Practical Power Supply Theory

04/21/2003

Special Notes:

1. Outline of Power Supplies
2. A 83Kb pdf file to print out, to be used as reference (or Test Questions) in this document.
3. A 110Kb pdf file (PwrCkts1) to print out, with basic variations to be used as reference diagrams in this document by appropriate Figure #'s indicated in Blue.
4. A 93Kb pdf file (PwrCkts2) to print out, with basic variations to be used as reference diagrams in this document by appropriate Figure #'s indicated in Blue.
5. Any Power Supply System has the task of supplying the necessary voltage and current into the given load without significant variations.
6. Diode rectifiers need to be evaluated not only in terms of their average current and peak current, but also in terms of the peak reverse voltage breakdown (PIV).
7. It is important to realize that the effectiveness of any filter is its ability to maintain the same degree of filtering under load variations.
8. Switching Power Supply Concepts will be covered following the Linear Power Section.

Rectifier Systems

• Basic Half-Wave Rectifier System (Figure #1) :
  • This is a simple system, but with it's simplicity it also has an inherent problem. It only operates half of the time, as in only every half cycle, to feed voltage and current into the filter system.
  • If the line frequency is 60 Hz then that means that this rectifier system will send pulses into the filter at 60 pulses per second. This will show up as slight variations in the output voltage called "Ripple Voltage" and a "Ripple Frequency" of 60 Hz.
  • This requires a rather heavy-duty rectifier diode to provide the necessary heavy pulses of current into the filter in order to be able to provide an average current into the load, without significant voltage variations across the load. The rectifier diode in fact, needs to be at least 2 times the average output current, and actually requiring up to a peak of 5 times the average output current.
  • This is however a suitable system for simple load requirements, allowing low cost and light weight construction.
• Basic Full-Wave Rectifier System
  • The diodes conduct alternately on the half cycles, giving twice the number of re-charge cycles.
  • This rectifier system provides an advantage over the Half-Wave by the fact that this filter is re-charged at twice the line frequency, and therefore could maintain a slightly lower variation in voltage between re-charge pulses.
• Full-Wave Center Rectifier Systems using a Center Tapped Transformer (Figure #2)
  • The power rating of the transformer is a primary consideration here, where transformers are rated in VoltAmperes, rather than simple Wattage. Wattage infers pure resistive loads, with no reactance elements, but since there may very well be some reactive components in the load, VoltAmperes allows consideration of those.
  • With only one half of the output winding of the transformer supplying current per half cycle, the power rating of the transformer is based on this factor.
  • Note however that this split output voltage of the transformer secondary allows only half the total across the secondary winding.
  • If you lost one of the diode rectifiers, the system reverted to a half-wave system.
• Full-Wave Bridge Rectifier System across the whole Transformer Secondary
  • In this Rectifier System, the full transformer secondary voltage is available, but at the cost of limiting the current capability to half that of the Full-Wave Center-Tapped System.
  • The answer to this is simple. I.e. Transformer ratings are based on VoltAmperes, which simply means if you get twice the voltage, you must limit the current to half to keep from burning up the transformer.
  • Since the Bridge-Rectifier System uses 4 diode rectifiers, with 2 diodes in series conduction at a time, each of the diodes need only be half the breakdown voltage voltage of the Full-Wave Center-Tap Rectifier System
  • Here also, if you lost just one of the 4 diode rectifiers, the system would revert to a half-wave system.
• Full-Wave Bridge Rectifier Component, Center-Tapped Transformer, Dual Voltage System (Figure #3) :
  • In this interesting system, even though a Bridge Rectifier Component is used, one half of the Bridge component is used as part of a Center-Tapped Transformer Rectifier System for one voltage polarity output, and the other half of the Bridge component is used in the same way for the opposite polarity output.
  • This method is an interesting way of obtaining a dual polarity output power supply. Even though a Bridge Component is used, it is actually used as if it were two separate pairs of rectifiers.

Filter Systems

Special Notes to help understand just how filters really work:

1. Capacitor Filters (Figure #1) :
   1. A very important theoretical point about capacitors is that they do NOT like changes in voltage. They react to any changes in voltage with either absorbing currents to keep the voltage from rising, or by giving up these stored currents to keep the voltage from dropping. It is the property that reacts this way is where we identify with "Capacitive Reactance:
   2. We need to remember that this is accomplished by a very important aspect. I.e. When the voltage begins to rise, ENERGY is stored in the Electrostatic Field, and when the voltage begins to fall, this stored ENERGY is delivered from that same Electrostatic Field back into the circuit.
2. Inductive Filters (Figure #2) :
   1. Here, a very important theoretical point about inductors is that they do NOT like changes in current. They react to any changes in current by counteracting the voltage changes to either opposing the supplied voltage to keep the currents from rising, or aiding the supplied voltage to keep the currents from falling.
   2. We need to remember that this is accomplished by a very important aspect. I.e. When the current starts to rise, ENERGY is stored in the Electromagnetic Field, and when the current begins to fall, this stored ENERGY is delivered from that same Electromagnetic Field back into the circuit.
3. Simply stated, Capacitors store energy in Electrostatic Fields, and Inductors store energy in Electromagnetic Fields. In both cases, this energy is stored and not lost. The faster these changes occur, the more the Capacitor or Inductor "React", hence the terms "Inductive Reactance", and "Capacitive Reactance".

• Basic Single Capacitor Filter: (called "Capacitive Input Filter")
  • It is important to realize that when this power supply system is first turned on, the capacitor is at the very first instant a DEAD SHORT. This means that the diode that rectifies the ac current to supply a DC current to the filter will need to be able to handle a surge current of up to 5 times the average output current.
  • Using a simple filter capacitor as the one and only filter, is only acceptable if light loads (and therefore small currents) are required. This is simply because this capacitor only charges at the line frequency of 60Hz, and as the load draws current off from the capacitor it will need to re-charge. The problem here is that the re-charge rate is by pulses, and this can cause serious voltage variations. Remember that we are still dealing with "Stored Energy" here.
  • A single low ohmic value resistor placed prior to this Single Capacitor as a Filter will resist the high initial surge currents, but at a lower overall output voltage, and of course a power loss in the resistive element.
• Basic Single Inductive Filter ("Choke Input"):
  • An Inductive Filter, due to the construction of the iron core, wire wound inductor, there are cost, weight, and size considerations.
  • The nature of the Inductive Filter is such that there are no heavy surge currents going through the rectifier diodes, however the price of a more stable current supply, is a slightly lower voltage.

• Combination Filter Systems:
  • Inductor-Capacitor ("LC L-Type Filter" - Figure #2 )
    • This system has the distinct advantage of very good current control under heavy load variations.
    • The Inductor as a component immediately after the diode rectifier allows current surges to be stored in the electromagnetic field, and voltage variations are stored in the electrostatic field of the capacitor that follows the inductor.
    • This type of Filter System is quite well suited for Radio Transmitters or Modulators, where heavy current variations are the norm.
  • Capacitive-Resistive-Capacitive ("RC Pi-Type Filter" - Figure #1 )
    • This very common system has the advantage of low cost, lightweight, and moderate size, for loads that remain fairly stable. There is a slight power loss however in the series resistive component.
    • This system also provides a slightly higher output voltage than the "LC L-Type" Filter, but may well exhibit greater ripple voltages as well. This is usually considered reasonable for low cost consumer products.
  • Capacitive-Inductive-Capacitive ("LC Pi-Type Filter")
    • This filter system is usually employed where greater power requirements exist, and the need for better current regulation allowed by the Inductor vs the Resistor.
    • An inherent problem still exists here though, in that with the capacitor filter as the lead element, you again have the problem of the high surge currents through the rectifier diode(s).
  • Inductive-Capacitive-Inductive-Capacitive ("Dual LC L-Type Filter")
    • The disadvantage of the LC Pi-Type Filter is overcome by the addition of a leading Inductive element, by absorbing (or limiting) the rectifier diode(s) surge current.
    • The advantage of this system is superior handling of heavy load variations, but at the cost of added size, weight, and of course cost.
  • The use of the Swinging Choke and the Smoothing Choke (another version of the "Dual LC L-Type Filter")
    • It was determined that when there were small current variations, a higher value of Inductance was needed to stabilize the current variations. In that same regard, it was also determined that less Inductance was needed for large current variations.
    • A special construction of an Inductor was developed, where the slope of the Inductance was variable based on the amount of current within the Inductor. A small amount of current through the Inductor would find quite a bit of Inductance, but as the current increased, the Inductor would begin to saturate, and the result would be less effective Inductance.... hence the term "Swinging Choke".
    • The "Swinging Choke" would be the leading component, with a conventional "Smoothing Choke" nestled in between the two Capacitive elements. For all purposes, this is simply using a "Swinging Choke" followed by a more conventional "Pi-Type LC Filter".

Regulation Considerations and Systems

Note: References to "Active Element" indicates amplification, where a reference Zener Diode or VR Tube is a "Passive Element", but that "Active Element" would be any Amplifier with a Gain of more than 1.0

Special Notes

1. The major aspect of Series-Pass Post-Regulation is that the Series Element increases or decreases conduction (or resistance) to alter the voltage drop across that Series Element and thereby maintaining a constant voltage at the output to the load as the load changes.
2. The major aspect of Shunt Post-Regulation is to maintain a constant load on the power source and thereby maintaining a constant voltage at the output to the load as the load changes.
3. In all cases describing the Regulator System, it is understood that the Power Source includes a suitable Filter System prior to the Regulator System.

• The simplest form of Shunt Post-Regulation is with the use of a Zener Diode and a series dropping resistor. It should be understood that the Zener Diode's role is to take up the slack or back off in conduction during load variations (Figure #2) .
  • The value of the Zener is in 2 major aspects:
    • The desired voltage, such as 9V.
    • The required wattage that the Zener must be able to dissipate.
  • The value of the series resistor is based on the minimal and maximum current flow and variation, and the necessary voltage drop between the power source and the output to the load.
    • If the desired output is 9V and the source voltage is 10.5V, then 1.5V must be dropped across the resistor.
    • The Zener Diode has a minimal current that must flow through it to begin regulation, and a maximum current to stay with safe operating condition.
    • The resistor value is calculated by simple ohm's law, using the required voltage drop of the 1.5V and the maximum required current to the load plus the minimal current through the Zener Diode. Don't forget that the required wattage of the resistor should be about twice the computed value.
    • The last important consideration here is that you need to take into account that if the load demand drops, the Zener will conduct more by that same amount to take up the slack. If the load drops it's demand too much, the Zener may conduct too much (trying to take up all that slack) and simply go out in a flash!
    • The advantage of this system is simplicity, lightweight, and low cost. The second advantage is that of any Shunt Regulation method, in that the overall load on the Power Source remains relatively constant.
    • However, this can also be a disadvantage in that there is no power conservation here, because there is a constant demand on the Power Source. Obviously, this is not normally a good choice where a battery system is used as the source, unless you like the idea of having a "handy-dandy-battery-runner-downer".

• Perhaps the simplest form of Series-Pass Post-Regulation is by the use of a single Series-Pass Controllable Element, such as a NPN Power Transistor between the Power Source and the varying Load, with a Zener Diode as a Reference Voltage in the Base Circuit (Figure #3) .
  • In this System the Collector is tied to the output of the Power Source and the Emitter directly feeds the varying Load.
  • With a Zener Diode in the Base as a Voltage Reference, the Emitter behaves as an "Emitter Follower", sometimes called a "Voltage Follower". The difference between the Base Voltage and that of the Emitter would be the normal 0.5V or 0.6V drop across the conducting Base-Emitter junction.
  • An example might be with a Source Voltage of +15V to the Collector, and a 12.6V Zener as a Reference Diode between the Base and ground. Current flowing from the Emitter to the Base would cause a 0.6V drop across the junction.
  • Please note that this Zener Diode must be held in conduction through a resistor to the Power Source, in order for the Zener to operate properly and establish a Reference Voltage for the Base.
  • To understand properly just how effective and simple this method is, consider what happens if the Load should increase, which will tend to pull down the Output Voltage. Let's assume that the drop tries to be only 0.1V.
    • The voltage at the Emitter will tend to become "less positive" by that 0.1V drop, which will increase the biasing voltage across the Emitter-Base junction to 0.7V, because the Zener Diode is still holding at +12.6V.
    • Suddenly, this Power Transistor is told to conduct a whole lot more than it was (which can be translated also as a lot less resistance and therefore less Voltage Drop across the Power Transistor).
    • In effect, this means that this Power Transistor is suddenly, and quite literally, pulling the Output Voltage back up to where it belongs, by it's increased conductive effort.
    • The advantage of this system is it's simplicity and relatively low cost, although a heat sink is usually required for the Series Power Transistor. After all, whatever current is required for the Load and it's variations must go through this Series-Pass-Element.
    • A distinct advantage of this method of Post-Regulation is that it will only supply current as needed. The only exception to that is that the Zener needs to kept alive with a small current.
  • Here is a short Quiz on this Section that I hope you find useful and perhaps even a little fun to do.
  • For an explanation of this circuit, see Series-Pass Regulation, using Reference/Sensor/Amplification

Reference & Active Series Element: (Figure #1 of the "Transistor Circuit Description" Sheet)

• In this slightly deceptive circuit, the Zener Diode (Z1) establishes the Base Bias Voltage for the Series-Pass Transistor (Q1). The Resistance Value (R1) in series with that Zener establishes the proper Bias Current for the proper operation of the Zener Diode.
• Even though the transistor Q1 may appear to be a "Common Emitter Configuration", it is not. Here we find the control for Q1 applied to the Base, and the Emitter is the output. Following that premise, that tells us that this is actually a "Common Collector Configuration", otherwise known as simply an "Emitter Follower".
• As an Emitter Follower, we know that the Emitter Output Voltage will simply "Follow" the Base Voltage, which stays established as a fixed voltage.
• However, we need to take into consideration that there is about 0.6V drop across the Base/Emitter Junction, and therefore the Emitter Voltage will be 0.6V lower that the Base Voltage of +5.6V, making the Emitter Voltage equal to +5.0V.
• What we have now established is that this is a very simple, but yet effective, "Constant Voltage Source" of +5V.

• Q1 functions as a Series-Pass Transistor, with the conduction of Q2 controlling the conduction of Q1.
• The 9V Zener (Z1) functions like a physical "Tie-Bar" of an automobile that maintains a constant distance between the 2 front wheels of a car, where if one wheel turns sideways - so does the other.
  • The 9V Zener in this circuit behaves like an "Electrical Tie-Bar", maintaining a 9V constant separation between the + Output Voltage, and yet allow any slight variation that happens at the output to be passed directly as a one-to-one relationship (mv to mv).
  • If the + DC Output starts to rise slightly, this slight rise in voltage is passed directly to the Base of Q3, which will cause Q3 increase conduction. This increase in conduction will draw down the Collector of Q3 due to the increased IR Drop across R2, which will decrease the voltage applied at the Base of Q2. This will cause the conduction of Q2 to decrease, which will directly cause the conduction of Q1 to decrease. This decrease in the conduction of Q1 (or increase the resistance of Q1) will cause the +DC Output Voltage to fall back to where it belongs.
  • In a nutshell, an slight increase in the Output Voltage will cause Q3 (which acts as an Inverter) to signal Q2 and Q3 to do the opposite, which will bring the Output Voltage back to where it belongs.
• The Variable Resistor R3 will affect the Emiter Bias of Q3, and therefore the conduction of Q3. Any change in this conduction of Q3 will have an Inverting effect on the conduction of Q2 and Q1. I.e. if Q3 conducts less, then Q2 and Q1 will conduct more, which will cause the Output Voltage to rise. We can use R3 to adjust the Output Voltage.
• In summary, the combination of the Zener and Q3 form the Sensor and First Amplifier. Q2 provides aditional Amplification and direct control for the Series-Pass Transistor Q1.
• In a final note: Although Q3 is Directly Coupled to Q2, only Q2 is Compound Connected to Q1, because it is the Collector Current of Q2 that is the Base Current of Q1.

What is wrong or missing in the design of this circuit in the area of Q2?

• In this circuit we find some similar comparisons to the cicuit described above as the "Series-Pass Regulation, using Reference/Sensor/Amplification" in the prior description.
• There are a couple of major differences however:
  • In this method of Output Voltage Control, we are using a "Shunt Conduction" Method, in conjuction with the current passing through a Fixed Resistance.
  • We need to maintain a constant current through that fixed resistance, in order to maintain a constant voltage drop, and therefore a constant Output Voltage.
• We sense slight changes in the Output Voltage (caused by changes in the Load) with the Zener (Z1) passing those changes directly to the Base of Transistor Q1.
• An increase in the Output Voltage (due to a "Lighter Load") will cause Q1 to increase conduction. This change is amplified by Q1, showing up as an increase in the Collector Current (and therefore IR drop across R1), causing an increase in the Base/Emitter Bias of Q2, causing the conduction of Q2 to increase it's conduction of the Collector current that is also the Base current of Q3. When Q3 now conducts more, it will drag down the Ouput Voltage to where it belongs. It does this by increasing it's conduction to compensate for the decresed conduction required by the change in the Load.
• In summary, remember that Shunt Regulation accomplishes Output Voltage Regulation by maintaining a constant current draw from the Source and therefore a Constant IR Drop across the fixed resistance R6.

3-Terminal Regulator (Devices)

Fixed Voltage Devices

• These are really marvelous devices, accomplishing in what appears to be a TO-220 Transistor, what it previously took an entire Regulator Circuit, such as these that we have been describing here. However, this is not a transistor, rather it is about 15 transistors, diodes, and resistors packaged together as a Regulator Circuit!
• They come in +5V, -5V, +9V, -9V, +12V, -12V, +18V, -18V, +24V, -24V, etc. Output Voltage Ratings.
• Moreover, they also come in various Power Ratings as well. One looks like a small Transistor (TO-39 or TO-92), but it can only handle a few milliamps(100ma with the LM78Lxx). The LM78Mxx (T-202) can handle up to 500ma, and The LM78xx in the TO-220 Package can typically handle up to 750ma to 1A with a good heat-sink.
• There is one in a TO-3 Package that can handle 1A easily with a good heat-sink, and there is a special version that can handle 5A!
• The use of the 78xx describes the Positive variations, where the 79xx describes the negative variations.
• The "xx" in the 78xx or 79xx refers to the Voltage Rating.
• Earlier variations of this Package was given as LM340-5, LM340-12, etc., where the the "LM" stood for "Linear-Module".
• There is a lot of good data available in "Linear Databooks", which are available from various manufactures.

Adjustable Voltage Devices

• Some folks have been able to get by with using a Fixed Voltage 3-Terminal Regulator, and by using an offset reference voltage at the ground leg, they are able to obtain a different Fixed Voltage.
• The LM723 is perhaps the best known of several available and common use. It is most comonly found as a DIP Chip, but is available in a metal can version as well.
  • Although this device can be used as an independant device, good for about 150ma as a Variable Voltage Device, it is more often found as the controller for Series-Pass Transistor Circuits for greater current capability.
  • It can be adjusted over a voltage range from 2V to 37V, which makes this a very desireable device.
  • By incorporating good design procedures, Current Limiting and control can be obtained as well.
  • By using a different circuit design, it is also possible to control a Shunt Regulation circuit, instead of Series-Pass Regulation.

Also note the SCS Circuit description for this purpose in the Switching Power Supply Section

Switching Power Supplies:

An interesting thought is that a very early (antique, actually) Power Supply System used for automobile tube type radios, is actually an early type of a "Switching Power Supply".

For more, see our "Switching Power Supplies" presentation