The Main features of this power supply using the IC SR087 are:
High efficiency without incorporating inductors.
Does not require high voltage capacitors for mains current dropping.
Can be used with 120V AC as well as 220V AC inputs
Output adjustable from 9V to 50VDC
Has an internal soft stat circuitry
Stand by consumption is less than 200mW
The Supertex SR087 is an transformerless switching regulator chip specially designed
to operate directly from a rectified 220V or 120V AC line.
The principle of operation is to switch ON a pass transistor each time the rectified AC reaches under the set output level, and switch it OFF as soon as the output level is sustained at the set level.
A internally set 5V linear regulator offers an additional 5V fixed output from the IC for operating devices requiring strict 5V inputs.
The IC also facilitates an external logic input triggered "disable" feature, which can be used for disabling the circuit when not in usee and keep the system in the standby mode.
WARNING! Galvanic isolation is not included in the design. Life threatening voltages and shocks may be floating when switched ON to the AC line. The designer employing the SR087 must ensure proper safety measures are employed to protect the end user from fatality.
The circuits described here are not guaranteed to fulfill surge and EMI conduction requirements.
The working of these
circuits might differ depending on a given application. The designer is advised to implement tests to verify compliance with the laid down world standards and
VIN - Should be connected across a 120/230VAC line. The AC input stgae of the circuit is safeguarded from surge currents by a 275V metal oxide varistor (MOV) and also a 1.25A
Never use a transformer at the input line. The high inductance might generate an inductive back EMF, overloading the
MOV and destroying it. Please note that the proposed 50V adjustable transformerless power supply is not design to operate through the supply input from a square wave uninterruptible which are also usually referred to as “modified sine wave”.
GND - This is the circuit common line. And because the circuit does not offer a galvanic isolation from mains 220V or 120V, connecting this common line to an earth-grounded equipment,
(such as an oscilloscope), could cause a short circuited the AC line, leading to an instant damage to the circuit or even the equipmet in use.
You may also want to note that GND may be at a raised voltage level with respect
to earth ground, even while the AC input is switched off. Be cautioned about this!
VOUT - This refers to the main output of the circuit stage.
The SR087 IC is designed to regulate the peak output voltage, and not the average value, therefore the
average voltage will show a tendency to decline when a load is attached.
VOUT can be adjusted from 9.0 to 50V by changing the value of R1 as per the formula given in the circuit diagram
VREG - It is the fixed 5V regulated output, from the IC. Since this output is derived from the 50V line any load on VREG might cause an equivalent current drop across VOUT.
VREG will require at least 4.0V of headroom
to generate the 5V, that is a minimum of 9V at VOUT.
Since the IC is typically a linear regulator, the SR087 will dissipate
power as curent on VREG output or VOUT goes up to around 460mW at 60mA.
ENABLE - If a logic low (<0.2V) is applied on this pinout it enables Q1
switching and the VOUT switched ON.
However a logic
high (>0.75 • VREG) on this pinout quickly disables Q1
, shutting the VOUT supply and also the VREG output.
However, if an external voltage exists across VOUT terminals in the disabled state, VREG will continue to function allow a 5.0V to be generated across the specified terminals.
The ENABLE input is equipped with 20kΩ pull-down resistor. In case it's not required or unused, it could be simply left unconnected or connected to ground.