Electronic Design

High-Power Class D Power Stage

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What is a Class D amplifier?
A Class D amplifier converts an audio signal to pulse-width modulation (PWM). Its power stage efficiently amplifies the PWM signal and filters it to drive the speakers. The Class D designation means that the power stage biases and operates its output devices as switches.

How do half-bridge and full-bridge Class D amplifiers differ?
A half-bridge power stage uses a pair of switches in a totem-pole arrangement (see the figure). The output filter and load refer to a midpoint voltage. You can build a full bridge from a pair of half bridges, connecting the load and filter between them. The two half bridges operate in antiphase, which, under the same supply conditions, doubles the supply-voltage utilization, quadrupling the potential power output.

Why is the power-stage design important to Class D amplifier performance?
Advances in Class D controllers have resulted in sonic performance that surpasses Class AB. Although the power stage seems simple compared to the controller, it must equal the controller's sonic performance and do so at significantly higher voltages and currents.

What are the criteria for output-filter components in a Class D amplifier?
At peak current, the inductor must maintain its inductance and not saturate. It must exhibit low magnetic and resistive losses. And, it often requires shielding. The filter capacitor must exhibit low losses and good temperature stability. Designers usually choose polyester or polypropylene capacitors for these traits.

What is shoot through? How can I prevent it?
Shoot through can occur when the half bridge changes state. Finite switch-transition times let the off-going switch's conduction overlap slightly with the on-going switch's conduction. During the overlap, current flows from one rail, through both devices, and to the opposite rail, reducing reliability and increasing total harmonic distortion (THD). Prevent shoot through by forcing a deadtime, a brief interval when neither switch conducts, at each switching transition.

Can I program the deadtime to optimize Class D power-stage performance?
Yes. Some gate-driver ICs provide programmable deadtimes, which minimizes the effects of noise in the circuit and provides a clean noise floor for systems that do not or cannot use feedback. This produces excellent THD performance in both open- and closed-loop amplifiers.

What protection methods do Class D power stages require?
The power stage's most important protection feature is overcurrent protection. Protection circuits can use a current-sensing resistor or, more economically but less accurately, the FETs' on-resistance. Other key protection features include overtemperature protection, which measures driver and FET temperatures, and fast shutdown, which quickly turns off the bridge during fault conditions.

What methods minimize EMI in a Class D power stage?
The reverse-recovery of the bridge-switches' freewheeling diodes is a major source of electromagnetic interference (EMI). To minimize these switching spikes, minimize the bridge-power circuit's pc-board trace impedances. Slowing the bridge-switches' rise and fall times by adding series gate resistance also reduces EMI but at the cost of greater switching losses.

Why is layout important for a Class D power stage?
In all high-frequency switching-power applications, the pc-board layout is the circuit. Layout impedances distinguish good designs from marginal or inoperative ones. Copper planes have lower impedances than traces. Minimize loop areas in high-current paths. Separate high dV/dt signals from sensitive nodes. Carefully choose where control- and power-circuit grounds connect.

What power-MOSFET characteristics are important for a Class D power stage?
Class D amplifiers typically switch at 400 kHz to keep switching noise out of the audio spectrum, so it's not sufficient to choose the FET with the lowest on-resistance. Total gate charge adds significantly to the overall losses at high frequencies, and it can dominate over conduction losses if the designer doesn't carefully choose the switch.

What does pumping mean for a Class D amplifier?
Half-bridge topologies can exhibit bus pumping, whereby the potential on the power-supply rail of opposite polarity to the output increases, or pumps up. Inductor energy transfers to this rail when the switch of lower duty cycle turns on. Amplifier gain is proportional to the rail voltage, so pumping introduces distortion. A full-bridge topology cancels pumping, but at the cost of additional components.

What are the test considerations for measuring a Class D amplifier's performance?
A Class D amplifier's output contains out-of-band switching artifacts, which can cause an incorrect reading from a conventional audio analyzer. An audio analyzer with an appropriate input filter, though, will produce the proper results.

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