Measuring Inrush Current

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In AC/DC power converters beyond a few watts, during the initial application of power an excessive inrush current will flow when the input capacitors are suddenly charged. If unhindered the inrush current can easily exceed 50 A at the peak of the AC cycle and severely stress the converter’s fuse and input rectifiers, thereby significantly reducing the reliability and life expectancy of the modules. Universal power supplies (supplies which accept a wide range of input voltages) are particularly susceptible to high inrush current since their input capacitors must be large enough to handle line voltages as low as 110 VAC, as well as voltages as high as 305 VAC at start-up. In these environments, a power-supply failure or a tripped circuit breaker can be inconvenient at best, and expensive or dangerous at worst.

A typical power supply (see Fig. 1) has combined equivalent impedance R1 from the common-mode choke, bridge rectifier, line cord, and wiring. This is typically a few ohms. C1 is used for EMI filtering and is usually around 0.1 μF. It does not hold enough energy to present much of an inrush-current problem. C2, the bulk storage capacitor, can be many hundreds of microfarads and is usually sized to meet a minimum holdup time and ripple-current rating. This figure shows that if the switch is closed near the top of the AC cycle, a large input current can occur, limited only by R1, making it quite possible to get an inrush-current spike of 50 A or more on a nominal 120-Vac line (170 V peak). In countries where the nominal line voltage is 240 Vac, the inrush current can exceed 100 A. This large inrush current degrades the performance and lifetime of the power supply in a number of ways:

• The arching of the switch contact leads to premature switch failure. It can also cause the line circuit breaker to trip, especially if there are multiple power supplies on the same circuit.

• The current can thermally overstress the input rectifiers, causing immediate power supply failure.

• High currents on the fuse cause heating, which can slowly degrade the fuse over time.

Figure 1: Inrush current of > 50A may occur with 120VAC input at the top of the AC cycle.

Inrush-current specs

Higher inrush-current specifications equate to greater stress on the rectifier and lower reliability. When looking at inrush-current specs, consider the following:

• Is the inrush current rated as average or peak? Many companies specify the half-cycle average inrush current. Peak current is 40% larger.

• Is the inrush current specified only at 120 VACor 240 VAC? You must consider the inrush current over the real line voltage, which can be as high as 277 VAC in universal supplies.

• Is the inrush-current-limiting technique effective over your entire expected operating temperature range? This can be problematic when using an NTC resistor for inrush- current limiting.

Inrush current waveform

A typical power system inrush current waveform is shown in Figure 2. It has two current peaks. The first “inrush spike” peak current occurs when the input voltage source is turned on. This peak current flows into the EMI filter capacitors and any capacitors on the input side of the AC-DC converter, charging them to their steady state value. The second current peak occurs when the AC-DC converter turns on. This peak current flows through the power transformer in the AC-DC converter to the output capacitor and into any load capacitance, charging them to their steady state value. There can be multiple occurrences if there is more than one AC-DC converter.

Figure 2. Typical inrush current waveform

Inrush spike current

The first current peak is often referred to as the inrush spike. Its peak value and shape are highly dependent on the characteristics of the input source, specifically the voltage rise time or dv/dt and source impedance. A fast rising input voltage waveform, such as from a mechanical switch or relay closure, will produce a very high and narrow current peak, limited only by series resistance and inductance. EMI magnetics are usually too small in value or quickly saturate under high peak currents. And the resulting peak is limited only by the source, line and parasitic resistances.

For a slower rising input waveform the peak value of the inrush current is determined by the equation, i=C*dv/dt. The capacitance is the total capacitance in the EMI filter and on the input side of the DC-DC converter. The dv/dt is the slope of the applied voltage waveform. Switching power converters typically have output voltage rise times on the order of a few milliseconds, solid state power controllers (SSPC) usually from 50us to 500us, and large capacitor banks cannot be charged in less than several milliseconds. These slow rise times will not produce excessively high current peaks and may meet system specifications without additional protection. While the peak inrush current must be tested for spec compliance, the i2t of the current waveform should also be evaluated as it could trip an upstream fuse, circuit breaker, or SSPC.

Turn On Current

The second current peak of Figure 2 is also considered part of the inrush current. This peak occurs when the DC-DC converter turns on and draws current from the source to charge its output capacitance and any load capacitance. Typical turn-on current waveforms are shown in Figure 3. The turn-on current is the same whether the converter is turned on by applying an input voltage or via an enable/inhibit signal. The turn-on current wave shape and peak value will be well-controlled as long as the converter has an output soft start feature. But it could require a higher peak current when starting with a large capacitive load.

Figure 3: Typical turn-on current waveform

Some manufacturers don’t show inrush current in their specs.  Some manufacturers declare, that their LED drivers have the lowest inrush current in the market, for example, below 0.5A. But after measuring  competitive LED driver in our R&D laboratory we got the inrush current waveforms as in Figure 4.

Figure 4. Inrush current waveform from one of competitor’s LED driver.

The first peak is 47.5A (1.3us)!!! Additionally the third or fourth peak only is ~0.5A.

This critical attribute of Aimtec product specifications is stated based on the first peak of inrush current for all of our AC-DC & LED drivers.