whitepaper see, feel, and hear the difference with gan

WHITEPAPER

See, Feel, and Hear the Difference with GaNClass-D Amplifier and Companion SMPS

LEARN MORE AT GANSYSTEMS.COM

https://gansystems.com/


Table of Contents

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Introduction

Amplifier and SMPS designs

Performance Test and Validation

Performance Characterization Results

Efficiency and Thermal Performance Results

Benefits of GaN-based Class-D Audio Hardware

Summary

IntroductionAs engineers within the design community read and learn about new products and topologies,

they become skeptical about the promises for improved operating characteristics. The promise of

a step-function improvement with GaN in audio systems could be classified as one of these

scenarios. However, with the GaN Systems Stereo Class-D Amplifier system solution, the audio

industry is pleasantly surprised to see that GaN is keeping its promise.

The GaN Systems solution consists of a GaN FET-based LLC SMPS with PFC and Stereo

200W/8-ohm Class-D Amplifier. The SMPS uses the GaN Systems GS66506T for both PFC

Front-End and LLC Half-Bridge Back-End and the GaN Systems GS61008P for each of the four

Half-Bridge Output Stages of the Class-D Amplifier.

This whitepaper reviews the GaN Systems audio evaluation kit using the standard set of industry

performance and validation tests. These tests include, but were not limited to:

• Performance Specification Testing – Power Output and Efficiency

• Performance Characterization Testing – THD vs Power, Frequency; Frequency Response; Noise Floor

• Efficiency and Thermal Performance Testing – Amplifier and SMPS

• Amplifier Robustness Testing – 4-ohm, 8-ohm Performance; 200W into 8 ohms, 300W into 4 ohms; Current-Limit Functionality

• Amplifier Sonic Quality Testing - Listening Test, Sonic Quality Assessment

See, Feel, and Hear the Difference with GaN Class-D Amplifier and Companion SMPS Design

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Amplifier and SMPS designsThe Class-D Amplifier is a two-channel Bridge-Tied-Load (BTL) design, composed of four

ground-referenced Half-Bridge Output Stages with evident support for scalability and

expandability. The design of the companion SMPS is a high-efficiency LLC design with PFC and

Universal Input support. These architectures and topologies are the most scalable. The Amplifier

and SMPS platforms are also readily modified to a wide variety of cost-effective Controllers.

AMPLIFIER DESIGNThe user guide for the Class-D evaluation kit shows the following specifications.

Power Output 200Wrms into an 8-ohm load

300Wrms into a 4-ohm load

THD+N (open loop) < 0.2% into an 8-ohm load, 1kHz

< 0.3% into a 4-ohm load, 1kHz

THD+N (closed loop) < 0.02% into an 8-ohm load, 1kHz

< 0.03% into a 4-ohm load, 1kHz

SNR > 115dB (open loop)

> 105dB (closed loop)

Frequency Response 20Hz to 20kHz, +/- 0.5dB, 6-ohm

Switching Frequency 400kHz (closed loop)

384kHz (open loop)

Efficiency > 95%

Power Supply Voltage < +/- 32VDC for 2x BTL and Ground-Referenced Output

The Amplifier provides the following interfaces which support the application and evaluation

requirements.

Input/Source

Analog 3.5mm Stereo Mini-Plug

Single-ended RCA Phono x 2, Unbalanced, 1Vrms

Digital Coaxial Digital (S/PDIF)

Optical Digital (S/PDIF)

Volume Control Encoder Digital – On Board

Power Supply input 6-Pin JST Latching Connector (+HV, -HV, Ground)

Output Power 4-Pin Phoenix Connector x 2 (+L, -L, +R, -R)


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SMPS DESIGNThe SMPS design includes an existing and proven high-efficiency LLC architecture and

universal PFC input with the following specifications.

AC Voltage Input 85VAC to 265VAC

50Hz/60Hz

DC Output +/- 32VDC @ 12A

The SMPS platform provides the following connectivity.

Input Power 5-Pin JST Latching Connector (Line, Neutral, Ground)

Output Power 6-pin JST Latching Connector (+HV, -HV, Ground)


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Performance Test and ValidationThe GaN Systems Class-D evaluation kit is tested and validated using industry-standard

measurements with recognized techniques and equipment. The Test Bench utilizes the following

equipment for bring-up, test, and evaluation:

• Tektronix DPO3034 Digital Oscilloscope (or equivalent)

• Audio Precision AP2700 System Two Cascade or APx555B w/Switcher & AES-17 Filter

The standard set of industry performance and validation tests were run using this Test

Bench. These tests include, but were not limited to:

Performance Specification Testing

Power Output (200W into 8 ohms)

Power Output (300W into 4 ohms)

Efficiency

Performance Characterization Testing

THD+N vs. Power/Level

THD+N vs. Frequency

Frequency Response (8-ohm, 4-ohm)

Limited by Audio Precision AES-17 Brick-Wall Filter

Noise Floor (SNR)

Efficiency and Thermal Performance Testing

Amplifier Efficiency Measurements

SMPS Thermal Performance

Amplifier Robustness Testing

4-ohm, 8-ohm Performance

200W into 8 ohms

300W into 4 ohms

Current-Limit Functionality

Amplifier Sonic Quality Testing

Listening Test/Sonic Quality Assessment


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Performance Characterization ResultsThe system characterization was performed on the complete Class-D Amplifier/SMPS evaluation

platform combination. To follow are the results of both the initial Characterization and the

Thermal Performance measurements that were performed on the GaN FET Class-D Amplifier

platform. Unless otherwise noted, the Characterization was performed with the companion GaN

Systems GaN FET LLC SMPS that allow for the specification of 200W/8-ohms and 300W/4-ohms.

This requirement resulted in Power Supply Voltage rails of +/-32VDC. The companion GaN

Systems LLC Power Supply provides up to 350W of clean power into 8 ohms. This Power Supply

was only altered in the Thermal Performance testing to allow for a more exact comparison with

other Class-D Amplifier Platforms.

This reference design accommodates both Open-Loop and Closed-Loop operation. Many Class D

amplifiers in the market utilize negative feedback from the PWM output back to the input of the

device. These Class D amplifiers typically use silicon MOSFETs which cause non-linearities in the

system. A closed-loop approach not only improves linearity, but also allows the system to have

power supply rejection. This contrasts with an open-loop amplifier, which inherently has minimal

(if any) supply rejection. Because the output waveform is sensed and fed back to the input of the

amplifier in a closed-loop topology, deviations in the supply rail are detected at the output and

corrected by the control loop. The advantages of a closed-loop design come at the price of

possible stability issues as is the case with all systems utilizing feedback.

GaN FETs have changed the paradigm. No longer do the assumptions above need to apply to

amplifier design. In the following figures, measurements with the system in both open-loop and

closed-loop are measured. From the THD+N vs. Power plot (Figure 1), it can be readily determined

that the low signal level THD performance for the Open-Loop amplifier exceeds that of the

Closed-Loop approach.

This is mainly due to the increase Noise contribution of the feedback and can easily be

understood by comparing this data to the Noise Floor performance illustrated below in Figure 3.

As the audio signal level is increased, and hence the output power increased, the benefit of the

Closed-Loop architecture is evident. However, the THD+N of the Open-Loop architecture

compares very favorably, mainly due to the excellent switch characteristics of the GaN Systems

transistors in the output stage.


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By using an Open-Loop architecture with the ability to tightly control the switching deadtime,

near Closed-Loop THD performance can be achieved. This is apparent in the THD+N vs. Frequency

figures below as well. The increase in THD+N with the Open-Loop architecture, and at the lower

frequencies is mainly due to the lack of Power Supply Rejection, and the contribution to the

system-level performance by the SMPS. However, as with the THD+N vs. Power measurements,

the Open-Loop architecture very quickly approaches the performance of the Closed-Loop

architecture in the upper-mid-range.

The THD vs. Level performance capture in Figure 1 indicates both Open-Loop and Closed-

Loop results for a 1kHz Sine Wave and an 8-ohm, non-inductive load.

Figure 1: THD+N vs. Power (Level) into 8 Ohms @ +/-32VDC


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Figure 2: THD+N vs. Frequency into 8 Ohms @ -9dBFS

Figure 2 illustrates the THD+N vs. Frequency at a measurement level of -9dBFS – once again into

an 8-ohm, non-inductive load. Both Open-Loop and Closed-Loop performances are captured

under the same SMPS measurement conditions.


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Figure 3: Noise Floor Relative to 0dBFS

Figure 3 provides a capture of the Noise Floor for both Open-Loop and Closed-Loop

configurations.


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Figure 4: Frequency Response into an 8-ohm Load @ -9dBFS

Figure 4 provides a capture of the Frequency Response of the GaN Systems Class-D Amplifier into

an 8-ohm load.

The slight peaking in the response is due to the alignment of the Output L/C Reconstruction

Filter. In this design, to allow the best trade-off between both 8-ohm and 4-ohm loads, the Output

Filter is optimized for a 6-ohm load. This optimization results in this small peak in the Frequency

Response for an 8-ohm load, and a corresponding mild roll-off into a 4-ohm load.


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Figure 5: Class-D Amplifier FFT With and Without Input Signal @ -60dBFS

To illustrate the content of the measured Noise contribution for the above plots, Figure 5 provides

a snapshot of the Fast Fourier Transform (FFT), both with and without Audio Input Signal.

Efficiency and Thermal PerformanceThe Efficiency and Thermal Performance characterization was performed on the complete

Class-D Amplifier/SMPS Demonstration platform combination and on the individual PCBAs to

assure the best capture of performance.

To capture correct and accurate efficiency measurements, it is first necessary to characterize the

‘idle’ current/power usage of the entire Amplifier, since most of this ‘idle’ dissipation is not

associated with the GaN FET Amplifier Power Stage, but is associated with the other peripheral

circuitry on the Amplifier PCBA. This ‘idle’ current requirement is usually reflected in a significant

reduction in efficiency as the output power level is reduced to the point where this ‘idle’

dissipation becomes an appreciable percentage of the overall dissipation. This resultant “Idle

Dissipation” was captured to assure an accurate measurement of amplifier efficiency.


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Figure 6: Closed-Loop Amplifier Efficiency vs. Output Power @ 8 Ohms

Figure 7: Open-Loop Amplifier Efficiency vs. Output Power @ 8 Ohms

In Figure 6 and Figure 7, the blue line is this “Idle Dissipation” which was subtracted from the

overall dissipation. This is the appropriate approach, as this “Idle Dissipation” does not factor into

the “Efficiency” of the GaN FET Power Stage. It also does not prove to be a significant contributor

to the overall efficiency at the power levels which would create the need for costly components

such as heat sinks and large copper mounting tabs. For comparison purposes, the red plot line of

Figure 6 and Figure 7 adds back the additional power loss that is characteristic of the total Class-D

Amplifier Test Platform, but does not include the peripheral dissipation associated with

components that are not a part of the Amplifier Platform.


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Hence, the blue curve in each plot indicates the efficiency of the Power Stage and should be used

when comparing the GaN FET Efficiency with other MOSFET or Integrated Power Device

solutions. The red curve in each plot indicates the efficiency of the GaN-based Class-D Amplifier

and should be used in comparison with other complete MOSFET Reference Designs (that are

measured equivalently) or Integrated Power Amplifiers (with equivalent Power Output

capability).

“Blue” Curve (Heat Sink consideration): With the 8-ohm Power Stage Efficiency, measured at 95%

to 96% efficient, this places the GaN FET Power Stage at 6% - 10% higher than a similar MOSFET

Class-D Power Stage of the same Power Output capability. For compact system solutions, this

increase in efficiency allows for the complete removal of any classical or historical heat sink

requirement. This elimination of the heat sink also reduces the potential contribution to radiated

EMI/EMC emissions. These two combined characteristics make the GaN FET Class-D Amplifier a

very desirable alternative to the incumbent MOSFET Class-D Amplifier architectures.

“Red” Curve (Power Supply consideration): With the 8-ohm Amplifier efficiency measured at 90%

to 96% efficient, this places the GaN FET Amplifier from 3% - 10% higher than a similar MOSFET

Class-D Amplifier of the same Power Output capability. An additional item of interest is that the

category of Integrated Power Amplifier could NOT play in the 8-ohm application that is defined

by this performance.

Since the companion SMPS is also based on GaN Systems devices, it is also desirable to consider

the increase efficiency and resulting cost reduction of this application. One of the driving factors

in this particular design is the desire for reduced size and complexity, even at the potential

sacrifice of some amount of efficiency. Other topologies are available for both the PFC

implementation and the LLC implementation. However, for this Class-D Amplifier SMPS targeted

performance, the additional complexity was not warranted.

A simple, single GaN FET design was used for the Front-End PFC, and a simple, dual-GaN FET

Half-Bridge was used for the Back-End LLC. This allows for a very low cost, 3-FET design that does

not need any massive external heat sink for full-power operation. As an illustration of the

Operating Temperature of the SMPS under the Load conditions, the following Thermal Data was

captured.

Figures 8 through 11 capture the thermal performance of the GaN Systems SMPS design under

varying amplifier load conditions, all continuous sine wave driven. These figures show the

temperature of the PFC Inductor (IndPFC), PFC GaN FET (FETPFC) and LLC Half-Bridge GaN FET

(FETLLC).


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Figure 8: Thermal measurements, no amplifier input

Figure 9: Thermal measurement, 2 Ch at -0dBFS

Figure 10 Thermal measurement at 1/4th power Figure 11: Thermal measurement at 1/2 power

Figure 8 shows the thermal measurements with no input to the amplifier. The thermal

measurements in Figure 9 are with both amplifier channels driven at -9dBFS, sine wave into an

8-ohm load. This 1/8th power load is typical for a Consumer-like content material load.

Figure 10 and Figure 11 represent the range of power which more closely represent a Professional

or Commercial continuous content material loading. Figure 10 is the low end of the range at 1/4th

power. Figure 11 represents a very heavily loaded SMPS, with both channels of the amplifier being

driven at -3dBFS, sine wave. Under these worst-case conditions, the temperature does not rise

above 100°C, well below the maximum safe operating condition. All of the thermal performance

was measured without any heat sinking on the devices. The thermal management of the GaN

FETs was provided entirely by the PCB copper traces and planes.


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Benefits of GaN-based Class-D Audio HardwareWhen it comes to implementing the best sounding, most efficient Class-D audio hardware, the

most compelling transistor switching platform choice is GaN Systems power transistors. In this

audio reference design evaluation kit from GaN Systems, the system features:

AUDIO AMPLIFIER

The GaN Systems design with GaN FETs provides the right combination of performance, size, and

power consumption for Class-D audio applications. With the selected design topology and

resulting thermal and sonic performance, the design is scalable in many ways. Derivative designs

include: increasing power by using a heatsink on the GaN Systems top-side cooled devices or

tying the two outputs together, or reducing power by going to half bridge without BTL or a single

rail design.

Complete Class-D Amplifier

• Multi Audio Signal Inputs

• Bridge-Tied Load Output

• On-Board Power Management

High-Performance Audio

• 200W/Ch (8Ω), 300W/Ch (4Ω)

• > 108dB SNR & Dynamic Range

• < 0.01% THD+N (8Ω, 1W)

• +/- 0.5dB Freq. Response (8Ω)

High-Performance Output

• 96% Efficiency

• Reduced thermals

Complete Audio LLC SMPS

• Universal AC Line Input

• Easy Integration w/GaN Amplifier

• Common-Mode AC Filter

High-Performance Audio

• 400W Continuous, 550W Peak

• +/-32VDC Dual-Rail Output

High-Performance Output

• No heatsinking, no cooling

• High efficiency, 40% loss decrease

• EMI/EMC friendly

LEARN MORE AT GANSYSTEMS.COM

SummaryThe measurements indicate that the performance

of this reference design is very good and system

designers now have a choice between Open-Loop and

Closed-Loop designs. The use of good Open-Loop topologies

allows for the elimination of complexities associated with a

Closed-Loop design counterpart. Additionally, the Open-Loop topology

allows for both “static” and “real-time” optimization of audio performance

vs. EMI/EMC profile vs. thermal performance. Lastly, for the Open-Loop design,

the Power Supply becomes an even more important component in the overall

audio system solution and the companion SMPS in this evaluation kit is a good pairing

with the amplifier.

Audio quality has become an important part of nearly every interaction people

have today. Ranging from your phone to your car to your home, everyone

wants good sound quality and can have it. No longer is good sound only

affordable for the audiophiles, expensive cars, or elaborate home sound

systems. GaN devices provide the performance, size, and cost benefits

to allow everyone to enjoy Class-D audio sound.

Visit Our Website:

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Contact Us >



https://gansystems.com/design-center/evaluation-boards/

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