12V/120mA switch mode power supply circuit.
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. Applications of a power supply based on this IC includes hand held devices, timers, small appliances, LED drivers, industrial gadgets etc.
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. Applications of a power supply based on this IC includes hand held devices, timers, small appliances, LED drivers, industrial gadgets etc.
LNK304 is a low component count,
efficient off-line switcher IC that can support buck, buck-boost and
flyback topologies. The IC has a built in auto start circuit for short
circuit and open loop fault protection. Other features of LNK304
includes low temperature variation, thermal shut down,high break down
voltage, good line & load regulation, high band width , wide input
voltage range (85 to 230V AC) etc. In general the LNK304 has a better
performance when compared to the many other discrete buck regulators.
LNK304 pin configuration and typical application
The pin configuration and the typical
application diagram of LNK304 are shown above. Drain (D) pin the drain
connection of the built in power MOSFET. The external by pass capacitor
(0.1uF) is connected to the BYPASS (BP) terminal. FEEDBACK (FB) pin
controls the switching of the built in power MOSFET. A current above
than 49uA delivered to this pin will inhibit the switching. The internal
power MOSFETs source is connected to the SOURCE (S) pin.
LNK304 based switch mode power supply circuit.
12V /120 switch mode power supply circuit
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.
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.
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.
The IC will auto restart if no cycles
are skipped during a 50mS time period and this limits the maximum output
power to 6% of the maximum over load power. That’s how over load
protection is attained. Resistor R4 serves as a small preload which
nullifies the effects of tracking error.
Notes.
- Assemble the circuit on a good quality PCB.
- LNK304 is a very high efficiency switching regulator IC that has a hand full of applications.
- LNK304 is commonly available in SMD package (DIP is also available) and care must be taken while soldering it.
- D1 and D2 are standard 1N4007 silicon rectifier diodes.
- D3 (UF4005) is a fast recovery diode.
- D4 (1N4005GP) is a glass passivated diode.
- C3 can be a ceramic capacitor.
- C1,C2 and C4 are polyester capacitors.
- C5 can be electrolytic or polyester capacitor.
- Voltage ratings of the capacitors are shown in the circuit diagram.
- R1 is a fusible, fire proof resistor.
- Maximum possible output current is 120mA.
- Input voltage range is 85 to 230V AC.