Features of the use of oxide capacitors in microprocessor power circuits

Encyclopedia of radio electronics and electrical engineering / Computers

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To increase the reliability of the computer, highly heated components (processors, chipset, power supply transistors) are provided with heat sinks, additional fans are installed in the system unit and on hard drives. But it turns out that the oxide capacitors of the power filters of these units are also fuel elements. Why this happens and what needs to be done to prevent their heating, is described in the article.

In the microproprocessor, millions of transistors of digital nodes are connected to the power bus, operating according to the algorithms specified by the programs, with a total power consumption reaching several tens of watts. In the first approximation, their connections to the power bus are random, therefore, in the future, to simplify the presentation, we will call them noise [1].

The duration of the key state change front in the microprocessor does not exceed 10-8 s, therefore, slightly underestimating the width of the spectrum of generated noise (currents), it is possible to determine its upper limit frp as more than 100 MHz (frp > 1/τf [2]), and the bandwidth frequencies - from 0 to more than 100 MHz. 90% of the generated noise power is concentrated in this range. Given the random (noise-like) nature of the processes, in reality this range is even wider.

Thus, microprocessors are complex loads for power supplies and generate currents of a wide spectral composition (hundreds of megahertz) and high power (up to 5 ... 20 W) in power circuits. Maximum currents are generated at 100% microprocessor load.

As an example, let's consider the microprocessor core power circuit diagram (Fig. 1) in the Abit BE6-II motherboard (it was announced as a processor overclocking board).

The supply voltage of 2,05 V through the inductor L1 and a filter of three oxide capacitors C1-C3 with a capacity of 1500 microfarads is supplied to the processor power outputs. The constructive capacitance Сm has a low self-inductance and therefore well shunts the high-frequency (more than 100 MHz) power components of the generated noise.

As C1-C3, high-quality gel oxide capacitors with a maximum operating temperature of +105 ° C are used, capable of dissipating power of 0,5 ... 5 W. Perhaps this allowed manufacturers to ignore their mode of operation.

Measurements showed that during long-term operation of a computer in which two case fans (in the power supply and an additional one), a Celeron processor with a Golden Orb fan and a video card with a fan are installed, the heating of the cases of the mentioned capacitors reached +60...80 °C. At high outdoor temperatures, two of the three filter capacitors failed in succession: first, the case of one of them was mechanically destroyed, after which the computer began to periodically “freeze” during operation, then the same thing happened with the second capacitor and the system began to fail already at BIOS processing stage. The reason for "freezes" is the appearance in the power circuits of voltage surges commensurate with the amplitude of the control signal pulses. Such glitches penetrate control or data circuits and compromise processor performance and data integrity.

According to the temperature of the cases of oxide capacitors, it can be concluded that they dissipate power of about 3 ... 5 W. What are the causes of heating? As is known, the heating of an oxide capacitor is determined by the power released in its volume, i.e., losses in the dielectric and metal elements. Losses are described by the loss angle tangent: tg δc = Rp/P = (Pm + Rd)/P = tg δM + tg δD, where Pp is the loss power; Pm - power loss in the metal; Rd is the power loss in the dielectric; tg δM and tg δD - loss angle tangent for metal and dielectric, respectively. The typical value of tg δС of an oxide capacitor is (1000...2000)-10-4 at a frequency of 50 Hz. With such its values, from 10 to 20% of the power of low-frequency currents pass into heat, and given that the spectrum of filtered currents (voltages) extends to tens of megahertz and tg δС increases with increasing frequency (tg δМ = Rп2πfС), more than 80% noise energy generated by the processor and filtered by the power circuits.

How does an increase in temperature affect the operation of an oxide capacitor?

Insulation resistance decreases by 10...1,26 times with increasing temperature by 2 °C, and when the temperature rises to the limit of +105 °C - by 7...350 times (minimum values ​​correspond to inorganic dielectrics, and maximum values ​​correspond to organic dielectrics) . The electrical strength of the capacitor decreases three times with an increase in the frequency of the applied voltage by a factor of 10 (at the rated power loss) [3].

All of the above suggests that it is unacceptable to use oxide capacitors in processor power circuits without taking special measures. Failure to comply with this condition leads to a decrease in the reliability of the motherboard and may cause their failure even in the operating temperature range.

A simple solution suggests itself: to prevent the penetration of high-frequency components (up to tens of megahertz) into the oxide capacitors, install a packageless ceramic capacitor with a capacity of 0,033 μF in the immediate vicinity of the processor pins, and, as an obstacle to low-frequency components (up to hundreds of kilohertz), turn on a ceramic capacitor with a capacity of 3,3 ...4,7 uF. Due to the small tan δС of such capacitors, the shunted energy does not turn into heat. The total reactive power of these capacitors is 30 VAr.

The modified scheme of the power circuit of the microprocessor core is shown in fig. 2.

The revision was carried out on this board, which led to a decrease in the temperature of the oxide capacitor cases to +20...30°C. The board successfully passed the tests in the hot period of the summer of 2002 at an air temperature in the room of +40...50 °C. In addition, the level of noise emitted by the computer has been reduced.

It is advisable to subject the system boards of computers used as servers, other computers operating with 100% load (for example, in distributed computing systems), as well as video cards, i.e. all nodes in which processors work with maximum load, to such refinement. . It is also useful in computers that are not used so intensively: a decrease in heat dissipation in the system unit by 10 ... 25 W will favorably affect the reliability of the system.

Literature

1. Ott Henry W. Noise reduction techniques in electronic system. - John Wiley & Sons, NY 1976.
2. Gonorovskiy IS Radio engineering circuits and signals. 4.1. - M.: Soviet radio, 1967.
3. Dulin V. Zh., Zhuk M. S. Handbook of elements of radio electronic devices. - M.: Energy, 1977.

Author: A.Sorokin, Raduzhny, Vladimir Region

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