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9/8/2005 The Shunt Regulator.doc 1/5 The Shunt Regulator R iL i + VS + - V0=VZK iZ RL - The shunt regulator is a voltage regulator. That is, a device that keeps the voltage across some load resistor (RL) constant. Q: Why would this voltage not be a constant? A: Two reasons: (1) the source voltage Vs may vary and change with time. (2) The load RL may also vary and change with time. In other words, the current iL delivered to the load may change. What can we do to keep load voltage VO constant? ⇒ Employ a Zener diode in a shunt regulator circuit! Jim Stiles The Univ. of Kansas Dept. of EECS 9/8/2005 The Shunt Regulator.doc 2/5 Let’s analyze the shunt regulator circuit in terms of Zener breakdown voltage VZK, source voltage VS , and load resistor RL. Replacing the Zener diode with a Zener CVD model, we ASSUME the ideal diode is forward biased, and thus ENFORCE vDi = 0 . ANALYZE: i R iL + v =0 i d VS − + - idi + VZK - + vz= VO RL - From KVL: v Z =VO = vDi +VZK =VZK From KCL: i = iDi + iL where from Ohm’s Law: i = Jim Stiles VS −VZK R The Univ. of Kansas Dept. of EECS 9/8/2005 The Shunt Regulator.doc and also : iL = Therefore: 3/5 VZK RL iDi = i − iL V −VZK VZK = S − R RL = VS VZK (R + RL ) − R RRL CHECK: Note we find that ideal diode is forward biased if: iDi = VS VZK (R + RL ) − >0 R RRL or therefore: VS VZK (R + RL ) − >0 R RRL VS VZK (R + RL ) > R RRL VS RL >VZK R + RL Hence, the Zener diode may not be in breakdown (i.e., the ideal diode may not be f.b.) if VS or RL are too small, or shunt resistor R is too large! Jim Stiles The Univ. of Kansas Dept. of EECS 9/8/2005 The Shunt Regulator.doc 4/5 Summarizing, we find that if: VS RL > VZK R + RL then: 1. The Zener diode is in breakdown. 2. The load voltage VO = VZK . 3. The load current is iL = VZK RL . 4. The current through the shunt resistor R is i = (VS −VZK ) R . 5. The current through the Zener diode is iZ = i − iL > 0 . We find then, that if the source voltage VS increases, the current i through shunt resistor R will likewise increase. However, this extra current will result in an equal increase in the Zener diode current iZ—thus the load current (and therefore load voltage VO) will remain unchanged! R iL i + VS Jim Stiles + - Extra current goes in here! V0=VZK iZ The Univ. of Kansas RL - Dept. of EECS 9/8/2005 The Shunt Regulator.doc 5/5 Similarly, if the load current iL increases (i.e., RL decreases), then the Zener current iZ will decrease by an equal amount. As a result, the current through shunt resistor R (and therefore the load voltage VO) will remain unchanged! R iL i + VS + - iZ Extra current comes from here! RL V0=VZK - Q: You mean that VO stays perfectly constant, regardless of source voltage VS or load current iL?? A: Well, VO remains approximately constant, but it will change a tiny amount when VS or iL changes. To determine precisely how much the load voltage VO changes, we will need to use a more precise Zener diode model (i.e., the Zener PWL)! Jim Stiles The Univ. of Kansas Dept. of EECS