1. Test the efficiency of the power supply
Efficiency is a very common test item in power supply testing, and efficient power supply performance is a goal that many manufacturers have been pursuing. In the chip specification book, efficiency curves for several common input and output applications are generally provided. When our actual application scope is different from that in the specification, or we make other changes based on the demo board, we need to re-run the efficiency test. This article will talk about how to conduct efficiency testing and some precautions. Corrections and additions are welcome.
1. Measured value
can be known according to the calculation formula of efficiency
When measuring efficiency, you need to measure the four values of Vin, Vout, Iin, and Iout (or Pout and Pin) and perform calculations to get the final result.
2. Four-meter method
The most common efficiency measurement method is the four-meter method, which uses four multimeters to measure the above four parameters. Common multimeters have both current and voltage settings.
The connection diagram is as follows: 
Tips:
- When using the current mode, the multimeter needs to be connected in series in the circuit, paying attention to the current flow direction; when using the voltage mode, it is connected in parallel, paying attention to the positive and negative poles.
- When using the current setting, first use the ampere setting. If the displayed digits are not accurate enough, then change to the milliamp setting for testing. When testing in the milliamp range, when increasing the load current, pay attention to whether it exceeds the milliamp range (usually 400mA). If the measurement range is accidentally exceeded, the fuse in the multimeter will burn out. Only after replacing the fuse can you continue to use the milliamp range for testing.
- Both voltmeters are connected to the board terminal, and the connecting wires should be as short as possible. Do not connect to the power supply end and load end to read data, this will take into account the loss generated on the connecting line and affect the test results.
- If you want to use an electronic load to directly read the data of the output part, you can use a ring connection line. The ring end is directly soldered to the demo board, and the other end is connected to the electronic load. This test produces less loss than a direct clip connection. But it is generally lower than the efficiency measured with a multimeter.
- If you encounter a situation that exceeds the range of the multimeter, you can use an electronic load reading, or you can use a power meter with a larger range to directly measure the output power.
3. Test steps
Taking a simple BUCK circuit as an example, the efficiency test steps are roughly as follows:
(1) Determine the conditions to be tested: input and output voltage and output current. In the light load current part, it is necessary to take a few more points; in the heavy load part, the point interval can be slightly larger. For example, Iout=0-6A, the test points can be: 0A, 0.1A, 0.3A, 0.5A, 0.8A, 1A, 1.5A, 2A, 2.5A, 3A, 3.5A, 4A, 4.5A, 5A, 5.5A , 6A.
(2) Confirm that the test board works normally under the test conditions, and the input and output voltages are correct. Observe the SW waveform. The SW waveform is normal under light load and heavy load, and there is no whistling or abnormal heating.
(3) Power off, according to the above schematic diagram, connect four multimeters to the circuit, and put the ammeter in the ampere position. After the connection is completed, power on again.
(4) After power-on, the load current can be adjusted slowly according to the test conditions. It is necessary to wait for the value on the multimeter to stabilize before recording the test data. The input voltage may drop below the test condition as the load current increases. At this time, it is necessary to increase the input voltage appropriately, and try to keep the data on the multimeter for testing the input voltage consistent with the test conditions.
4. Report form
In addition to filling the tested Vin Vout Iin Iout into the form, the corresponding calculation results are obtained. In order to show the results more intuitively and compare them with other chips, an efficiency curve is generally drawn.
The figure below is the efficiency result measured by MP4581 under the conditions of Vin=24V/36V/48V, Vout=12V, Iout=1mA-800mA:
The efficiency of a general DCDC power supply is low at light loads, and the highest efficiency point appears at heavy loads. The efficiency curve is relatively smooth. If there is a sudden upward or downward point in the drawn efficiency curve, you can retest the efficiency at that point to confirm the correctness of the data.
The results of efficiency tests are sometimes unsatisfactory. When the efficiency does not reach the expected value, what methods can be used to optimize it? The following are listed here for reference, and additions and corrections are welcome.
1. Replace components.
In the basic DCDC circuit (here we still take the BUCK circuit as an example), many components will produce losses, thus affecting efficiency:
a. Inductor DCR: refers to the DC resistance of the inductor (i.e. the coil's Resistor), it can be regarded as an energy-consuming component like an ordinary resistor. When current flows, losses will occur. Therefore, choosing an inductor with a smaller DCR can reduce losses and improve efficiency.
b. Capacitance ESR: Since all devices are not ideal components, actual capacitances have parasitic parameters ESR (equivalent series resistance). When current flows, the greater the ESR, the greater the power loss in the resistor, which will not only affect the efficiency, but also affect the life of the capacitor. Therefore, reducing the capacitor ESR can also achieve the effect of improving efficiency. Multiple capacitors can also be connected in parallel to reduce ESR.
c. MOSFET: MOSFET is present in commonly used synchronous DCDC converters. Rdson refers to the on-resistance of the transistor (can be found in the device specification sheet). This specification directly determines the power efficiency of the MOSFET. A small Rdson value is beneficial to reducing the loss generated during device conduction.
In addition to conduction losses, the switching losses of the device also affect efficiency. The switching loss comes from the overlapping area of the current and voltage curves at the switching moment of the device (as shown in the figure below). Therefore, increasing the switching speed and driving speed of the MOS tube can also improve efficiency. 
Nowadays, due to the pursuit of high integration, many power supply chips have integrated switching tubes inside the chip. At this time, there is no separate selection of MOS tubes, but a direct selection of the power supply chip. whaosoft aiot http://143ai.com
2. Frequency
reduction The reduction of switching losses can be achieved not only by increasing the speed of the switching tube, but also by reducing frequency. At light load, the switching loss is almost unchanged, and the efficiency drops a lot because the output power is low.
When the power supply outputs light load current, it works in PFM (pulse-frequency modulation) mode; when it outputs heavy load, it works in PWM (pulse-width modulation) mode.