Are you expecting inrush current in your circuit? or have you heard you heard about the surge current? Have you ever dealt with surge current? Are you a beginner and don't know what component/circuit to use? Then after reading this article you must be able to decide after need to be done. Also, if we see our SMPS designs, inrush current limiter is commonly used.
To start this discussion, let us take a scenario when we power on the circuit. Initially, there will be a surge current that flows. This is because of the capacitor bank that is present in the circuit which draw a significant current instantaneously to charge themselves. Also, we can put it the other way, when the current drawn by the load is greater than steady state current of the circuit, it is called surge current. Surge current may not be significant in some cases. This depends on the underlying circuitry used. If the surge current is of small duration such that the circuitry doesn't get affected by it, then we need not have any protection. Also, the affect of surge current may not be straight forward, it may reduce the life time of a component. The graph here shows the surge current drawn in a circuit over a time.
The tradition way of limiting the current is to use a fuse to limit the current. But if the current exceeds the fuse limit, then fuse blows off and only way to recover the circuit functionality is to replace the fuse with a new one. But this is a tedious job. One way to avoid fuse blow is to use some other circuit which handles the inrush current. Thermistor is one such component. we always use resistors in circuits to limit the current, for example, if we take the case of LED, it is always preferable to use a series resistor. But series resistor will not suffice if the inrush current is in amperes range and in fact the resistor may blow away. Also, resistors doesn't have the property to change their resistance unlike thermistor. Thermistors are better suited for these applications.
Thermistors are of type positive temperature co-efficient (PTC) and negative temperature co-efficient (NTC). NTC type are used as inrush current limiters. NTC thermistors are designed to be used in inrush limiter circuits. The principle of NTC is that resistance decreases with increase in temperature. so, initially, thermistor has very high resistance. As the current starts flowing, the resistance decreases as drop/temperature increases.
Using thermistor is also a cheaper option rather than having a complex circuitry (Active circuit) to limit the current. NTC is the most commonly used in the industry.
General specifications to be checked while selecting a NTC/PTC as current limiter:
1. Resistance offered vs temperature
2. Hold current (current at which thermistor provides low resistance)
3. Trip current (the current at which the thermistor offers high resistance)
4. Recovery time (for thermistor to come back to high resistance after the circuit has switched off)
Advantages of using thermistors:
1. Reliable
2. Stable
3. Lower cost
4. Lesser footprint
5. Easy to include in the circuit and easy to select
6. Longer life time
Some of the disadvantages of using inrush current protection:
1. When thermistor is used, there will be a voltage drop across it.
2. For a small form factor boards, with sizable power consumption (current draw by load), we need to include a bigger size thermistor which may be a limiting factor
3. High recovery time for thermistors.
Important points of thermistor as Inrush current limiter:
1. Resistance drops to milli ohms within in milliseconds
2. The resistance offered by thermistor during idle state will be in hundreds of ohms.
How to select the resistance of inrush current limiter?
Let us assume that we have a circuit with input voltage 12V and load current of 1A. In such cases, the value of thermistor is selected such that it allows min. 1A and cuts off above that current. So, here resistance if V/I = 12 ohms. So, have a thermistor of at least 12 ohms.
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