Sunday, February 9, 2014

Simple Circuit LNK304 as 12V/120mA Switch Mode Power Supply Circuit

Robometricschool. - There are many electronic equipment or device that using power supply without using transformer component. We think this is one of development of electronic power supply technology that can transform high voltage 110 volt- 220 volt into low voltage like 12 volt or 18 volt.

In this time we want to show you one of simple electronic circuit schematic switch mode power supply for 12V/120mA without transformer component that using IC LNK304 like in figure 1 below. As we know that Transformer less switch mode power supplies have become very popular these days. The circuit shown below is of a 12V/120mA output, 85 to 230V AC input transformerless switch mode power supply using LNK304 IC. 

There are many applications of a power supply based on this IC includes hand held devices, timers, small appliances, LED drivers, industrial gadgets etc now.

Circuit Schematic 

Figure 1. LNK304 as 12V/120mA Switch Mode Power Supply Circuit (Circuitstoday)

Component List

All of the component of 12/120mA switch mode power supply circuit like in figure 1 above. The main component of this circuit is LN304 as a low component count, efficient off-line switcher IC that can support buck, buck-boost and flyback topologies.


To know more work of this circuit, you can read the description of this circuit as follow:

  1. The circuit diagram of a practical 12V/120mA transformerless switch mode power supply is shown above. Resistor R1, capacitors C1 and C2, diodes D1 and D2 and inductor L1 forms the input stage. D1 and D2 forms the rectifier section while C1 and C2 are input filters. Resistor R1 which is a fusible resistor limits the inrush current, increases differential mode noise attenuation and also serves as an input safety fuse. 
  2. The next stage is the regulator stage which consists of IC LNK304, diodes D3 and D4, capacitors C3, C4 and C5, resistors R3, R4 and R5 and inductor L2. D3 is the freewheeling diode while L2 is the output choke. C5 is the output filter capacitor which limits the output ripple voltage to a value as low as possible. The IC LNK304 is so configured that the power supply operated in the most discontinuous mode and that’s why a fast recovery diode (UF4005) is used as the freewheeling diode (D3). UF4005 has a reverse recovery time of around 75nS and it is well enough for the given situation. 
  3. The voltage drop across diodes D3 and D4 are practically same and so the voltage across C4 tracks the output voltage and this voltage is picked by the network comprising of resistors R2, R3 and is fed to the feedback pin. R2 and R3 sets the output voltage and for 12V output the voltage at the feedback pin must be 1.65V DC. The circuit attains regulation by skipping the switching cycles. When the output voltage rise, the current at the feedback pin also rises and when the current rises above the threshold value, subsequent cycles are skipped until the current at the feedback pin goes below the threshold and thus a constant output voltage is maintained. (Source:circuitstroday blog)


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