When an electrical device is first powered on, inrush current is the surge or momentary burst of current that flows into it.
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Think of a oto sitting on flat pavement, parked in neutral, engine off. To lớn get it moving without using the engine, a human would need lớn give it a substantial push, probably a vigorous leg drive. Once moving, though, the car"s wheels roll more cooperatively with less physical oomph required.
That initial, emphatic leg drive is the equivalent of inrush current. The subsequent, easy-rolling motion equals the steady-state current flow that occurs in a motor once its gears và rotors have been jolted out of inertia và into motion.
To measure the inrush current, technicians can use either a hard-jawed clamp meter or a flexible current probe. Only meters that offer an inrush button can measure inrush current. Here are steps for measuring it, in this case when using the Fluke 381 (see illustration above):
With the device khổng lồ be tested turned off, turn the meter"s dial to .Center the jaw or flexible probe around the device"s live wire.Push the inrush button, on the face of the meter.Switch on the device. The inrush current (spike) is displayed in the meter"s display.Why does this measurement matter? Newer, high-efficiency motors draw more running current than their predecessors. Knowing the value of inrush current can help a technician locate a startup problem, whether it"s in the motor or in the starting circuit. Inrush measurements are usually recorded in a preventative maintenance log for future reference.
To provide repeatable motor inrush measurements, advanced clamp meters (such as the Fluke 370 series or the Fluke 381) use a "triggered" mode that synchronizes measurements with the starting current.
Technicians "arm" a meter by pressing its inrush button. The meter is then triggered by the inrush current. Once triggered, this inrush function takes approximately 400 samples over a 100-millisecond period và calculates the actual starting current.
Inrush current may cause a meter"s display to lớn show a value that is higher than the rating of the circuit breaker, yet the breaker does not trip. Why so?
Inrush current can be 4 lớn 10 times greater than the normal running current, depending on the type of motor. So if a motor"s running current is eight amperes và its circuit breaker is rated for 20 amperes, how is it possible that a clamp meter could display a measurement of 40 amps?
The reason the breaker or overload unit do not trip is because both devices work on a time-vs.-current curve. This curve (see chart) indicates how much current passes through the breaker, và for what length of time, without opening the circuit.
The durability & dependability of an electronic circuit are highly dependent on how well it is designed considering all odds, which could practically occur when the sản phẩm is actually in use. This is particularly true for all nguồn supply units like AC-DC Converters, or SMPS Circuits because they are connected directly to lớn the AC mains và a varying load which makes them susceptible lớn overvoltages, voltage spikes, overloading, etc. This is why designers include many types of Protection circuits in their design, we have already covered a lot of popular protection circuits namely
We previously discussed Inrush current, in this article we will discuss how lớn design an inrush current limiter circuits, to protect your nguồn supply designs from inrush currents. We will first understand what inrush current is & the reason why it is generated. Then we will discuss the different types of circuit design that can be used to protect inrush current và finally conclude with some tips khổng lồ protect your device against inrush current. So, let’s get started.
What is Inrush Current?
As the name suggests the term “inrush current” indicates that when a device is turned on during the initial stage a huge amount of current rushes into the circuit. By definition, it can be defined as the maximum instantaneous input đầu vào current drawn by an electrical device when it is turned on. This behavior can be well observed in AC inductive loads like Transformers và Motors, where the inrush current value will normally be twenty or thirty times more than the nominal values. Even though the value of inrush current is very high it occurs only for a few milliseconds or microseconds hence cannot be noticed without a meter. Inrush current can also be called as Input surge current or Switch-on surge current based on convenience. Since this phenomenon is more with AC loads, AC Inrush current limiter is more used than its DC counterpart.
Each và every circuit draws current from a source depending on the state of the circuit. Let’s assume a circuit that has three states, that is idle state, normal working state, và maximum working state. In idle state consider, the circuit draws 1m
A of current, in a normal working state the circuit draws 500m
A of current & in the maximum working state it can draw 1000m
A or 1A of current. Therefore, if the circuit mostly works in a normal state, we can say that 500m
A is the steady-state current for the circuit, whereas 1A is the peak current drawn by the circuit.
This is fairly true, easy khổng lồ work with and simple math. But, as told earlier there is another state exists where the current drawn by the circuit can be trăng tròn or even 40 times larger than the steady-state current. It is the initial state or power nguồn on stage of the circuit. Now, why this high current is suddenly drawn by the circuit as it is rated for low current application? Such as the previous example, 1m
A to the 1000m
A.
What causes Inrush Current in a device?
To answer the questions we have lớn get into the magnetics of inductor and motor coils, but to start let’s consider that, it’s like moving a huge cupboard or pulling a car, initially, we need high energy, but as things start moving, it became easier. Exactly the same thing happens inside a circuit. Almost every circuit, especially power supplies, uses large value capacitors and inductors, chokes, & Transformers (a huge inductor) all of which draw a huge initial current lớn develop the magnetic or electric field required for their operation. Thus the input đầu vào of the circuit suddenly provides a low resistance (impedance) path which allows a large value of current to flow into the circuit.
Capacitors & inductors behave differently when they are in a fully charged condition or discharge condition. For example, a capacitor when it is in a fully discharged condition acts as a short circuit due khổng lồ the low impedance, whereas a fully charged capacitor smoothens out the dc if connected as a filter capacitor. However, it is a very small span of time; in few milliseconds the capacitor gets charged. You can also read about the ESR and ESL values of a capacitor khổng lồ better understand how it works in a circuit.
On the other side, Transformers, motors, và inductors (all coil related stuff) generates back emf during startup, also requires very high current during the charging state. Normally, few current cycles are required to lớn stabilize the input current to a steady-state. You can also read about DCR value in the inductor to better understand how inductors work in a circuit.
In the above image, a current vs. Time graph is shown. The time shown in milliseconds but that can be in microseconds too. However, during the startup, the current start to lớn increase và the maximum peak current is 6A. It is the inrush current that exists for a very short time span. But after the inrush current, the current flow gets stable at a value of .5A or in 500m
A. This is the steady-state current of the circuit.
Therefore, when the input đầu vào voltage is applied to lớn the power supply or in a circuit that has very high capacitance or inductance or both, inrush current occurs. This initial current as shown in the inrush current graph gets very high to lớn cause the input đầu vào switch melting or blown-up.
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Inrush Current Protection Circuits – Types
There are many methods khổng lồ protect your device from inrush current and different components are available to lớn protect the circuit from inrush current. Here is the danh sách of effective methods khổng lồ overcome inrush current-
Resistor limit method
There are two ways khổng lồ design inrush current limiter using the resistor limit method. The first one is to showroom a series resistor to lớn reduce the current flow in the circuit line and the other one is to lớn use line filter impedance in AC supply input.
But this method is not an efficient way to add across a high output current circuit. The reason is obvious because it includes resistance. The inrush current resistor gets heated up during normal operation và reduces efficiency. The resistor wattage depends on the application requirement, typically ranges between 1W to lớn 4W.
Thermistor or NTC based current limiter
The Thermistor is a temperature coupled resistor that changes resistance depending on the temperature. In an NTC inrush, current limiter circuit is similar khổng lồ resistor limiting method, Thermistor or NTC (Negative temperature coefficient) is also used in series with the input.
Thermistors have characteristics of changed resistance value at different temperatures, specifically, at low-temperature Thermistor behaves lượt thích a high-value resistor, whereas at high temperatures, it provides low-value resistance. This property is used for the Inrush current limiting application.
During the initial startup of the circuit, the NTC provides high-value resistance which decreases the inrush current flow. But during the circuit goes into the steady-state condition, the temperature of the NTC starts increasing which further resulted in low resistance. NTC is a very effective method of controlling inrush current.
Soft Start or Delay circuit
Different type of voltage regulator DC/DC converters uses the soft start or delay circuit khổng lồ reduce the inrush current effect. Such type of functionality enables us lớn change the output rise time which effectively reduces the đầu ra current when connected to a high-value capacitive load.
For example, 1.5A Ultra-LDO TPS742 from Texas Instruments offers programmable soft-start pin where the user can configure Linear Start Up using a simple external capacitor. In the below circuit diagram, an example circuit of TPS742 is shown where the soft-start time is configurable using the SS sạc pin by using the CSS capacitor.
Where and why we need to consider Inrush Current Protection Circuit?
As discussed before, the circuit where high-value capacitance or inductance exists, an inrush current protection circuit is required. The inrush current circuit stabilizes the high current requirement in the initial starting stage of the circuit. An inrush current limiter circuit limits the input đầu vào current & keeps the source và the host device safer. Because a high inrush current increases the failure chances of the circuit & that needs to be rejected. Inrush current is harmful because of the following reasons-
High inrush current affects the source power supply.Often high inrush current drops the source voltage và results in a brownout reset for microcontroller-based circuitry.In few cases the amount of current supplied to lớn the circuit gets beyond the acceptable maximum voltage of the load circuit, causing permanent damage to the load.In high voltage AC motors, the high inrush current causes the power nguồn switch to lớn trip or sometimes burned out.The PCB board traces are made khổng lồ carry a specific value of current. The high current could potentially weaken the PCB board traces.Therefore, lớn minimize the effect of inrush current, it is important to provide an inrush current limiter circuit where the input capacitance is very high or has a large inductance.
How to lớn measure Inrush Current:
The main challenge of measuring inrush current is the fast time span. Inrush current occurs for a few milliseconds (or even microseconds) depending on the load capacitance. The value of the time span generally differs from 20-100 milliseconds.
One easiest way is lớn use the dedicated clamp meter which has the option to measure the inrush current. The meter gets triggered by the high current and takes multiple samples lớn get the maximum inrush current.
Another method is lớn use a high-frequency oscilloscope but this process is a bit tricky. One needs lớn use a very low-value shunt resistor & requires two channels to lớn connect across the shunt resistor. By using the different functions of these two probes one can get the maximum peak current. One needs lớn be careful while connecting the GND probe, the wrong connection across the resistor could lead to lớn a short circuit. The GND needs khổng lồ be connected across the circuit GND. The below image is the representation of the above-mentioned technique.
Factors to consider while designing an Inrush Current Protection Circuit:
Some different factors and specifications are needed khổng lồ be taken into the account before choosing the inrush current limiting method. Here is a danh mục of few essential parameters –
1. The capacitance value of the load
The capacitance of the load is essential parameters lớn select the specification of inrush current limiting circuit. High capacitance requires a high transient current during startup. For such a case an effective soft start circuit is required.
2. Steady-state current rating
Steady-state current is a huge factor for the efficiency of the current limiter. For example, the high steady-state current could lead to lớn increased temperature và poor efficiency if the resistor limit method is used. NTC based current limiting circuit can be a choice.
3. Switching time
How fast the load gets on or off during a given time frame is another parameter to choose the inrush current limiting method. For example, if the switching on/off time is very fast then the NTC could not protect the circuit from inrush current. Because, after a first cycle reset, the NTC does not get cooled down if the load circuit is turned off và on in a very short time span. Therefore the initial start resistance couldn’t be increased và the inrush current gets bypassed through the NTC.
4. Low voltage and Low current operation
In specific cases, during circuit design, if the power source and the load is existing inside the same circuit it is wiser to use voltage regulator or LDOs with soft start facility lớn reduce the inrush current. In such a case, the application is a low voltage low current application.
