The regulated power supply may use zener diode as the voltage controlling device as shown in fig. 4. The output voltage is determined by the reverse breakdown voltage of the zener diode. This is nearly constant for a wide range of currents. The load voltage can be maintained constant by controlling the current through zener.
The zener diode regulator has limitations of range. The load current range for which regulation is maintained, is the difference between maximum allowable zener current and minimum current required for the zener to operate in breakdown region. For example, if zener diode requires a minimum current of 10 mA and is limited to a maximum of 1A (to prevent excessive dissipation), the range is 1 - 0.01 = 0.99A. If the load current variation exceeds 0.99A, regulation may be lost.
To obtain better voltage regulation in shunt regulator, the zener diode can be connected to the base circuit of a power transistor as shown in fig. 5. This amplifies the zener current range. It is also known as emitter follower regulation.
This configuration reduces the current flow in the diode. The power transistor used in this configuration is known as pass transistor. The purpose of CL is to ensure that the variations in one of the regulated power supply loads will not be fed to other loads. That is, the capacitor effectively shorts out high-frequency variations.
Because of the current amplifying property of the transistor, the current in the zenor dioide is small. Hence there is little voltage drop across the diode resistance, and the zener approximates an ideal constant voltage source.
Operation of the circuit:
The current through resistor R is the sum of zener current IZ and the transistor base current IB ( = IL / β ).
IL = IZ + IB
The output voltage across RL resistance is given by
VO = VZ – VBE
Where VBE » 0.7 V
Therefore, VO= constant.
The emitter current is same as load current. The current IR is assumed to be constant for a given supply voltage. Therefore, if IL increases, it needs more base currents, to increase base current Iz decreases. The difference in this regulator with zener regulator is that in later case the zener current decreases (increase) by same amount by which the load current increases (decreases). Thus the current range is less, while in the shunt regulators, if IL increases by ΔIL then IB should increase by ΔIL / β or IZ should decrease by ΔIL / β. Therefore the current range control is more for the same rating zener.
The simplified circuit of the shunt regulator is shown in fig. 6.
In a power supply the power regulation is basically, because of its high internal impedance. In the circuit discussed, the unregulated supply has resistance RS of the order of 100 ohm. The use of emitter follower is to reduce the output resistance and it becomes approximately.
RO = ( Rz + hie ) / (1 + hfe)
Where RZ represents the dynamic zener resistance. The voltage stabilization ratio SV is approximately
SV = ∂ Vo / ∂ VI = Rz / (Rz + R)
SV can be improved by increasing R. This increases VCE and power dissipated in the transistor. Other disadvantages of the circuit are.
- No provision for varying the output voltage since it is almost equal to the zener voltage.
- Change in VBEand Vz due to temperature variations appear at the output since the transistor is connected in series with load, it is called series regulator and transistor is allow series pass transistor.