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Linear Voltage Regulators

In an analog power supply's voltage regulators come into play. After rectification, the dc voltage is filtered using large capacitors but it normally still has a ripple voltage component due to the fact that it makes little sense to select exceedingly large capacitors. In order to suppress the ripple effect and obtain an output voltage which remains stable and constant at variable loads or fluctuating line voltages, voltage stabilisation (regulation) is needed. There are two basic categories of voltage regulation, namely:

  • line regulation
  • load regulation

  

Line Regulation:

When the voltage at the dc input (line voltage, usually after rectifier and filter) fluctuates, the voltage regulator must maintain a nearly constant voltage output. Line regulation can be defined as the percentage change in the output voltage for a given change in the input (line) voltage. It is usually expressed in units of %V. For example, a line regulation of 0.05%V means the output voltage changes 0.05 percent when the input voltage increases or decreases by one volt. Line regulation can be calculated using the following formula (Δ means "a change in", for example, voltage at time 2 minus voltage at time 1).

line-regulation.jpg

Sample calculation of line regulation:

If the input to a voltage regulator decreases from 10 to 5 V, the output decreases by 0.25 V. The nominal output is 15 V. Determine the line regulation in %/V.

line-regulation1.jpg

Load Regulation:

When the amount of current flowing through the load changes due to a varying load resistance, the voltage regulator must maintain a nearly constant output voltage across the load. Load regulation can be defined as the percentage change in output voltage for a given change in load current. It can be expressed as a percentage change in output voltage from no-load (NL) to full-load (FL) as follows:

load-regulation.jpg

Alternatively, the load regulation can be expressed as a percentage change in output voltage for each mA change in the load current. For example, a load regulation of 0.01%/mA means that the output voltage changes 0.01 percent when the load current increases or decreases 1 mA.

Sample calculation of percent load regulation:

A voltage regulator has a 12 V output when there is no load (IL = 0). When there is a full-load current of 10 mA, the output voltage is 11.95 V. Express the voltage regulation as a percentage change from no-load to full-load and also as a percentage change for each mA in load current.

and expressing as %/mA...

Percentage Regulation:

Percentage regulation specifies the performance of a voltage regulator. It can be expressed in terms of input (line) regulation or load regulation. Percentage line regulation specifies how much change occurs in the output voltage for a given change in input voltage.

Line regulation is usually expressed as a percent change in Vout for a 1 V change in VIN (%V): 

percentage-regulation.jpg

Percentage load regulation specifies how much change occurs in the output voltage over a certain range of load current values, usually from minimum current (no load) to maximum current (full load). It is usually expressed as a percentage and can be calculated with the following formula, which is actually the same as the one shown above.

where VNL is the output voltage at no load, and VFL is the output voltage at full (maximum) load.

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