Types of Power Supplies
Hybrid (SMPS followed by linear) power supply:
- A comparison of linear and switched mode power supplies tells about the advantages and disadvantages of the two. Linear power supply is highly inefficient if it has to work over large variations in input voltage, is more bulky because of the use of low frequency transformer and filter elements (inductors and capacitors).
- On the other hand linear power supplies give better output voltage regulation. It may sometimes be required to have output voltage regulation similar to the one provided by linear supplies and compactness and better efficiency of a switched mode supply. For this, the linear power supply may be put in tandem with a switched mode supply.
- Let us consider a case where one needs an isolated and well-regulated 5 volts output while input power is drawn from utility supply that has large voltage fluctuation. In such a situation one may generate an isolated 7.5 volts from an SMPS and follow it by a 5 volts linear power supply set to work with 7.5 volts input. The input to linear power supply must be few volts more than the required output (for proper biasing of the switches) and hence SMPS tries to maintain around 7.5 volts input.
- It can be seen that the linear power supply now does not have large input voltage variation in spite of large variations in the utility rms voltage. The SMPS portion of the power supply efficiently performs the job of voltage isolation and conversion from widely varying utility voltage to fairly regulated 7.5 volts dc.
- Under the given condition it may not be difficult to see that the overall efficiency of this hybrid power supply will lie between that of a SMPS and a linear supply. The overall cost may or may not increase even though two supplies in tandem are used.
- It is to be kept in mind that to achieve the same output voltage specification by an SMPS circuit alone, the control and filtering circuit may become more costly and complex (than the one used in the hybrid power supply unit). Similarly if the linear supply has to be designed for larger fluctuation in input voltage the component ratings, including heat-sink ratings, will be higher and may cost as much as the hybrid unit.
Multiple output SMPS:
- A single power supply unit may need to output several different voltages. The individual output voltages may have different ratings in terms of output current, voltage regulation and ripple voltages. These outputs may need isolation between them.
- Generally a common high frequency transformer links the input and output windings and in spite of output voltage feedback all the outputs cannot have same regulation because of different loads connected to different outputs and hence different ohmic (resistive) drops in the output windings (loads are generally variable and user dependent).
- Also the coupling between the different secondary windings and the primary winding may not be same causing different voltage drops across the respective leakage inductances. Barring this mismatch in the voltage drops across the resistances and leakage inductances of the secondary windings their output voltages are in proportional to their turns ratios.
- The turns ratios are properly chosen to give fairly regulated individual output voltages (even if only one output voltage feedback is used for SMPS switch control).
- The output that needs to have tighter voltage regulation may be used for output voltage feedback. In case another output needs to have similarly tight regulation then that particular output may be passed through an additional linear regulator circuit as in the case of hybrid power supply circuit.
Resonant Mode Power Supplies:
- Resonant mode power supplies are a variation over SMPS circuits where the switching losses are significantly reduced by adapting zero-voltage or zero-current switching techniques.
- In non-resonant mode SMPS circuits the switches are subjected to hard switching (during hard-switching, both the voltage and current in the switch are of considerable magnitude resulting in large instantaneous switching power loss).
- Efficiency of resonant mode power supplies is generally higher than non-resonant mode supplies.