A brief discussion on the causes of inrush current
and over current protection testing methods of power supplies
Surge Mode application of electronic load
Keywords: Inrush current, capacitive load, over current protection (OCP)
There are two types of inrush currents in power supplies. The first is the inrush current generated instantaneously when the AC input supplies AC power; the second is the inrush current generated instantaneously when the DC output of the power supply is connected to a capacitive load. A power supply may not be able to operate normally due to damage or a blown fuse caused by an inrush current at the AC input.
The other scenario is that the over current protection mechanism may be triggered due to an inrush current at the output and the power supply may not be able to smoothly supply power to the electrical equipment (motherboard, etc.). The worst impact of both is failure to supply power smoothly. Hence, ensuring that the power supply can withstand an inrush current and has appropriate over current protection mechanisms becomes important in design considerations.
Causes of inrush current:
A transient voltage on the capacitor will generate a current. Although the capacitor does not allow the voltage to change instantaneously in order to suppress dV/dt from reaching infinity, the transient current is still related to the slew rate of the transient voltage. Furthermore, the resonance caused by the inductance on the circuit will increase the transient ringing current. There are many circuit mechanisms that can be used to suppress the inrush current at the input. We will not discuss that here.
In the test, the AC power supply is turned on and off at a designated angle, so that the peak voltage of the AC voltage is turned on or off to efficiently measure the inrush current. The Imax hold measurement function of the AC power supply can also be used to measure the inrush current. Maximum current data can be obtained without an oscilloscope and current probe.
Figure 1: ASR-6000 series AC/DC power supply designated phase startup waveform,
which can quickly verify the input inrush current
The cause of inrush current at the output is also due to capacitive load and inductive resonance. However, the handling mechanism is different. The output of the power supply has an over current protection function. The over current protection mechanism includes protecting the power supply from being damaged.
Another mechanism is to protect the load from being damaged due to over current. Asmentioned above, when the power supply is connected to a capacitive load, the over current protection mechanism may be triggered instantly and the power supply cannot smoothly supply power to the electrical equipment (motherboard, etc.), so the test at the output is to test the over current protection mechanism.
To put it simply, the transient maximum current (Surge I), transient current time (Time), steady-state current (Normal I), steps from maximum current to steady-state current (Step) can be simulated through simple settings to obtain a transient inrush current. Power supply test is conducted to ensure that it will not be damaged by the transient inrush current and will not accidentally trigger the over current protection (OCP) and shut down, and finally return to the steady-state current load operation. This is the complete process of using an electronic load to simulate the inrush current at the output. You only need to set four variables for testing. GW Instek electronic loads with the Surge Mode icon have the above-mentioned functions.
Commercially available products also have another way to simulate the output inrush current, which requires first using a precision impedance analyzer to measure the motherboard's equivalent parallel capacitance (CL), equivalent parallel resistance (RL), and equivalent series inductance (LS), equivalent series resistance (LS).
After obtaining these values, simulate the inrush current. This method is too complicated for testing the over current protection mechanism, and the equivalent circuit requires additional instruments to obtain. Whether or not the test lead connecting the load to the power supply needs to be included in the equivalent circuit is also an issue. The best testing solution is that Surge Mode directly addresses the core issue of over current protection through intuitive settings.
Figure 2: Icon of GW Instek Surge Mode
The recommended instruments for verifying server power supply inrush current are as follows:
Figure 3: PEL-5000G electronic load with Surge Mode
Figure 4: ASR-6000 series AC/DC power supply
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