Discussion on Switching Power Supply Test

introduction
    In the electronic environment, electromagnetic interference will have a certain impact on the work of the power supply. The stability of the power supply with small source effect and load effect is better. Electronic engineers should consider these aspects in the power supply design.
    1. Test items
    The items to be tested include switching power supply no-load output, voltage and current output at rated load, source effect, load effect, ripple, withstand voltage and insulation resistance, short circuit protection (or overcurrent protection point).
    The test refers to the detailed parameter manual given by each switching power supply.
    For the more important power supply or the power of more than tens of watts, its efficiency (or the operating temperature of the internal power device) directly determines its reliability and failure rate, and should be tested; in addition, there are many other indicators that should be tested according to different It is required to arrange tests, such as the instantaneous drop of the output voltage of a sudden load and its recovery time, the input power factor and waveform peak ratio of the AC/DC power supply, various EMC indicators of the power supply, temperature coefficient, and time stability.
    2. Test requirements
    1. The tester needs to be able to use the digital multimeter correctly, identify the pin diagram of the switching power supply, be able to adjust the output voltage of the power supply, and have electrical knowledge.
    2. The test instrument requires the use of instruments with high precision and high resolution as much as possible, and the instrument should be selected according to the actual situation.
    3. The general routine test is tested under normal temperature and pressure. Those with special requirements for test conditions need to be tested under the required conditions (for example, some need to simulate the environment of the work site, such as outdoor, rainy, and sun exposure).
    3. Test method and process
    3.1 No-load output voltage
    Adjust the input voltage of the switching power supply to the rated voltage of the switching power supply, and use a multimeter to test the output voltage of the switching power supply. In order to reduce the error, you can measure several sets of data (the power switching power supply in the figure represents the tested switching power supply).

    Figure 1 Schematic diagram of no-load wiring

    3.2 Switching power supply output under rated load
    This step of the test includes the test of rated output voltage and current. First, determine the rated load of the switching power supply. Generally, a resistor is selected as the load. Note that the power of the selected resistor must be much greater than the output power of the switching power supply to reduce the heat generated by the resistor. You can also add some heat dissipation measures, such as placing an exhaust fan.
    Rated load calculation formula:
    R0=U2/P
    Note: In the formula, R0 is the rated load resistance value, U is the nominal output voltage value, and P is the rated power.
    After determining the rated load, connect the rated input voltage of the switching power supply, switch on the load circuit of the switching power supply, and string an ammeter in the load circuit (for a safety meter, it is recommended to use a precision shunt resistor in series to measure the voltage drop and convert it to Current value), test the current in the loop, use a multimeter to test the output voltage of the switching power supply. And record the voltage and current values. The wiring diagram is shown in 2, where R0 is the rated load.

    Figure 2 Schematic diagram of rated load wiring

    3.3 Source effect (i.e. voltage regulation rate)
    The source effect is the change in the output voltage relative to the nominal output when the input voltage changes from low to high within the input voltage range of the switching power supply.
    Adjust the input voltage of the switching power supply to the lower limit and upper limit of the range, and use a multimeter to measure and record the output voltage of the switching power supply.

    Figure 3 Source effect test

    The calculation formula is: [(Vo1-Vo2)/Vo]*100%
    Note: Vo1 is the output voltage value measured at the input voltage, Vo2 is the output voltage value measured at the input voltage, and Vo is the nominal output voltage.
    3.4 Load effect (i.e. current regulation rate)
    The load effect is the change in the output voltage of the switching power supply relative to the nominal value when the load changes from rated load to half load (or 20% load) when the switching power supply is working at the rated voltage.
    The important task of this step is to determine the load. The percentage of the load is calculated based on the current, that is, the percentage of the half-load (or 20% load) current to the rated current. According to the calculated current value, the resistance value is estimated for selection.
    Load calculation formula under half load:
    R1=(U2/P)*2
    Note: R1 is the load resistance at half load, which is twice the rated load.
    In the figure, R1 is the estimated equivalent load.

    Figure 4 Load effect test

    The calculation formula is: [(Vo’-V amount)/Vo]*100%
    Note: Vo' is the output voltage of the switching power supply measured after connecting the resistance equivalent in percentage to the output circuit of the switching power supply. V is the output voltage of the switching power supply measured under rated load, and Vo is the nominal output. Voltage.
    3.5 ripple
    Switching power supply from no-load to full-load, multi-point or continuous uniform change (generally test the ripple in the three cases of no-load, half-load, and full-load) under the rated input voltage condition, adjust the oscilloscope to 20MHZ, AC coupling mode, suitable During the scan period, clamp the ground clip of the oscilloscope to the GND terminal of the switching power supply output, and touch the Vo terminal of the switching power supply output with a test pen to read the peak-to-peak value of the ripple in the oscilloscope.

    Figure 5 Ripple test

    For AC/DC power supply, it should be observed under full load and the oscilloscope scanning speed can display several AC cycles. The "bandwidth attenuation" of the oscilloscope should be turned off (not attenuated).
    3.6 Withstand voltage and insulation resistance
    The withstand voltage is tested with a withstand voltage tester. According to the technical data given by the switching power supply, find out the reference value of the withstand voltage, turn on the power supply of the withstand voltage tester, and set the parameters, including AC/DC, range, leakage current and time After setting, start the withstand voltage tester and observe the over-leakage alarm. If the over-leakage alarm, the leakage current is selected to be small, increase the leakage current or reduce the test voltage. The withstand voltages between 1, 4 and 1, 2 and 3 and 4 need to be tested respectively.

    Figure 6 Withstand voltage test

    Use a megohmmeter to test the insulation resistance. Clamp the two ends of the megohmmeter with appropriate working voltage to the two ends to be tested, shake the handle quickly until the clutch slips, and read the meter value. You can also use an electronic megohmmeter to test.
    3.7 Short-circuit protection characteristics (or overcurrent protection point)
    This depends on the description given in the technical data. For example, the description of the switching power supply is: the short-circuit protection characteristic is long-term self-recovery. You can use a wire to connect to the output terminal of the switching power supply for testing. Observe for a long time (determined as needed), the voltage at the time of short-circuit Output and the output of the switching power supply after the short circuit is removed.

    Figure 7 Reference circuit for over-current protection point test

    In the figure, R3 represents a current that can generate twice the rated load (that is, the resistance of R3 at this time is half of the rated load), and VO+ and VO- are respectively connected to the output positive and negative terminals of the switching power supply.
    The meaning of the overcurrent protection point is that when the current in the loop reaches a certain value, the switching power supply cuts off the output (electronic component technology network prompts: note that the overcurrent protection of some switching power supplies is not a cut-off type but may be a current-limiting type). A variable load is connected in series in the output loop of the switching power supply (the variable range is required to be large enough), and the current in the loop is adjusted by adjusting the variable load. During the current rise, pay attention to the reading of the ammeter, and read when the current changes to 0 (Or a very small, smaller value) The previous value is the overcurrent protection point of the switching power supply (at this time, attention should be paid to the heat dissipation of the resistor, because the resistor generates more heat than the rated output in the case of overcurrent).
    Four, test records and data processing
    Each step of the test must record the data and abnormal conditions in detail, and analyze the reasons if there are abnormal conditions. Data records are used for calculation parameters and evaluation of switching power supply.
    Data processing:
    1. Average processing
    2. Source effect calculation
    The formula is:
    [(Vo1-Vo2)/Vo]*100%
    Note: Vo1 is the output voltage value measured under the input voltage, Vo2 is the output voltage value measured under the input voltage, and Vo is the nominal output voltage.
    3. Calculation of load effect
    The formula is:
    [(Vo’-V amount)/Vo]*100%
    Note: Vo' is the output voltage of the switching power supply measured after a resistance equivalent to a percentage is connected to the output loop of the switching power supply. V is the output voltage of the switching power supply measured under rated load, and Vo is the nominal output. Voltage. In addition, electronic engineers can pay more attention to the electronics exhibition and understand the product technology in the industry, which is still good for them.