The foldback current limiting technique in linear power supplies is a technique used to keep the power dissipation under safe limits.
The folding back of the output current in power supplies potentially reduces the thermal, electrical, and mechanical stresses in circuit components, extending the life of these components.
Under constant current limiting mode, the power supply never gets tripped down to zero. The foldback current limiting method trips down the power supply and the voltage output will be equal to zero under this condition.
The major share of our voltage requirements are fulfilled by Switch-Mode Power Supplies (SMPS). While the lightweight and highly efficient features of SMPS make it a constant element in consumer electronics, its counterpart, linear power supply, is more often found in radio frequency (RF) circuits. A linear power supply is highly reliable and immune to electromagnetic interference, especially RF interference, making it an excellent choice for RF and microwave circuits. When discussing linear power supply, we can't ignore the linear voltage regulators and the associated power dissipation. The foldback current limiting technique in linear power supplies is a technique used to keep power dissipation under safe limits. It also protects the linear supplies from faults and overload conditions.
Foldback Current Limiting Technique in Linear Power Supplies
The occurrence of overcurrent, overload, and short circuit conditions can damage the linear power supplies and affect the total useful life of the power source. Several current limiting techniques are commonly employed to protect the linear power supplies from such faults. Foldback current limiting method is one such overcurrent protection scheme. Foldback current limiting technique reduces the output voltage and output current linearly during overload and overcurrent conditions, bringing the power dissipation under limits.
Consider a linear power supply with input voltage Vi, output voltage Vo and output current Io. The power dissipation is the linear regulator is given by the following equation:
When the load connected increases, more output current is drawn from the supply, causing an increase in power dissipation. The linear power supply tries to maintain a constant voltage until the output current value reaches the knee point as shown in the image above. This is the point where the foldback current limiting circuit comes into action. Once the knee-point is reached, the foldback current limiting circuit decreases the output current linearly with the output voltage. The output current is folded back to a lower value and the output voltage moves linearly with the current towards a zero value. When the current value equals Isc, the power supply voltage output will be null.
As the voltage Vo and the current Io are following a decreasing trend with foldback current limiting circuit operation, so is the power dissipation in the linear power supply. This folding back of the output current potentially reduces the thermal, electrical, and mechanical stresses in the circuit components and thereby extends its life. The recovery from the foldback current limiting mode can be automatic or after a time delay when the overload is taken off.
Comparing Foldback Current Limiting with Constant Current Limiting
Power supplies are intended to supply constant voltage to the load. When the power supply is overloaded, the output current increases and can go beyond the maximum current rating of the supply. When the power supply comes with the constant current limiting feature, the power supply moves from constant voltage to constant current mode under overloads. It keeps decreasing the voltage under constant current limiting mode. However, the power supply never gets tripped down to zero in the constant current limiting method. The foldback mode trips down the power supply and the voltage output will be equal to zero under this condition.
Similar to foldback current limiting, the constant current limiting technique also comes with a limiting current value which determines when the power supply should be switched to constant current mode. Constant current limiting is frequently seen in SMPS, whereas the foldback method is the most common protection in linear regulated supplies. In the constant current limiting protection scheme, the supply will move back to constant voltage mode once the overload is removed.
Under short-circuiting conditions, the current drawn from the power supply is not linearly reduced in the constant current limiting method, and this leads to large power dissipation. The power supply gets damaged when the constant current limiting scheme keeps the current at maximum value until the short circuit is cleared. The foldback current limiting scheme reduces the short circuit current and provides the required current at rated output voltage to the load. The power dissipation in foldback current limited power supplies is comparatively low at short circuit conditions.
Variants of Current Foldback Circuits
The overcurrent protection scheme employed in power supplies can be of different types. There are schemes where the current limiting and current foldbacks are combined. The main 3 types of current foldback circuits are:
1) Current Limiter, Then-Current Foldback
The lack of accuracy of the current foldback circuit can be corrected by including a current limiting scheme in the circuit. In this current protection, whenever the current crosses the threshold limit, it moves to constant current limiting mode. When the voltage reaches a certain value in constant current limiting mode, the foldback circuit takes charge of protection. In the image above, the curve plotted in red shows the characteristics of this circuit.
2) Current Limiter, Then-Current Limiter
This protection scheme blends the current limiting method with current foldback limiting twice. First, the current is limited until the knee-point current and corresponding threshold voltage is reached simultaneously. Then the circuit folds back the current to a lower value. Once the current coordinate matches the second threshold limit, it's the second turn for the current limiter to reappear and start protecting the circuit. This current limiting-foldback circuit produces more heat than the foldback circuit. The circuit characteristics of this particular type of foldback circuit are plotted in green in the image above.
3) Current Foldback, Then-Current Limiter
In this type of overcurrent protection, the current is folded back to a lower value and is then limited. When the folded current reaches a certain limit, the current limiter circuit starts to operate. The image above presents the characteristics of this protection scheme in blue.
Power supplies are vulnerable to overcurrents under overload conditions. Our investment in power supplies is critical, and it remains important to provide overcurrent protection circuits along with the power supplies. The foldback current limiting technique is one of the best power supply protection schemes to guard the supply and devices connected from overcurrents and short circuits.