How inrush current limiters can improve power supplies notes CIRCUIT PROTECTION How inrush current...
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+44 (0) 845 270 2780 135 www.rhopointcomponents.com www.rhopointgermany.com www.rhopointnordic.com +49 6173 318 5562 +45 4362 0794 Application notes CIRCUIT PROTECTION How inrush current limiters can improve power supplies Figure 1 – ‘Inrush current limiter’ in the circuit Inrush current limiters are NTC thermistors made from a specially formulated metal-oxide ceramic material which has the characteristic of sharply decreasing electrical resistance with increase in temperature. If current flows through these devices they self-heat and the resistance falls. Thus, they are used in equipment where there is a need to limit a surge of current when power is first applied. They have many applications in the electronics industry from power supplies to lighting; the one described here is typical and shows step by step the way inrush current limiters are specified for a power supply. The advantages of limiting the surge current are: • Lower rated and therefore a cheaper on/off switch can be used • A lower rated, lower cost rectifier bridge may be used • A standard fuse can be used • A reduction of power sag noise The main reason for fitting an inrush current limiter in a power supply is to protect the diode rectifier bridge while the input capacitor is charging. (This is almost a short circuit in its uncharged state.) The inrush current limiter in the circuit (figure 1) will offer a high resistance at first but will quickly self heat (one to two seconds) and its resistance will drop to a relatively low resistance, thus protecting the bridge while the capacitor charges. Consider a 400W power supply whose maximum input current is 4.5A ( Imax) at 100volts, and whose rectifier has a maximum over current of 10A for one cycle. Here we would want a cold resistance (R 25 ) of 100/10 = 10 ohms Of the available models, the SL12 10006 (10 ohms 6A ) and SL22 30005 (30 ohms 5A) are closest. The selection of the best device for the application does not stop here, as there are other things to be taken into consideration. If the inrush current limiter is to operate at high ambient temperatures, the ‘cold resistance’ specified at 25°C is affected. If the maximum ambient temperature is 50°C, the ‘cold resistance’ will be approximately half. This would rule out the SL12 10006 Each inrush current limiter has a limit to the amount of surge energy that it can tolerate. Each device has an energy rating in joules. The energy rating of the circuit is given by the equation E = 0.5*CV 2 . If in this circuit the capacitor being charged was 470uF then the energy rating would be about 2.4J ( well within the energy rating for the SL22 30005). The voltage drop across the device when hot can be calculated using the Resistance Curve (see figure 2). By using this chart the multiplying factor M can be found to determine the hot resistance of the device, this figure should be multiplied by the resistance at maximum current figure. So the hot resistance is approximately 1.25x0.3= 0.4Ω. So the voltage drop is approximately 1.8V. One final consideration is the Imax versus the ambient temperature. A derating curve is given in Figure 3. It shows that Imax reduces to zero at 150˚C. Note that if the maximum operating temperature is 100˚C, then the ‘Inrush current limiter’ should only be used at only 58 per cent of its rated maximum current. Inrush current limiters may get hot in operation. Use the following guidelines when mounting for best results. • Do not place on the PCB near temperature sensitive components. • Use a high temperature solder on the PCB. • If equipped, use a ceramic spacer to stand off the device from the PCB or contact us for pre-formed devices. • Keep away from wiring looms. MOUNTING INRUSH CURRENT LIMITERS
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Application notesCIRCUIT PROTECTION
How inrush current limiters can improve power supplies
Figure 1 – ‘Inrush current limiter’ in the circuit
Inrush current limiters are NTC thermistors made froma specially formulated metal-oxide ceramic materialwhich has the characteristic of sharply decreasing electrical resistance with increase in temperature. If current flows through these devices they self-heat andthe resistance falls. Thus, they are used in equipmentwhere there is a need to limit a surge of current whenpower is first applied.
They have many applications in the electronics industryfrom power supplies to lighting; the one described hereis typical and shows step by step the way inrush currentlimiters are specified for a power supply.
The advantages of limiting the surge current are:
• Lower rated and therefore a cheaper on/off switch can be used
• A lower rated, lower cost rectifier bridge may be used
• A standard fuse can be used
• A reduction of power sag noise
The main reason for fitting an inrush current limiter ina power supply is to protect the diode rectifier bridgewhile the input capacitor is charging. (This is almost ashort circuit in its uncharged state.) The inrush currentlimiter in the circuit (figure 1) will offer a highresistance at first but will quickly self heat (one to twoseconds) and its resistance will drop to a relatively lowresistance, thus protecting the bridge while thecapacitor charges.
Consider a 400W power supply whose maximum inputcurrent is 4.5A ( Imax) at 100volts, and whose rectifierhas a maximum over current of 10A for one cycle.
Here we would want a cold resistance (R25) of 100/10= 10 ohms
Of the available models, the SL12 10006 ( 10 ohms 6A) and SL22 30005 (30 ohms 5A) are closest. Theselection of the best device for the application doesnot stop here, as there are other things to be taken intoconsideration.
If the inrush current limiter is to operate at highambient temperatures, the ‘cold resistance’ specified at25°C is affected. If the maximum ambient temperatureis 50°C, the ‘cold resistance’ will be approximatelyhalf. This would rule out the SL12 10006
Each inrush current limiter has a limit to the amountof surge energy that it can tolerate. Each device has anenergy rating in joules. The energy rating of the circuitis given by the equation E = 0.5*CV2.
If in this circuit the capacitor being charged was470uF then the energy rating would be about 2.4J ( well within the energy rating for the SL22 30005).
The voltage drop across the device when hot can becalculated using the Resistance Curve (see figure 2).By using this chart the multiplying factor M can befound to determine the hot resistance of the device,this figure should be multiplied by the resistance atmaximum current figure. So the hot resistance isapproximately 1.25x0.3= 0.4Ω. So the voltage drop isapproximately 1.8V.
One final consideration is the Imax versus the ambienttemperature. A derating curve is given in Figure 3. Itshows that Imax reduces to zero at 150˚C. Note that ifthe maximum operating temperature is 100˚C, thenthe ‘Inrush current limiter’ should only be used at only58 per cent of its rated maximum current.
Inrush current limiters may get hotin operation.Use the following guidelines whenmounting for best results.• Do not place on the PCB near
temperature sensitive components.• Use a high temperature solder on
the PCB.• If equipped, use a ceramic spacer to
stand off the device from the PCBor contact us for pre-formed devices.
• Keep away from wiring looms.
MOUNTING INRUSH CURRENT LIMITERS
Application notes CIRCUIT PROTECTION
How inrush current limiters can improve power supplies (continued)
Figure 2 – ‘Inrush current limiter’ Resistance Curve
Figure 3 – Derating Curve
To summarise, there are five steps to take to select thecorrect ‘Inrush current limiter’ for a power supply:
1. Calculate the maximum continuous current (Imax)
2. Calculate the resistance needed to limit the surge current at 25˚C
3. Select an ‘Inrush current limiter’ that is specified to handle the required energy of the circuit
4. Calculate the ‘Inrush current limiter’ resistance at the operating current, using the M curve
5. Consider if derating is required for operating temperatures over 65˚C
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CROPPING AND FORMING
Rhopoint Components can crop and/or formInrush current limiter leads to your
specification (subject to minimum quantitytypically 1000 pcs).
For further information please contact us
DESIGN TIPS FOR –INRUSH CURRENT LIMITERS
The SL22 series of inrush limiters isCSA & UL approved. Other SL series parts are CSAapproved. However the following designguidelines should be followed whenusing these devices.
1. The body should not touch othercomponents on the PCB.
2. A fuse rated at the maximum currentrating of the device (allowing for any derating that may be needed for ambienttemperatures in excess of 65˚C) or lowerthan the maximum current rating of thedevice should be used.
