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Design problem from AoE

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stoica adrian:
Hello, i work on this design problem from AoE and i need your help, can you please help me with some advice, on how to resolve this problem, Thanks.

Design a +10V regulated supply for load currents from 0 to 100mA; the input voltage is +20 to +25V. Allow at least 10mA zener current under all(worst case) conditions.

A. what power rating must the zener have?

B. With the same specifications use a sener emitter follower. Calculate worst-case dissipation in transistor and zener.

iMo:
My advice to you would be to download the LTspice and draw the circuit in. You will learn how to design and simulate your AoE circuits.

todorp:
Hi, a long time ago I found solutiuons to some of the exercises for the AoE 2nd edition. Look after page 17 (Bipolar transistors) in the attached pdf file. Some of the exercises in the 3rd edition are the same. Mind it is a good idea to first try to do the exercises as you have done and than use the solutions :-)

T3sl4co1l:
Incidentally, 100 ohms is not a "best fit" common value -- that will violate Iz(min).  Note that Iz(min) / Iz(max) is a factor of 10%, and 100 ohms is larger than 90.9 by about 10%.  Prefer smaller resistances (and adjust power ratings upward as needed).  91 and 82 ohms should be easy enough to find, otherwise 100 || 1k can be used.

P_R is correct given the assumed resistance (100 ohms), but obviously it's a bit higher with the calculated resistance.  By about 20%, or 2.7W.  Because "10% more" (a ratio of 1.1) squared is about 1.2 ("20% more"); or exactly, 1.21.

Pzmax is simply Izmax * Vz or 1.1W.

For B, first, show the circuit.  Consider the zener: what bias does it need*?  Does it depend on the transistor characteristics?  If so, which ones do we need to know to solve it?  Should we begin selecting components immediately, or further consider the problem before looking up parts?

*Note that Vz varies with Iz, and diodes are rated at a given Iz.  So, if we're varying Iz, Vz will vary some as well.  For sake of simplicity, let's just assume that we can select a diode which has the correct Vz at whatever Iz we're using in the circuit.  We can worry about that later (when it comes to component selection).

The transistor dissipation will be closely related to the voltage drop and load current; indeed for common components, it's fair to assume (because the error will be less than, say, 10%) that this is all of it, so we can immediately solve for the approximate power dissipation as simply
(Vin,max - Vout) * Iout,max
or about 1.5W.

Tim

iMo:
Your Rc calculation is not good, imho. The transistor has to get some Vce, say 3-4V.

PS: also your calculation has to involve the transistor too. You need some Ibe to allow for some Ice.

1) I_load = Ice + Ibe

2) Vce = 3V (for example)

3) Vload = (Ice + Ibe) * Rload  = 10V  (it will never be precise 10V with a zener)

4) Ibe = Ice / beta      (where the beta is an amplification coefficient of the transistor at specific Ice and Vce, 50 - 200 for example)

5) I_Rb = Iz + Ibe

6) Iz = (Vin - Vz) / Rb

7) 0 = Vin - Ice*Rc - Vce - Vload

8 ) Vload = Vz - Vbe = Vz - 0.7V  (aprox)

PPS: indexed the equations

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