In this article we will learn about the Switched Mode Power Supply a.k.a. SMPS & its design working & applications.
What is Switched Mode Power Supply (SMPS)?
SMPS stands for switched mode power supply. It is known by a wide range of names like power supply, supply unit, regulator, or switcher in an electronic power supply. It incorporates a switching regulator to convert electrical power efficiently. It is mainly used for obtaining a controlled dc power supply as output.
It is used to convert power (voltage) using switching devices that are turned on and off alternatively at high frequencies. It uses storage components like inductors or capacitors to supply power when the switching device is in its non-conduction state (off-state). SMPS possesses high efficiency and is widely used in various electronic equipment such as computers, battery chargers, and other sensitive equipment requiring a stable and efficient power supply.
Design & Working
The working & design of SMPS is divided into various sections and stages.
1: Input Stage
The AC input supply of frequency (50-60) Hz feds directly to the rectifier and filter circuit. Its output contains many variations and the capacitance value of the capacitor should be higher enough to handle the input fluctuations. Finally, the unregulated dc is given to the central switching section of SMPS in order to regulate it. This section does not contain any transformer for the step down in input voltage supply.
2: Switching Section
It consists of fast switching devices like a Power transistor or a MOSFET, which switches ON and OFF according to the variations in the voltage. The output obtained is given to the primary of the transformer which is present in this section.
The transformer used here is a much smaller, lighter, and highly effective one that steps down voltage. These are much efficient compared to other step-down methods. Hence, the power conversion ratio is higher.
3: Output Stage
The output that is derived from the switching section is again rectified and filtered. It uses a rectification and filter circuit to get the desired DC voltage. The obtained regulated output voltage is then given to the control circuit.
4: Control Unit
This unit is all about feedback, which has many sections contain in it. Lets see the brief information about this section.
The inner control unit consists of an oscillator, amplifier, sensor, etc. The sensor senses the output signal and feedback to the control unit. All the signals are isolated from each other so that, any sudden spikes should not affect the circuitry. The reference voltage is given as one input along with the signal to the error amplifier. The amplifier is a comparator that compares the signal with the required signal level.
The next stage is Controlling the chopping frequency. The final voltage level is controlled by comparing the inputs given to the error amplifier, whose output helps to decide whether to increase or to decrease the chopping frequency. The oscillator produces a standard PWM wave with a fixed frequency.
The SMPS is mostly used where switching of voltages is not at all a problem, but where the efficiency of the system really matters. The design and working of SMPS of based on the same concept.
Types of SMPS
1: Non-isolated
Non-isolated converters are mostly used when the change in the voltage is comparatively small. The non-isolated SMPS are the ones whose input and output circuitry are not isolated from each other. The major disadvantage is that it cannot provide protection from high electrical voltages and it poses more noise. They are of 3 types.
I: Buck
In a typical non-isolated step-down (buck) converter the output voltage VOUT depends on the input voltage VIN and the switching duty cycle of the power switch.
II: Boost
It is used to boost voltage and it uses the same number of passive components but arranged to step up the input voltage so that the output is higher than that of the input.
III: Buck-Boost
This converter allows the input voltage to be either stepped-up or stepped-down, depending on the duty cycle. The output voltage is given by the relation
VOUT = -VIN *D/ (1-D)
2: Isolated
Isolated SMPS are the ones where there is isolation maintained between the input and output circuitry. The supplies make use of a transformer to separate the switching from the output. The secondary winding of the transformer acts as the energy storing element.
I: Fly-back Converter:
The working of this converter is similar to the buck-boost converter of the non-isolating category. The only difference is that it uses a transformer to store energy instead of an inductor in the circuit.
II: Forward Converter
The working of this converter makes use of the transformer to send the energy, between the input and output in a single step.
Application of Switched Mode power supply (SMPS)
• It is used in servers, power stations, and personal computers.
• It is used in vehicles for charging batteries.
• It is used in factories and industries for power.
• It is used in the railway system, security system.
• It is also used in mobile and also as lighting.
Advantages & Disadvantages of SMPS
Advantages
• Smaller in size and light-weighted.
• Better power efficiency of around 60 to 70 percent.
• Strong anti-interference.
• Wide range of output.
• Produces less heat.
Disadvantages
• The SMPS design & working is more complex.
• Has higher output ripple and its regulation is not satisfactory.
• Mostly limited to the step-down regulator.
• Has high-frequency electrical noise.
• Leads to harmonic distortion.
