Posted on: July 1, 2014

Direct off line power supply

A switching power supply that is isolated by the high frequency transformer and has no 60 Hz transformer at the front end is called direct-off-line.

A direct off line power supply operation is similar to that of a normal SMPS, however, it eliminates the bulky step down transformer and therefore, lighter, cheaper, more efficient at about 90% and has less heat output. The direct-off-line ac-dc converters are also known as off-line converters or off-line power supplies.

Basic operation of a direct-off line supply

The 240V or 120 Volts line voltage passes through the RMI/RFI line filter and the fuse. The line filter prevents interferences getting into, or out of, the power unit. This ac input is then rectified using a full-wave bridge rectifier after which the dc voltage is filtered using two high-voltage capacitors.

What is Direct off Line

Credit Image – Texas instruments

The high frequency switching is controlled by integrated circuits, which controls the speed at which the transistors switch on and off, as well as the conduction time of each transistor. By controlling the timing, these pulse width modulation ics are able to control the output voltage.
The switching on and off of the transistors causes the transformer’s primary winding to charge and discharge and hence induce power to the secondary winding.

The high frequency output voltage pulses from the transformer secondary winding are then rectified using high speed diodes to produce the d.c voltage which is then filtered to remove the high frequency components. Voltage regulation is achieved by feeding part of the output voltage back to the switching IC which then compares against a preset value and adjusts the switching frequency accordingly.


The high frequency transformer isolates the output voltage from the ac line voltage through the mutual conduction.

Inrush Current Limiting

A current limiting mechanism is usually employed to avoid damage by the high peak inrush currents which may develop during the supply power turn-on. At turn-on, the filter capacitors will starts charging, initially, they present a low impendence to the ac line which can cause high current to flow into the circuit.

Current limiting is achieved by introducing impendence to the ac-line to limits the inrush current and avoids damage to the components of power supply.

Resistor triac circuit

The resistor is connected in series with the ac line and then shunted by the triac. Once the capacitors are fully charged a trigger circuit fires the triac which in turn shorts the resistor. Care is taken to ensure that the Triac can handle the full input current when in its on condition.

Using NTC thermistors

The Negative temperature coefficient (NTC) thermistors are connected in series with the ac line of the d.c buses immediately after the rectifier. On switch on, the thermistors provides the impendence to the ac, hence limiting the inrush current.

The direct off power supplies are widely used in many power supply units used for computers, laptops, telecommunication, gaming and other electronic equipment.