How Zener Diode works as a Voltage Regulator?
Zener Diode Voltage Regulator:
Power supply Design doesn't mean designing complicated Switch Mode Power Supply only, even a Zener based Linear Power Supply(LPS) is a kind of power supply design. It is also called as Zener Diode Voltage regulator or simply Zener Voltage regulator.
In this post, we will see How to design the zener shunt regulator and How to select the components, Constraints, Disadvantages etc.
It is recommended to know about Zener Diode: basics, Operation, Characteristics for proper understanding Zener Diode Voltage Regulator.
One of the simple Linear Power Supply is the Zener Shunt regulator. This type of regulator is typically used for very low voltage regulation for less than 200mW of a load. A series resistance(R) is placed between a higher voltage(Vin) and it is used to limit the current to the load and zener diode. The zener diode compensates for the variation in load current.
The diodes used as voltage regulators are called zener diodes. A regular diode will not work effectively as a zener diode,and vice versa. A simplified typical zener regulating circuit is shown below.
- Assume zener diode is rated at VZ = 18V.
- The zener's function is to limit the voltage across the load, to nearly 18V, if the supply voltage is 18V or more.
- With the supply voltage below 18V, the zener will not conduct. The output voltage will be equal to supply voltage minus voltage drop of the resistor R. (ie, VS - VR).
- If the supply voltage is turned up to exceed 18V, the zener will start to conduct.
- The zener voltage VZ will drift with temperature. It is the disadvantage of this linear power supply. The drift characteristics are given in many zener diode data sheets.
- The Zener diode must have an adequate power rating to avoid the burn out.
- The limiting voltage depends upon the particular zener diode used. These diodes are available in the market for any reasonable power supply voltage. These voltage regulators are available in the form of ICs.
Vin(min) > Vout+3
Vz = Vout
R = Vin(min)/1.1 Iout(max)
PD(R) = (Vin(max) - Vout)2R
A zener diode voltage regulator is inefficient when the supply is used with equipment that draws high current. When a supply must deliver a lot of current, a power transistor is used along with the Zener diode as shown below.
Vin(min) > Vout+2.5V
R = Vin(min). hFE(min)/1.2 Iout(max)
Vz = Vout + 0.6V
The series-pass transistor (or pass-transistor) is operating in the linear region of its voltage-current characteristics. ie., It acts like a variable resistor. Thus the transistor drop voltage across itself. This excess voltage is known as ‘headroom’ of the linear regulator which has the value of Vo<Vin. Therefore the linear regulators are in principle always step-down in nature.
Low drop-out regulators:
The linear regulators with Vin is barely exceeding Vo is known as low dropout regulator(LDO)