Published:2011/8/4 21:49:00 Author:Phyllis From:SeekIC
By Ton Giesberts
Coil winding
Winding the output inductors is not difficult, but you must pay careful attention to the winding method. With the selected wire diameter of 1.5 mm (16 SWG), the 29 turns will not fit on the selected core in a single layer. To keep the internal capacitance as small as possible, the coil is wound progressively in approximately seven sections. This means that after the first three turns are wound, the fourth turn is placed on top of the third turn, and the fifth turn is then wound directly on the core next to the third turn. It is followed by turns six and seven, with turn eight again being placed on top of turn seven, turn nine again next to turn seven, and so on (see Figure 3).
The relatively thick wire doesn’t make the job any easier. Depending on how neatly and tightly you manage to wind the coil, you may well have to place a few more turns on top of other ones.
IC and output transistors
Most people will probably be reluctant to solder the IC to the circuit board, so we searched for a 48-pin IC socket with very high quality. We finally decided on a type having turned contacts with 30-micron gold plating and rated at 3 A. Here we can’t be satisfied with anything less.
A very important detail in fitting the output transistors is the material for the electrical insulators. As these transistors have a metal cooling surface that is electrically connected to the drain (T2 and T4), the capacitance to the heat sink (which is connected to ground) would be too large if they were fitted using standard insulators made from mica, silicone rubber, silicone foam or even fancy stuff like Kapton. We tried this, and at maximum output power there were large parasitic currents that could not be suppressed.
There is only one good solution to this problem, which is to use ceramic (AI2O3) insulators that are several millimetres thick. The insulator we use here is the Fischer type AOS220SL, which is 4.5 mm thick and is actually intended to be used with a TO-220 package, instead of the larger TO-247 package. Despite being a bit too small, the insulator fully covers the metal cooling surface of the transistor. It also keeps the parasitic capacitance extremely small.
For the heat sink, we found a type with a surface large enough to mount the circuit board parallel to the surface. The selected type (from Marston) is 160 mm wide arid 150 mm deep, and it even provides a bit of clearance at the edges. Eight holes tapped for 3-mm screws can be drilled in the base, which is 10 mm thick, for fastening the circuit board and the four output transistors. We recommend that you first centre the board on the heat sink and mark the four corner holes. Next, bend the leads of the output transistors exactly where they become thinner, slide them in place and mark the positions of the four fastening holes for the transistors.
Cylindrical standoffs (metal types with a threaded end) with a length of approximately 10 mm should be used to attach the circuit board to the heat sink. The threaded end will probably be too long to be fully screwed into the heat sink. This can be solved by first fitting a nut and lock washer on the threaded end. This causes the mounting height of the circuit board to be just right, and the leads of the output transistors will pass through the matching holes in the circuit board with length to spare.
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__(4).html
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