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Technical Working of SMPS
In our last blog, we looked at why SMPS is an important aspect of your control panel. In this blog, we will be looking at the technical aspect of SMPS and its functionality.
A quick recap:
A switch-mode power supply is an electronic power supply with a switching regulator that converts electrical power efficiently by switching the incoming signal between low and high with very low energy dissipation rate.
The most ideal switch-mode power supply has no power dissipation. Regulated output voltage regulation is achieved by varying the duty cycle. In contrast, a linear power supply regulates the output voltage by continuously dissipating power in the transistor. What makes switch-mode power supply better than the linear power supply is its power conversion efficiency extremely high. Switch-mode power supplies are more compact and lighter in weight. Switching mode power supplies are known to convert power from AC mains into DC efficiently, and the same goes for DC. If the power supply is designed for 230VAC and has no voltage selector switch, the required DC voltage would be 325 VDC (230 × √2).
Now let us explain each stage of the SMPS and help you to understand how the SMPS works
Input Filter Stage-
The input filter interference used in SMPS protects it from electromagnetic interference. SMPS is designed in a manner that input voltage does not affect the transformer’s performance as it has a wide output range.
This filter prevents the passing of high-frequency voltage through the SMPS as it may cause the power supply failure. A passive L-C filter is an ideal solution that has the characteristic to achieve both filtering requirements.
Input rectifier stage-
In the rectification process if the SMPS has an AC input, then it converts to DC. In an SMPS voltage doublers are used as rectifier circuits. This feature allows input voltage to be universal from 90 to 270 VAC. An unregulated DC voltage is then sent to a large filter capacitor through a rectifier.
Active PFC Circuit-
The current drawn from the mains supply by this rectifier circuit occurs in short pulses around the AC voltage peaks. These pulses have significant high-frequency energy which reduces the power factor. To rectify this; the SMPS uses a special power correction circuit that makes the input current follow the sinusoidal shape of the AC input voltage, correcting the power factor.
The switching stage converts DC to AC (step signal) by running it through a power oscillator. The frequency is usually chosen to be above 20 kHz as it is inaudible to humans. The switching is implemented as multistage to achieve/gain better performance.
Any switch-mode power supply that gets its power from an AC power line requires a transformer for galvanic isolation. Some DC-to-DC converters may include a transformer, although isolation may not be critical in these cases. SMPS transformers run at high frequency. Most of the cost savings (and space savings) in off-line power supplies result from the smaller size of the high-frequency transformer compared to the 50/60 Hz transformers formerly used.
A feedback circuit monitors the output voltage and compares it with a reference voltage. Depending on design and safety requirements, the controller may contain an isolation mechanism (such as an optocoupler) to isolate it from the DC output.
Rectifier and Output Protection-
The output of the high frequency is in AC forms, this signal needs to be rectified, a bridge rectifier is employed for the rectifier. This rectified output is then smoothened by a filter consisting of inductors and capacitors. For higher switching frequencies, components with lower capacitance and inductance are needed.
Above we have tried to explain the basic functioning of the SMPS. Each SMPS is designed in the same way by using different components or different technologies.