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APPLICATION SPECIFIC POWER SUPPLIES CLASS 'D' AUDIO POWER AMPLIFIERS Power requirements vary considerably dependent on the application hence the need for application specific power supplies to optimise the system performance. Under-designed power supplies result in the amplifier not meeting performance specifications. Over-designed power supplies increase product cost. 1. POWER AMPLIFIER DESIGN CONSIDERATIONS The choice of power supply for any given application depends on a number of factors including input voltage range (global input) line voltage regulation output voltage(s) voltage tolerance output voltage regulation ripple and noise number of outputs average and peak output power peak output current efficiency standby power consumption size and weight EMI and regulatory requirements safety and other regulatory requirements cost 1.1 POWER SUPPLY OUTPUT VOLTAGE AND CURRENT Factors that affect the required power supply output voltage include amplifier output power; rated amplifier power required at minimum supply output voltage load (speaker) impedance amplifier output configuration; single-ended or BTL (bridge-tied load) output amplifier maximum achievable duty cycle parasitic output path resistance under-voltage lockout; to avoid poor performance at too low an input voltage output voltage 'pumping' BUS PUMPING In Half-Bridge (single ended) Class D switching stages energy transfer between the power supply output and the load (speaker) is bi-directional resulting in an interval of energy that was stored in the amplifier's low pass filter inductor being returned to the power supply. The power supply voltage buses of halfbridge circuits exhibit voltage fluctuations that exceed the nominal values resulting in the terminology 'bus pumping'. Bus voltage fluctuation creates distortion due to the gain of a Class D amplifier stage being directly proportional to the bus voltage. Large decoupling capacitors at the amplifier's DC voltage input limit the transient dV/dt. Bus pumping does not occur in Full-Bridge circuits because inductor current flowing into one of the halfbridges flows out of the other creating a local current loop that minimally disturbs the power supply (energy returned back from one side is used in the other side and not returned to the power supply). EET423 POWER ELECTRONICS Class D power supply 1 Prof R T Kennedy 2008-2009 AMPLIFIER OUTPUT POWER The maximum power that an amplifier can deliver to the load without clipping: HALF-BRIDGE FULL BRIDGE PHB , av PFB , av ( Dsw, max VCC ) 2 Rload 8 RT 2 ( Dsw, max VCC ) 2 Rload 2 RT 2 PFB , av 4 4 PHB , av RT : sum of all of the DC resistances in series with the load: RT = Rload + Rdson + Rind + Rpcb + Rps,out Rload : loudspeaker resistance: Rdson : mosfet on -state resistance: HB Rdson FB 2 Rdson Rind : filter inductor DC resistance Rpcb : board traces, connectors, and wires Rps,out : power supply output impedance (use the component's resistance at its maximum operating temperature). Dsw,max: amplifier maximum output duty cycle. (Dsw is also referred to as the modulation index (M); MMAX is the maximum modulation factor) Vload, pk pkFB 2 Vload, pk pkHB for the same power supply voltages and load impedance. Amplifier peak output power occurs at the loudspeaker's peak voltage or peak current PHB , pk HALF BRIDGE FULL BRIDGE PFBpk I out 2 Rload 2 I out Rload Vout 2 Rload (2 RT ) 2 Vout 2 Rload RT 2 Vout 2 Rload 4 RT 2 2 PHB 2 PHF Vout : amplifier output voltage (loudspeaker voltage) Iout : amplifier output current PEAK versus AVERAGE OUTPUT POWER Regulatory agencies (e.g. U.S. Federal Trade Commission:(FTC) require manufacturers of audio products to specify the average output power rather than the peak power for consumer audio products. 2 Pav,sine 2 V pk Ppk Vrms R 2R 2 undistorted sinewave Amplifier manufacturers sometimes advertise a higher output power at a particular THD level. Pav (THD 10%) 1.28 Pav (THD 1%) EET423 POWER ELECTRONICS Class D power supply 2 Prof R T Kennedy 2008-2009 POWER SUPPLY OUTPUT POWER REQUIREMENTS Power supply design depends on the target market. Amplifiers for professional recording studios or laboratory applications may need to provide full output power on a continuous basis; an expensive requirement only to be used when needed! Consumers tend to operate amplifiers around 40% continuous power rating; with the outputs only approaching full power for short periods of time during music peaks. The difference between maximum available power and typical usage is the basis for the power rating requirements of regulatory agencies. Regulatory agencies specify a pre-conditioning warm-up period (0 1 hr) at 1/8 the continuous power rating followed by a short period of time (5 10 min) at the continuous rated power output as a sufficient test of an amplifier's capability resulting in less expensive amplifier and power supply designs. 1/8 power is regarded as a fair indicator of the average music content of a typical CD however; rock and heavy metal CDs range around 20% continuous power rating classical CDs vary widely from very little power up to higher levels than the rock CDs The full-power operational time depends on the amplifier and its power supply heat dissipation capability. Higher efficiency amplifiers and power supplies and good thermal management are essential for long periods at continuous rated output power. note: (i) Each channel is tested individually, while all other channels run at 1/8 power. (ii) All channels in the same frequency range tested at full power POWER SUPPLY OUTPUT VOLTAGE The required power supply voltage is determined at the lower limit of the power supply's output voltage tolerance based on the amplifier's rated output power with unclipped output voltage. HALF BRIDGE FULL BRIDGE VCCHB , min VCCHB , nom VCCFB, min VCCFB, nom EET423 POWER ELECTRONICS Class D power supply 8PHB ( RT ) 2 ( Dsw max ) 2 Rload 2 RT Dsw max 2 PHB Rload RT 2 PFB Rload VCCHB , min 1 tolerance 2 PFB ( RT ) 2 ( Dsw max ) 2 Rload Dsw max VCCFB, min 1 tolerance 3 Prof R T Kennedy 2008-2009 EFFICIENCY PPSout Pout max ( amp) max ( amp) High efficiency requirements come at a cost; high efficiency switching power supplies use low onresistance mosfets and synchronous rectification. Consumer products tend to keep the power supply on, even when the product is supposedly 'off ' in order to respond to remote controls and use low-voltage front panel power controls. 'Standby' and 'off ' state power consumption accounts for a large proportion of the consumption of natural resources and energy and have a number of other important environmental impacts Maximum levels (2 W 0.5 W) are now an important regulatory issue and are covered by the Energy-using Products (EuPs) Directive. CURRENT LIMIT A current limit is set to avoid the power supply limiting the output current during normal operation. A current limit threshold lower than the maximum required output current results in increased distortion due to a clipped amplifier output. I limit ,min Dsw,max VCC ,min RT REGULATED versus UNREGULATED POWER SUPPLY Once the power supply voltage and current requirements have been calculated the designer needs to decide whether to use a regulated or unregulated power supply. Power supply output voltage ripple can produce amplifier output distortion and audible hum; especially if the amplifier is an open loop (no output feedback) output stage design. The amplifier design therefore affects the power supply type with regulated supplies recommended for open-loop amplifiers. UNREGULATED POWER SUPPLIES Unregulated power supplies can be less expensive for amplifiers with low power requirements but the cost (and the size and weight) of the transformer will increase as power requirements go up Unregulated power supply line voltage and output load changes produce ±20% (or more) output voltage variations resulting in the VPSout,max 1.5 VPSout,min. Higher cost higher voltage components are required thereby reducing some of the savings gained by not using a regulated power supply. REGULATED POWER SUPPLIES LINEAR REGULATORS Linear regulators are NOT recommended for input line regulation due to their low efficiency and higher cost of line transformers and thermal management. Linear regulators are used as post regulators for auxiliary low-voltage outputs (±12V, 5V, 3.3V ). EET423 POWER ELECTRONICS Class D power supply 4 Prof R T Kennedy 2008-2009 SWITCHING REGULATORS Line and load regulation of a switching power supply depends on the type of feedback Secondary-side feedback (regulator + opto-isolator) provides the best output regulation as it senses the power supply output voltage(s) directly and provides feedback to a primary side regulator. Primary-side feedback using a transformer primary side winding to regulate the output voltage, rather than sensing the output voltage directly, is a lower cost poorer performance approach. Power supply design should take care to ensure that variations in the amplifier's power supply voltage due to load variations on the auxiliary outputs do not create audible amplifier noise. POWER SUPPLY COMPONENT REQUIREMENTS Switching power supply magnetic components (transformers and inductors) require a peak load current capability to avoid saturation. Low ESR power supply output capacitors are required to minimize losses due to the high-frequency ripple current. Low capacitor ESR also provides a lower source impedance (looking back from the amplifier) and affects the amplifier's overall damping and efficiency. OUTPUT PROTECTION Regulatory agency's safety requirements require output protection to prevent damage, overheating and fire due to short circuits, internal component failures or other abnormal conditions. Switching power supplies usually have inherent current limiting and current shut down pwm controllers whereas unregulated power supplies tend to rely on fuses or circuit breakers. EET423 POWER ELECTRONICS Class D power supply 5 Prof R T Kennedy 2008-2009 DESIGN EXAMPLE: SINGLE-CHANNEL FULL- BRIDGE AMPLIFIER PFB,max amplifier maximum output power 20 W ηmax amplifier maximum efficiency 90% Dswmax amplifier maximum duty cycle 0.8 Rload amplifier loudspeaker impedance 8Ω RT total output resistance 8.2 Ω power supply output voltage tolerance ± 10 % 2 PFB ( RT ) 2 ( Dsw max ) 2 Rload 2 PFB Rload VCC min 8.2 2 20 0.8 8 22.919 V VCCnom VCC min 1 tolerance 22.919 25.466 V 1 0.1 Dsw max VCCnom RT 0.8 25.466 8.2 I limit min PPSout Pout max (amp ) max ( amp ) EET423 POWER ELECTRONICS Class D power supply RT VCC min ( Dsw max 2.485 A 20 22.22 W 0.9 6 Prof R T Kennedy 2008-2009 load voltage, load current, peak power (instantaneous power), power. All the power delivered to the load comes from the power supply and from the decoupling capacitors. Note that the frequency of the instantaneous power delivered to the load is twice the frequency of the audio input signal. The frequency of power supply output current is also twice that of the audio signal . Instantaneous Power and PSU current EET423 POWER ELECTRONICS Class D power supply 7 Prof R T Kennedy 2008-2009 Single-Ended Outputs Many designs use amplifiers with single-ended outputs because they only require half as many transistors as a full-bridge output, and integrated amplifiers with single-ended outputs only require one output pin instead of two. Single-ended amplifiers also have a few disadvantages compared to amplifiers with bridge-tied load (BTL) outputs. Single-ended amplifiers require either split positive and negative power supplies or DC blocking capacitors. If DC blocking caps are used they need to be large in order to prevent them from affecting the low-frequency performance of the amplifier. DC blocking caps can also cause audible pops as they charge up to Vcc/2 when the amplifier is turned on. A resistor divider from Vcc to ground can be used to charge the capacitor up to Vcc/2 at a relatively slow rate when the power is turned on, minimizing or eliminating the pop. BTL Outputs Amplifiers with BTL outputs are popular because they do not require DC blocking caps even when operating with a single positive power supply. DC blocking caps limit the low-frequency response of the amplifier and can be quite large. BTL amplifiers have another advantage over amplifiers with single-ended outputs - the maximum peakto-peak voltage that a amplifier with BTL outputs can apply to the speaker is twice the power supply voltage, which in turn means that up to four times as much output power can be delivered to the load compared to a single-ended amplifier. This can be a big advantage in applications where the power supply voltage is limited, especially in portable applications where the amplifier is operating off of a battery. . EET423 POWER ELECTRONICS Class D power supply 8 Prof R T Kennedy 2008-2009