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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Reducing the heating of filter parts in the processor power circuits. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers When "reviving" a computer with an ATHLON AMD K7-600 processor and a GIGABYTE GA-7IXE motherboard, the author of the article was unpleasantly surprised by the strong heating of oxide capacitors and processor power filter chokes - the temperature clearly exceeded an acceptable value. But the board was new, under warranty, and I had to come to terms with this fact. During subsequent periodic checks, a clear overheating of capacitors and chokes reminded that this issue should be sorted out as well. But, as often happens in life, there was not enough "starting impulse" to do this work. They became, as the author writes, the article by A. Sorokin "Features of the use of oxide capacitors in microprocessor power circuits", published in Radio, 2003, No. 1. It is known that aluminum oxide capacitors have a large self-inductance, proportional to their capacitance, and cannot operate normally at high frequencies (HF). Therefore, in circuits with a broadband signal, almost non-inductive ceramic capacitors must be installed in parallel with them. This is how the filters in the power circuits of radios and TVs are made, and for developers all this has long become a truism. Let's give specific data on the processor power supply circuits for the board on which the revision was carried out. This information will not only help the reader to better understand the essence of the changes made, but will also serve as a guide when carrying out similar work on other types of motherboards. The 1,6 V processor power filter consists of five 1200 uF x 6,3 V oxide capacitors connected in parallel and two inductors also connected in parallel, and a 5 V voltage filter consists of four such capacitors and one inductor. The board has surface-mount ceramic capacitors shunting oxide capacitors, but they seem to be ineffective. The purpose of the first stage of work was to "unload" oxide capacitors from the RF component. The best option is to install ceramic capacitors directly on the printed circuit board on which the microprocessor is mounted, but this complicates the work and there is a risk of damage. Therefore, I had to limit myself to a somewhat less effective measure - mounting leadless capacitors on oxide leads. In total, six capacitors with a capacity of 2,2 μF with a nominal voltage of 16 V were installed: four in the 1,6 V supply circuit and two in the 5 V supply circuit. Since the distance between the terminals of the oxide capacitor is greater than the length of the ceramic one, one end of the latter was soldered directly to the output of the oxide, and the other - through an insert from a piece of tinned wire bent in half with a diameter of 0,5 ... 0,6 mm. After the revision, the heating of oxide capacitors decreased significantly, and three more leadless capacitors added a little later (one for each remaining oxide capacitor) practically did not change the picture. The next task is to reduce the heating of the chokes. In the 1,6 V power circuit, they had three turns of enameled wire with a diameter of 1,7 mm, wound on a ring core with an outer diameter of 12,7 mm, and in the 5 V power circuit - five turns of wire with a diameter of 1,4 mm on the same core. The material of the cores is unknown, but it can be assumed that it is ferrite. The reasons for the heating of chokes are well known. This is the release of power on the active resistance of the winding wire (Joule heat) and the so-called skin effect, causing an increase in this resistance for the RF components. Since the active resistance of the winding does not exceed a fraction of an ohm (it is impossible to measure it with conventional instruments), the influence of the first component is small and, as a first approximation, it can be neglected. The largest "contribution" is made by the second component. In addition, due to the saturation of the core with a large current, the inductance of the inductor is insufficient for good filtering of the variable component. The simplest refinement of the throttle is the introduction of a gap in the core. To do this, the inductor is unsoldered from the board and a cut about 1 mm wide is made with a diamond saw in the place where it does not affect the wire. In this case, the inductance of the inductor decreases somewhat, but it is not difficult to restore it by increasing the number of turns. Reducing the influence of the skin effect is a more difficult task, since it requires replacing the winding wire with a bundle with the same cross-sectional area, twisted from thinner wires. The thinner they are, the less the skin effect, the smaller the diameter of the bundle (due to the higher fill factor) and, in addition, it becomes softer and easier to wind. However, a large number of wires complicates the manufacture of the bundle, so the wire PEV-2 0,35 was chosen. A bundle of 5 wires 16 mm long was used to wind the inductor in the +180 V power circuit, and chokes in the core power circuit - from 25 wires 160 mm long. Making harnesses is not difficult, although it is very laborious. First, one of the ends of each wire is released from insulation over a length of 5 ... 8 mm and tinned, then the wires are folded with tinned ends together and, having aligned the ends, twisted into a bundle. Since its diameter in any case turns out to be larger than the diameter of the wire to be replaced, a segment of the latter (previously stripped and tinned) is inserted into the end of the bundle, the junction is wrapped with a thin tinned wire and carefully soldered. Then the tourniquet is wound on a core with a pre-cut. To compensate for the decrease in inductance due to the introduction of a gap, the number of turns is increased to 9 and 5, respectively. After winding, the second end of the bundle is shortened to the required length and prepared for installation in the same way as described above. New chokes cannot be installed close to the board in the same way as the unmodified ones, but this is even better, since the gap that has appeared improves the cooling conditions for the board and the chokes themselves. As a result, a double effect is obtained - a decrease in the temperature of the chokes and an improvement in the conditions for their cooling. Checking the modified power filters showed the following. After turning on the computer and loading the operating system, the heating of capacitors and chokes is barely noticeable. When the processor is heavily loaded (solving a complex problem), the heating of the inductors becomes noticeable, but it is much less than before the rework. In conclusion - a few tips for those who, after reading this article, will think about improving the reliability of their computer. First of all, it is necessary to find out the real need for the described refinement. In modern processors, such as the ATHLON 1700, decoupling ceramic capacitors are mounted directly on the board on which the microprocessor is installed. In addition, due to the high computing power in many cases (for example, when typing in the Word editor), the processor actually "rests", so it and the power filter elements do not heat up much. Heating can increase significantly when the processor is loaded with complex mathematical problems (by the way, games like 3D shooters also belong to them). And if in this mode the computer is used for a long time, it makes sense to modify it. In any case, it is advisable to shunt oxide capacitors with ceramic ones. In chokes, try to limit yourself to cutting the core, and if this is not enough, replace the single wire with a bundle. Author: A. Grishin, Moscow
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