How to Make a Variable Current SMPS Driver Circuit

This article discusses a method through which any ready made SMPS can be converted into a variable current smps circuit using a few external jumper links.

In one of the previous articles we learned how to make a variable voltage SMPS circuit by employing a simple shunt regulators stage, in the present hack also we employ the same circuit stage for implementing a variable current output feature.

The Function of Opto-coupler in SMPS

In the above linked post we discussed how an opto coupler played an important role in providing the crucial constant output feature for any SMPS.

The function of the opto coupler may be understood with the following brief explanation:

The opto coupler possesses an inbuilt LED/photo-transistor circuitry, this device is integrated with the SMPS outputs stage such that when the output tends to rise above the unsafe threshold, the LED inside the opto lights up forcing the phototransistor to conduct.

The photo-transistor in turn is configured across a sensitive "shut down" point of the SMPS driver stage wherein the conduction of the photo-transistor forces the input stage to shut down.

The above condition results in the SMPS output to also instantaneously shut down, however the moment this switching initiates, it corrects and restores the output to the safe zone and the LED inside the opto deactivates which once again switches ON the input stage of the SMPS.

This operation keeps on cycling rapidly from On to OFF and vice versa ensuring a constant voltage at the output.

Adjustable Current SMPS Modification

In order to achieve a current control feature inside any SMPS we yet again seek the help of the opto coupler.

We implement a simple modification using a BC547 transistor configuration as shown below:

Referring to the above design we get a clear idea regarding how to modify or make a variable current SMPS driver circuit.

The opto coupler (indicated by red square) will be present by default for all SMPS modules, and assuming that the TL431 is not present then we may have to configure the entire configuration associated with opto coupler LED.

If the TL431 stage is already a part of the SMPS circuit, in that case we just have to consider integrating the BC547 stage which becomes solely responsible for the proposed current control of the circuit.

The BC547 can be seen connected with its collector/emitter across the TL431 IC's cathode/anode, and the base of BC547 can be seen connected with the output (-) of the SMPS via a group of selectable resistors Ra, Rb, Rc, Rd.

These resistors being in between the base and emitter of the BC547 transistor begin functioning like current sensors for the circuit.

These are appropriately calculated such that by shifting the jumper connection across the relevant contacts, different current limits are introduced in the line.

When the current tends to increase beyond the set threshold as determined by the values of the corresponding resistors, a potential difference is developed across the base/emitter of the BC547 which becomes sufficient to turn ON the transistor, shorting the TL431 IC between the opto LEd and ground.

The above action instantly lights up the LED of the opto, sending a "fault" signal to the input side of the SMPS via the opto's in-built photo transistor.

The condition immediately tries to execute a shut down across the output side which in turn stops the BC547 from conducting and the situation fluctuates from ON to OFF and ON rapidly ensuring that the current never exceeds the predetermined threshold.

The resistors Ra...Rd may be calculated by using the following formula:

R = 0.7/cut-of current threshold

For example if suppose we want to connect an LED at the output having a current rating of 1 amp.

We can set the value of the corresponding resistor (selected by the jumper) as:

R = 0.7/1 = 0.7 ohm

Wattage of the resistor can be simply gotten by multiplying the variants, i.e. 0.7 x 1 = 0.7 watts or simply 1 watt.

The calculated resistor ensures that the output current to the LED never crosses the 1 amp mark, thereby safeguarding the LED from damage, other values for the remaining resistors may be appropriately calculated for getting the desired variable current option in the SMPS module.