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Dec 27 2017

Principle and Applicaions of OverCurrent Protection Circuit

 

Warm hints: The word in this article is about 2800 and reading time is about 15 minutes.

Summary

Many electronic devices have a rated current. Once the device exceed the rated current, it will burn out the device. Therefore, these devices do a current protection module, when the current exceeds the set current, the device automatically power off to protect the device, which is over-current protection. Such as the USB interface on the computer motherboard, USB overcurrent protection generally have to protect the motherboard is not burned. This article will show you what is overcurrent protection;OverCurrent Protection types; its Principle and Applicaions.


Catalog

Summary

Catalog

I What is OverCurrent Protection

II How Overcurrent Protection Works?

III Types of Overcurrent Protection

IV Application Examples of Overcurrent Protection Circuit

4.1 Transformer

4.2 Motor

V Over-current Protection Circuit in the Switching Power Supply Design

VI Conclusion


I What is OverCurrent Protection

OverCurrent Protection (Over Current Protection) is the current protection device action when the current exceeds a predetermined maximum. When the current flowing through the protected original exceeds a preset value, the protective device is activated and the timing is used to ensure the selectivity of the action, tripping the circuit breaker or giving an alarm signal.

Many electronic devices have a rated current. Once the device exceed the rated current, it will burn out the device. Therefore, these devices do a current protection module, when the current exceeds the set current, the device automatically power off to protect the device, which is over-current protection. Such as the USB interface on the computer motherboard, USB overcurrent protection generally have to protect the motherboard is not burned.

A power supply circuit with overcurrent protection function

A power supply circuit with overcurrent protection function

Overcurrent protection include short circuit protection and overload protection. Short-circuit protection is characterized by larger setting current and instantaneous action. Electromagnetic current release (or relay), fuses are often used as short-circuit protection components. Overload protection is characterized by smaller setting current, inverse time action. Thermal relay, delay-type electromagnetic current relays commonly used as overload protection components.

Fuses are also commonly used as overload protection components without much impact current.

In the TN system, When use fuses for short circuit protection, the melt rated current should be less than 1/4-phase short-circuit current. with the circuit breaker protection, the circuit breaker instantaneous action or short delay action overcurrent release’s setting current should be less than 2/3 of the single-phase short-circuit current

learn the knowledge about overcurrent protection more intuitively: 


How to Protect Circuits from Overcurrent Spikes

II How Overcurrent Protection Works?

In the case of a phase-to-phase short-circuit fault or abnormal load increase in the power grid, or a decrease in the insulation level, the current will suddenly increase and the voltage will suddenly drop. Overcurrent protection is to set the operating current of the current relay according to the requirements of line selectivity. When the fault current in the line reaches the action value of the current relay, the current relay acts according to the selective requirements of the protection device, selectively cutting off the fault line, and starting the time relay through its contacts. After a predetermined delay, the time relay touches The point is closed, the circuit breaker trip coil is turned on, the circuit breaker is tripped, the fault line is cut off, and the signal relay is activated at the same time, the signal board falls, and the light or sound signal is turned on.

When there are unexpected conditions such as load short circuit, overload or control circuit failure, it will cause excessive current flowing through the switching transistor in the regulator, which will increase the power consumption of the tube and generate heat. If there is no overcurrent protection device, a high-power switching transistor It may be damaged. Therefore, overcurrent protection is commonly used in switching regulators. The most economical and convenient method is to use a fuse. Due to the small heat capacity of transistors, ordinary fuses generally cannot provide protection. Fast-blow fuses are commonly used. This method has the advantage of easy protection, but it is necessary to select the fuse specifications according to the requirements of the safe working area of ​​the specific switching transistor. The disadvantage of this overcurrent protection measure is the inconvenience of frequent fuse replacement.

An inverter overcurrent protection circuit

An inverter overcurrent protection circuit

Current limiting protection and current cut-off protection commonly used in linear regulators can be applied in switching regulators. However, according to the characteristics of the switching regulator, the output of this protection circuit cannot directly control the switching transistor, but the output of the overcurrent protection must be converted into a pulse command to control the modulator to protect the switching transistor. In order to achieve over-current protection, it is generally necessary to use a sampling resistor in series in the circuit, which will affect the efficiency of the power supply, so it is mostly used in small-power switching regulators. In high-power switching regulated power supplies, considering power consumption, the sampling resistor should be avoided as much as possible. Therefore, the overcurrent protection is usually converted to over and under voltage protection.

A protective device is provided at the origin of the circuit concerned (see the following picture)

Circuit protection by circuit breakerCircuit protection by fuses

Acting to cut-off the current in a time shorter than that given by the I2t characteristic of the circuit cabling

But allowing the maximum load current IB to flow indefinitely

The characteristics of insulated conductors when carrying short-circuit currents can, for periods up to 5 seconds following short-circuit initiation, be determined approximately by the formula:

I2t = k2 S2

which shows that the allowable heat generated is proportional to the squared cross-sectional-area of the condutor.

where

t = Duration of short-circuit current (seconds)

S = Cross sectional area of insulated conductor (mm2)

I = Short-circuit current (A r.m.s.)

k = Insulated conductor constant (values of k are given in Figure 5)

For a given insulated conductor, the maximum permissible current varies according to the environment. For instance, for a high ambient temperature (θa1 > θa2), Iz1 is less than Iz2 (see Fig. 5). θ means “temperature”.

I2t characteristic of an insulated conductor at two different ambient temperatures

Note:

ISC = 3-phase short-circuit current

ISCB = rated 3-ph. short-circuit breaking current of the circuit-breaker

Ir (or Irth)[1] = regulated “nominal” current level; e.g. a 50 A nominal circuit-breaker can be regulated to have a protective range, i.e. a conventional overcurrent tripping level similar to that of a 30 A circuit-breaker.

III Types of Overcurrent Protection

  • Complex type: a variety of protection in line.

  • Limited power type: limited output of the total power

  • Rewound type: the initial current is constant, the voltage drops to a certain value current began to decrease.

  • Playing type: over-current, the current voltage dropped to 0, and then began to rise again and again.

  • Constant current: constant current, voltage drop

  • Comparison of several over-current protection methods

Several overcurrent protection methods are listed in Table 1.

Circuit mode

Components used

Debugging difficulty

Protection performance

Power consumption

Impact on efficiency

Resistor primary current limit circuit

few

easy

Poor

large

larger

Base drive current limit circuit

less

easier

worse

larger

large

No power current limit circuit

more

easier

better

smaller

smaller

555 timer current limit circuit

many

easy

good

small

small

IV Application Examples of Overcurrent Protection Circuit

Overcurrent protection is when the fault current in the circuit reaches the action value of the current relay, the current of the current relay is set according to the requirements of the line selectivity. PTC thermistors for overcurrent protection reduce the residual current by limiting the consumption of the entire line by a sudden change in their resistance. They Can replace the traditional fuse, widely used in motors, transformers, switching power supplies, electronic circuits, over-current thermal protection, the traditional fuse can not be restored after the line is blown, and over-current protection with PTC thermistor after the fault is removed Can be restored to the pre-protection state, when the fault occurs again can achieve over-current protection function.

4.1 Transformer

The primary voltage of the voltage transformer is 220V, the secondary voltage is 16V, the secondary current is 1.5A, the primary current of the secondary abnormality is about 350mA, the protection state should be entered in 10 minutes, the working environment temperature of the transformer is -10-40 ℃, 15 ~ 20 ℃, PTC thermistor installed near the transformer, please select a PTC thermistor for primary protection.

  • Determine the maximum operating voltage

Given transformer voltage is 220V, considering the power fluctuations, the maximum operating voltage should reach 220V × (1 + 20%) = 264V

PTC thermistor maximum operating voltage selection 265V.

  • Determine no action current

After calculation and actual measurement, the primary current of the transformer is 125mA when it works normally. Considering that the ambient temperature of the PTC thermistor is up to 60 ℃, it can be determined that the non-operating current should be 130 ~ 140mA at 60 ℃.

  • Determine the operating current

Taking into account the PTC thermistor mounting position of the ambient temperature can reach -10 ℃ or 25 ℃, the operating current can be determined at -10 ℃ or 25 ℃ should be 340 ~ 350mA, the operating time of about 5 minutes.

  • Determine the nominal zero power resistor R25

PTC thermistor in series in the primary, the resulting voltage drop should be as small as possible, PTC thermistor itself, the heating power should be as small as possible, the general PTC thermistor voltage drop should be less than 1% of the total power, R25 Calculated:

220V × 1% ÷ 0.125A = 17.6 Ω

  • Determine the maximum current

The actual measurement, the transformer secondary short-circuit, the primary current up to 500mA, taking into account the primary coil short-circuit when there is a greater part of the current through, PTC thermistor to determine the maximum current above 1A.

  • Determine Curie temperature and dimensions

Taking into account the PTC thermistor ambient temperature  of the mounting location can go up to 60 ℃, selected Curie temperature should be on the basis of 100 ℃. But taking into account the low cost and the PTC thermistor is not installed in the transformer line package, the higher surface temperature will not produce adverse effects to transformer. So the temperature can be selected Curie temperature 120 ℃, so that the PTC thermistor can reduce the diameter, and the cost can be reduced.

  • Determine the PTC thermistor model

According to the above requirements, consult specification sheet, selected standard as below: 

Namely: the maximum working voltage is 265V, the rated zero power resistance is 15Ω ± 25%, the operating current is 140 mA, the operating current is 350 mA, the maximum current is 1.2A, the Curie temperature is 120 ℃ and the maximum size is 11.0mm.

Overcurrent protection applied in motor

4.2 Motor

When the motor starts, press the lock button SBi, the start is over (after the motor speed stabilizes), press SBi again, and the protection circuit is put into operation. For motors with short starting time (such as several seconds), SBi can also use ordinary buttons, as long as the SBi is kept pressed during the starting process.

When the motor is running normally, the secondary induced potential of the current transformer TAi~TA3 is small, and it is not enough to trigger the thyristor V. As shown below.

Application in motor

V Over-current Protection Circuit in the Switching Power Supply Design

A switching power supply commonly used over-current protection circuit.

  • Current sensor for current detection

Through the converter secondary current obtained by the converter I / V converts to voltage. After the voltage becomes DC form, it is compared with the set value by the voltage comparator . If the DC voltage is greater than the set value, the identification signal is issued . However, this detection sensor is generally used to monitor the load current induction power supply. So we should take the following measures. Since the start-up current is several times as the rated current at the start-up of the inductive power supply and is much larger than the current at the end of start-up. in the case of simply monitoring the current battery, the necessary output signal should be obtained at the start of the inductive power supply. We must use a timer to set prohibit time, so that induction power supply does not get an unnecessary signal output before the end of start-up. After the end of the timer, the power supply will turn into the scheduled monitoring state.

  • Start inrush current limit circuit

Switching power supply will generate high inrush current when power is on. Therefore, soft start device to prevent inrush current must be installed at the input end of the power supply to effectively reduce the inrush current to the allowable range. Inrush current is mainly caused by the filter capacitor charging, the capacitor on the exchange showed a lower impedance at the beginning of the switch is turned on. In the absence of any protective measures, the inrush current can approach hundreds of A.

Rectifier filter circuit

 

Switching power supply input generally use capacitor filtering circuit shown in Figure 6, the filter capacitor C can use low-frequency or high-frequency capacitors, low-frequency capacitor need to be parallel with the capacity of high-frequency capacitors to bear the charge and discharge current. In the figure, the current limiting resistor Rsc, which is inserted between the rectification and the filtering, is to prevent the impact of the inrush current. Closing Rsc limits the charging current of capacitor C. And after a period of time, the voltage on C reaches the preset value or the voltage on capacitor C1 reaches the operating voltage of relay T, and Rsc is shorted. At the same time, SCR can also be used to circuit Rsc. When closing, due to the SCR cut-off, the capacitor C is charged through Rsc. After a period of time, the SCR is turned on, short-circuiting the current-limiting resistor Rsc.

  • Current limiting circuit applied in base drive circuit

The current limit circuit diagram drawn in the figure below is suitable for power supplies of various circuits. The output part of this circuit shares the ground with the control circuit.

The working principle is: Under normal working conditions, the Il flowing into Rsc will not produce a large voltage drop, then Q1 will not be turned on. If the load current is large enough, a voltage will be generated on Rsc to make Q1 Conduction. If Q1 is in the OFF state and C1 will be fully discharged when Ic1=0, Q2 will also be in the OFF state. If the Il current gradually increases, then Il*Rsc=VbeQ1+Ib1R1

At this time, a current Ic1 will flow through the collector, and the following time constant will charge C1 T=R2*C1

Then the voltage on C1 is: Vc1=Ib2R3+VbeQ2

In order to minimize the load effect of the capacitor voltage, we can use a Darling stool tube with higher HFE instead of Q2, so that the base current can be limited to microamps. When choosing resistor R4, we must Much larger than R3. In this way, when the current is overloaded, the C1 capacitor will quickly discharge.

The value of   R2 is as follows:

IBL=(V1-VBEQ1)/R1

and Ic1=HfeQ1IBLMAX

So, R2 "=(V1-VCEMAX) R1/(V1-VBEQ1)

With proper circuit design, VCE can quickly reach its voltage value and bias the Q2 transistor to the on state, so that the drive signal of the regulator can be turned off.

When the overload is removed, the circuit will automatically return to working status. If an IC PWM control circuit with a fixed current limit comparator is used, the circuit shown in Figure 1B, we put the current limit resistor RSC on the positive terminal of the output, and a good current limit effect can be obtained.

Current limiting cicuit applied in based driver

  • By detecting the Vce of IGBT

When the power output has an overload or short circuit, IGBT Vce value becomes larger. According to this principle, we can take protective measures on the circuit. This usually uses a dedicated drive EXB841, whose internal circuit can be done well down the gate and soft-shutdown, and has an internal delay function. You can eliminate the interference caused by malfunction. Its working principle is shown in Figure 8, Vce with IGBT overcurrent information is not sent directly to the collector voltage monitoring pin 6 of the EXB841, but is rapidly recovered by the diode VD1. Then connected to the pin 6 of the EXB841 through the comparator IC1 output Elimination of the forward voltage drop varies with the current situation, the use of threshold comparator to improve the accuracy of current detection. In the event of an overcurrent, the driver: The EXB841's low-speed shutdown circuit will turn off the IGBT slowly to prevent collector current spikes from damaging the IGBT devices.

Using IGBT overcurrent principle circuit


VI Conclusion

In recent years, the switching power supply is widely used, and its reliability also has higher requirements. Once the electronic product fails, if the input end of the electronic product is short-circuited or the output end is open, the power supply must turn off its output voltage to protect the power MOSFET and the output end device from being damaged. Otherwise, the electronic product may be further damaged or even cause Electric shock and fire of operators. Therefore, the switching power supply over-current protection must be improved.

 

 


Book Recommendation

  • Electric Power System Protection and Coordination: A Design Handbook for Overcurrent Protection

A guide to the implementation of electric power protection in both new and existing systems in individual and commercial facilities. Focusing on systems in the low to medium volt range, the book helps in the solution of protection and co-ordination problems by use of microcomputers as well as more traditional methods. The text provides step-by-step procedures for quick problem solving. It shows how to engineer intelligent switchgear and includes critical information on setting up a protection and co-ordination workstation. The text should help meet the requirements of the National Electrical Code and the National Standards Institute.

--Michael A. Anthony (Author)

 


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