN offers the following advantages for the LLC stage:

1. The peak efficiency is achieved when the switching frequency is equal to the resonant frequency (fsw= fr) (i.e., the peak efficiency is reached).
   • Considering the same switching frequency and same dead time (tdead), GaN transistors provide higher magnetizing inductance (Lm) or equivalently lower magnetizing current on the primary side of the LLC transformer. Consequently, lower reverse conduction loss is observed during the deadtime. As a result, GaN transistors provide higher efficiency compared to Si/SiC devices.
   • If the same dead time (tdead) and same magnetizing inductance (Lm) are considered, higher switching frequencies can be achieved, which results in increased power density.
   • If the same switching frequency and same magnetizing inductance (Lm) are considered, a significantly shorter lead time is required with GaN transistors to achieve ZVS. In particular, shorter dead time means lower reverse conduction loss for the transistor. Therefore, higher efficiency is achieved with GaN.

2.  When the switching frequency is greater than the resonant frequency (fsw> fr), the LLC converter operates in step-down mode. In this mode, the switching frequency is
   increasing. As a result, the turn-off high switching losses become dominant. This is where GaN helps by offering the lowest turn-off losses.

3. When the switching frequency is less than the resonant frequency (fsw< fr), the LLC the converter operates in step-up mode, in this mode, there is a significant primary circulating current, as mentioned earlier, GaN enables the utilization of a larger magnetizing inductance (Lm) with ZVS, or equivalently, lower magnetizing current. This reduces the circulating conduction loss on the primary side and improves efficiency.

Test Results

In this section, some test results of the charger reference design are presented. For more information and results, please contact us.
4.1 Test Equipment

• Oscilloscope: Tektronix MDO3054
• AC power source: Chroma 6530
• Electronic load: Chroma 6312A
• Power meter: HIOKI PW3335
• Multi-meter: UNI-T UT61E

4.2 Efficiency
For the efficiency test, the output voltage is directly measured by the multimeter from the output port on the PCBA board, and the output current is measured via the E-load. In
addition, the input power is measured with the power analyzer. To ensure accurate efficiency measurements, the input voltage is directly measured from the AC input port on the PCBA when the output voltage is equal to 19V. The efficiency values are measured for various load and input voltages, shown in Figure 4.

4.3 Standby Power
In Figure 5, the no-load standby power is measured from 90V to 264V, and the maximum standby power of 100mW occurs at the 264V input, which meets the CoC V5 2019/1782 standard requirement with standby power less than 150mW, as shown in Figure 5.

4.4 Electromagnetic Interference (EMI)
The charger reference design board is measured based on EN55032 CE Class B standard for EMI conduction and EN55032 RE Class B standard for EMI radiation. The test results show that this adapter passes the EMI conduction and EMI radiation tests with at least 10dB margin and 3dB margin, respectively. Figure 6 shows the conduction EMI results at 230V under full load conditions (20V/5A). Moreover, Figure 7 demonstrates the radiation EMI results at 230V under full load conditions (20V/5A).

4.5 Thermal performance
The decreased size of the adapters (i.e., the direct outcome of the ultra-fast switching capability) will bring thermal challenges. Besides, the new standards (e.g., IEC 62368-1) require that the adapter case touch temperature not exceed 77 ℃ at an ambient temperature of 25 ℃. This temperature requirement is more stringent than the former standards (e.g., IEC 60950). The maximum allowable heat dissipation within the adapter enclosure must be limited to follow the case temperature limits. In addition, if the heat inside the adapter is not appropriately managed, it can impact the reliability and electrical characteristics of the critical components. These emphasize the importance of a proper thermal design and optimization for power adapters. Therefore, thermal stack-up layers are crucial parts of the adapter design. Moreover, for EMI purposes, it is essential to have copper shielding around the adapter. After running the setup for an hour, and at an ambient temperature of 25 ℃, the following measurements are obtained:

Figure 8 shows the adapter’s surface temperature at an ambient temperature of 25℃. According to the IEC 62368-1 standard, the surface touch temperature at an ambient temperature of 25℃ should not exceed 77℃. Accordingly, the thermal images of the 250W adapter show that the surface temperature meets the standard requirements.

4.6 Electrical Waveforms
Figure 9(a) shows the key waveforms of the full-load steady-state waveforms at different input AC voltages. In particular, CCM is appropriate for heavy loads as it offers a lower peak current. Therefore, as the load decreases, the controller changes the mode of the circuit to the mixed CCM/DCM waveform shown in Figure 9(b). Various experimental waveforms of the 250w adapter reference design are demonstrated in this section in more detail.

Boost PFC Results:

Half-Bridge LLC Results:

Conclusion

The GaN-based 250W adapter reference design is introduced in this technical manual. The reference design achieves the following best-in-class features and performance:

References

1. GaN Systems Application Note GN010, EZDriveTM Solution for GaN Systems E-HEMTs, 2020,
   [Online]. Available: https://gansystems.com/wp-content/uploads/2020/07 /GN010_EZDriveSolution-for-GaN-Systems-GaN-Transistors-_20200715.pdf

2. GaN Systems Reference Design, GS-EVM-CHG-65WQR-GS1, 65W Type-C PD QR Charger
   Reference Design [Online]. Available: https://gansystems.com/evaluation- boards/gs-evm-chg65wqr-gs1/

3. GaN Systems Reference Design, GS-EVM-CHG-100WPFCQR-GS1, 100W PFC QR USB PD Charger with 2 Type-C Ports Reference Design [Online]. Available:
   https://gansystems.com/evaluation-boards/gs-evm-chg-100wpfcqr-gs1/

Reference Design Important Notice
GaN Systems Inc. (GaN Systems) provides the enclosed reference design under the following AS-IS conditions:

This reference design is intended for use for ENGINEERING DEVELOPMENT, DEMONSTRATION, and OR EVALUATION PURPOSES ONLY and is not considered by GaN Systems to be the design of a finished end-product fit for general consumer use. As such, the reference design provided is not intended to be complete in terms of the required design, marketing, and/or manufacturing-related protective considerations, including but not limited to product safety and environmental measures typically found in end products that incorporate such semiconductor components or circuit boards. This reference design does not fall within the scope of the European Union directives regarding electromagnetic compatibility, restricted substances (RoHS), recycling (WEEE), FCC, CE or UL, and therefore may not meet the technical requirements of these directives, or other related regulations.

No License is granted under any patent right or other intellectual property right of GaN Systems whatsoever. GaN Systems assumes no liability for applications assistance,  customer product design, software performance, or infringement of patents or any other intellectual property rights of any kind.

GaN Systems Inc.
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Ottawa, Ontario, Canada K2K 3G8
T +1 613-686-1996| GaN Systems Ltd., German Branch
Terminalstrasse Mitte 18,
85356 München, Germany
T +49 (0) 8165 9822 7260| GaN Systems Corp.
2723 South State Street, Suite 150,
Ann Arbor, MI. USA 48104
T +1 248-609-7643

www.gansystems.com

Important Notice – Unless expressly approved in writing by an authorized representative of GaN Systems, GaN Systems components are not designed, authorized or warranted for use in lifesaving, life-sustaining, military, aircraft, or space applications, nor in products or systems where failure or malfunction may result in personal injury, death, or property or environmental damage. The information given in this document shall not, in any event, be regarded as a guarantee of performance. GaN Systems hereby disclaims any or all warranties and liabilities of any kind, including but not limited to warranties of non-infringement of intellectual property rights. All other brand and product names are trademarks or registered trademarks of their respective owners. Information provided herein is intended as a guide only and is subject to change without notice. The information contained herein or any use of such information does not grant, explicitly, or implicitly, to any party any patent rights, licenses, or any other intellectual property rights. General Sales and Terms Conditions apply.

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https://gansystems.com/evaluation-boards/gs-evm-chg-250wpfcllc-gs1/

GS-EVM-CHG-250WPFCLLC-GS1 abridged TM Rev.220119
© 2022 GaN Systems Inc
Please refer to the Reference
Design Kit Important Notice on page 18

References

• GaN Systems
• GaN Systems
• GS-EVM-CHG-100WPFCQR-GS1 Evaluation Board | GaN Systems
• GS-EVM-CHG-100WPFCQR-GS1 Evaluation Board | GaN Systems

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