Switch Mode Power Supply
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SWITCH MODE POWER SUPPLY Guided By: Prepared By: Mr. M. A. Mulla Rakesh (U06EE542) Tech IV, EED, SVNIT
Raj B.
Overview…… Ø Introduction. Ø A Brief History Of Development. Ø Factors Behind The SMPS Evolution. Ø Common Topologies. Ø Principle Of Operation. Ø Steady State Analysis. Ø Advantages. Ø Drawbacks. Ø Areas Of Application. Ø
Introduction to SMPS…… Switch Mode Power Supplies i.e. SMPSs are the consequence of the never ending urge of smaller and lighter yet efficient power supply to our electrical and electronic devices.
The majority of electronic DC loads are supplied from the standard power sources. Unfortunately, standard source voltages may not match the levels required by motors, microprocessors, LEDs, or other loads.
Battery-powered devices are prime examples of the problem: the typical voltage of a standard Li+ cell is either too high or low or drops too far during discharge to be used.
Considering the multiple DC voltage levels required by many electronic devices, we need a way to convert standard powersource potentials into the voltages dictated by the load.
SMPS i.e. Switch Mode Power Supply for the
is the solution
A Brief History Of Development… Earlier developed models of SMPSs were highly ineffective. In the mid 1960s, it was popular to say that the switch mode power supplies were one microsecond away from disaster.
Even the manufacturers did not completely understand the various failure mechanisms of their new bipolar power transistors. And the users tend to worsen the problem by doing things like connecting these devices in parallel for
Designs that seem to be perfect in lab failed in field. On the other hand if the design did well in the field, the designers have no idea why it did.
In fact they may not have been so called power supply designers at all, but rather general purpose engineers who have to design their own power supplies as a necessary evil along with their other more important modules. Or they may have been young engineers who were handed power
Therefore, despite the apparent size, weight and efficiency advantages of SMPSs, it took many years for them to be generally accepted.
But by contrast, today the high reliability of SMPSs is taken for granted and are being extensively used everywhere.
Factors Behind The SMPS Evolution Ø Materials used for the manufacturing are better. Ø The manufacturing techniques are far superior and precise. Ø An overall improved design i.e. well electrical, mechanical and thermal design. Ø The devices are better and their general understanding has improved too. Ø Now the designers are more equipped with several simulation softwares and have a sound background with specialization in power electronics devices.
Common Topologies…… Presently numerous topologies are being used according to the requirement of the specific device. But there are three basic topologies based upon the function of conversion.
Ø Buck i.e. Step Down. Ø Boost i.e. Step Up. Ø Buck- Boost i.e. Inverter.
Buck i.e. Step Down…… The buck converter is a step-down converter that changes a higher input voltage to a lower output voltage.
Fig 1. Circuit diagram of Buck converter
Boost i.e. Step Up…… The Boost converter is similar to Buck but instead of stepping down the input voltage, the output voltage is higher than the input voltage.
Fig 2. Circuit diagram of Boost converter
Buck Boost i.e. Inverter…… This topology is used where we need to step up and step down the output voltage simultaneously.
Fig 3. Circuit diagram of Buck Boost converter
Principle Of Operation…… All three fundamental topologies include a MOSFET switch, a diode, an output capacitor, and an inductor. The MOSFET, which is the actively controlled component in the circuit, is interfaced to a controller . The controller applies a pulse width modulated (PWM) square-wave signal to the MOSFET's gate, thereby switching the device on and off.
Doing so it varies the duty cycle D of the square wave signal which directly affects the output voltage of the SMPS.
D = TON /TS
….(1)
To maintain a constant output voltage, the controller senses the SMPS output voltage and varies the duty cycle (D) of the square-wave signal, dictating how long the MOSFET is on during each switching period (T ). S
The on and off states of the MOSFET divide the SMPS circuit into two phases: a charge phase and a discharge phase, both of which describe the energy transfer of the inductor. Energy stored in the inductor during the charging phase is transferred to the output load and capacitor during the discharge phase.
Fig 4.Voltage and current characteristics for a steady-state inductor
The capacitor supports the load while the inductor is charging and sustains the output voltage. This cyclical transfer of energy between the circuit elements maintains the output voltage at the proper value, in accordance with its topology.
The inductor is central to the energy transfer from source to load during each switching cycle. Without it, the SMPS would not function when the MOSFET is switched.
Energy stored in the inductor L is given by,
E = 0.5 L*I2
….(2)
Thus the change in energy of inductor depends upon the change in its current (ΔI ) which depends upon L voltage (VL) across the inductor.
•
ΔIL = VL * ΔT/ L
….(3)
During the charge phase, the MOSFET is on, the diode is reverse biased and energy is transferred from the voltage source to the inductor. Inductor current ramps up because V is positive. Also, the output capacitance transfers L the energy it stored from the previous cycle to the load in order to maintain a constant output voltage.
During the discharge phase, the MOSFET turns off, and the diode becomes forward biased and, therefore, conducts. As the source is no longer charging the inductor it swaps the polarity and discharges energy to the load and replenishes the capacitor.
The inductor current ramps down as it imparts energy, according to the transfer relationship given by eqn (3).
The charge/discharge cycles repeat and maintain a steady state switching condition. During the circuit's progression to a steady state, inductor current builds up to its final level, which is a superposition of DC current and the ramped AC current (or inductor ripple current) developed during the two circuit phases.
So, in summary, energy is shuttled between the source, the inductor, and the output capacitor to maintain a
To deliver the true DC current to output, we need to filter the ripple current. This is done by the output capacitor which let the high frequency AC to pass through it. The unwanted output ripple current passes through the output capacitor, and maintains the capacitor's charge as the current passes to ground. So it stabilizes output voltage also.
Steady State Analysis…… To be in a steady state, a variable that repeats with period TS must be equal at the beginning and end of each period.
As the inductor current is periodic, due to the charge and discharge phases described previously, the inductor current at the beginning of the PWM period must equal inductor current at the end. This means that the change in inductor current during the charge phase (ΔI CHARGE ) must equal the
Equating the change in inductor current for the charge and
discharge phases, an interesting result is achieved, which is
also referred to as the volt second rule:
|ΔICHARGE | = |ΔIDISCHARGE |
⇒
|VCHARGE *D* TS/L| =
|VDISCHARGE
*(1 – D)*TS/L|
Applying Kirchhoff’s voltage law we have,
VCHARGE
VDISHARGE
=
= VIN - VOUT
&
-VOUT
Thus we have,
|VIN - VOUT | * D = |-VOUT | * (1 – D)
⇒
….(4)
VOUT /VIN =
D
Also for an ideal circuit,
PIN =
=
POUT
⇒
VIN *IIN
VOUT *IOUT
IIN /IOUT =
Topology Buck
D
….(5)
VC Ratio CC Ratio
D
D
Boost
1/(1-D)
1/(1-D)
Inverter
D/(1-D)
D/(1-D)
Why do we prefer SMPS ? The linear regulators can do the same but still we prefer the SMPS because:
Ø Higher Efficiency. Ø Compactness and Light Weight. Ø Easier PFC support. Ø Less Thermal Management Requirement. Ø Enhanced Lifetime and Reliability. Ø Ø
Ø SMPS
has higher efficiency, almost 90% which is too high as compared to 50% efficiency of linear regulators.
SMPS has higher efficiency, almost 90% which is too high as compared to 50% efficiency of linear regulators. While a linear regulator maintains the desired output voltage by dissipating excess power in a pass power transistor, the SMPS switches a power transistor between saturation and cut off region. Thus saving a lot of power as
Ø SMPSs
are smaller and light weight as compared to line regulators. It switches at a much higher frequency (tens to hundreds of kHz). So the low frequency transformers which are bulky and heavy weight are eliminated, reducing the size of SMPS.
Ø Linear
regulators can only step down the voltage but SMPS can be selected to fit any output voltage i.e. they can be used for step up, step down or in inverter mode.
Ø
Ø Thermal
management requirements of SMPS are comparatively lesser due to the low power loss. PFC is the process that insures that the input voltages and currents from the AC power line into a power supply are in phase to achieve a “Unity Power Factor”. PFC is very costly to achieve in a linear power supply.
Ø All
these factors like less losses, higher efficiency, lesser thermal footprints, considered together make the SMPS much reliable and increases their lifetime.
Drawbacks Of SMPS…… ØSMPS
radiates EM interference and conduct noise.
Electric fields are caused by the rapid changing of voltage at the inductor node while the fast-switching currents of the charge/discharge loops produce magnetic fields. Noise is propagated to input and output circuits when SMPS capacitances and PCB parasitics present higher impedances to switching currents.
Ø SMPSs
an
can be quite complex and require additional external components, both of which can equate to increase in overall cost of the power supply.
Ø But good component placement and PCB layout techniques
take good care of the EMI and noise problems.
Choosing correct components according to the datasheet of
the SMPS ICs may keep the complexities away.
Areas Of Application …… SMPS’s are having wide range of applications. Some of them are….
Ø Machine tool industries. Ø Security systems (Close Circuit Cameras). Ø In computers and other electronic accessories. Ø Support supplies with PLC’s. Ø ESPs of power plants.
•
THANK YOU
Raj B.
Overview…… Ø Introduction. Ø A Brief History Of Development. Ø Factors Behind The SMPS Evolution. Ø Common Topologies. Ø Principle Of Operation. Ø Steady State Analysis. Ø Advantages. Ø Drawbacks. Ø Areas Of Application. Ø
Introduction to SMPS…… Switch Mode Power Supplies i.e. SMPSs are the consequence of the never ending urge of smaller and lighter yet efficient power supply to our electrical and electronic devices.
The majority of electronic DC loads are supplied from the standard power sources. Unfortunately, standard source voltages may not match the levels required by motors, microprocessors, LEDs, or other loads.
Battery-powered devices are prime examples of the problem: the typical voltage of a standard Li+ cell is either too high or low or drops too far during discharge to be used.
Considering the multiple DC voltage levels required by many electronic devices, we need a way to convert standard powersource potentials into the voltages dictated by the load.
SMPS i.e. Switch Mode Power Supply for the
is the solution
A Brief History Of Development… Earlier developed models of SMPSs were highly ineffective. In the mid 1960s, it was popular to say that the switch mode power supplies were one microsecond away from disaster.
Even the manufacturers did not completely understand the various failure mechanisms of their new bipolar power transistors. And the users tend to worsen the problem by doing things like connecting these devices in parallel for
Designs that seem to be perfect in lab failed in field. On the other hand if the design did well in the field, the designers have no idea why it did.
In fact they may not have been so called power supply designers at all, but rather general purpose engineers who have to design their own power supplies as a necessary evil along with their other more important modules. Or they may have been young engineers who were handed power
Therefore, despite the apparent size, weight and efficiency advantages of SMPSs, it took many years for them to be generally accepted.
But by contrast, today the high reliability of SMPSs is taken for granted and are being extensively used everywhere.
Factors Behind The SMPS Evolution Ø Materials used for the manufacturing are better. Ø The manufacturing techniques are far superior and precise. Ø An overall improved design i.e. well electrical, mechanical and thermal design. Ø The devices are better and their general understanding has improved too. Ø Now the designers are more equipped with several simulation softwares and have a sound background with specialization in power electronics devices.
Common Topologies…… Presently numerous topologies are being used according to the requirement of the specific device. But there are three basic topologies based upon the function of conversion.
Ø Buck i.e. Step Down. Ø Boost i.e. Step Up. Ø Buck- Boost i.e. Inverter.
Buck i.e. Step Down…… The buck converter is a step-down converter that changes a higher input voltage to a lower output voltage.
Fig 1. Circuit diagram of Buck converter
Boost i.e. Step Up…… The Boost converter is similar to Buck but instead of stepping down the input voltage, the output voltage is higher than the input voltage.
Fig 2. Circuit diagram of Boost converter
Buck Boost i.e. Inverter…… This topology is used where we need to step up and step down the output voltage simultaneously.
Fig 3. Circuit diagram of Buck Boost converter
Principle Of Operation…… All three fundamental topologies include a MOSFET switch, a diode, an output capacitor, and an inductor. The MOSFET, which is the actively controlled component in the circuit, is interfaced to a controller . The controller applies a pulse width modulated (PWM) square-wave signal to the MOSFET's gate, thereby switching the device on and off.
Doing so it varies the duty cycle D of the square wave signal which directly affects the output voltage of the SMPS.
D = TON /TS
….(1)
To maintain a constant output voltage, the controller senses the SMPS output voltage and varies the duty cycle (D) of the square-wave signal, dictating how long the MOSFET is on during each switching period (T ). S
The on and off states of the MOSFET divide the SMPS circuit into two phases: a charge phase and a discharge phase, both of which describe the energy transfer of the inductor. Energy stored in the inductor during the charging phase is transferred to the output load and capacitor during the discharge phase.
Fig 4.Voltage and current characteristics for a steady-state inductor
The capacitor supports the load while the inductor is charging and sustains the output voltage. This cyclical transfer of energy between the circuit elements maintains the output voltage at the proper value, in accordance with its topology.
The inductor is central to the energy transfer from source to load during each switching cycle. Without it, the SMPS would not function when the MOSFET is switched.
Energy stored in the inductor L is given by,
E = 0.5 L*I2
….(2)
Thus the change in energy of inductor depends upon the change in its current (ΔI ) which depends upon L voltage (VL) across the inductor.
•
ΔIL = VL * ΔT/ L
….(3)
During the charge phase, the MOSFET is on, the diode is reverse biased and energy is transferred from the voltage source to the inductor. Inductor current ramps up because V is positive. Also, the output capacitance transfers L the energy it stored from the previous cycle to the load in order to maintain a constant output voltage.
During the discharge phase, the MOSFET turns off, and the diode becomes forward biased and, therefore, conducts. As the source is no longer charging the inductor it swaps the polarity and discharges energy to the load and replenishes the capacitor.
The inductor current ramps down as it imparts energy, according to the transfer relationship given by eqn (3).
The charge/discharge cycles repeat and maintain a steady state switching condition. During the circuit's progression to a steady state, inductor current builds up to its final level, which is a superposition of DC current and the ramped AC current (or inductor ripple current) developed during the two circuit phases.
So, in summary, energy is shuttled between the source, the inductor, and the output capacitor to maintain a
To deliver the true DC current to output, we need to filter the ripple current. This is done by the output capacitor which let the high frequency AC to pass through it. The unwanted output ripple current passes through the output capacitor, and maintains the capacitor's charge as the current passes to ground. So it stabilizes output voltage also.
Steady State Analysis…… To be in a steady state, a variable that repeats with period TS must be equal at the beginning and end of each period.
As the inductor current is periodic, due to the charge and discharge phases described previously, the inductor current at the beginning of the PWM period must equal inductor current at the end. This means that the change in inductor current during the charge phase (ΔI CHARGE ) must equal the
Equating the change in inductor current for the charge and
discharge phases, an interesting result is achieved, which is
also referred to as the volt second rule:
|ΔICHARGE | = |ΔIDISCHARGE |
⇒
|VCHARGE *D* TS/L| =
|VDISCHARGE
*(1 – D)*TS/L|
Applying Kirchhoff’s voltage law we have,
VCHARGE
VDISHARGE
=
= VIN - VOUT
&
-VOUT
Thus we have,
|VIN - VOUT | * D = |-VOUT | * (1 – D)
⇒
….(4)
VOUT /VIN =
D
Also for an ideal circuit,
PIN =
=
POUT
⇒
VIN *IIN
VOUT *IOUT
IIN /IOUT =
Topology Buck
D
….(5)
VC Ratio CC Ratio
D
D
Boost
1/(1-D)
1/(1-D)
Inverter
D/(1-D)
D/(1-D)
Why do we prefer SMPS ? The linear regulators can do the same but still we prefer the SMPS because:
Ø Higher Efficiency. Ø Compactness and Light Weight. Ø Easier PFC support. Ø Less Thermal Management Requirement. Ø Enhanced Lifetime and Reliability. Ø Ø
Ø SMPS
has higher efficiency, almost 90% which is too high as compared to 50% efficiency of linear regulators.
SMPS has higher efficiency, almost 90% which is too high as compared to 50% efficiency of linear regulators. While a linear regulator maintains the desired output voltage by dissipating excess power in a pass power transistor, the SMPS switches a power transistor between saturation and cut off region. Thus saving a lot of power as
Ø SMPSs
are smaller and light weight as compared to line regulators. It switches at a much higher frequency (tens to hundreds of kHz). So the low frequency transformers which are bulky and heavy weight are eliminated, reducing the size of SMPS.
Ø Linear
regulators can only step down the voltage but SMPS can be selected to fit any output voltage i.e. they can be used for step up, step down or in inverter mode.
Ø
Ø Thermal
management requirements of SMPS are comparatively lesser due to the low power loss. PFC is the process that insures that the input voltages and currents from the AC power line into a power supply are in phase to achieve a “Unity Power Factor”. PFC is very costly to achieve in a linear power supply.
Ø All
these factors like less losses, higher efficiency, lesser thermal footprints, considered together make the SMPS much reliable and increases their lifetime.
Drawbacks Of SMPS…… ØSMPS
radiates EM interference and conduct noise.
Electric fields are caused by the rapid changing of voltage at the inductor node while the fast-switching currents of the charge/discharge loops produce magnetic fields. Noise is propagated to input and output circuits when SMPS capacitances and PCB parasitics present higher impedances to switching currents.
Ø SMPSs
an
can be quite complex and require additional external components, both of which can equate to increase in overall cost of the power supply.
Ø But good component placement and PCB layout techniques
take good care of the EMI and noise problems.
Choosing correct components according to the datasheet of
the SMPS ICs may keep the complexities away.
Areas Of Application …… SMPS’s are having wide range of applications. Some of them are….
Ø Machine tool industries. Ø Security systems (Close Circuit Cameras). Ø In computers and other electronic accessories. Ø Support supplies with PLC’s. Ø ESPs of power plants.
•
THANK YOU
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