Linear regulated power supply
Linear Regulated Power Supply:
Fig: Schematic linear voltage regulator Fig: schematic switched mode dc to dc chopper circuit
Fig: Typical voltage o/p from rectifier-capacitor combination
- In most of the basic block for a linear power supply operating from an unregulated dc input, the unregulated dc voltage is most often derived from the utility ac source. The utility ac voltage is first stepped down using a utility frequency transformer, then it is rectified using diode rectifier and filtered by placing a capacitor across the rectifier output.
- The voltage across the capacitor is still fairly unregulated and is load dependent. The ripple in the capacitor voltage is not only dependent on the capacitance magnitude but also depends on load and supply voltage variations.
- The unregulated capacitor voltage becomes the input to the linear type power supply circuit. The filter capacitor size is chosen to optimize the overall cost and volume. However, unless the capacitor is sufficiently large the capacitor voltage may have unacceptably large ripple.
- For proper operation of the voltage regulator, the instantaneous value of unregulated input voltage must always be few volts more than the desired regulated voltage at the output. Thus the ripple across the capacitor voltage (difference between the maximum and minimum instantaneous magnitudes) must not be large or else the minimum voltage level may fall below the required level for output voltage regulation.
- The magnitude of voltage-ripple across the input capacitor increases with increase in load connected at the output. The step down transformer talked above should be chosen such that the peak value of rectified voltage is always larger than the sum of bare minimum voltage required at the input of the regulator and the worst-case ripple in the capacitor voltage.
- Thus the transformer turns ratio is chosen on the basis of minimum specified supply voltage magnitude. The end user of the power supply will like to have a regulated output voltage (with voltage ripple within some specified range) while the load and supply voltage fluctuations remain within the allowable limit.
- To maintain the allowable limit the unregulated dc voltage is fed to a voltage regulator circuit. The circuit in Fig. shows, schematically, a linear regulator circuit where a transistor is placed in between the unregulated dc voltage and the desired regulated dc output.
- Difference between the instantaneous input voltage and the regulated output voltage is blocked across the collector -emitter terminals of the transistor.
- In such circuits the lowest instantaneous magnitude of the unregulated dc voltage must be slightly greater than the desired output voltage (to allow some voltage for transistor biasing circuit).
- The power dissipation in the transistor and the useful output power will be in the ratio of voltage drops across the transistor and the load (here the control power dissipated in the base drive circuit of the transistor is assumed to be relatively small and is neglected).
- The worst-case series voltage drop across the transistor may be quite large if the allowed variation in supply magnitude is large. Worst-case power dissipation in the transistor will correspond to maximum supply voltage and maximum load condition (load voltage is assumed to be well regulated). Efficiency of linear voltage regulator circuits will be quite low when supply voltage is on the higher side of the nominal voltage.
Questions of this topic
An 18V (rms), 50 Hz supply is rectified using a full bridge diode rectifier and is followed by a capacitor filter. The load connected across the capacitor is a simple resistor of 30 ohm. What should be the value of filter capacitor to get only 5 volts peak to peak ripple across the load voltage? Neglect voltage drop across conducting diode.