Switching power supply test solution

Central topics:

• Discussion on Switching Power Supply Test Scheme

Solution:

• Testing with a digital multimeter
• Use high-precision and high-resolution instruments

Switching power supplies are widely used by electronic companies because of their high efficiency and small size. The following are the test methods and test items summarized by myself in my work. If these aspects can be fully considered when designing a power supply, then its products will be used by more and more customers.

Today's electronic products have higher requirements for power supplies. Most of them are concerned about its stability, input voltage range, output stability, and output harmonic size. It is found that some power supplies have large ripples at work, which causes the equipment to not work. Therefore, this is also an aspect that affects the development of power supplies. When

the power supply actually works in the circuit, the most concerned thing is the source effect (voltage regulation rate). Because after the circuit is determined, the load has been basically determined, and the impact of the load is significantly smaller than the impact of the input voltage on the power supply. In addition, electromagnetic interference should be paid attention to. In the electronic environment, electromagnetic interference will have a certain impact on the work of the power supply. The power supply with small source effect and load effect has better stability. The editor of I Love Solution Network believes that electronic engineers should consider these aspects in power supply design. 1. Test items The items to be tested include the no-load output of the switching power supply, the voltage and current output at rated load, source effect, load effect, ripple, withstand voltage and insulation resistance, and short-circuit protection (or overcurrent protection point). The test is carried out with reference to the detailed parameter specifications given by each switching power supply. For more important power supplies or those with a power of more than tens of watts, their efficiency (or the operating temperature of the internal power device) directly determines their reliability and failure rate, and should be tested; in addition, there are many other indicators that should be arranged for testing according to different requirements, such as the instantaneous drop of the output voltage under sudden load and its recovery time, the input power factor and waveform peak ratio of the AC/DC power supply, the various EMC indicators of the power supply, as well as the temperature coefficient and time stability. 2. Test requirements 1. The tester must 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 relevant knowledge of electricity. 2. Test instruments require the use of instruments with high precision and high resolution as much as possible, and choose to use instruments according to actual conditions. 3. Generally, conventional tests are tested at normal temperature and pressure. Those with special requirements for test conditions need to be tested under required conditions (for example, some need to simulate the environment of the work site, such as outdoor, rainy, exposed to the sun, etc.). 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 switching power supply under test).

3.2 Switching power supply output under rated load

This step of testing includes the rated output voltage and current test. First, the rated load of the switching power supply must be determined. 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 heating of the resistor. Some heat dissipation measures can also be added, 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, connect the load circuit of the switching power supply, connect an ammeter in series in the load circuit (for safety, it is recommended to use a precision shunt resistor in series to measure its voltage drop and convert it into a current value), test the current in the circuit, and use the voltage range of the multimeter to test the output voltage of the switching power supply. And record the voltage and current values. The wiring diagram is shown in Figure 2, where R0 is the rated load.

3.3 Source effect (i.e. voltage regulation rate)

Source effect is the change in 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 and upper limits of the range respectively, and use a multimeter to measure the output voltage of the switching power supply and record it.

The calculation formula is: [(Vo1-Vo2)/Vo]*100%

Note: Vo1 is the output voltage value measured at the upper limit of the input voltage, Vo2 is the output voltage value measured at the lower limit of 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 the rated load to half load (or 20% load) under rated voltage operation. 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 inferred for selection. The load calculation formula under half load: R1=(U2/P)*2 Note: R1 is the load resistance under half load, which is twice the rated load. In the figure, R1 is the inferred equivalent load.

The calculation formula is: [(Vo'-V rated)/Vo]*100%

Note: Vo' is the output voltage of the switching power supply measured after the equivalent percentage resistance is connected to the output circuit of the switching power supply, V rated is the output voltage of the switching power supply measured under the rated load, and Vo is the nominal output voltage.

3.5 Ripple

The switching power supply changes from no-load to full-load, multi-point or continuous uniform change (generally test the ripple under no-load, half-load and full-load conditions) Under the rated input voltage condition, adjust the oscilloscope to 20MHZ, AC coupling mode, appropriate scanning cycle, clamp the ground clamp of the oscilloscope to the GND end of the switching power supply output, touch the Vo end of the switching power supply output with the test pen, and read the peak-to-peak value of the maximum ripple in the oscilloscope.

For AC/DC power supply, it should be observed under full load and at a scanning speed that can display several AC cycles. The "bandwidth attenuation" of the oscilloscope should be set to the off (no attenuation) state.

3.6 Withstand voltage and insulation

resistance Withstand voltage is tested with a withstand voltage tester. According to the technical data provided by the switching power supply, find the reference value of the withstand voltage, turn on the power of the withstand voltage tester, set the parameters, including AC/DC, range, leakage current and time setting, and after setting, start the withstand voltage tester to observe the over-leakage alarm. If the over-leakage alarm occurs, the leakage current is selected to be small, increase the leakage current or reduce the test voltage. It is necessary to test the withstand voltage between 1,4 and 1,2 and 3,4 respectively.

The insulation resistance is tested with a megohmmeter. 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 table value. An electronic megohmmeter can also be used for testing. 3.7 Short-circuit protection characteristics (or overcurrent protection points)

This depends on the instructions given in the technical data. For example, the switching power supply is described as: the short-circuit protection characteristic is long-term self-recovery. You can use a wire to connect to the output end of the switching power supply for testing, and observe for a long time (determined according to needs), the voltage output during short circuit and the output of the switching power supply after the short circuit is eliminated.

In the figure, R3 represents a current that can generate twice the rated load (that is, the resistance of R3 is half of the rated load at this time), and VO+ and VO- are connected to the positive and negative output terminals of the switching power supply respectively.

The overcurrent protection point means that when the current in the circuit reaches a certain value, the switching power supply cuts off the output (Electronic Component Technology Network Tips: Note that some switching power supplies are not cut-off type but may be current limiting type). Connect a variable load in series in the output circuit of the switching power supply (the variable range is required to be large enough), and adjust the current in the circuit by adjusting the variable load. During the current rise, pay attention to the reading of the ammeter, and read the value before the current changes to 0 (or a very small, small value), which is the overcurrent protection point of the switching power supply (at this time, pay attention to the heat dissipation of the resistor, because the resistor heats up more in the case of overcurrent than at rated output). 4. Test records and data processing Each step of the test should record the data and abnormal conditions in detail, and analyze the reasons if there are abnormal conditions. Data records are used for calculating parameters and evaluating switching power supplies. Data processing: 1. Average value processing 2. Source effect calculation The formula is: [(Vo1-Vo2)/Vo]*100% Note: Vo1 is the output voltage value measured at the upper limit of the input voltage, Vo2 is the output voltage value measured at the lower limit of the input voltage, and Vo is the nominal output voltage. 3. Load effect calculation The formula is: [(Vo'-V rated)/Vo]*100% Note: Vo' is the output voltage of the switching power supply measured after the equivalent percentage resistance is connected to the output circuit of the switching power supply, V rated is the output voltage of the switching power supply measured under the rated load, and Vo is the nominal output voltage. In addition, electronic engineers can pay more attention to the electronics exhibition to understand the latest products and technologies in the industry, which is still good for themselves.