Application Note AN30.01.en

Power supplies with significantly reduced inrush current by Michael Raspotnig

When electrical and electronic ted. Devices can differ devices are switched on, they here through the use of load the supply network momen- different concepts. tarily with high current pulses. MCBs and breakers have to be Inrush current limiting designed for these peak currents by means of NTC resi- to prevent nuisance trips or con- stors tact welding. Besides this, such This is undoubtedly the high current pulses produce volta- simplest form of inrush ge dips in the supply voltage, current limiting. The char- which in turn can cause mal- ging current is limited by functions in other consumer cir- an NTC and with cuits. Often, this inrush current is an NTC resistor, the resi- not given sufficient consideration stance value decreases However, this simple method has dis- or is overlooked when selecting with rising temperature. On initial advantages, too: power supplies, which can lead to -on the resistor is cold, has a unexpected circumstances later on. high-resistance and limits the char- Due to the nature of the compo- ging current effectively. After a relati- nents, the effect of the inrush cur- PULS, the power supply manufacturer, vely short time, the resistor heats up to rent limiting is heavily dependent has developed an effective method around 110°C due to its own losses on the ambient temperature. Low for reducing inrush current. The and the resistance decreases by a fac- temperatures (minus range) can method is unique up to now and, for tor of 15. As a result, the losses in ser- lead to starting problems and at the first time, almost completely vice stay within limits. high temperatures the inrush cur- eliminates high inrush currents. rent limiting is inadequate. Another serious disadvan- What causes the inrush current? tage is that the inrush cur- Limiter Modern drive systems, inverters and rents are only partially DC power supplies convert electrical + limited after momentary Inrush- energy by means of pulsed technology. current mains failures (a few 100 The supply voltage is filtered, conver- Converter ms). The electrolytic capa- ted to DC by means of a bridge recti- citor discharges during fier and smoothed with a large elec- AC Input this interuption, but the trolytic . The voltage only DC NTC resistor has stored Electrolytic reaches the actual conversion stage capacitor the heat, stays at a low after that and the charging current of resistance and is virtually Simplified schematic of a switch-mode this large electrolytic capacitor causes power supply ineffective. the inrush current and must be limi-

February 2006, Rev. 1 page 1/3 www.pulspower.com Order number: AN30.01.en The NTC resistor generates a power rents. If these input devices are used The premium solution: pulsed loss and reduces the efficiency by on switch mode power supplies, they charging of the input capacitor approx. 1%. This is equivalent to have precisely the opposite effect. This method, newly developed by 15% more losses with a power sup- They would always switch on at the PULS GmbH, “soft” charges the elec- ply rated at 92% efficiency. worst possible time and cause high trolytic capacitor. The electrolytic The data sheet information of dif- peak currents. capacitor is pulse charged with low ferent equipment manufacturers is losses by means of a switch, inductor very difficult to compare. The para- Inrush current limiting by means and free-wheeling diode. Parameters meters used as a basis are different, of fixed such as peak current and charging but have a decisive effect on the With this method, a power resistor is delay can be precisely calculated and result. used to limit the charging current. suitably incorporated. On the high- The resistor is bypassed after the elec- performance units of the PULS External inrush current limiters trolytic capacitor is fully charged. Dimension Q series, the electrolytic High inrush currents are also known Relays, triacs, IGBTs or other compo- capacitor is intentionally charged when are switched on. nents can be used to bypassed the after a delay. This helps the restart However, the effect is different in this resistor. This method is significantly after mains failures, when lots of con- case. When a is switched more expensive than on at a zero crossing of the sinusoidal inrush current limiting Electrolytic capacitor input voltage, the magnetization of with NTC resistors and is DC the magnetic core of the transformer normally only used at + is driven into an unbalanced state rating classes above during the first few cycles. As a result, 250W. The advantages Converter the magnetic core saturates on each are that the charging AC half-cycle. The magnetizing current, current is independent of Input DC which is only limited by the leakage temperature and there is inductance, increases dramatically and significantly less power causes the high inrush currents. There loss. Simplified schematic of the Dimension Q-Series are special input devices for transfor- Inrush current limitation with pulsed charging of mers that switch on at the voltage the input capacitor. maximum and prevent inrush cur-

29A approx. 3A Input Input Current Current 10A / DIV 2A / DIV

Delay

Input Voltage 230V Input Voltage 230V

Output Voltage 24V Output Voltage 24V

“Cold start” of a 24V, 10A power supply with 24V, 10A power supply from the PULS Dimension NTC resistor inrush current limiting series with pulsed charging of the input capacitor typ. peak inrush current = 29A typ. peak inrush current = 3A

February 2006, Rev. 1 page 2/3 www.pulspower.com Order number: AN30.01.en sumer circuits are trying to meet their summate and load the MCBs and the Advantages of inrush current limiting starting current requirements simulta- system voltage heavily. The delay in by means of pulsed charging: neously when the mains supply is the PULS Dimension Q series anticipa- Peak inrush current is only restored. In this case, the peak inrush tes this initial current surge and swit- marginally higher than the maxi- currents of all the consumer circuits ches on a few 100 ms later. This gives mum operating current a softer restart and pre- MCBs do not have to be rated for Limiter vents unnecessary the peak inrush current, but can be DC + blowing or MCB trips. designed for the operating current Inrush Inrush current is independent of temperature, input voltage or other Converter conditions AC Low power loss Input DC Delayed charging process mitigates Electrolytic peak currents after the supply vol- capacitor tage is switched on Inrush current limiting by means of fixed resi- No starting problems at low stors which will be bypassed after a certain period of time temperatures

Evaluation of the individual methods of inrush current limiting

++ very good + good - not so good NTC Resistor with Pulsed shorting circuit charging

Limiting of the inrush current, cold start + + ++ Inrush current after momentary mains failures - + ++ Temperature dependency - + ++ Input voltage dependency - - ++ Power loss - ++ ++ Suitability at low temperatures (< -10°C) - + ++

February 2006, Rev. 1 page 3/3 www.pulspower.com Order number: AN30.01.en