Electronic circuits - SMPS
Tutorial on electronic circuits
Electronic Circuits - SMPS
The topics covered so far represent different sections of the power supply. Together, these sections make up the linear feed . This is the conventional method of obtaining direct current from the input AC power supply.
Linear power supply
Linear power supply (LPS) is the regulated power supply which dissipates a lot of heat in the series resistor to regulate the output voltage which has low ripple and low noise. This LPS has many applications.
A linear power supply requires larger semiconductor devices to regulate the output voltage and generate more heat, resulting in lower power efficiency. Linear power supplies have transient response times up to 100 times faster than others, which is very important in some specialist areas.
Aadvantages of LPS
- The power is continuous.
- The circuit is simple.
- These are reliable systems.
- This system responds dynamically to changes in load.
- The resistances of the circuit are changed to regulate the output voltage.
- As the components operate in a linear region, the noise is low.
- The ripple is very low in the voltage output.
Disadvantages of LPS
- The transformers used are heavier and bigger.
- The heat dissipation is more.
- Linear power efficiency is 40-50%
- Electricity is wasted as heat in LPS circuits.
- Single voltage output is obtained.
We have already gone through different parts of a linear power supply. The block diagram of a linear power supply is shown in fig.next ure.
Despite the above disadvantages, linear power supplies are widely used in low noise amplifiers, test equipment, control circuits. In addition, they are also used in data acquisition and signal processing.
All power systems requiring simple regulation and where efficiency is not a concern, LPS circuits are used. As the electrical noise is lower, LPS is used to power sensitive analog circuits. But to overcome the disadvantages of linear power system, switched mode power supply (SMPS) is used.
Switched Mode Power Supply (SMPS)
The disadvantages of LPS such as lower efficiency, need for large capacitors to reduce ripples and heavy and expensive transformers etc. are overcome by the implementation of power supplies to dclipping .
Operation of SMPS is simply understood knowing that the transistor used in LPS is used to control the voltage drop while the transistor in SMPS is used as a controlled switch .
The operation of SMPS can be understood by the following figure.
Let's try to understand what is happening at each step of the SMPS circuit.
AC input power 50Hz signal is given directly to the combination rectifier and filter circuit without u singing any what a transformer. This output will have many variations and the capacitance value of the capacitor must be higher to handle the input fluctuations. This unregulated direct current is fed to the central switching section of the SMPS.
A fast switching device such as a power transistor or MOSFET is used in thisthis section, which turns on and OFF according to the variations and this output is given to the primary of the transformer present in this section. The transformers used here are much smaller and lighter unlike those used for a 60Hz power supply. These are much more efficient and therefore the power conversion ratio is higher.
The output signal of the switching section is again rectified and filtered to obtain the required DC voltage. This is a regulated output voltage which is then given to the control circuit, which is a feedback circuit. The final output is obtained after considering the feedback signal.
This unit is the feedback circuit which has many sections. Let us have a clear understanding about this from
The figure above explains the internal parts of a unitcontrol. The output sensor detects the signal and connects it to the control unit. The signal is isolated from the other section so that no sudden spikes affect the circuits. A reference voltage is given as an input with the signal to the error amplifier which is a comparator which compares the signal with the required signal level.
By controlling the switching frequency, the final voltage level is maintained. This is checked by comparing the data inputs to the error amplifier, the output of which helps decide whether to increase or decrease the switching frequency. The PWM oscillator produces a fixed frequency standard PWM wave.
We can get a better idea of how SMPS actually works by looking at the following figure.
SMPS is mainly used where switching voltages is not a problem at all and where system efficiency really matters. There are few points to note concerningant SMPS. They are
The SMPS circuit works by switching and therefore the voltages vary continuously.
The switching device operates in saturation or cutoff mode.
The output voltage is controlled by the switching time of the feedback circuit.
The switching time is adjusted by adjusting the duty cycle.
The efficiency of SMPS is high because, instead of dissipating excess energy as heat, it continually switches its input to control the output .
There are few disadvantages in SMPS, such as
- Noise is present due to high frequency switching.
- The circuit is complex.
- It produces electromagnetic interference.
Advantages of SMPS include,
- Efficiency isas high as 80-90%
- Less heat generation; less wasted energy.
- Reduction of harmonic return in the power supply network.
- The device is compact and small in size.
- The manufacturing cost is reduced.
- Provision to supply the required number of voltages.
There are many applications of SMPS. They are used in the motherboard of computers, cell phone chargers, HVDC meters, battery chargers, central power distribution, automotive vehicles, consumer electronics, laptops, control systems. security, space stations, etc.
Types of SMPS
SMPS is the switching power supply circuit which is designed to get the regulated DC output voltage from unregulated DC or AC voltage . There are four main types of SMPS such as
- DC to DC converter
- AC to DC converter
- Fly back Converter
- Front converter
The part AC-DC converter in the input section makes the difference between AC-DC converter and DC-DC converter. The Fly back converter is used for low power applications. There are also Buck and Boost converters in the SMPS types which decrease or increase the output voltage as needed. The other type of SMPS includes self-oscillating fly-back converter, Buck-boost converter, Cuk, Sepic, etc.