Introduction to Transformerless Power Supply

A transformerless power supply is an electronic power supply that does not use a transformer to convert AC mains voltage to a lower AC voltage for rectification. Instead, it uses capacitive or resistive voltage dividers to step down the high voltage AC mains to a lower voltage suitable for the low voltage circuitry.

Transformerless power supplies offer several advantages over traditional transformer-based power supplies, including:

1. Lower cost
2. Smaller size and weight
3. Higher efficiency
4. Reduced electromagnetic interference (EMI)

However, transformerless power supplies also have some disadvantages, such as:

1. Limited power output capacity
2. Require more careful design to ensure safety and reliability
3. May not provide galvanic isolation between the input and output

Despite these limitations, transformerless power supplies are widely used in many applications where cost, size, and efficiency are critical factors. Some common applications include:

• LED lighting
• Home appliances
• Consumer electronics
• Industrial control systems

How Transformerless Power Supplies Work

The basic principle behind a transformerless power supply is to use a capacitive or resistive voltage divider to step down the high voltage AC mains to a lower voltage suitable for the low voltage circuitry. Here is a simplified schematic of a typical transformerless power supply:

  AC Mains                  Rectifier   Filter    Regulator
  -----+                    +-----+    +----+    +------+
       |                    |     |    |    |    |      |
       +--||---+-----|>|----+     +----+    +----+      +---+ DC Output
       |       |            |     |    |    |    |      |
       +-------+            +-----+    +----+    +------+
         Voltage Divider

The AC mains voltage is first stepped down by the voltage divider, which can be either capacitive or resistive. The stepped-down voltage is then rectified by a full-wave bridge rectifier to convert it to pulsating DC. The rectified voltage is then filtered by a capacitor to smooth out the ripple and produce a steady DC voltage. Finally, the DC voltage is regulated by a linear or switching regulator to produce a stable and precise output voltage.

Capacitive Voltage Divider

A capacitive voltage divider uses two capacitors connected in series to step down the AC mains voltage. The capacitance values are chosen so that the reactance of the capacitors at the AC mains frequency is much higher than the impedance of the load. This ensures that most of the voltage drop occurs across the capacitors and not the load.

The voltage across each capacitor is given by:

V1 = Vin * (C2 / (C1 + C2))
V2 = Vin * (C1 / (C1 + C2))

Where:
– V1 is the voltage across the first capacitor
– V2 is the voltage across the second capacitor
– Vin is the input AC mains voltage
– C1 and C2 are the capacitance values of the two capacitors

The total capacitance required for a given load power and voltage can be calculated using the following formula:

C = (2 * P) / (2 * pi * f * V^2)

Where:
– C is the total capacitance in farads
– P is the load power in watts
– f is the AC mains frequency in hertz
– V is the desired output voltage in volts

For example, to power a 10W load at 12V from a 120V/60Hz AC mains, the total capacitance required would be:

C = (2 * 10) / (2 * pi * 60 * 12^2) = 46 uF

So two 22uF capacitors could be used in series to provide the required capacitance.

Resistive Voltage Divider

A resistive voltage divider uses two resistors connected in series to step down the AC mains voltage. The resistance values are chosen so that the current through the resistors is limited to a safe value and the power dissipation in the resistors is within their ratings.

The voltage across each resistor is given by:

V1 = Vin * (R2 / (R1 + R2))  
V2 = Vin * (R1 / (R1 + R2))

Where:
– V1 is the voltage across the first resistor
– V2 is the voltage across the second resistor
– Vin is the input AC mains voltage
– R1 and R2 are the resistance values of the two resistors

The total resistance required for a given load current and voltage can be calculated using Ohm’s law:

R = V / I

Where:
– R is the total resistance in ohms
– V is the desired output voltage in volts
– I is the load current in amperes

For example, to power a 100mA load at 12V from a 120V AC mains, the total resistance required would be:

R = 120 / 0.1 = 1200 ohms

So two 600 ohm resistors could be used in series to provide the required resistance. However, the power dissipation in each resistor would be:

P = I^2 * R = 0.1^2 * 600 = 6W

So 10W resistors or higher would be needed to safely handle the power dissipation.

Advantages of Transformerless Power Supplies

Lower Cost

One of the main advantages of transformerless power supplies is their lower cost compared to transformer-based power supplies. This is because transformers are typically the most expensive component in a power supply, due to their large size, weight, and complex construction.

By eliminating the transformer, transformerless power supplies can significantly reduce the bill of materials (BOM) cost and the overall manufacturing cost. This makes them an attractive option for cost-sensitive applications such as consumer electronics and home appliances.

Smaller Size and Weight

Another advantage of transformerless power supplies is their smaller size and weight compared to transformer-based power supplies. This is because transformers are bulky and heavy components that take up a lot of space and add significant weight to the power supply.

By eliminating the transformer, transformerless power supplies can be made much smaller and lighter, which is important for applications where space and weight are critical factors, such as portable devices and LED lighting fixtures.

Higher Efficiency

Transformerless power supplies can also achieve higher efficiency than transformer-based power supplies, especially at low power levels. This is because transformers have inherent losses due to their windings, core, and leakage inductance, which reduce the overall efficiency of the power supply.

By eliminating the transformer, transformerless power supplies can avoid these losses and achieve higher efficiency, typically in the range of 80-90% depending on the design and components used. This higher efficiency translates to lower power consumption, longer battery life, and reduced heat generation, which are all important benefits for many applications.

Reduced EMI

Finally, transformerless power supplies can have reduced electromagnetic interference (EMI) compared to transformer-based power supplies. This is because transformers can act as antennas that radiate EMI, especially at high frequencies, which can interfere with other electronic devices and cause compliance issues with EMC regulations.

By eliminating the transformer, transformerless power supplies can avoid this source of EMI and have a cleaner electromagnetic spectrum. However, careful design is still needed to minimize EMI from other sources such as the switching components and PCB layout.

Disadvantages of Transformerless Power Supplies

Limited Power Output Capacity

One of the main disadvantages of transformerless power supplies is their limited power output capacity compared to transformer-based power supplies. This is because the voltage divider components (capacitors or resistors) have to handle the full input voltage and current, which limits their maximum power rating and size.

Typically, transformerless power supplies are limited to power levels of a few watts to a few tens of watts, depending on the input voltage and the type of voltage divider used. For higher power levels, transformer-based power supplies are usually a better choice due to their ability to efficiently step down the voltage and current.

Require More Careful Design

Another disadvantage of transformerless power supplies is that they require more careful design to ensure safety and reliability. This is because the voltage divider components are directly connected to the AC mains, which poses several safety hazards such as electric shock, fire, and component failure.

To mitigate these risks, transformerless power supplies need to use safety-certified components with appropriate voltage and current ratings, as well as proper creepage and clearance distances between the high voltage and low voltage sections. They also need to include safety features such as input fuses, transient voltage suppressors (TVS), and earth ground connections to protect against overvoltage and overcurrent conditions.

In addition, the design of transformerless power supplies needs to account for the variations in the AC mains voltage and frequency, which can affect the output voltage and ripple. This requires careful selection of the voltage divider components and the use of regulation and filtering circuits to maintain a stable and clean output voltage.

May Not Provide Galvanic Isolation

Finally, transformerless power supplies may not provide galvanic isolation between the input and output, which can be a safety and noise issue in some applications. Galvanic isolation refers to the electrical separation between two circuits that prevents current flow between them, while allowing the transfer of power or signals through electromagnetic or optical means.

Transformer-based power supplies inherently provide galvanic isolation due to the physical separation between the primary and secondary windings of the transformer. This isolation helps to protect the load from high voltages, transients, and noise on the AC mains, as well as preventing ground loops and common mode noise from affecting the output.

Transformerless power supplies, on the other hand, do not have this inherent isolation, since the voltage divider components are directly connected to the AC mains. This means that any noise, transients, or ground potential differences on the AC mains can couple into the output and affect the load.

In some applications, such as medical devices or industrial control systems, galvanic isolation is a mandatory safety requirement to protect human operators and sensitive equipment from electrical hazards. In these cases, transformer-based power supplies or isolation transformers may need to be used instead of transformerless power supplies.

Applications of Transformerless Power Supplies

LED Lighting

One of the most common applications of transformerless power supplies is in LED lighting, especially for low-power bulbs and fixtures. LEDs are highly efficient and long-lasting light sources that require a constant current source to operate properly. Transformerless power supplies are well-suited for this application due to their small size, low cost, and high efficiency.

A typical LED driver using a transformerless power supply consists of a capacitive voltage divider, a rectifier, a Filter capacitor, and a constant current regulator. The capacitive voltage divider steps down the AC mains voltage to a lower voltage suitable for the LED string, typically in the range of 12-24V. The rectifier and filter capacitor convert the AC voltage to DC, and the constant current regulator maintains a stable current through the LED string regardless of variations in the input voltage or the LED forward voltage.

Some examples of LED lighting applications that use transformerless power supplies include:

• LED bulbs for residential and commercial lighting
• LED tubes for fluorescent lamp replacement
• LED downlights and spotlights
• LED strip lights and signage
• LED grow lights for indoor farming

Home Appliances

Another application of transformerless power supplies is in home appliances, such as fans, humidifiers, and air purifiers. These appliances typically have low power requirements, in the range of a few watts to a few tens of watts, and need a simple and reliable power supply to operate.

Transformerless power supplies are a good fit for these applications due to their low cost, small size, and high efficiency. They can be easily integrated into the appliance’s housing and provide a stable DC voltage to power the motor, control circuitry, and sensors.

Some examples of home appliances that use transformerless power supplies include:

• Table and ceiling fans
• Humidifiers and dehumidifiers
• Air purifiers and ionizers
• Coffee makers and electric kettles
• Vacuum cleaners and floor polishers

Consumer Electronics

Transformerless power supplies are also widely used in consumer electronics, such as mobile phone chargers, laptop adapters, and set-top boxes. These devices have higher power requirements than home appliances, typically in the range of a few tens of watts to a few hundred watts, and need a more sophisticated power supply design to meet the requirements of the load.

Transformerless power supplies for consumer electronics often use a combination of capacitive and resistive voltage dividers, as well as active power factor correction (PFC) and switch-mode regulation techniques to achieve high efficiency, low harmonic distortion, and compact size.

Some examples of consumer electronics that use transformerless power supplies include:

• Mobile phone and tablet chargers
• Laptop and notebook adapters
• Set-top boxes and media players
• Printers and scanners
• Audio and video equipment

Industrial Control Systems

Finally, transformerless power supplies are also used in some industrial control systems, such as programmable logic controllers (PLCs), sensors, and actuators. These systems have specific power requirements, such as wide input voltage range, low noise, and high reliability, and need a rugged and efficient power supply to operate in harsh environments.

Transformerless power supplies for industrial control systems often use high-voltage capacitors and resistors, as well as protective devices such as fuses, varistors, and Tvs Diodes to withstand the surge and transient voltages common in industrial settings. They also use advanced regulation and filtering techniques to provide a clean and stable output voltage to the sensitive control circuitry.

Some examples of industrial control systems that use transformerless power supplies include:

• PLCs and remote I/O modules
• Sensors and transmitters
• Actuators and solenoid valves
• Motor drives and servo controllers
• Process monitors and data loggers

Frequently Asked Questions (FAQ)

1. What is a transformerless power supply?

A transformerless power supply is a type of power supply that does not use a transformer to step down the high voltage AC mains to a lower voltage suitable for the load. Instead, it uses capacitive or resistive voltage dividers to directly convert the AC mains voltage to a lower DC voltage.

2. What are the advantages of transformerless power supplies?

The main advantages of transformerless power supplies are:

• Lower cost due to fewer components and simpler manufacturing
• Smaller size and weight due to the absence of a bulky transformer
• Higher efficiency due to reduced losses in the voltage conversion process
• Reduced EMI due to the absence of a transformer that can act as an antenna

3. What are the disadvantages of transformerless power supplies?

The main disadvantages of transformerless power supplies are:

• Limited power output capacity due to the rating of the voltage divider components
• Require more careful design to ensure safety and reliability due to direct connection to AC mains
• May not provide galvanic isolation between input and output, which can be a safety and noise issue

4. What are some common applications of transformerless power supplies?

Some common applications of transformerless power supplies are:

• LED lighting, such as bulbs, tubes, and fixtures
• Home appliances, such as fans, humidifiers, and air purifiers
• Consumer electronics, such as mobile phone chargers, laptop adapters, and set-top boxes
• Industrial control systems, such as PLCs, sensors, and actuators

5. How do I choose a transformerless power supply for my application?

To choose a transformerless power supply for your application, you need to consider the following factors:

• Input voltage range and frequency of the AC mains
• Output voltage and current requirements of the load
• Power rating and efficiency of the power supply
• Size and weight constraints of the application
• Safety and reliability requirements, such as isolation, protection, and compliance with standards
• Cost and availability of the components and manufacturing process

It is recommended to consult with a power supply designer or manufacturer to select the best transformerless power supply for your specific application and requirements.

Conclusion

Transformerless power supplies are a cost-effective and space-saving alternative to traditional transformer-based power supplies for low-power applications. By using capacitive or resistive voltage dividers instead of a transformer to step down the AC mains voltage, they can achieve lower cost, smaller size, higher efficiency, and reduced EMI.

However, transformerless power supplies also have some limitations and challenges, such as limited power output capacity, more careful design for safety and reliability, and lack of galvanic isolation in some cases. Therefore, they are not suitable for all applications and need to be selected and designed carefully based on the specific requirements and constraints of the application.

Despite these challenges, transformerless power supplies have found widespread use in many applications, such as LED lighting, home appliances, consumer electronics, and industrial control systems. With advances in component technology and design techniques, they are expected to continue to grow in popularity and performance in the future.