Published:2011/8/4 21:18:00 Author:Phyllis From:SeekIC
By Ton Giesberts
A pulse-width-modulated output stage with hi-fi characteristics is something rather special. We already described various aspects of this design in the June 2004 issue. Following that introduction, in this installment we continue with a description of how to build this powerhouse.
The pulse-width modulated amplifier we’re talking about here is based on the Tripath TA3020 driver IC. The term ’pulse-width modulated’ may call up negative associations, but they are fully out of place here. The specifications are outstanding, and the amplifier is certainly as good as quite a few models in the upper end of the commercial range.
To make construction as easy as possible for Elektor Electronics readers, we can supply the printed circuit board for this project with all of the SMD components already nicely soldered in place, all at a very attractive price. And don’t forget that a single circuit board houses a complete 2 x 300-W stereo amplifier.
Special components
Before you start enthusiastically buying components, we’d like to emphasize that you can’t simply conjure up a dual 300-watt amplifier from of a box of standard parts. When this amount of power is involved, even the power supply must meet special requirements. On top of that, we’re dealing with a switch-mode amplifier here. That makes the circuit board layout and the quality of the components especially important. To avoid potential problems, you need to know the requirements imposed on the various components before you start building the circuit board for this final amp.
Most of the components can be obtained from Farnell as stock items, and several merchants including C-I Electronics and Geist Electronics will be offering complete parts kits for this amplifier. It’s essential for some of the components to be SMD types, due to the characteristics of SMDs and the short signal paths that can be obtained with them. However, this doesn’t mean that soldering has to be a problem, since all of the SMDs are already fitted to the board. The ferrite cores for the inductors are also included with the printed circuit board (two per board). Winding the inductors with 1.5-mm enameled copper does require a pair of sturdy fingers, but we have more to say about that later on.
Decoupling
The issue that requires the most attention with this amplifier is the interference that can be generated by quickly switching large currents. The circuit board has therefore been designed such that tracks that carry large currents have the least possible amount of coupling with the rest of the circuit. In addition, the supply voltages are always decoupled locally, in order to keep the loops in subcircuits conducting large currents as small as possible. In particular, this decoupling is provided by C5, C18, C32, C33, C36 and C37 for the output transistors. We selected 250-V MKT types for these decoupling capacitors, since they can better withstand extremely high switching currents.
Capacitors C6 and C19 also deserve special attention. For these two electrolytics, it is extremely important to have the lowest possible self-inductance and effective series resistance, as well as a good thermal rating.
The snubber networks (C4/R12 and C17/R33) help remove HF overshoots. To save space, R12 and R33 are mounted vertically. When fitting them, keep the loop as small as possible in order to keep their parasitic self-inductance as low as possible. As 1-W resistors are used here, you must allow for a somewhat larger diameter than usual (see Figure 1). For the capacitors (C4 and C17), we have selected 200-V ceramic types. This is because the maximum voltage across these capacitors can be nearly the full supply voltage (approximately 110 V between the positive and negative rails), or even more with any overshoots that may be present.
Reprinted Url Of This Article: http://www.seekic.com/blog/project_solutions/2011/08/04/Clarity_2x300W_Class_T_Amplifier_Part_2__building_amplifier_board__(1).html
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