D.C. to D.C. converters and D.C. to A.C. Converters belong to the category of Switched Mode Power Supplies (SMPS). The various types of voltage regulators, used in Linear Power Supplies (LPS), fall in the category of dissipative regulator, as they have a voltage control element usually transistor or zener diode which dissipates power equal to the voltage difference between an unregulated input voltage and a fixed supply voltage multiplied by the current flowing through it. The switching regulator acts as a continuously variable power converter and hence its efficiency is negligibly affected by the voltage difference. Hence the switching regulator is also known as ‘non-dissipative regulator’. In a SMPS, the active device that provides regulation is always operated in cut-off or in saturation mode.
The input D.C. Supply is chopped at a higher frequency around 15 to 50 kHz using an active device like the BJT, power MOSFET or SCR and the converter transformer. Here the size of the ferrite core reduces inversely with the frequency. The lower limit is around 5 kHz for silent operation and an upper limit of 50 kHz to limit the losses in the choke and in active switching elements. The transformed wave form is rectified and filtered. A sample of the output voltage is used as the feedback signal for the drive circuit for the switching transistor to achieve regulation.
Block Diagram of SMPS Working
The oscillator in above figure allows the control element to be switched ON and OFF. The control element usually consists of a transistor switch, an inductor, and a diode. For each switch ON, energy is pumped into the magnetic field associated with the inductor which is a transformer winding in practice. This energy is then released to the load at the desired voltage level.
By varying the duty cycle or frequency of switching, we can vary the stored energy in each cycle and thus control the output voltage. Higher efficiency is obtained since only the energy required is pumped to maintain the load current hence no power dissipation.
The major feature of SMPS is the elimination of physically massive power transformers and other power line magnetic. The net result is smaller, lighter package and reduced manufacturing cost, reducing primarily from the elimination of the 50 Hz components. The basic concept of switching regulator in a simple form is shown in this figure below.
Types of SMPS:
· D.C. to D.C. Converter:
· Forward Converter:
· Flyback Converter:
· Self-Oscillating Flyback Converter:
D.C. to D.C. Converter:
The block diagram of D.C. to D.C. converter (SMPS) is shown here.
Here, the primary power received from AC main is rectified and filtered as high voltage DC. It is then switched at a huge rate of speed approximately 15 kHz to 50 kHz and fed to the primary side of the step-down transformer. The step-down transformer is only a fraction of the size of a comparable 50 Hz unit thus reliving the size and weight problems. The output at the secondary side of the transformer is rectified and filtered. Then it is sent to the output of the power supply. A sample of this output is sent back to the switch to control the output voltage.
SMPS rely on PWM to control the average value of the output voltage. The average value of the repetitive pulse waveform depends on the area under the waveform. As load increases, output voltage tends to fall. Most switching power supplies regulate their output using the method called Pulse – Width Modulation (PWM). The power switch which feeds the primary of the step-down transformer is driven by the PWM oscillator. When the duty cycle is at 50%, then the maximum amount of energy will be passed through the step-down transformer. As the duty cycle decreases the power transmitted is less hence low power dissipation.
The Pulse Width signal given to the switch is inversely proportional to the output voltage. The width or the ON time of the oscillator is controlled by the voltage feedback from the secondary of the rectifier output and forms a closed loop regulator. Since switching regulator is complex, modern IC packages like Motorola MC 3420/3520 or Silicon General SG 1524 can be used instead of discrete components.