A linear power supply uses a transformer, rectifier, filter and linear regulator to produce the required DC output. An SMPS (Switched-Mode Power Supply) uses high-frequency switching, energy-storage components and control circuitry to regulate the output.
The major differences include working principle, efficiency, size, weight, switching frequency, heat generation, noise, regulation and applications.
- What is a Power Supply?
- What is a Linear Power Supply?
- How Does a Linear Power Supply Work?
- What is SMPS?
- How Does SMPS Work?
- Linear Power Supply vs SMPS
- Efficiency Difference
- Heat Generation
- Ripple and Noise
- Transformer and Frequency
- Applications
- Advantages and Disadvantages
- Which Power Supply Should You Use?
- Important Exam Points
- Frequently Asked Questions
- Conclusion
What is a Power Supply?
A power supply is an electrical or electronic system that provides a suitable voltage and current to another circuit or device.
Many electronic circuits require a stable DC voltage even though the available source may be AC mains electricity or another voltage level.
A power supply may perform several operations, including:
- Voltage conversion
- Rectification
- Filtering
- Voltage regulation
- Current limiting
- Protection against abnormal electrical conditions
What is a Linear Power Supply?
A linear power supply is a power supply that uses a linear voltage regulator to maintain the required output voltage.
A traditional linear supply commonly contains:
- Transformer
- Rectifier
- Filter capacitor
- Linear regulator
The regulator controls the output by dissipating excess voltage as heat.
How Does a Linear Power Supply Work?
1. Transformer
The transformer changes the AC voltage to a more suitable AC voltage level and provides electrical isolation in appropriately designed supplies.
2. Rectifier
The rectifier converts AC into a pulsating DC waveform. A bridge rectifier is commonly used in many designs.
3. Filter
Capacitors and other filtering components smooth the rectified waveform and reduce voltage variations.
4. Linear Regulator
The regulator maintains a relatively constant output voltage. The excess voltage is converted primarily into heat.
Ploss ≈ (Vin − Vout) × Iout
This explains why a linear regulator can become hot when the input voltage is substantially higher than the required output voltage.
What is SMPS?
SMPS stands for Switched-Mode Power Supply.
Unlike a linear supply, an SMPS uses switching devices that rapidly turn electrical energy on and off. Energy-storage components and control circuits then regulate the output.
SMPS technology is widely used because it can provide efficient voltage conversion in relatively compact designs.
Common Components of an SMPS
- Input rectifier or conversion stage
- Filter capacitors
- Switching transistor or other switching device
- High-frequency transformer in isolated designs
- Inductors and capacitors
- Control circuit
- Feedback circuit
- Output rectification and filtering
How Does SMPS Work?
The exact SMPS topology depends on the design. A simplified isolated AC-to-DC SMPS can be represented as:
The switching stage operates at a much higher frequency than the 50/60 Hz mains frequency used by conventional mains transformers.
Higher switching frequency allows magnetic components such as transformers and inductors to be made substantially smaller for a given power-transfer requirement, although the overall design also depends on topology, power level and operating conditions.
Difference Between Linear Power Supply and SMPS
The following table provides a detailed parameter-based comparison of a traditional linear power supply and a switched-mode power supply.
| Parameter | Linear Power Supply | SMPS |
|---|---|---|
| Full Form | Linear Power Supply | Switched-Mode Power Supply |
| Basic Principle | Regulates voltage using a linear pass element. | Regulates power using high-frequency switching. |
| Switching Operation | Normally does not use high-frequency power switching as the primary regulation method. | Uses high-frequency switching devices. |
| Typical Input Transformer | Traditional designs commonly use a mains-frequency transformer. | Isolated designs commonly use a high-frequency transformer. |
| Operating Frequency | Traditional transformer operates at mains frequency. | Power conversion operates at much higher switching frequencies. |
| Efficiency | Usually lower when there is a significant voltage drop across the regulator. | Usually higher because switching devices can operate with lower average power loss. |
| Heat Generation | Can be significant, especially at high current and large voltage drop. | Usually lower for a comparable efficient design. |
| Size | Traditional designs can be relatively large. | Can be relatively compact. |
| Weight | Often heavier because of the mains-frequency transformer and heatsink. | Often lighter because high-frequency magnetic components can be smaller. |
| Noise | Generally produces low high-frequency switching noise. | Can produce high-frequency electromagnetic and switching noise. |
| EMI | Usually easier to control with respect to high-frequency switching EMI. | Requires careful EMI filtering, layout and shielding where necessary. |
| Ripple | Can provide very low output ripple with appropriate design and filtering. | Switching ripple can be present and must be filtered appropriately. |
| Regulation | Can provide excellent regulation depending on regulator design. | Can also provide excellent regulation using feedback and control circuits. |
| Complexity | Generally simpler. | Generally more complex. |
| Design | Relatively straightforward for many low-power applications. | Requires careful control, switching, magnetic and EMI design. |
| Power Density | Generally lower. | Generally higher. |
| Voltage Conversion | Linear regulation is especially suitable when input voltage is already close to the desired output. | Can efficiently step voltage up, down or perform other conversion functions depending on topology. |
| Efficiency at Large Voltage Drop | Can become poor because excess voltage is dissipated as heat. | Can remain efficient because energy is transferred through switching and storage components. |
| Cooling Requirement | May require a substantial heatsink. | Often requires less heat dissipation for the same output power, but still requires thermal design. |
| Output Response | Can have excellent performance for suitable loads and designs. | Depends strongly on the control loop and topology. |
| Acoustic Noise | Transformer hum may occur in some designs. | Some designs can produce audible coil or transformer noise. |
| Maintenance | Simple designs can be easier to troubleshoot. | More components and control circuitry can make troubleshooting more involved. |
| Cost | Can be economical for simple low-power designs. | Can be economical for high-volume and higher-power applications, but design complexity varies. |
| Typical Applications | Low-noise electronics, laboratory supplies, audio equipment and simple circuits. | Computers, televisions, chargers, adapters, industrial electronics and many modern electronic devices. |
| Main Advantage | Simplicity and low high-frequency switching noise. | High efficiency, compact size and high power density. |
| Main Limitation | Lower efficiency and greater heat dissipation when voltage drop is large. | Greater circuit complexity and switching noise. |
Efficiency Difference Between Linear Supply and SMPS
Efficiency is one of the most important differences between linear power supplies and SMPS.
Linear Regulator
For an idealized linear regulator, the approximate efficiency can be expressed as:
For example, if a linear regulator receives 12 V and provides 5 V at the same current:
≈ 41.7%
The remaining input power is primarily dissipated as heat in the regulator, ignoring other losses.
SMPS
A properly designed SMPS can achieve substantially higher efficiency because its switching devices spend much of their operating time in states where their power dissipation can be relatively low.
Actual efficiency depends on:
- Topology
- Switching frequency
- Load level
- Switching losses
- Conduction losses
- Magnetic losses
- Rectifier losses
- Control circuitry
Heat Generation
Linear regulators dissipate the voltage difference between their input and output as heat.
Consider a 12 V input, 5 V output and 1 A load:
= 7 W
Seven watts of heat is substantial for a small regulator and may require a heatsink.
An SMPS transfers energy using switching and storage components, so the same voltage conversion can generally be performed with much lower losses in a well-designed system.
Ripple and Noise
Linear power supplies are often selected when very low high-frequency switching noise is important.
SMPS circuits generate switching waveforms and therefore can introduce:
- Switching ripple
- Electromagnetic interference (EMI)
- High-frequency noise
Good SMPS designs use appropriate filtering, grounding, PCB layout and shielding techniques to control these effects.
Transformer and Frequency
Linear Power Supply
A conventional linear supply commonly uses a transformer operating at the mains frequency, such as 50 Hz or 60 Hz.
A transformer designed for low-frequency operation requires relatively large magnetic components for a given power level.
SMPS
An SMPS switches at a much higher frequency. In isolated designs, this permits the use of a much smaller high-frequency transformer.
Voltage Regulation
Both linear power supplies and SMPS can provide regulated output voltage.
The difference is primarily in how the regulation is achieved.
| Feature | Linear Supply | SMPS |
|---|---|---|
| Regulation method | Controls a pass element in its linear region. | Controls switching operation and energy transfer. |
| Feedback | Common in regulated linear supplies. | Commonly used to control output voltage. |
| Heat due to voltage drop | Can be significant. | Generally much lower in efficient designs. |
Linear Power Supply vs SMPS: Circuit Complexity
A simple linear supply can be constructed using relatively few components:
An SMPS generally contains additional components and requires careful engineering:
The additional complexity provides benefits such as higher efficiency, smaller size and flexible voltage conversion.
Applications of Linear Power Supplies
- Laboratory power supplies
- Low-noise analog circuits
- Audio electronics
- Measurement equipment
- Simple low-power electronics
- Applications where low switching noise is particularly important
Applications of SMPS
- Desktop computer power supplies
- Laptop adapters
- Phone chargers
- Televisions
- Monitors
- Networking equipment
- Industrial electronics
- LED drivers
- Servers
- Many modern electronic devices
Why Computers Commonly Use SMPS
Computers require multiple regulated voltage rails and significant power while maintaining reasonable size, weight and thermal performance.
SMPS technology is well suited to this requirement because it can provide efficient power conversion at high power density.
A computer PSU may convert AC mains power into several regulated DC outputs required by the motherboard, CPU, storage devices and other components.
Advantages and Disadvantages of Linear Power Supply
Advantages
- Simple circuit structure.
- Low high-frequency switching noise.
- Good output regulation is possible.
- Suitable for sensitive analog applications.
- Easy to understand and troubleshoot in simple designs.
Disadvantages
- Lower efficiency when input voltage is much higher than output voltage.
- Produces more heat under significant voltage drop and load.
- Traditional transformers can be large and heavy.
- Lower power density.
Advantages and Disadvantages of SMPS
Advantages
- High efficiency is possible.
- Compact size.
- Lower weight.
- High power density.
- Can support step-up and step-down conversion depending on topology.
- Suitable for medium- and high-power applications.
Disadvantages
- More complex circuit design.
- Produces switching noise.
- EMI control can be challenging.
- Requires careful PCB layout.
- Troubleshooting can be more complicated.
Which Power Supply Should You Use?
The appropriate power-supply architecture depends on the application.
| Requirement | Commonly Suitable Choice | Reason |
|---|---|---|
| Very low switching noise | Linear supply | Does not rely on high-frequency power switching for regulation. |
| High efficiency | SMPS | Efficient switching conversion is possible. |
| Small size | SMPS | High-frequency magnetic components can be smaller. |
| Low-cost simple low-power design | Linear supply may be suitable | Simple circuits can require fewer components. |
| Computer PSU | SMPS | High power density and efficiency are important. |
| Audio or sensitive analog application | Linear supply may be suitable | Low switching noise can be valuable. |
| Large voltage conversion | SMPS | Can perform efficient voltage conversion using suitable topology. |
Linear Power Supply vs SMPS: Simple Difference
| Linear Power Supply | SMPS |
|---|---|
| Uses linear regulation. | Uses switching regulation. |
| Usually less efficient for large voltage drops. | Usually more efficient for significant voltage conversion. |
| Produces more heat under large voltage drops. | Generally produces less heat for the same conversion when efficiently designed. |
| Usually larger and heavier in traditional designs. | Usually smaller and lighter. |
| Low high-frequency switching noise. | Produces switching noise that must be controlled. |
| Simple design. | More complex design. |
| Common in low-noise applications. | Common in computers and modern electronic equipment. |
Important Exam Points
- SMPS stands for Switched-Mode Power Supply.
- A linear power supply uses a linear regulator for voltage regulation.
- SMPS uses high-frequency switching.
- Linear supplies can dissipate significant power as heat when the voltage drop is large.
- SMPS can achieve high efficiency through switching-based energy conversion.
- Traditional linear supplies commonly use mains-frequency transformers.
- Isolated SMPS designs commonly use high-frequency transformers.
- Linear supplies are generally simpler than SMPS.
- SMPS generally has higher power density.
- SMPS can generate switching noise and EMI.
- Linear supplies are often useful for low-noise analog applications.
- SMPS is widely used in computers, chargers, televisions and adapters.
- Actual efficiency depends on the specific design and operating condition.
- Both linear supplies and SMPS can provide regulated output voltage.
Frequently Asked Questions
1. What is the difference between linear power supply and SMPS?
A linear power supply regulates voltage using a linear pass element, while an SMPS regulates power using high-frequency switching and energy-storage components.
2. What does SMPS stand for?
SMPS stands for Switched-Mode Power Supply.
3. Which is more efficient, linear power supply or SMPS?
SMPS is generally more efficient when significant voltage conversion is required, although actual efficiency depends on the specific design and operating conditions.
4. Why does a linear power supply produce heat?
A linear regulator dissipates the difference between input and output voltage as heat. The approximate loss is (Vin − Vout) × I.
5. Why is SMPS smaller than a traditional linear power supply?
SMPS operates at much higher switching frequencies, allowing magnetic components such as transformers and inductors to be smaller for a given power-transfer requirement.
6. Which has less noise, linear supply or SMPS?
Linear supplies generally have less high-frequency switching noise because they do not use high-frequency power switching as their primary regulation method. However, actual output noise depends on the complete design.
7. Is SMPS used in computers?
Yes. Computer power supplies commonly use switched-mode power conversion because of its efficiency, compactness and power-density advantages.
8. Is a linear power supply better for audio?
Linear supplies can be useful in audio and sensitive analog applications where low high-frequency switching noise is important. The appropriate choice still depends on the complete circuit design.
9. Does SMPS always have higher efficiency?
No technology guarantees a particular efficiency. A well-designed SMPS can be highly efficient, but efficiency varies with topology, load, components and operating conditions.
10. What is the main disadvantage of a linear power supply?
Its main limitation is that significant voltage drops across the regulator can result in substantial power dissipation and heat.
11. What is the main disadvantage of SMPS?
SMPS circuits are generally more complex and generate switching noise and EMI that must be properly controlled.
12. Does an SMPS use a transformer?
Many isolated SMPS designs use a high-frequency transformer, but not every SMPS topology requires a transformer.
13. Does a linear power supply use a transformer?
Traditional AC-input linear power supplies commonly use a mains-frequency transformer, although the exact architecture depends on the application.
14. Which power supply is lighter?
SMPS designs are generally lighter because high-frequency operation allows smaller magnetic components and can reduce the size of heat-dissipation components.
15. Which power supply is simpler?
A basic linear power supply is generally simpler than a comparable SMPS.
16. Why is switching frequency important in SMPS?
Higher switching frequency can allow smaller magnetic components, although increasing frequency also introduces additional switching and electromagnetic losses that must be managed.
17. Can a linear regulator be used after an SMPS?
Yes. A linear regulator can sometimes be placed after an SMPS to provide additional filtering or very low-noise regulation, provided the voltage, current and thermal requirements are suitable.
18. What is power density?
Power density describes how much power a power-conversion system can handle relative to its physical size or volume. SMPS designs commonly achieve higher power density than traditional linear supplies.
Conclusion
The main difference between a linear power supply and SMPS is their method of voltage regulation and energy conversion.
A linear power supply uses a linear regulator and can provide very clean output with relatively simple circuitry, but it can dissipate substantial heat when the input-to-output voltage difference is large.
An SMPS uses high-frequency switching and energy-storage components to achieve efficient power conversion. It is generally smaller, lighter and more efficient, but its design is more complex and requires careful control of switching noise and EMI.
Linear Power Supply → Simple + Low switching noise + More heat
SMPS → High efficiency + Compact + Switching noise/EMI
This is why SMPS technology is widely used in modern computers, chargers, adapters, televisions, networking equipment and many other electronic devices, while linear supplies remain useful in applications where simplicity and low switching noise are important.
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