The circuit can also be inverted and used as a current limiting device on the positive rail with a P-Channel MOSFET operating with a resistor or in the ohmic region. If there are space constraints, then Rlimit can be omitted and the channel resistance of the MOSFET used as current limiting, although this is not as well controlled as with a separate resistor. The circuit shown in Figure 7 uses a low-cost N-Channel MOSFET, but has the disadvantage that a high-wattage current limiting resistor R limit is needed. R2 both limits the maximum voltage on the gate to safe levels by acting as a potential divider with R1 and also discharges C1 when the power is removed, to reset the protection. When the voltage exceeds the gate voltage of the MOSFET, it turns on and bypasses the current limiting resistor. The capacitor C1 is slowly charged up via the resistor R1. On power-on, the transistor Q1 is off and the converter is supplied via the current limiting resistor Rlimit. Soft-start circuit using an N-channel MOSFET to bypass a current limiting resistor This is bulkier and more expensive solution, but more efficient and can also react quickly to power interruptions as the thermistor is cold during normal operation.įigure 5 shows two different RECOM products (RACM60 and RACM550) that use the thermistor and thermistor plus relay methods.įig 7. It is also ineffective if the AC input is removed and reapplied before it has cooled down.Ī more efficient solution is to use an NTC thermistor and then short circuit it with a relay contact or triac once the converter is operational. Although a cheap and compact solution, the thermistor runs very hot during normal operation reducing converter efficiency. The thermistor then quickly heats up due to the operating current flowing through it and becomes low resistance, allowing the converter to deliver its full power. On power-up, the device is high resistance and the inrush current is limited. The solution is to add a series resistance to limit the input current until the converter has started up.Ī negative temperature coefficient (NTC) thermistor is a device that has high resistance when cold and lower resistance when hot. In many AC/DC power supplies, a high inrush current can cause nuisance-tripping of fuses or over-current protectors. Reducing input inrush current (AC/DC power supplies) Effect of different input capacitors on the input voltage at the end of a long cable The first image is the input voltage measured at the converter without an input capacitor (peak=71V), the second with a 100♟ capacitor (peak=55V) and the last with a 220♟ capacitor (over-damped to 48V).įig. In the following example, a DC/DC LED driver was powered from 48V through a 15m long cable. The high capacitance absorbs the over-voltage spike and the high ESR helps damp out the oscillation. Electrolytics have a high capacitance and a relatively high equivalent series resistance (ESR). The simplest method of reducing the over-voltage transitions at the end of long cables is to add an electrolytic capacitor across the terminals of the DC/DC converter. Then the over-voltage peak can exceed the voltage rating of the converter and damage it. Typically, the input inrush peak current is more of a problem than the input over-voltage, except when the supply cable is long or the main power supply is not a low-impedance source. After a few back-and forth cycles, the input voltage stabilizes to the supply value.įig.3: Comparison of simplified power connection to a converter and its transmission-line equivalent As the input voltage rises up, the current drawn from the supply increases, but after the input current has peaked and goes negative, current now flows back out of the converter and into the supply, with the effect that the input voltage keeps on rising. The effect of this ‘negative’ current is that the input voltage can momentarily be higher than the supply voltage! Figure 2 shows the input voltage curve. This oscillation means that current is periodically flowing back out of the converter into the supply. At t >1, the exponential is zero and the input current is the same as the operating current.Īn additional artefact can be observed in Figure 1 after the initial surge current peak, the input current does not drop back down to the operating current, but goes negative for a short time and oscillates before settling. At t=0, the exponential is unity so the only limitation on the input current is the resistance R and the current capability of the power supply. Where I in(t) is the capacitor current (time dependent), V in is the supply voltage, R is the output resistance of the supply plus ESR of the capacitor and any interconnection resistance, and C is the input capacitance.
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