Introduction to power supply pdf

This gives an introduction to the Power supply circuits that we come across in our daily life. Any electronic device consists of a power supply unit which provides the required amount of AC or DC power supply to various sections of that electronic device. There are many small sections present in the electronic devices such as Computer, Television, Cathode ray Oscilloscope etc.

Instead one or more sections may need a 12v DC while some others may need a 30v DC. In order to provide the required dc voltages, the incoming v AC supply has to be converted into pure DC for the usage. The Power supply units serve the same purpose. From the diagram above, it is evident that the transformer is present at the initial stage. A transformer has a primary coil to which input is given and a secondary coil from which the output is collected.

This is explored in the following example. The circuit of Figure above is a Zener divider Zener diodes are discussed in chapter 3. By substituting a reverse-biased Zener diode in place of R2 in the previous circuit, the shifting Zener impedance above a certain reverse current knee point can be exploited to guarantee a stable output voltage over different line and load conditions.

Keeping in mind that Zener diodes can only be constructed with certain reverse voltages, the closest stable output to 5V is chosen, giving a Zener voltage of 5.

At no load, all available current will be passed through the Zener diode. By choosing this load current to be slightly over mA at maximum load say by 10mA , regulation can be ensured even when the the full load current is delivered to the load. R1 is selected for all voltages within the tolerance of the input voltage range: the worst case, at 12V, requires that:.

As long as the Zener diode has current through it, a load of In theory, this design should therefore be capable of meeting all the requirements. A closer examination of the affected parameters offers some caveats. The Zener regulator efficiency differs depending on input voltage and output loading.

The best case efficiency for any input voltage is at full load, and the best case efficiency for any load is at the minimum input voltage. This is better than the resistor divider, but not by much, and only at one extreme corner of operation. At the other corner the results are less impressive:.

At no load, the full operating current of the Zener regulator must travel through the Zener diode. Best case, this is always more than the maximum output current; worst case, it can be much greater. At 17V, this regulator consumes almost double the maximum output current! Since the Zener regulator quiescent current is always greater than the maximum operating current, the worst case power dissipation leads to a great deal of heat dissipated in both R1 and in the diode. However, as the load current increases, the Zener diode dissipates less and less power, since the current and therefore the power must be diverted from the diode to the output load.

Meanwhile, R1 power dissipation remains almost constant across loading, but benefits from a lower input voltage. If for any reason the output current exceeds the quiescent current such as during a short circuit , the power dissipation in R1 increases above the typical worst case operating point, requiring a larger component or better cooling to endure this stress. Even under normal operating conditions, R1 still dissipates enough to require a large wirewound resistor and probably some form of active cooling:.

It is worth noting, in passing, that at worst case the Zener diode must dissipate close to 5W; while there exist Zener diodes capable of this, 5W is an uncommonly large value for a Zener diode. With smaller maximum load current requirements, low power Zeners may be used at substantially decreased costs. From an ideal standpoint, the Zener voltage is always 5. In reality, however, the Zener diode has some temperature related effects which cause the Zener voltage to change.

Worse still, the temperature effects do not all act in the same direction. Low voltage Zener diodes behave predominantly according to the Zener effect, an electron tunneling process, which has a negative temperature coefficient Zener voltage decreases with increasing heat.

Higher voltage Zener diodes behave predominantly according to the avalanche effect, a form of current multiplication that has a positive temperature coefficient Zener voltage increases with increasing heat.

At around 4V to 6V, and dependent on the Zener current, the temperature coefficients of these two mechanisms will combine and can occasionally cancel out almost entirely. Unfortunately, there is still some effect at 5. A basic approximation of this effect can explain the difficulty. With a 15V supply voltage, at no load the Zener current and power are found to be:.

Assuming the change in Zener voltage is up to 0. Iteration shows this change in Zener voltage with temperature eventually stabilizes; still, the output is far from its ideal value.

As load current increases, Zener current decreases, returning the output voltage to a lower value. Line and load regulation are difficult to estimate precisely, since the exact location of the Zener knee, the effect of process variation on the temperature coefficient, and the variation of the temperature coefficient with Zener current cannot always be predicted.

Both are frequently verified experimentally or with a spice simulation. Broadly speaking, with a maximum specified regulation swing of about 0.

For the small signal analysis, voltage sources are shorted. The impedance looking into the output is just R1 RZ. Interestingly, from this equation we can discover that the output impedance increases as a function of increasing load, to a maximum of R1 at a dead short across the output. The output is regulated because the output impedance continuously changes to match the level of loading.

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So friends that are complete post about the power supply, if you have any queries about this post ask in comments. See you in the next interesting post. Have a good day. I am a professional engineer and graduate from a reputed engineering university also have experience of working as an engineer in different famous industries.