Designing Practical High Performance Class D Audio Amplifier www.irf.com Contents Chapter 1 • Class D Amplifier Introduction Theory of class D operation, Points of design Chapter 2 • The latest Digital Audio MOSFET, DirectFET® MOSFET Importance of layout and packaging Optimized MOSFET for no-heat sinking Chapter 3 • Designing Dead-time and Overload Protection with Digital Audio Gate Driver IC Designing with built-in dead-time generation How to design OCP. Tj estimation. Chapter 4 • Design Example No-heat sink 100W x 6ch compact Class D amplifier www.irf.com Chapter 1 Class D Audio Overview www.irf.com Review: Traditional Class AB Amplifier • Class AB amplifier uses linear regulating transistors to modulate output voltage. Feed back ＋Vcc ＋ － Error amp • A loss in the regulating transistor in Class AB amplifier is proportional to the product of the voltage across the device and the current flowing through it. Bias ＋ －Vcc η = 30% at temp rise test condition. PC = VCE ⋅ I C Î Independent of device parameter www.irf.com Class D Amplifier Feed back Triangle +VCC Nch Level Shift ＋ － COMP Dead Time Nch Error Amp ＋ -VCC •Class D amplifier employs MOSFETs which are either ON or OFF state. Therefore ideally 100% efficiency can be achieved. •PWM technique is used to express analog audio signals with ON or OFF states in output devices. •A loss in the switching device caused by 1)finite transition speed, 2)ON state resistance and 3)gate charge. P = Psw + Pcond + Pgd TOTAL Î dependent of device parameter Î can be improved further! www.irf.com Basic PWM Operation The output signal of comparator goes high when the sine wave is higher than the sawtooth. COMP Class D switching stage LPF Using fPWM=400kHz to modulate 25kHz sinusoidal waveform www.irf.com Major Causes of Imperfection Perturbation Zo Bus Pumping Pulse width error Quantization error Audio source +VCC Gate Driver PWM -VCC Dead time Delay time An ideal Class D amplifying stage has no distortion and no noise generation in the audible band, along with providing 100% efficiency. However, as shown, practical Class D amplifiers have imperfections that generate distortions and noise. Nonlinear inductance / Capacitance DC Resistance Finite Rds(on) Vth and Qg Body diode recovery Stray inductances RDS(ON) ON delay OFF delay 0.01% of non-linearity corresponds to 10mV out of 100V DC bus Finite dV/dt or 0.25ns out of 400kHz. www.irf.com Three Difficulties in Class D Design • PCB Layout • Dead-time Generation Direct-FET, Half-bridge MOSFET can eliminate influences from stray inductances. Integrated Gate Driver IC can make things easier • Overload Protection www.irf.com Chapter 2 DIGITAL AUDIO MOSFET The right power switch for Class-D audio amplifiers www.irf.com Digital Audio MOSFET introduction • Digital Audio MOSFET is specifically designed for Class-D audio amplifier applications • Key parameters such as RDS(on), Qg, and Qrr are optimised for maximizing efficiency, THD and EMI amplifier performance • Low internal RG distribution guaranteed for better dead time control • New and innovative packages offer greater flexibility and performance • These features make IR Digital Audio MOSFETs the right power switches for Class-D audio amplifiers!! www.irf.com IRF6665 DirectFET® The best MOSFET for Mid-Power Class-D amplifier applications www.irf.com IRF6665 Digital Audio MOSFET • IRF6665 Digital Audio MOSFET combines the latest IR medium voltage trench silicon with the advanced DirectFET® package • Key parameters, such as RDS(on), Qg, Qsw, and Qrr are optimized for mid-power Class-D audio amplifier applications • IRF6665, has all the characteristics to be the best power switch for mid-power amplifiers!! www.irf.com DirectFET® device technology Multiple gold wirebonds SO-8 copper drain leads DirectFET® passivated die die attach copper ‘drain’ material clip gate connection copper source leads • Drain/source leads and wirebonds contribute to both package resistance and inductance • Majority of heat transferred through leads to PCB board copper track on board source connection • Remove wirebonds from package and replace with large area solder contacts • Reduced package inductance and resistance • Copper can enables dual sided cooling www.irf.com ® DirectFET : low inductance package for audio • Lower inductance at frequency than SO-8, D-Pak, MLP and D-Pak • TO-220 inductance package is ~ 12nH www.irf.com Advantages of DirectFET®: Reduce ringing • • • Inductance related ringing reduced compared to SO-8 Example below for DirectFET® and SO-8 switching 30A at 500kHz Silicon of the near identical active area, voltage and generation used in both packages DirectFET® waveform SO-8 waveform www.irf.com IRF6665 for class-D audio applications Key Parameters Parameter Min Typ V(BR)DSS 100 RDS(ON) @ VGS = 10V 53 Qg 8.4 Qgd 2.8 Qsw 3.4 RG (int) 1.9 VGS(TH) 3 - • Max 62 13.0 2.9 5 Units V mOhms nC nC nC Ohms V Refer to IRF6665 datasheet for further details www.irf.com IRF6665 DirectFET® Evaluation Board Spec: Power Supply ±35.0V Output Power 150W+150W, 4Ω MOSFET Gate Driver IRF6665 IR2011S DirectFET® IRF6665 IR2011S www.irf.com Efficiency Data Test Conditions: Half-Bridge Configuration, Vbus = +/- 35V, fswitching = 395kHz, finput = 1kHz, Rload = 4 and 8 Ohms Rload = 8Ω Rload = 4Ω Rload (Ω) Efficiency @ 1%THD 4 94.8 8 96.0 www.irf.com THD+N Data Test Conditions: Half-Bridge Configuration, Vbus = +/- 35V, fswitching = 395KHz, finput = 1KHz, Rload = 4 and 8 Ohms Rload (Ω) THD + N @ 1/8 Pout 4 0.0057 8 0.0031 Rload = 8Ω Rload = 4Ω www.irf.com • • VDS Switching Waveforms DirectFET® package shows cleanest and fastest (approx. three times faster) switching waveforms than amplifier with TO-220 package. Same IRF6665 MOSFET die is tested in both packages DirectFET® Package Blue : VGS Pink : VDS TO-220 package www.irf.com EMI Data @ 1/8 Pout Condition (12.5W) • • • • • DirectFET® and TO-220 with the same IRF6665 silicon die MosFET devices with no heatsink No shielded room Over 2MHz, DirectFET® amplifier shows approximately 9dB lower noise than TO-220 amplifier Under 2MHz, background noise is dominant DirectFET® Frequency (MHz) TO-220 Frequency (MHz) CISPR13 CISPR13 Quasi-Peak Limits Quasi-Peak Limits Average Limits Average Limits www.irf.com Thermal Performance No Heatsink Typical Case Scenario After 10 minutes IRF6665 case temperature = 80.1°C @ 100W/8Ω without heatsink (ΔTc = 55.1°C) 120 C a s e T e m p e ra tu re (°C ) Test Conditions: 100W/8Ω, 1% THD, +/- 45Vbus, fsw=400KHz, TAMBIENT ~ 25°C Worst Case Scenario Amplifier specs: 100W/8Ω 100 80 full power Estimated Plosses=1.6W 60 40 20 1/8 power Estimated Plosses=0.6W 0 T AMBIENT ~ 20 °C 0 100 200 300 400 500 Time (s) Full Power: TC=104°C @ 5min (ΔTC=83°C) 1/8 Power: TC=58°C @ 5min (ΔTC=39°C) www.irf.com Assembling IRF6665 in audio Class-D circuits • • • Stencil on solder paste Pick and place devices onto pads Re-flow devices If additional heatsink is needed for higher power, • • • • • Place thermal interface material over devices Place heatsink over device/thermal interface stack Secure heatsink in place with screws PCM burned in to wet out interface between can and heatsink Screw torques reset when assembly has cooled www.irf.com Thermal Performance with Heatsink • Individual DirectFET® MOSFET audio reference boards assembled with 3 different phase change materials • Heatsink applied to assembly – Fischer SK04, 3.8”X0.6”, 0.6” extrusion, black anodised, 3°CW-1 • Constant power applied to device junctions to simulate 100W amplifier operation: – Normal operation conditions (1/8 full output power) into 4Ω and 8Ω – Full output power into 4Ω and 8Ω • Case temperature was monitored before and during application of power to the junctions with thermocouples applied between can and heatsink www.irf.com ΔTCASE versus time 100 100W into 4Ω Power/device* = 2.4W Temperature difference (°C) 90 80 70 100W into 8Ω Power/device* = 1.6W 60 50 40 30 12.5W into 4Ω & 8Ω (1/8 of 100W) Power/device* = 0.6W 20 No significant difference between the PCM materials used 10 0 0 100 200 300 400 500 Time (s) Amplifier Conditions Plosses* per device Temperature rise (°C) after 5 min Material A 0.6 22.1 - Material C 12.5W (1/8) into 4 & 8Ω - Material B 24.8 23.4 100W into 8Ω 1.6 52.7 58.2 55.1 2.4 77.1 100W into 4Ω (*) Estimated Plosses @ worst case scenario 82.8 76.4 - www.irf.com Half-Bridge Full-Pak Another Innovative Package for Class D Audio Amplifier Application www.irf.com Half-Bridge Full-Pak D1 G1 D2/S1 G2 S2 ⇒ TO-220 Full-Pak 5 PIN • 55V, 100V, 150V and 200V devices to be released on Q3 ‘06 55V: IRFI4024H-117 100V: IRFI4212H-117 150V: IRFI4019H-117 200V: IRFI4020H-117 www.irf.com Half-Bridge Full-Pak Features • • • • • Integrated Half-Bridge Package Reduces the part count by half Reduced package inductance improves EMI performance Facilitates better PCB layout Enables single layer PCB layout in combination with IR2011S • Low RG(int) distribution for better dead time control • Lead-Free package Reduce devices number, stray inductance and facilitates layout and assembly www.irf.com IRFI4024H-117 + IR2011S Disturbance Power Half-Bridge Full-Pak vs. TO-220 (single die) • Half-Bridge Full-Pak amplifier shows better performance than TO-220 amplifier. Approximately 10dB lower disturbance power level. 180VAC @ 50Hz, Speaker Output Half-Bridge Full-Pak TO-220 Same MOSFET silicon die in both packages www.irf.com Summary • DirectFET® devices are ideal candidates for use in ClassD audio amplifier applications • Evaluations of IRF6665 in Class-D audio amplifier demonstrated improved efficiency, THD, and EMI • Utilising DirectFET® technology reduces EMI compared to TO-220 packages • Thermal evaluations demonstrated that IRF6665 can deliver up to 100W per channel into 8Ω with no heatsink • Half-Bridge Full-Pak features make this device an excellent option for Class-D audio amplifier applications — Integrated half-bridge package reduces the part count by half — Low package inductance improves EMI performance • Digital Audio MOSFET is the right switch for Class-D audio amplifiers!! www.irf.com Conclusion 1 • With IR’s DirectFET® technology, Class D amplifier reaches the point where 100W amplifier can be built without heat sink • Optimum package provides the best audio performance along with minimum EMI emissions • Optimum silicon design provides the best efficiency over 95% www.irf.com Chapter 3 DIGITAL AUDIO Gate Driver www.irf.com Protected DIGITAL AUDIO Gate Driver IC IRS20124 •Programmable Discrete Deadtime (PAT.Pending) •Programmable Bi-directional Over Current Sensing (PAT.Pending) VSUPPLY 200V max IO +/-1.0A / 1.2A Selectable Dead-time 15/25/35/45nS Prop Delay time 70ns •200V high voltage ratings to deliver up to 1000W output power in Class D audio amplifier applications •Simplifies design due to integrated deadtime generation and bi-directional over current sensing •Optimized and compensated preset deadtime selections for improved THD performances over temperature and noise •Shutdown function to protect devices from overloaded conditions 14pin SOIC •Operates up to 1MHz •3.3V/5V logic compatible input www.irf.com VB OCSET1 High Voltage Level Shifter OCSET2 UVLO UV S Q R HO OC Current Sensing DT/SD VS High Side Floating Well Vcc Comparator UVLO IN Dead-time Gen Shutdown Logic Matching Delay LO COM IRS20124 www.irf.com Discrete Dead-time IRS20124 The discrete dead-time method sets a dead-time by selecting one of the preset values from outside of the IC. Comparing with previous program method, the discrete dead-time can provide a precise dead-time insertion, regardless of noise injection in DT pin. Thus, the dead-time setting value can be set tighter, which is highly beneficial for the THD performance in Class D applications. >0.5mA Vcc R1 DT/SD R2 COM Operation Mode 15nS 25nS 35nS 45nS Shutdown 0.23xVcc 0.36xVcc 0.57xVcc 0.89xVcc Vcc Noise injection VDT www.irf.com IRS20124 THD+N Performance VCC: ±35.0V Gate Driver: IRS20124 MOSFET: IRFB4212 fPWM = 400kHz Note that low THD+N characteristic shows quiet noise floor due to clean and stable switching timings. Dead-time Settings: DT1 = 15ns DT2 = 25ns DT3 = 35ns DT4 = 45ns www.irf.com Overload Protection in Class AB B+ Ov erload X1 Equivalent circuit TIP3055 Q1 2.2k 220m R1 220m R2 TIP2955 Q2 220m R6 Q2N3904 Q3 R3 10k R4 RLOAD D1 D1N4148 1K R5 Overload 2.2k R7 Q2N3904 Q4 D2 D1N4148 RLOAD 8 R8 10k R9 B- When a Class AB amplifier has a shorted load, the load current can ramp up rapidly. The voltage across the device is fixed to the bus voltage. ÎThe loss in the device is enormous amount Î The power devices can not be protected with over current detection method Therefore an impedance bridge has been commonly used, which has following drawback; • Reactive components in the load impedance, which is common in realistic loudspeakers, causes false protection www.irf.com Overload Protection in Class D Load Over Current Detection When a Class D amplifier has a shorted load, there still is a LPF inductor in between the load and the amplifier. Therefore, the load current ramps up at a rate of Vo/L. The loss in the device is determined by the RDS(ON) and the load current. ÎOver current detection works very good in Class D Benefits from over current detection; • Trip level is independent of phase shift in the load current • The amplifier can sustain any instantaneous low load impedance until the load current reaches the trip level www.irf.com Load Load Load Why Bi-Directional CS? Inductor Current Load current flows through the low-side MOSFET unless the high duty cycle reaches 100%, where no conduction period exists in the low-side MOSFET. Since 100% duty cycle is not allowed due to highside bootstrap power supply operation, the amount of current sensed from the low side MOSFET covers full cycle of an audio signal. High Side ON Low Side ON www.irf.com Benefits of Bi-Directional CS Conventional Current Sensing More than just cost savings… Bi-directional current sensing provides following technical benefits. 100n C2 50 V2 IRF530 Q1 22u L1 Q4 IRF530 •Minimum stray inductance in power stage current path due to no additional current sensing components in the path •No influences in measured current from gate charge current and reverse recovery charge current •Positive Temp/Co in RDS(ON) reduces the trip level at high junction temperature in real time Shunt Resistors With IRS20124 5 V1 470n C1 W=1u L=1u Q3 W=1u L=1u Q2 8 R2 50 V3 20m R1 L2 50n Shunt Resistor 100n C3 •Adding a shunt resistor causes ringing by adding stray inductances. •The current includes gate charge/discharge current and reverse recovery charge current. •Imbalance of effective RDS(ON) in high and low sides causes distortion. www.irf.com What happens in the Short Circuit Event? 5 V1 Vo V di = O dt L L Zo 100m 18u R1 L1 C 0.47u C1 RL 8 R2 Short VO ZO Class D amplifier ITRIP 5 V1 100m 18u R1 L1 IL 0 With Shutdown IL Shutdown Short Circuit Now the Class D amplifier is driving an inductor in the LPF. Note that the audio frequency components induce quite large volt-second feeding into the inductor, causing excessive inductor current in the event of short circuit load. When short circuit occurs, the load current starts to ramp up quickly with a gradual rate of Vo/L. Since an inductor is in between the amplifier and the load, short current may not exceed the trip level. Without Shutdown Inductor discharge A junction temperature at the end of the protection event can be estimated by using a thermal transient model in IR’s Digital Audio MOSFET to ensure the functionality. After the shutdown, the energy stored in the inductor discharges to the power supply, only the waveform of the current contributes the loss in the MOSFET. www.irf.com How to Design with OC Function IRS20124 IRS20124 PWM input Gate Driver PWM input Gate Driver SD Vs Sensing O C SD Ext. Latch OC Vs Sensing With the OC pin connected to DT/SD pin, the output signal out of OC at the event of over current will be removed by its output itself when the IRS20124 goes into shutdown mode. This could cause a hiccup in the protection sequence. One simple way to assure shutdown upon an over current detection is to attach a latch circuitry onto the OC pin, as shown. www.irf.com IRS20124 OC Functionality In Negative Half Cycle In Positive Half Cycle 1kHz, 20W OC pin Inductor discharge Switching node Inductor current The bi-directional current sensing can capture over current conditions at either positive or negative current direction. In this demonstration, the threshold for Vs is set to be ±1V by setting OCSET1=1V and OCSET2=3V, which can be translated into 15A trip level with 70mΩ RDS(ON) . Short circuit occurred Shutdown by OC www.irf.com Load IRS20124 OC Functionality - Vs Waveform Vs These are close up shots of overload protection with magnified waveform of Vs. At the instance voltage at Vs reaches trip level, which is ±1V in this setting, OC pin shuts down the switching and the MOSFET is protected. Red: Vs node, 1V/div Red: Vs node, 1V/div Green: OC pin w/10kΩ pull-up, 5V/div Green: OC pin w/10kΩ pull-up, 5V/div 10µS/div 10µS/div www.irf.com Tj Estimation in Short Circuit Event All the DIGITAL AUDIO MOSFET have thermal equivalent circuit on the datasheet for transient thermal analysis. The junction temperature of the MOSFET at the end of the over current protection event can be estimated using this model along with the waveform from the bench evaluations. Transient Thermal Impedance (Excerpt from IRF6665 Datasheet) current V(n1)*V(n2) ARB2 N1 OUT N2 Duty V3 0.85 V1 V(n1)*(V(n2)*0.007692+0.808) ARB1 Shutdown N1 OUT N2 Duty Rds 0.051 V2 V(n1)*V(n2) ARB3 N1 OUT N2 Rds Vs LO Vds N2 N1 Id(av e) dI/dt Peak current IL Tj Tj Duration Pd 667.6m 1.0462 1.56111 29.2822 25.455 R1 R2 R3 R6 R7 98.9u IC=25 0.856m IC=25 C1 C2 Commuted to the other side ARB4 (-1)*V(n1)*V(n1)*V(n2) 2.81m IC=25 23.4m IC=25 1.257 IC=25 C3 C5 C6 An Example of Tj Simulation using a Circuit Simulator www.irf.com Tj Estimation Simulation Result 3 2.5 2 TJ=50ºC V ID=43A 45 VDS 1.5 1 0.5 40 0 35 Tj -0.5 0 20 40 60 80 Time/µSecs 25 100 20µSecs/div 20 64mΩ 64 ID 15 62 10 60 5 58 0 10 Time/µSecs 20 30 40 50 100uS 60 70 80 90 10µSecs/div mV V 30 RDS(ON) 56 54 52 50 51mΩ 48 0 Time/µSecs 20 40 60 80 100 20µSecs/div www.irf.com Chapter 4 Design Example www.irf.com 100W x 6 Channels Design Specs: Supply Voltage: ±35V Output Power: 6ch x 100W into 8 ohm Protections: OCP, DC, OTP, OVP IR devices: IRS20124, IRF6665 Dimensions: 295mm x 95mm x 40mm(H) CH-2 CH-3 100W+100W MODULE CH-4 CH-5 100W+100W MODULE CH-6 100W+100W MODULE POWER SUPPLY PROTECTION CH-1 Single layer board www.irf.com Design: 100W+100W Module IRS20124 • • • • IR’s latest technology allows continuous 100W+100W audio outputs with no heat sink attached. All the critical current paths are included in the module so that a single layer PC board can be used. All the tricky functions, such as deadtime generation and over current protection, are included inside the module. Over temperature protection Digital Audio Direct-FET IRF6665 and Digital Audio gate driver IRS20124 placed back to back are a perfect combination to obtain minimal stray inductances. 65mm 30mm • IRF6665 Direct-FET www.irf.com Design Test Results Switching Waveforms (10nS/div, 20V/div) THD+N Performance, ±35.0V, 4 ohm load 120W @THD=1% 170W @THD=10% fSW=430kHz THD=0.01%@50W Noise=62uVrms (IHF-A) S/N=110dB www.irf.com Conclusion 2 By using IR’s Digital Audio Gate Drivers and MOSFETs, • Class D audio amplifier design is no longer a do-it-by-feel trial and error process. • Class D amplifier is now entering a new age of do-it-yourself design with superior efficiency, performance and ruggedness. Visit IR’s Audio Website for more information: http://www.irf.com/product-info/audio/ www.irf.com