Circuit analysis

Electricity is all around us, since the industrial revolution and Edison’s lamp we now have a world that is based on the consumption of electric energy. Over all the years we saw the electric network grow and become more important, all because of the higher efficiency that was necessary to please the growing population. The electricity network was being standardized, all you need to do these days to get your bread toasted is plug in the mains connector and wait until the toasts jump out; hot and all.

Next to the industrial revolution was the digital revolution, with the introduction of the computer as a driving force. We no longer need to write a letter and bring it to the post office so that it can be sent to your dearest beloveds, no... all we need to do is open our digital/virtual mailbox and hit the "send message" button, few seconds later the message is all ready received by who ever it is being send to, wherever he lives on our planet. Modern computers differ much from the ancestors who were build in the late 1950's, today we have standards and everything is being made plug-and-play; again because of the efficiency and the demand for it.

The progress in efficiency introduced the transistor and with it low voltage digital circuits. These circuits weren't able to run at the 110V/220V voltage that was available on the net so the game of power conversion became a big industry with great manufacturers like Texas Instruments, National Semiconductor, Philips, ... coming out of it. For today’s computers we need three common voltage sources being 12V, 5V and 3,3V. All of these sources are converted from the 110V/220V voltage available on the net; it’s the power supply which transforms/converts the high voltage electricity to lower levels.

Power conversion in ATX PSU's hasn't really changed over the past 10 years, even the modern pc power supply is somehow based on the same TL494 driver IC which Texas Instruments released a decade ago. This type of circuit is called a 'half bridge converter' and makes use of fast switching high-side transistors to make the power supply highly efficient compared with linear power conversion. In the power supply I have the manufacturer choose to use the SG6105 regulator produced by System General (SG).



Let us have a look at how the SG6105 is put into use for converting electronic power; here is an overview of a typical half bridge converter circuit:


Original picture courtesy of Elektor International Media B.V.
Elektor n° 476: "DC-power nodig? Neem een PC-AT-voeding"

In modern computer supplies we can clearly notice two parts in the PCB design, one side being the primary part of the power supply which can be quite dangerous to work on since it's a high voltage circuit and the secondary part which contains the various low voltage circuits and control unit. In our picture above the high side primary part is found at the left. Both sides of the PSU are separated by the T1 transformer which makes both parts galvanic separated (improving safety). In our example we have a 220V AC mains voltage, instead of transforming it straight to low voltage levels we find a rectifying circuit. Yes that's right, the AC net voltage is first being rectified through the bridge rectifier and C1/C2 capacitors. If you ever have opened your PSU you may clearly notice these two capacitors since they are about the largest components you will ever see inside a PSU. The 320V DC is then being switched on and off by the high side Q1 and Q2 FET's (transistors used as a switch). The current on the primary side of the T1 transformer will then switch polarity and thus create a current on the secondary side equivalent to the type of transformer being used. You may question why we don't go straight to lower level voltage instead of rectifying the high voltage first? Well, the output voltage of the PSU is being controlled by the switching signal of the primary FET's, the longer the FET's conduct the higher the output voltage will be, but controlling the output voltage can only work correctly when the voltage remains at the same level, so that's why we convert the 220V AC into 320V DC first.

On the secondary side of our PSU, after the T1 transformer we get low level voltage which is again being rectified, for the 12V line for example diodes D1 and D2 will conduct depending on how the transformer is polarized. The DC voltage is further stabilized and smoothed out by the linear coils L1(a,b,..) and the filter capacitors. You now have a full functioning power supply, though applying different loads will make the voltage levels swing and thus there is a need to control the PSU via a control unit, being the SG6105 in our example. You know that the Q1 and Q2 FET's duty cycle (duty cycle = time on versus time off) decide how much the output voltage will be in our PSU. The SG6105 decides how long the FET's will be active and inactive, the FET's are being driven through the low power T2 transformer, although that an opto-coupler may also be used. Again, a transformer is being used to make sure that there is galvanic isolation between high and low voltage sides of the PSU, this because of the higher safety. The SG6105 also reads the output voltage of the PSU, if a high load is applied then the output voltage may tend to drop, but the SG6105 will then increase the duty cycle of the high side FET's in order to keep the output voltage at constant level. Okay theoretically, in practice there is always small transient noise and ripple voltage.