The inrush current of a power supply is often much higher than the nominal current but is mentioned only in passing in many data sheets. That can lead to difficulties.
Usually, only a small line in the specification sheet of a power supply describes the Inrush Current. There are sorrowful experiences for this. Why do users often have such immense problems with the effects of inrush current? How are the terms understood and what are the consequences? What does cold start mean? It should be pointed out which difficulties arise, how these are to be evaluated and which measures can be useful to obtain a sustainable solution.
The inrush current is often much higher than the rated current
The inrush current is the electric current that flows immediately after switching on an electrical load. It is often much higher than the rated current. Inrush currents occur mainly in transformers, motors, heating coils, incandescent lamps, DC / DC converters and generally in power supplies.
Causes of high inrush current
The main cause of the inrush current is the charging current flowing into the electrolytic capacitor C1 (Figure 2). This has a high capacity. A discharged capacity represents a short circuit for the feeding network. Also, a charging current flows into the filter capacitors (filter C). This is less important because the filter capacitors have a significantly lower capacitance than the electrolytic capacitor C1.
How to evaluate an inrush current
To be able to evaluate an inrush current, the melting integral I 2 t, among other things, is decisive. To easily calculate the melting integral for normal inrush current surges, there is the rule of thumb ½. I max 2 .T 50 . In this case, the maximum amplitude of the inrush current is used for I max. T 50 describes the pulse duration in which I max / two is exceeded. Using this formula, it becomes clear that the current spike caused by the filter capacitors can be neglected. This has a significantly lower energy content than the inrush current caused by the electrolytic capacitor.
Why is the inrush current so significant?
Due to high inrush current, the feeding network and its components are heavily loaded. It can lead to power-induced voltage dips that affect other devices, among other things. Cables, switches and relays must be able to handle these high currents without being damaged. Probably the most important feature of high inrush currents is the unintentional triggering of fuses and circuit breakers.
Circuit breakers and fuses
There are two different mechanisms for triggering circuit breakers. The first mechanism is the overload protection, which works with a thermo-bimetallic release. Warming triggers it, that is, it is time and current dependent. The second mechanism is the short-circuit protection. This can be triggered exceptionally quickly with the aid of an electromagnetic release, which is only current-dependent. When switching on power supplies, only the short-circuit protection is decisive for triggering the fuse. For fuses, care should be taken that the melt integral I 2 t is sufficiently high.
Parallel connection of several devices
In the case of a parallel connection of several power supplies to a fuse, special care must be taken. The inrush currents of each power supply add up and often lead to the unintentional triggering of the fuse. The fast-growing LED market is a good example of this. Each LED light requires an LED driver. Since usually several lights are operated in parallel, attention must be paid to the inrush current of the driver. Another example is building automation when, for example, all the shutters in a building are controlled simultaneously.
Passive inrush current limiting with NTC
Probably the most common method of inrush current limiting is the use of an NTC. An NTC is a temperature-dependent resistor (thermistor). This reduces its resistance value with increasing temperature. The NTC is implemented in the power path and limits the inrush current in the cold state. This heats up over time and reduces its resistance, causing fewer losses during normal operation. The advantages of this solution are the simple implementation, low component costs and the associated low costs. Also, the NTC is a very robust solution. The disadvantages, in turn, are the temperature-dependent inrush current, a worse efficiency and poor limitation after a start of the power supply in the event of a power failure (warm start).
The active inrush current limitation with NTC
The active inrush current limitation using NTC eliminates the disadvantages of passive limiting. The NTC limits the inrush current when the power supply is turned on. So that it does not cause losses during normal operation, it is bridged with a switch. For this purpose, for example, a relay can be used. Also, the inrush current is again efficiently limited after a power interruption, since the NTC again has time to cool down during normal operation.
The disadvantages here are the increased circuit complexity and the associated costs. Instead of an NTC with active bypass, fixed resistors or PTCs (PTC thermistors) can also be used. For bridging, a MOSFET, thyristor or triac can be used instead of a relay. There are many other types of inrush current limiting, such as pulsed charging of the input capacitors by an additional circuit, minimizing the input capacitance, etc.
External inrush current limiters
There are external inrush current limiters on which several devices can be bundled. The advantage is that you only have to realize an inrush current limiter once. The downside is the increased performance in normal operation. The maximum number of devices per limiter must be taken into account.
Datasheet information may be misleading
The data sheet specifications of various manufacturers about the inrush current must be considered carefully. These can be misleading and uninformative. It is important to pay attention to the conditions under which the inrush current is specified. It is also important to specify only a typical or maximum value. The real value might be higher than the typical value. To understand the requirements of the desired limiter, to be able to advise the user adequately and implement a suitable type of limitation in the power supply, a manufacturer of custom power supplies would ask the following questions:
- Is there a maximum inrush current you want?
- What expectations do you have of the inrush current limiter? (Clarification understanding of cold and warm start)
- Which fuses, switches, relays are installed on your device?
- What backups, switches, relays are permitted for the end user for your device?
- Are several devices connected in parallel? If so, how many maximum?
Measurement of the maximum inrush current
The measurement aims to measure the maximum inrush current. It must be paid to certain things. For each measurement, make sure that the input capacitors are completely discharged. Operating the power supply on a load can help. Regardless of whether you want to measure a cold or warm start, it must be ensured that the voltage maximum of a sine half-cycle is switched on. As a rule, the inrush current is independent of the output load. Also, attention must be paid to the ambient temperature and component temperature.
For a cold start, the NTC should be cold, for example, and hot for a warm spring. Before a cold start, the NTC should have enough time to cool down. A warm start can be guaranteed by operating the power supply for a long time under full load. Also, the differences between different feeding networks/sources must be taken into account. For an AC source, the maximum peak current (peak) that the source can run may not be less than the maximum inrush current of the power supply to be measured. Since an AC source has low impedance, low to no voltage drops occur at high peak currents.
If means of an isolation transformer measures the inrush current, lower inrush currents and larger voltage drops will occur as it has a relatively high impedance. This may vary depending on the isolation transformer used. When measuring on the direct supply network, it can lead to strong switch bounce when plugging in the power supply. This makes it almost impossible to take a reasonable and reproducible measurement. A suitable switch that does not bounce can help. This leads to voltage dips in the supply network since the network has a relatively high impedance. This depends very much on the distance between the measuring socket and the distributor. Also, special attention must be paid to safety when measuring on the direct network.
Using an oscilloscope and incorrectly connecting the probes may destroy the oscilloscope. It can also be brought to the mains voltage on the housing and thus there is a risk of electric shock. To obtain a safe and usable measurement, suitable background knowledge and suitable measuring equipment are required.
Specification Sheets: If there are any uncertainties, check with the inrush current
Although the description of the inrush current of power supplies in specification sheets is rather short, it may well be more significant to the user or the overall system. In case of doubt, it is advisable to ask the supplier for extended information and to be given appropriate advice. Manufacturers of custom power supplies advise and solve open questions, instead of leaving the customer with data sheets alone. Generally, one should also be able to trust the manufacturer or even at such inconspicuous points in a power supply specification.