Computer power supply for XNUMX/XNUMX operation. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Computers

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A complete description of the PSU from computers and modes of operation.

In this article, only facts that have been verified and tested by time have been used. The author of the article does not aim to convince the reader of anything. And even more so, it does not bear any responsibility for your experiments with your own equipment. The information is valid for power supplies costing much less than $40. So. Let's get back to our... er... file servers and routers. As a rule, in home networks, such a thing does not belong to a specific person, but is assembled from common components or with common money. They try to make it reliable and cheaper (CPU - Intel, Memory - no more, no less, Network cards - NE2000 ISA 10Base2/T). For complete reliability and speed, Unix is ​​installed on all this hardware. Oh yes!!! totally forgot. We still need UPS here.

Without the cost of UPS, a modest system will cost $50..70 + the cost of a HDD for a file server. Naturally, a power supply in such a system cannot cost $40.

Someone will object: "And we have an old case from the branded i486." Yes. How old is this BP? And how many more years will he have to work? Will all this work long and bug-free? So:

Typical diagram of the power supply unit AT 200W

Rice. 2. Combined load 50% (2 or more HDDs)

On fig. 2 section No. 1 - static load. Section No. 2 - HDD in read / write mode. Dips in the supply voltage + 12V are typical. The magnitude and duration of the dip depends on the parameters filter power supply and HDD power. Consequence: Due to the instability of the +12V power rail, the hard drive begins to bang its heads on the "pancakes". Buds appear. Glitches in devices powered by +12V bus (ISA cards, COM ports)

How to deal with it

Consider filter power supply.

Rice. 3. Filter (what it is)

 In most AT units, a filter for the power rail + 5V consists of two electrolytic capacitors 1000uFx10V. For power rail + 12V one capacitor 1000mkFx16V. For switching power supplies, the capacitance of filtering capacitors is taken from the calculation 500..1000uF on 1А load current. In our case, we get for the bus + 5V the maximum load current will be 4А.For power rail + 12V the maximum load current will be 2A.In most cases, an emergency does not occur. But when using even one HDD of the IBM DPTA 7200RPM type (or with similar power consumption), the above glitches were observed.

Rice. 4. Filter. (what should it be)

For this circuit (Fig. 4), the following parameters are valid: Bus + 5V - maximum dynamic load current 20A. Tire + 12V - maximum dynamic load current 8A. Electrolytic capacitors eliminate current instability. Ceramic (2.2uF 3..6pcs) eliminate voltage surges. A series with low resistance for impulse currents is recommended (it seems to be called that) Each company marks them in its own way. From what you can get in St. Petersburg - for example Hitano, Series EXR, operating temperature up to 105 Celsius. For + 5V - two things 2200uF or 3300uF 6,3 or 10V (you need to look at the dimensions, the PSU manufacturers squeeze the place very much) I can’t advise anything with ceramics. From what I saw, only TKE and accuracy differ (for example, + 80 -50%). I think in filters of this kind it is not important. The larger the capacity, the better. It's probably better to take SMD (unpackaged) and solder from the back of the board directly to the conductors. About the coils in the output filters. If there is no winding experience, it is better not to experiment. If you have the opportunity to buy, then you can try. Or solder from a dead PSU. With coils at the output - you need to be very careful. Check the block only by loading the resistors. After upgrading the filter, we look at the oscillogram.
Fig5. Static load 30% (bus +5V)

 This is how the “surface” of the voltage of a branded power supply looks like under load. There are voltage surges, but they are insignificant (much less than the permissible norm) and practically do not increase with increasing load. The total capacitance (my version) of electrolytic capacitors is 6800uF. Ceramic capacitors 1.5uF. (everything that was at hand). For interest, the power supply was tested ATX of firms power man from the case InWin A500- the oscillogram is similar, but there are no voltage spikes.

Fig6. Combined load 50% (2 or more HDDs)

On fig. 6 section 2 corresponds to the dynamic load.

Filter capacity - one capacitor 4700mkFh25V (HDD in read / write mode). The maximum interference is not more than 100mV. power unit ATX of firms power man showed about the same result.

Safety / reliability of the high-voltage part of the PSU

Rice. 7. Oscillogram of mains voltage. Ideal

... Again, someone turned off the light.

Rice. 8. Oscillogram of mains voltage. Operation of several PCs without filter

Someone will say: "well, we don't care if our PC is on the network or not. Well, the cunning Chinese saved on network filter, so what. "Perhaps the following waveform will convince you.

Rice. 9. Network operation (220V) of some powerful consumers

On fig. 9. Section number 1 - the work of a powerful puncher. Section number 2 - the inclusion of a powerful inductive consumer. (For example, a refrigerator or a vacuum cleaner). Switching on an inductive load is always accompanied by a powerful voltage surge. The transient voltage is calculated using the following formula.

Rice. 10. Network filter + rectifier. What they should be.

 Of all the elements in the filter circuit of a conventional PSU, there is only a PS405L thermistor and a fuse. (the essentials) Sometimes they put a symmetrical transformer (in the diagram - 5mH). Of course - an RS205 rectifier and a high-voltage DC filter (2 electrolytes 220x200V)

Increase in efficiency

1. Replacing powerful key transistors.

We will change imported bipolar KSE13007 (or NT405F, 2SC3306) to our Soviet field worker KP948A.

Rice. 11. Typical circuit for switching on a field-effect transistor.

This option is suitable for ATX power supplies, because. the block is started from a separate low-power power source. Such a scheme is not suitable for AT blocks. Therefore, I left the transistor wiring as is, adding a 15V zener diode. (As shown in the diagram in Fig. 11) It is not necessary to install zener diodes, because the direct voltage at the gate does not exceed 1v (direct diode), and its reverse breakdown voltage is not more than 10v. Capacitors 1 * 50v (Fig. 12.) radiator) is the main reason for the failure of the power supply, since the power transistors are not locked sharply enough.

Rice. 12. Recommended switching circuit KP948A for the AT unit.

I don't know why, but it works for me. The power drop across the transistors is reduced by 3..5W. Although I still left the zener diodes. As a result, it stops heating up.

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2. Rectifier diodes

We put powerful rectifier diodes on normal radiators. Suitable radiator from the CPU - cut in half. One half for + 5V rectifier. The second one is for the +12V rectifier. It is also recommended to replace the power diode assemblies with our Soviet KD2998A diodes.

Radiators - enlarge. All! Now the fan can be thrown out of the PSU. In this case, the normal heat exchange inside the case is disturbed. But if this is a PSU for a router, then there is nothing special to warm up inside the case. If this is a file server - then at your own peril and risk. Although Manowar Manowar'ych claims that he has a converted ATX power supply loaded with 2HDD 7200RPM + ULF and all this stuff works without a fan.

Publication: cxem.net

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