Switched Mode Power Supply, Physics tutorial

Switched-mode power supply operation:

Rapid switching of pass transistor of the switched mode power supply is dependable for operation. The Switched mode power supply operates by regulating either output voltage or current by switching ideal storage elements like inductors and capacitors, into and out of different electrical configurations. This is explained with ideal storage element being inductor. When visualized as the ideal circuit element, pass transistor operates outside the active region and has no resistance when switch is closed while it carries no current when switch is open. Theoretically there is no energy dissipation in ideal switch as product of voltage across switch and current through switch at any given time is zero. All input power is delivered to load and this converter operates at 100% efficiency.

The Direct Current source, inductor, a switch, and corresponding electrical ground are placed in series and switch is driven by the square wave where peak-to-peak voltage of waveform estimated across switch can exceed input voltage from DC source.

The inductor responds to changes in current by inducing own voltage to counter change in current that it adds to source voltage while switch is open. If the diode-and-capacitor combination is placed in parallel to switch, peak voltage can be stored in capacitor, and the capacitor can be utilized as the DC source with the output voltage greater than DC voltage driving circuit. Output current flow depends on input power signal; storage elements and circuit topologies utilized, and on waveform pattern in the switched mode power supply and quite frequently a pulse-width modulation with the adaptable duty cycle are utilized to drive switching elements.

In general, the output parameter like output voltage can be handled by varying duty cycle, frequency or phase shift of switching of active circuit element while output filters average the energy transfer rate and guarantee continuous power flow in load. DC gain of converter is estimated based on fact that in steady state, net volt-seconds across the inductor over one switching cycle should be zero. Typical frequency range of the offline Switched Mode Power Supply is 50 kHz to 500 kHz whereas DC-DC converters with low-voltage input operate up to numerous MHz High operating frequency results in smaller size of switched mode power supplies as usually size of power transformers, inductors and filter capacitors is inversely proportional to frequency and Switched mode operation also decreases energy losses while it increases efficiency.

Rectifier, filter and inverter:

Input rectifier and filter stage converts input to AC to DC voltage through rectification that produces the unregulated DC voltage while pulsating DC is applied across terminals of the large filter capacitor.

Current drawn from input supply by rectifier circuit takes place in short pulses around AC voltage peaks. These pulses have important high frequency energy that decreases power factor and special control methods can be used by chopper stage to force average input current to follow sinusoidal shape of AC input voltage to correct power factor.

Switched mode power supplies with DC input don't need input rectifier and filter stage and switched mode power supply that is designed for AC input can be operated from the DC supply if it doesn't have the input transformer as DC passes through rectifier stage unchanged.

The inverter stage converts Direct Current to Alternating Current through a power oscillator, whose output transformer is very small and that operates at frequency above 20 kHz to make it inaudible to humans but that may be as high as few MHz In practical designs, output voltage is optically coupled to input and therefore very tightly controlled and is electrically isolated from mains potential.

Switching is practically implemented as a multistage high gain MOSFET amplifier because of the MOSFET's low on-resistance and high current handling capacity.

Converter and output rectifier:

In mains power supplies output of the Switched Mode Power Supply is needed to be isolated from input and inverted Alternating Current is utilized to drive primary winding of the high-frequency transformer. This converts voltage up or down to required output level on its secondary winding. Output transformer in block diagram serves this function.

If the DC output is subsequently required, Alternating Current output from transformer is rectified and but for output voltages above ten volts ordinary silicon diodes are usually utilized; lower voltages though need Schottky diodes that have benefits of faster recovery times than silicon diodes and lower voltage drop when conducting. The special case arises when even lower output voltages are needed and when MOSFETs are utilized as synchronous rectifiers. MOSFETs compared to Schottky diodes have even lower conducting state voltage drops. Rectified output is passed through the filter comprising of inductors and capacitors. When higher switching frequencies are utilized, components with lower capacitance and inductance can be utilized that decreases size.

The feedback circuit including optical isolation monitors output voltage and compares it with the reference voltage to attain regulation. There are two main kinds of regulation; on one hand are Open-loop regulators that don't have feedback circuit but instead depend on feeding the constant voltage to input of the transformer or inductor with assumption that output will be correct. On the other hand are Closed Loop regulators that compensate for impedance of the transformer or coil and for the magnetic hysteresis of the core in Monopolar designs. As feedback circuit needs power to operate before it can produce establish control, the additional non-switching power-supply is required.

Merits and demerits:

In the switched-mode power supply, switching transistor dissipates small power outside active region making power supply very proficient; which is a main advantage. This is so because transistor serves like the switch and either has negligible voltage drop across it or negligible current through it resulting in very low power dissipation by switching transistor.

Another benefit is smaller size and lighter weight of switched mode power supplies which is direct consequence of elimination of low frequency transformers that have substantial size drawback, a weight drawback and cost disadvantage. Switched mode power supply due to its high switching frequency uses light weight high efficiency ferrite core or air core transformers. These produces less heat, have reduced hysteresis loss and are compact in size not to mention the comparatively low cost.

Drawbacks of switched mode power supplies comprise greater complexity, generation of high-amplitude, high-frequency energy which low-pass filter should block to avoid electromagnetic interference and the ripple voltage at switching frequency and its higher harmonic frequencies.

Very low cost switched-mode power supplies may couple electrical switching noise back onto mains power line, causing interference with Audio Visual equipment connected to same phase while non-power factor corrected switched mode power supplies cause harmonic distortion of output in case of AC to AC or DC to AC supplies.

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