Zener diodes for voltage regulation

2.3.3 · Hardware › Diodes & Applications

Zener diode KYA hai?

Woh ek fixed voltage par breakdown KYU karta hai?

Zener effect (low $V_{Z}$ , ≲ 5 V): heavy doping → bahut thin depletion region → strong internal E-field electrons ko bonds se khींch leta hai (tunneling).
Avalanche effect (high $V_{Z}$ , ≳ 6 V): carriers field se accelerate hokar chain reaction mein aur carriers ko knock out karte hain.

Dono ek hi behaviour dete hain: current tezi se badh sakta hai jabki voltage muskil se hilta hai. Woh steep, near-vertical part of the reverse curve wahi hai jis par hum jiite hain.

WHY yeh regulate karta hai: flat curve

$V_{Z} (actual) = V_{Z 0} + I_{Z} r_{Z}$

$V_{Z 0}$ = ideal knee voltage, $r_{Z}$ = breakdown region ki slope (kuch Ω, ideally 0).
YEH form KYU? Ek straight line $V = V_{in t er ce pt} + (slope) \times I$ . Chhota $r_{Z}$ ⇒ achha regulator.

Shunt regulator KAISE banate hain (scratch se derive karo)

Hum chahte hain ek fixed output $V_{o u t} = V_{Z}$ ek unregulated, wobbly input $V_{in}$ se, jo ek load $R_{L}$ ko feed kar raha ho.

Figure — Zener diodes for voltage regulation

Circuit: $V_{in} \to$ series resistor $R_{S} \to$ node (output). Node par, ek Zener ground tak jaata hai (reverse biased) load $R_{L}$ ke parallel mein.

1. Series current (Ohm's law across $R_{S}$ ): $I_{S} = \frac{V _{in} - V _{Z}}{R _{S}}$ Yeh step KYU? $R_{S}$ ke across poora voltage drop $V_{in} - V_{o u t}$ hai, aur $V_{o u t} = V_{Z}$ jab Zener conduct karta hai.

2. Load current: $I_{L} = \frac{V _{Z}}{R _{L}}$ KYU? Load ke across exactly $V_{Z}$ hai (woh Zener ke parallel mein hai).

3. Node par Kirchhoff's current law: $I_{S} = I_{Z} + I_{L} \Rightarrow I_{Z} = I_{S} - I_{L}$ KYU? Current in ( $I_{S}$ ) Zener branch aur load branch mein split hota hai.

Yeh actually "regulate" KAISE karta hai?

Agar $V_{in}$ badhe → $I_{S}$ badhega → extra current Zener ke through dump hoga ( $I_{Z}$ up), $V_{Z}$ muskil se hilega. Zener ek shunt ki tarah kaam karta hai (ek controllable bypass).
Agar $R_{L}$ ghate (load zyada draw kare) → $I_{L}$ up → $I_{Z}$ down, Zener apna kuch share chhod deta hai, output hold karta hai.

Zener ek pressure-relief valve ki tarah hai: woh slack soak kar leta hai taaki output flat rahe.

Do limits jinhe aapko ZAROOR respect karna hai

Design ke liye worst cases:

Minimum $I_{Z}$ tab hota hai jab $V_{in}$ sabse low ho aur load current sabse high ho.
Maximum $I_{Z}$ tab hota hai jab $V_{in}$ sabse high ho aur load remove ho ( $I_{L} = 0$ ).

Worked example 1 — basic operation

Worked example 2 — load remove hai (danger check)

Worked example 3 — $R_{S}$ size karna

Line & load regulation (yeh kitna achha hai?)

Common mistakes (steel-manned)

Active recall

Recall Pehle predict karo, phir verify karo

Jab load disconnect ho toh Zener ke through kitna current bahta hai? → Poora $I_{S}$ (kyunki $I_{L} = 0$ ).
Minimum $I_{Z}$ ke liye worst case kaunsa hai? → Min $V_{in}$ , max load.
Line regulation kitni achhi hai yeh kya decide karta hai? → Ratio $r_{Z} / (R_{S} + r_{Z})$ .

Recall Feynman: ek 12-saal ke bachhe ko samjhao

Socho ek paani ka tank jise tum exactly same level par rakhna chahte ho, chahe aane wala hose kabhi tezi se bahay aur kabhi dhire. Tum us height par jahan tum chahte ho ek overflow drain lagate ho. Agar bahut zyada paani aata hai, woh bas drain se nikal jaata hai — level wahi rehta hai. Agar log bahut paani le jaate hain, kam drain ke neeche girata hai. Zener woh overflow drain hai, lekin electricity ke liye: woh extra current dump karta hai taaki voltage "level" $V_{Z}$ par rahe.

Flashcards

In what bias region does a Zener regulate?

Reverse breakdown (cathode positive side ki taraf).

Write the three master equations of a Zener shunt regulator.

I_{S} = (V_{in} - V_{Z}) / R_{S}

I_{L} = V_{Z} / R_{L}

I_{Z} = I_{S} - I_{L}

What happens to

I_{Z}

when

V_{in}

increases?

I_{S}

badhta hai, extra current Zener ke through shunt hota hai,

V_{o u t}

~constant rehta hai.

When is Zener current maximum (danger)?

Sabse high

V_{in}

ke saath no load (

I_{L} = 0

), isliye

I_{Z} = I_{S}

When is Zener current minimum (regulation may fail)?

Sabse low

V_{in}

ke saath maximum load current.

Formula for the largest allowable series resistor?

R_{S, ma x} = (V_{in, min} - V_{Z}) / (I_{Z min} + I_{L, ma x})

Why does the Zener give a constant voltage?

Iska reverse breakdown I–V curve lagbhag vertical hai; bada

Δ I

bahut chhota

Δ V

deta hai.

What is Zener (dynamic) resistance

r_{Z}

Breakdown region ki choti si slope;

V_{o u t}

V_{Z 0} + I_{Z} r_{Z}

ki tarah drift karta hai.

Line-regulation factor for output vs input change?

Δ V_{o u t} = Δ V_{in} \cdot r_{Z} / (R_{S} + r_{Z})

Zener vs avalanche breakdown — which is which by voltage?

Zener (tunneling) ~5 V se neeche dominate karta hai; avalanche ~6 V se upar.

How do you find the power dissipated in the Zener?

P_{Z} = V_{Z} I_{Z}

; diode ki rating se neeche rehna chahiye.

Connections

Diodes & Applications
PN junction and reverse breakdown
Ohm's law and Kirchhoff's current law
Voltage dividers
Linear voltage regulators (series pass, LDO)
Power dissipation and thermal ratings
Rectifiers and filtering (unregulated DC source)