Power Supply

~~----Power-Supplies----~~


	Half Wave Rectifier Since a capacitor input filter only draws current from the rectification circuit in short pulses, the frequency of the pulses is half that of a full -wave circuit, therefore the peak current of those pulses is so high that this circuit would not be recommended for DC power more than 1/2 watt.
	60 Hz Ripple Depth of ripple slope is dependent on Capacitance and Load.

	Full Wave Center Tapped A full-wave rectifier uses only one-half of the transformer winding at a time. The transformer secondary rated current should be 1.2 times the DC current of the power supply. The transformer secondary voltage should be approximately 0.8 times the DC voltage of the unregulated power supply per side of the center tap or the transformer should be 1.6 times VDC center tapped.
	120 Hz Ripple Depth of ripple slope is dependent on Capacitance and Load.
	Full Wave Bridge The full-wave bridge rectification circuit is the most cost effective because it requires a lower VA rated transformer than a full-wave tapped rectifier. In a full-wave bridge, the entire transformer secondary is used on each half cycle, unlike the full-wave center tapped which only uses one- half the secondary on each half cycle. The transformer secondary rated con-rent should be 1.8 times the DC current of the power supply. The transformer secondary voltage should be approximately .8 times the DC voltage of the unregulated power supply.
	Dual Voltage Supply A dual complementary rectifier is used to supply a positive and negative DC output of the same voltage. In most cases, the negative current is significantly less than the positive current requirements so the AC voltage and current relationship to the DC voltage and current should be the same as the full-wave center tapped described earlier.

Unregulated Linear Power Supply Unregulated power supplies contain four basic components: transformer, rectifier, filter capacitor, and a bleeder resistor. This type of power supply, because of Us simpticity, is the least costly and most reliable for low power requirements. The disadvantage is that the output voltage is not constant It will vary with the input voltage and the load current, and the ripple is not suitable for electronic applications. The ripple can be reduced by changing the filter capacitor to an LC (inductor-capacitor) filter but the cost to make this change would make use of the regulated linear power supply a more economical choice.

Regulated Linear Supply A regulated linear power supply is Identical to the unregulated linear power supply except that a 3-terminal regulator is used in place of the bleeder resistor. The regulated linear power supply solves alt of the problems of the unregulated supply, but is not as efficient because the 3-terminal regulator will dissipate the excess power in the form of heat which must be accom-modated In the design of the supply. The output voltage has negligible ripple, very small load regulation, and high reliability, thus making it an ideal choice for use in low power electronic applications.

Switch Mode Power Supplies The switch mode power supply has a rectifier, filter capacitor, series transistor, regulator, transformer, but is more complicated than the other power supplies that we have discussed. The schematic above is a simple block diagram and does not represent all of the components in the power supply. The AC voltage is rectified to an unregulated DC voltage, with the series transistor and the regulator. This DC is chopped to a constant high frequency voltage which enables the size of the transformer to be dramatically reduced, and allows for a much smaller power supply. The disadvantages of this type of supply are that ail of the transformers have to be custom-made and the complexity of the power supply does not lend itself to low production or economical low power applications.

PLEASE READ & HEED BEFORE GOING ANY FARTHER!

Television Sets Are Dangerous!!
This fact cannot be overstated.

Within the average color TV set, Lethal voltages
range from 150_Volts; 1500_Volts;
with the Highest Voltage at 25,000_Volts (25_KV).

ALL OF THESE VOLTAGES CAN KILL YOU!

SEE: Working with HOT CHASSIS

Typical TV Receiver Power Supply

Secondary Power Supplies

Flyback Switching Technology

Primary Power Supply, Line Rectifier Sections

Line Rectifier Circuits: Line EMI Filter, Automatic Degaussing Coil

Hot Chassis

Line Rectifier Circuits: Power Transformer, Line EMI Filter, Automatic Degaussing Coil

Isolated Chassis

The power supply is often the part of the equipment which converts alternating to direct current. The filter circuit, which includes the ininductor, smooths out the fluctuating or pulsating direct current until it is nearly pure direct current. There are two types of filler chokes: the smoothing choke and the swinging choke. The swinging choke is one in which the I and E laminates are butted together so that there is a minimum air gap between them. This makes the amount of inductance vary with the amount of current. A typical swinging choke may be rated 20 H at 50 mA and 5 H at 200 ma. The smoothing choke frequently has a small (0.1 mm air gap between the I and E laminates. This makes the inductance less dependent on the amount of current because air does not saturate as easily as iron.

One more thing to consider about chokes: the "Q" or quality of the inductor has an effect on its efficiency. As previously stated, the inductor should appear as a short circuit to the DC power it is carrying, and a high impedance to any AC, i.e., no series "R." In the practical world this isn't feasible. However, if heavy current carrying chokes are required, then the choke must have higher "Q," i.e., less wire which means lower "R." This can be achieved by using chokes with ferrite cores, which need considerably less wire for the same value of inductance: it is truly a multiplier of "Q." Also ferrite beads, i.e., very small donut or tubular shaped ferrite, are regularly used for circuit isolation, effectively preventing parasitic oscillations, etc. The down-side of ferrite, is that it will change inductance as the current or flux changes. In the case of large currents, it can saturate. However, by correct component choice -- frequency, AC and DC current, etc. -- ferrite is great tool for the designer.

1...Decoupling is used where the supply voltage cannot be lowered, i.e., if one needed a noise-free +12 volts on a PC bus, say. One could get a "clean" +12 volts with a voltage regulator... if only there was +15 volts or higher to start with. But such is not the case. So you use a high "Q" inductor (RFC choke) along with the proper bypass capacitor to effectively lowpass filter the +12 volt supply rail. For a real noisy supply you can use more than one inductor: a "pie" network for example.

2...One of the most efficient inductors is the ferrite toroid. It has high "Q" -- low "R" -- and because of its toroidal shape its fields are confined, and therefore has little stray fields. The super star of high "Q" inductors or transformers is the pot core. And of course, don't forget the ferrite bead. Thread the wire through the bead once or several passes and it may be just what the doctor ordered.

3...Decoupling is only as good as the components that you use. The capacitor part of the network should be high "Q" and minimum inductance: the noise is dropped across the inductor, and the capacitor must exclude the remaining noise. Another way of saying it: in a perfect world the inductor is an open circuit to noise (AC) and the capacitor is a dead short -- Zero, Nada, Caput, Zilch; "This here parrot is dead." The slightest inductance in series with that capacitor, and some very high frequency noise will come through like Gang Busters!.... Anyway nuff said.

4...SMT or chip capacitors made of ceramic are best. Also, sometimes in critical circuits, several size caps in parallel are appropriate, e.g., 1ufd || .1ufd || .001ufd, etc. The reason for this is as the capacitors become smaller in value, they also get physically smaller, hence less inductance. However this is less the case with SMT caps: consult your capacitor data sheets for the impedance verses frequency plots. Didn't he just say that?

Linear Voltage Regulators as Decoupling Devices



Simple Shunt Regulator	Simple Pass Regulator	Pass Regulator with Gain

Fig. 4_a

Fig. 4_b

Simplifier Linear Voltage Regulator IC

Example of 7805
The 1N4002 is for protection (optional)

NOTE: The following waveforms, figs 5 through 10, are shown where the top waveform is the initiating signal, or stimulus, and represents the current drawn by that circuit.

The bottom waveforms are of the voltage variations on the regulated D.C. output. These voltages are measured at ~500 millivolts per division. See Test Circuit

Introduction	Discussion	Symptoms	Simplified VR

Waveforms	Refresher:Bypassing	Animation	Addendum

Fig. 5

Fig. 6

LVR Recovering from load changes C = 0.33 µF

LVR Recovering from load changes C = 0.10 µF

Fig. 7

Fig. 8

Overshoot & Ringing
LVR Recovering from load changes C = 0.033 µF

Damped Oscillations
LVR attempting to oscillate C = 0.005 µF

Fig. 9

Fig. 10

Squegging *
LVR oscillating or Squegging due to OPEN
bypass cap C = 0.00 µF

Ripple
Caused by squegging oscillation

Linear Regulators

1..Read the data sheet. The needes and capabilities of the regulator are in there somewhere; they might not jump out and bite you right away, but they are there.

2..The use of three terminal linear voltage regulators, like the 78xx and 79xx devices, is fairly straightforward. However, there are a few things to remember: Always bypass -- there's that word again! -- the input pin and the common pin with a ceramic capacitor no smaller than 0.33 ufd, and use absolutely the shortest leads possible (there are some transistors with pretty high f_t in that regulator, and if you furnish enough reactance of the wrong kind, Mr. Oscillation will visit you).

3..If your regulator is furnishing power to a capacitive load, and the primary power is removed -- like unplugging a PC card, or disconnecting an experimental setup -- the charge in that capacitive load will cause the secondary or output of the regulator to be more positive than the primary or input. If this reverse voltage exceeds the regulator's ratings it will blow up. To prevent this sort of failure, a diode is placed between the input and output, such that, when reverse voltages are present, the diode conducts preventing damage. (see Figure)

4..There will come a day (or night) when you may need an eight volt regulator, and all you have is a 7805, five volt regulator. By inserting a voltage equal to the difference in the common lead, "Voila," you have 8 volts. You can do this by inserting a zener diode or a low resistance voltage divider (or a pot for variability). If all else fails, insert a series of silicon diodes (cathodes toward Grd.) @ .6 volts per, until you have the desired output.

5..These regulators don't need an output capacitor per se, but a minimum of 1 ufd is recommended to prevent fast load pulses from causing needless error correction by the regulator. As for the primary or input capacitance, it depends on the ripple content from the primary voltage: If the voltage is straight from the rectifier, then obviously large capacitors are required -- assuming a large load on the regulator's output. The greater the difference between the input voltage and the output voltage, the less stringent the capacitor requirements.

6...In the data sheet -- you know, that funny looking piece of paper that causes you to squint, and makes your head feel funny -- In the data sheet, there is information on forward drop, Vfwd, of the regulator at some current. This means that if the primary voltage is near the desired secondary voltage at some current, you may be in "Deep Dudu." The greater the difference between the input voltage and the output voltage, the easier life is: if the rating of the regulator is a 1.1 volt drop at 500 ma, and you have a 5 volt margin -- say -- you are in fairly good shape; if you have, on the other hand, a 10 volt margin, you're in great shape!


Click image to Enlarge	Click image to Enlarge

Squegging *
Uncontrolled, unwanted or parasitic oscillation, varying in amplitude from some peak value to very low, or completely off. The frequency of this oscillation is high compared to the rate at which its varying.

Suggestions are Solicited, P l e a s e !

: a
BASIC POWER SUPPLY APPLICATION GUIDE
There are four basic types of power supplies used:
1) Unregulated Linear
2) Regulated Linear
3) Ferroresonant
4) Switch Mode
The differences between the four types include constant voltage output* cost, efficiency, size, weight, and ripple. We will explain each type of supply, describe the principle of operation, and the advantages and

3) Ferroresonant Power Supplies
A ferroresonant power supply is very similar to an unregulated power supply except for the characteristics of the ferroresonant transformer.

The ferroresonant transformer will supply a constant output voltage over a wide variation of the transformer input voltage. The problems with using a ferroresonant power supply include that it is very sensitive to slight changes in line frequency and would not be swrtchable from 50 Hz to 60 Hz, and that the transformers dissipate more heat than conventional transformers. These power supplies are heavier and will have more audible noise from the transformer resonance than regulated linear power supplies.

RECTIFICATION CIRCUITS FOR REGULATED LINEAR POWER SUPPLIES
From our previous description, a regulated linear power supply is the most economical design for lower power, low ripple, and low regulation which is suitable for electronic applications. In this section we will explain the four basic rectification circuits that are used:
1) Half Wave
3; Full Wave Bridge 4) Oual Complementary