Class D Chips

Class D Chips which pave the way

As mentioned in a prior article, Class D amplification is nothing new, modern switching components and topologies paving the way to resilient and high quality amplification circuits.

We can also draw a parallel between modern switched mode power supply design, H-Bridge motor control and that of Class D, all benefiting from newer ultra fast switching transistors, the bug-bear of the first class D and switching power supplies.

Class A and Class AB output stages are biased to draw current when there is no signal  or at idle (quiescent) to reduce cross over distortion and to keep the output devices operating within their linear region. Class A audio amplifiers are the least efficient, at about 25% and AB at about 50% to 78.5% depending on design operating point.  (the time at which conduction in the output transistors take place, the relationship between driver and output and even pre-driver to output). At 80% we are looking at Class C which is where conduction only takes place in one half of the sine-wave cycle,  depending on the type of transistor used i.e. NPN or PNP.

Class D in it’s broadest sense defines a PWM or non-linear switching amplifier where the output stage efficiency is often boasted of as being higher than 90%.  This is due to the transistors switching to either on or off, high current low voltage (low power dissipation) or high voltage low current (low power dissipation) through formula P = I * V.

Class D audio showing PWM
Class D PWM

On the face of it Class-D  follows a simple, tried and trusted formula much of it found under Further Reading below. The facts are that they were the most difficult to build amplifier types for DIYer types until recently.  (components, PCB layout, dead-time switching and EMI).

When it comes to power delivery most manufacturers have opted to keep their amplifiers light-weight using both Class-D amplification as well as switching supplies.  The IRS20124S Class D audio gate driver from International Rectifier is well suited to DIY projects. The data sheet can be found here. Using the IRF6645 as output devices we have a schematic of the output switching stage here , the original link for Designing High Powered Class-D amplifiers here.

NXP’s offering, the TDA8953 – a 2 x 210W audio amplifier

NXP TDA8593J
NXP TDA8593J

TDA7498 from STMicroelectronics delivers 160+160W into 4 Ohms using single rail PSU. Technical Doc here.

TDA7498
TDA7498   (About R150.00 for two channels of 100W on 36V).

Further Reading:

Build your own Class-D Amp – All About Circuits.com

Class D amplifiers What, How and Why – analog.com

Designing with Class D Amplifier chips  – ElectronicDesign.com

Differentiating Cost Competitive Class-D Designs – powerguru.org

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