Transformerless 12V Switching Power Supply Circuit

A Transformerless 12V Switching Power Supply Circuit is a small and smart choice instead of the usual transformer based power supplies.

It changes high voltage AC into a steady low voltage DC output by using fast switching techniques.

This way, it gets rid of big transformers making the whole setup light and perfect for low power uses like embedded systems, IoT gadgets and industrial controls.

The circuit here is for a power supply that gives 12V and 120mA output working with 85 to 230V AC input and it uses the LNK304 IC.

The LNK304 is a simple and efficient off line switcher IC that can work with buck, buck-boost and flyback designs.

It also has a built in auto start feature to protect against short circuits and open loop faults.

LNK304 usually comes in an SMD package, so you need to be careful when soldering it.

Circuit Working:

Parts List:

This circuit works like a switched mode power supply SMPS using the LNK304 chip.

This chip has a special high voltage MOSFET and control parts that help change power efficiently.

It takes an AC input supply of 85V to 250V which gets changed into a smooth DC after being rectified and filtered.

R1 is a special resistor that can melt if it gets too hot keeping things safe.

D1 and D2 are diodes that work together as a bridge rectifier to turn AC into pulsating DC.

C1 and C2 are input filters that help make the DC voltage nice and smooth.

L1 helps reduce any high frequency noise.

L2 is another choke that helps keep the output voltage steady by reducing ripple.

The LNK304 chip makes sure the power conversion is super efficient.

C3 is a capacitor that helps keep the chip working well.

R2 and R3 help set the right conditions for the output voltage, which is 12V.

C4 smooths out the output from the switching.

D3 is a diode that helps fix the high frequency AC from the switching part.

C5 capacitor gives the final smoothing touch.

R4 acts like a load resistor.

Diode D4 makes sure everything is connected the right way and keeps it safe.

In the end the circuit gives out 12V DC at 200mA perfect for small devices that run on DC power.

Formulas with Calculations:

Below are the formulas with calculations for Transformerless 12V Switching Power Supply Circuit:

Input Power Calculation:

P_input = V_input × I_input

where,

• V_input is 230V AC
• Efficiency is 75%
• Output Power Required is:

P_out = 12V × 0.2A = 2.4W

Now, let us calculate the input power:

P_in = P_out / Efficiency

P_in = 2.4W / 0.75

P_in = 3.2W

Current Limiting Resistor R1 Calculation:

I = V_input / R

where,

• R1 is 8.2Ω
• V_input is 230V AC

Maximum possible current:

I = 230V / 8.2Ω

I = 28A

(Note: This is the initial surge current, actual current is limited by circuit operation.)

Inductor Selection L1, L2:

Formula for Inductor Current Ripple:

ΔI_L = (V_out × (1 – D)) / (f × L)

where,

• L1 is 1mH Switching Frequency
• (f) is 65kHz
• Duty Cycle D is 0.5
• Output Voltage is 12V

Substituting values:

ΔI_L = (12V × 0.5) / (65,000 × 1mH)

ΔI_L = (6V) / (65,000 × 0.001)

ΔI_L = 6 / 65

ΔI_L = 92mA

Output Capacitor Selection:

Formula for Ripple Voltage:

V_ripple = I_load / (f × C)

where:

• C5 is 100µF
• Load Current (I_load) is 200mA = 0.2A
• Switching Frequency (f) = 65kHz

Substituting values:

V_ripple = 0.2A / (65,000 × 100µF)

V_ripple = 0.2 / (65,000 × 0.0001)

V_ripple = 0.2 / 6.5

V_ripple = 30mV

How to Build:

To build a Transformerless 12V Switching Power Supply Circuit follow the below mentioned steps for connections and asseblying:

• Assemble all the components as mentioned in the above circuit diagram
• Connect the S(source) pin of IC1 LNK304 to the output supply of 12V
• Connect capacitor C3 between BP pin and Source pin of IC1
• Connect resistor R2 and R3 in series between source pin of IC1 through the junction of capacitor C4 and diode cathode of diode D4
• Connect cathode of diode D3 from source pin of IC1 and anode of diode D1 connect to GND.
• Connect capacitor C5 positive from source pin and negative of capacitor connect to GND.
• Connect a inductor coil L2 between capacitor C4 and capacitor C5.
• Connect resistor R4 from source pin of IC1 to GND.
• Connect the D drain pin of IC1 to input supply of 85V to 250V AC
• Connect positive of capacitor C2 from drain pin and negative of capacitor connect to GND.
• Connect coil inductor L1 between capacitor C1 and C2
• Connect positive of capacitor C1 from drain pin and negative of capacitor connect to GND.
• Connect one end of R1 to input supply and other nd of R1 connect to anode of diode D1
• Connect the cathode of D2 to other AC input supply

Conclusion:

This Transformerless 12V Switching Power Supply Circuit is a small, cost effective and efficient option compared to larger transformer based models.

It is perfect for low power uses like LED drivers, Internet of Things (IoT) gadgets and embedded circuits.

The IC LNK304 component helps it run safely and efficiently while keeping everything stable.

References:

Datasheet IC LNK304