Practical Power Supply Theory

04/21/2003

Considerations:

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 folowing the Linear Power Section.
  9. Switching Power Supply Sample (pdf file)


 Rectifier Systems

 Filter Systems

 Regulation Systems


Rectifier Systems:

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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 case, 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".



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Regulation Considerations and Systems:

  Pre-Regulation

Series Pass Regulation 

Shunt Regulation  
Manual settings   Manual Series Pass  Zener Diode (Passive Element)
Triac   Reference & Active Element  Reference & Active Element
 Saturable Reactor  Amplified Active Element  Amplified Active Element

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.



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Pre-Regulation methods:

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.

  • Series-Pass Regulation, using Reference/Sensor/Amplification: (Figure #3)

  • 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?


    Shunt Regulation, using Reference/Sensor/Amplification (Figure #4):

  • 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:

    Adjustable Voltage Devices:


    "Over Current" Contol and Protection (Figure #5):

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


    3-Phase Rectifier Systems:


    Regulation Systems using Controllable Saturable-Reactors:


    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".

    "Switching Power Supplies" presentation:


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