Current limiting
Encyclopedia
Current limiting is the practice in electrical or electronic
circuit
s of imposing an upper limit on the current
that may be delivered to a load
with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load. This term is also used to describe the ability of an overcurrent
protective device (fuse or circuit breaker) to reduce the peak current in a circuit, by opening and clearing the fault in a sub-cycle time frame.
. As the current exceeds the fuse's limits it blows thereby disconnecting the load from the source. This method is most commonly used for protecting the house-hold mains. A circuit breaker
is another device for mains current limiting.
Compared to circuit breakers, fuses attain faster current limitation by means of arc quenching. Since fuses are passive elements, they are inherently secure. Their drawback however is the single operation principle: once blown, they need to be replaced or reset.
is a device or group of devices used to limit inrush current. Negative temperature coefficient
(NTC) thermistors and resistors are two of the simplest options, with cool-down time and power dissipation being their main drawbacks, respectively. More complex solutions can be used when design constraints make simpler options infeasible.
Electronic circuits like regulated DC power supplies and power amplifiers employ, in addition to fuses, active current limiting since a fuse alone may not be able to protect the internal devices of the circuit in an over-current or short-circuit situation. A fuse generally is too slow in operation and the time it takes to blow may well be enough to destroy the devices.
A typical short-circuit/overload protection scheme is shown in the image. The schematic is representative of a simple protection mechanism employed in regulated DC supplies and class-AB power amplifiers‡.
Q1 is the pass or output transistor. Rsens is the load current sensing device. Q2 is the protection transistor which turns on as soon as the voltage across Rsens becomes about 0.65 V. This voltage is determined by the value of Rsens and the load current through it (Iload).
When Q2 turns on, it removes base current from Q1 thereby reducing the collector current of Q1. Neglecting the base currents of Q1 and Q2, the collector current of Q1 is also the load current. Thus, Rsens fixes the maximum current to a value given by 0.65/Rsens, for any given output voltage and load resistance.
For example, if Rsens = 0.33 Ω, the current is limited to about 2 A even if Rload becomes a short (and Vo becomes zero). With the absence of Q2, Q1 would attempt to drive a very large current (limited only by Rsens, and dependent on the output voltage Vo if Rload is not zero) and the result would be greater power dissipation in Q1.
If Rload is zero the dissipation will be much greater (enough to destroy Q1). With Q2 in place, the current is limited and the maximum power dissipation in Q1 is also limited to a safe value (though this is also dependent on Vcc, Rload and current-limited Vo).
Further, this power dissipation will remain as long as the overload exists, which means that the devices must be capable of withstanding it for a substantial period. For example, the pass-transistor in a regulated DC power supply system (corresponding to Q1 in the schematic above) rated for 25 V at 1.5 A (with limiting at 2 A) will normally (i.e. with rated load of 1.5 A) dissipate about 7.5 W for a Vcc of 30 V‡‡ (1).
With current limiting, the dissipation will increase to about 60 W if the output is shorted‡‡ (2). Without current limiting the dissipation would be greater than 300 W‡‡ (3) - so limiting does have a benefit, but it turns out that the pass-transistor must now be capable of dissipating at least 60 W.
In short, an 80-100 W device will be needed (for an expected overload and limiting) where a 10-20 W device (with no chance of shorted load) would have been sufficient. In this technique, beyond the current limit the output voltage will decrease to a value depending on the current limit and load resistance.
‡ – For class-AB stages, the circuit will be mirrored vertically and complementary devices will be used for Q1 & Q2.
‡‡ – The following conditions are considered for determining the power dissipation in Q1, with Vo = 25 V, Iload = 1.5 A (limit at 2 A), Rsens = 0.33 Ω (for limiting at 2A) and Vcc = 30 V —
This slows the edge rate which improves electromagnetic compatibility
.
Some devices have this "slew rate
limiting" output resistor built in; some devices have programmable slew rate limiting. This provides overall slew rate control.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
s of imposing an upper limit on the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
that may be delivered to a load
External electric load
If an electric circuit has a well-defined output terminal, the circuit connected to this terminal is the load....
with the purpose of protecting the circuit generating or transmitting the current from harmful effects due to a short-circuit or similar problem in the load. This term is also used to describe the ability of an overcurrent
Overcurrent
In electricity supply, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load,...
protective device (fuse or circuit breaker) to reduce the peak current in a circuit, by opening and clearing the fault in a sub-cycle time frame.
Mains power
The simplest form of current limiting for mains is a fuseFuse (electrical)
In electronics and electrical engineering, a fuse is a type of low resistance resistor that acts as a sacrificial device to provide overcurrent protection, of either the load or source circuit...
. As the current exceeds the fuse's limits it blows thereby disconnecting the load from the source. This method is most commonly used for protecting the house-hold mains. A circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
is another device for mains current limiting.
Compared to circuit breakers, fuses attain faster current limitation by means of arc quenching. Since fuses are passive elements, they are inherently secure. Their drawback however is the single operation principle: once blown, they need to be replaced or reset.
Inrush current limiting
An inrush current limiterInrush current limiter
An inrush current limiter is a component used to limit inrush current to avoid gradual damage to components and avoid tripping the supply's fuse or circuit breaker. Negative temperature coefficient thermistors and fixed resistors are often used for this. Less often, other components are also used...
is a device or group of devices used to limit inrush current. Negative temperature coefficient
Temperature coefficient
The temperature coefficient is the relative change of a physical property when the temperature is changed by 1 K.In the following formula, let R be the physical property to be measured and T be the temperature at which the property is measured. T0 is the reference temperature, and ΔT is the...
(NTC) thermistors and resistors are two of the simplest options, with cool-down time and power dissipation being their main drawbacks, respectively. More complex solutions can be used when design constraints make simpler options infeasible.
In electronic power circuits
A typical short-circuit/overload protection scheme is shown in the image. The schematic is representative of a simple protection mechanism employed in regulated DC supplies and class-AB power amplifiers‡.
Q1 is the pass or output transistor. Rsens is the load current sensing device. Q2 is the protection transistor which turns on as soon as the voltage across Rsens becomes about 0.65 V. This voltage is determined by the value of Rsens and the load current through it (Iload).
When Q2 turns on, it removes base current from Q1 thereby reducing the collector current of Q1. Neglecting the base currents of Q1 and Q2, the collector current of Q1 is also the load current. Thus, Rsens fixes the maximum current to a value given by 0.65/Rsens, for any given output voltage and load resistance.
For example, if Rsens = 0.33 Ω, the current is limited to about 2 A even if Rload becomes a short (and Vo becomes zero). With the absence of Q2, Q1 would attempt to drive a very large current (limited only by Rsens, and dependent on the output voltage Vo if Rload is not zero) and the result would be greater power dissipation in Q1.
If Rload is zero the dissipation will be much greater (enough to destroy Q1). With Q2 in place, the current is limited and the maximum power dissipation in Q1 is also limited to a safe value (though this is also dependent on Vcc, Rload and current-limited Vo).
Further, this power dissipation will remain as long as the overload exists, which means that the devices must be capable of withstanding it for a substantial period. For example, the pass-transistor in a regulated DC power supply system (corresponding to Q1 in the schematic above) rated for 25 V at 1.5 A (with limiting at 2 A) will normally (i.e. with rated load of 1.5 A) dissipate about 7.5 W for a Vcc of 30 V‡‡ (1).
With current limiting, the dissipation will increase to about 60 W if the output is shorted‡‡ (2). Without current limiting the dissipation would be greater than 300 W‡‡ (3) - so limiting does have a benefit, but it turns out that the pass-transistor must now be capable of dissipating at least 60 W.
In short, an 80-100 W device will be needed (for an expected overload and limiting) where a 10-20 W device (with no chance of shorted load) would have been sufficient. In this technique, beyond the current limit the output voltage will decrease to a value depending on the current limit and load resistance.
‡ – For class-AB stages, the circuit will be mirrored vertically and complementary devices will be used for Q1 & Q2.
‡‡ – The following conditions are considered for determining the power dissipation in Q1, with Vo = 25 V, Iload = 1.5 A (limit at 2 A), Rsens = 0.33 Ω (for limiting at 2A) and Vcc = 30 V —
- Normal operation: Vo = 25 V at a load current of 1 A. So Q1 dissipates a power of (30 - 25) V * 1.5 A = 7.5 W. The transistor used must be a 10-20 W device to account for ambient temperature (i.e., deratedDeratingDerating is the operation of a machine at less than its rated maximum power in order to prolong its life. The term is commonly applied to electrical and electronic devices and to internal combustion engines.-In electronics:...
) and must be mounted on a heat-sink. - Output shorted, with limiting at 2A: The dissipation is given by (30 - 0.65) V * 2 A = 58.7 W. The 0.65 V is the drop across Rsens. In practice, if the power supply Vcc is not able to provide the maximum short-circuit current it will collapse thereby reducing dissipation in Q1. However this is dependent on how "stiff" the supply is. A stiffer supply will sustain the voltage for a heavier current draw before collapsing. Further, the transistor used must be a 80-100 W device to account for ambient temperature (i.e., derated) and must be mounted on a heat-sink.
- Output shorted, and no limiting: A shorted load will mean that only Rsens is present as the load. With this, the circuit will attempt to put 25 V across Rsens (0.33 Ω) - here the output voltage has to be measured at the emitter of Q1 since Q1 is connected as an emitter-follower and the lower end of Rsens is effectively grounded due to the short. Thus the load current (and collector current of Q1) becomes nearly 76 A, and the dissipation in Q1 becomes (30 - 25) V * 76 A = 380 W. This is a very large power to dissipate, since in normal circumstances Q1 will only be required to dissipate about 7.5 W (60 W at worst with limiting), and even a 100 W transistor will not withstand a 380 W dissipation. Without Rsens (i.e., Q1 emitter is directly connected to the load) the situation is even worse — Q1 becomes a dead short across 30 V and will draw current limited only by its internal resistance. In practice, the dissipation will be less because the supply (Vcc) will collapse under such a condition. However the dissipation will still be enough to destroy Q1.
Slew rate control
Many electronics designers put a small resistor on IC output pins.This slows the edge rate which improves electromagnetic compatibility
Electromagnetic compatibility
Electromagnetic compatibility is the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to the unwanted effects that such energy may induce...
.
Some devices have this "slew rate
Slew rate
In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
limiting" output resistor built in; some devices have programmable slew rate limiting. This provides overall slew rate control.