The type of current limiting used depends primarily on the safe operating area of the pass element used. The three basic current limiting techniques are series resistor, constant current, and fold-back current limiting.
This is the simplest method for short-circuit protection. The short-circuit current is determined by the current-limiting resistor RCL, illustrated in Figure 5-28.
The primary drawback of this technique is error introduced by the voltage dropped across RCL under varying load conditions. The % error, as illustrated by the following equations, depends on the RCL and RL values. Maintaining RCL at a level which is an order of magnitude less than the nominal load impedance minimizes this effect.
Maintaining RCL at level which is an order of magnitude less than the nominal load impedance minimizes this effect.
This also yields a short-circuit current that is an order of magnitude greater than the normal operating load current.
This technique is obviously inefficient since it requires using a regulator or pass element with current capabilities in excess (11X) of its normal operating capabilities. The performance characteristics of a series resistance current limited regulator are illustrated in Figure 5-29.
Constant-current limiting is the most popular current limiting technique in low-power, low-current regulator circuits. The basic configuration is illustrated in Figure 5-30.
Note that this method requires access to the control element and remote voltage sense capabilities. By sensing the output voltage beyond the current limiting resistor, the circuit allows the regulator to compensate for the voltage changes across RCL.
If an external pass transistor is used, its base current may be starved to accomplish constant-current limiting, as illustrated in Figure 5-31. Current limiting takes effect as the voltage drop across RCL approaches the potential required to turn on transistor QI, As QI is biased on, the current supplying the base of Q2 is diverted, thus decreasing the drive current to Q3, the regulator’s pass transistor. The performance characteristics of a constant-current limited regulator are illustrated in Figure 5-32.
It should be noted that short-circuit conditions are the worst conditions that can be imposed on the pass transistor since it has to survive not only the short-circuit current, but it has to withstand the full input voltage across its collector and emitter terminals.
This normally requires the use of a pass transistor with power handling capabilities much greater than those required for normal operation i.e.:
This requirement may be reduced by the application of fold-back current limiting.
Fold-Back Current Limiting
Fold-back current limiting is used primarily for high-current applications where the normal operating requirements of the regulator dictate the use of an external power transistor. The principle of fold-back current limiting provides limiting at a predetermined current (IK). At this predetermined current, feedback reduces the load current as the load continues to increase (RL decreasing) and causes the output voltage to decay.
The fold-back current-limiting circuit of Figure 5-34 behaves in a manner similar to the constant-current limit circuit illustrated in Figure 5-31. In Figure 5-33, the potential developed across the current limit sense resistor (RCL) must not only develop the base-emitter voltage required to turn on QI, but it must develop sufficient potential to overcome the voltage across resistor RI.
As the load current requirement increases above IK, the output voltage (VO) decays. The decreasing output voltage results in a proportional decrease in voltage across Rl. Thus, less current is required through RCL to develop sufficient potential to maintain the forward-biased condition of Q1. This can be seen in the above expression for IK, As Vo decreases, IK decreases. Under short-ircuit conditions (VO = 0) IK becomes:
The approach illustrated in Figure 5-34 allows a more efficient design because the collector current of the pass transistor is less during short-circuit conditions than it is during normal operation. This means that during short-circuit conditions, when the voltage across the pass transistor is maximum, the collector-emitter current is reduced. As illustrated in Figure 5-35, fold-back current limiting fits closer to the typical performance characteristics of the transistor, thus allowing a better design match of the pass transistor to the regulator.