“The step-down power supply chip is an indispensable core component in the design of Electronic products. The power supply module composed of the power supply chip provides various power supplies used by electronic products. For automotive products, the voltage of the car battery will vary between 9V-16V. In this case, the power module needs to adapt to the changing conditions of the battery voltage and load. How to test the power supply is very important. Many companies The test of the power module of the electronic products of the company is based on the reference circuit and layout example given by the Datasheet, and the ripple of the output voltage is simply tested, and the power efficiency is over.
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1 Overview
The step-down power supply chip is an indispensable core component in the design of electronic products. The power supply module composed of the power supply chip provides various power supplies used by electronic products. For automotive products, the voltage of the car battery will vary between 9V-16V. In this case, the power module needs to adapt to the changing conditions of the battery voltage and load. How to test the power supply is very important. Many companies The test of the power module of the electronic products of the company is based on the reference circuit and layout example given by the Datasheet, and the ripple of the output voltage is simply tested, and the power efficiency is over. It is believed that all parameters of the power mode Datasheet have been marked, and there is nothing to measure, and no parameters can be changed. The actual buck circuit has many parameters that need to be tested. The picture below is a TI buck power chip LM60440, which meets the on-board requirements of AECQ-100.
In the Datasheet of LM60440, parameters such as efficiency curve graph, start-up timing graph, load transient response graph, ripple voltage graph and EMC test graph are provided. These test data are based on the EVM board test of LM60440. Tested in good condition. The actual electronic products should be tested according to specific customer requirements and system requirements. The LM60440 was used in a product we developed to step down to obtain an output voltage of 3V3. After completing the design of the schematic diagram, after getting the Demo sample by playing Allegro, the test of the power module is mainly completed from the following seven dimensions:
1. Input voltage performance test, including cold start test, enable voltage threshold test;
2. Output voltage performance test, including output voltage ripple test, load transient test, loop stability test;
3. Timing test: boot sequence, shutdown sequence;
4. Protection function test: overvoltage protection, overcurrent protection, short circuit protection, overtemperature protection;
5. Efficiency test;
6. PWM switching frequency test;
7. Withstand voltage test of key components, mainly including MOSFET, DIODE, Inductor, input capacitor, and output voltage;
Completing the above seven tests can basically reflect the performance of the power module. The following will make a comparative analysis of the measured data of the seven aspects in the test and the data in the Datasheet for discussion, and see how many test contents other friends have in the power supply design.
2. Analysis of test cases
2.1 Input voltage performance test
2.1.1 Cold start test
The cold engine start test is generally called the cold-trank test. This is because when the engine starts, a large current of the battery is required to work, resulting in a rapid drop in the battery voltage. There are corresponding test specifications for such working conditions, such as ISO 7637-2 (test pulse 4), as shown in the figure below. During the cold-trank test, even if the battery voltage drops, the on-board electronic products, such as navigation, entertainment, dashboard and other equipment, still need to maintain safe and reliable functions, which puts forward higher requirements for the power module.
According to customer requirements, the following test specifications are used in our products, which are relatively more stringent.
Regarding this test, it is not reflected in the LM60440 Datasheet. In our product testing, it meets the testing requirements. After the input voltage drops, the output voltage also drops, but it can be output again within a certain time (less than 10ms), which is in line with the definition of system requirements.
2.1.2 Enable Threshold Test
The enable (EN-Pin) of the LM60440 controls the startup and shutdown of the chip. The input voltage of this pin is controlled by a precise threshold value and is controlled by other external power supply voltages (usually 3.3V LDO control is used). In the actual product test, the threshold voltage consistent with that in the Datasheet was obtained.
2.2. Output voltage performance
2.2.1 Output voltage ripple test
Voltage ripple (ripple voltage) is basically tested, and the data tested under no-load, light-load and heavy-load conditions are different. Different selection of inductor and output capacitors will also cause different test data of voltage ripple. How to reduce the voltage ripple will not be expanded here, and many materials have been introduced. Here we mainly discuss how to test the power module after the design is completed. When the output current is 3A, the tested ripple voltage is 38mV, which is smaller than the parameter 60mV in the Datasheet.
2.2.2 Load transient test
When the load suddenly drops and loads, it will cause the phenomenon of output voltage overshoot and drop. In the actual test, for this kind of test, the output voltage overshoot and drop amplitude are required, which cannot exceed 5%*Vout. The output current is varied from 0.28A to 2.8A, the overshoot and sag voltages are 85mV and 115[size=14.6667px]mV, all are less than 3.3*0.05=165[size=14.6667px]mV. At no load and light load, the output voltage ripple will be smaller.
2.2.3 Loop Stability Test
The power supply system is generally negative feedback. After the loop is not designed, the output voltage will fluctuate. In the engineering design, the phase margin is required to be greater than 65 degrees and the gain margin is greater than 10dB. When the load current is 2.8A, use Bode100 The loop of the power supply system is tested, and the phase margin is 82 degrees and the gain margin is 15dB. The loop system is stable. Regarding this test item, it is not reflected in the Datasheet, and it is necessary to test the actual product.
Many chips are dedicated loop compensation comp-pins to compensate when the loop is unstable. The LM60440 does not have a loop compensation pin. If the loop is unstable, a compensation network can be added to the feedback loop. As shown in the Cff in the schematic diagram above, a resistor can also be connected in series.Adjust according to the specific circuit
2.3 Switching sequence
2.3.1 Boot sequence
In power products, the boot sequence is strictly corresponding to that in the Datasheet, but the soft-start time can be adjusted to meet the needs of different projects. The data obtained from the test is consistent with the Datasheet.
2.3.2 Shutdown Sequence
The shutdown sequence is not as strict as the startup sequence, so there is no test in the Datasheet, but our actual product has completed the test, and then compared with the shutdown sequence of the EVM provided by the LM60440, which is consistent.
2.4 Protection function test
2.4.1 Overvoltage Protection Test (OVP)
The power supply chip cannot be used with overvoltage. Exceeding the maximum input voltage of the power supply chip will cause the chip to be damaged by overvoltage.
Some power chips have OVP threshold voltage, the maximum absolute value voltage (this voltage is the overvoltage damage value of the chip), LM60440 has no OVP threshold voltage, another power chip used before, when the input voltage exceeds OVP threshold, the output voltage is reduced to 0, and the flag bit of the diagnostic pin is flipped.
2.4.2 Overcurrent Protection Test (OCP)
Using the electronic load to apply more than the maximum output current of the power chip will generate OCP to ensure that the output current and output voltage are within the normal working range;
2.4.3 Short circuit protection test
Using the test fixture, short the output. The power chip detects an instantaneous overcurrent and turns off the output. Ensure the normal operation of the power module.
2.5 Efficiency Test
The power module will generate various losses, such as MOSFET switching loss, conduction loss, inductor DC loss, AC loss and so on. This results in a change in output efficiency. Therefore customers will demand the lowest efficiency ratio. Under different input voltage conditions, the LM60440 can basically achieve an efficiency of more than 90% under heavy load.
In the actual measurement, the efficiency will be lower than that of the Datasheet.When the output current increases, the efficiency will be higher
2.6 PWM switching frequency test
The working mode of the LM60440 will change with the change of the load, and it will switch between PFM, DCM, and CCM modes to meet the needs of the load current. The PWM switching frequency also changes accordingly.
2.7 Withstand voltage test of key components
The LM60440 has a built-in MOSFET, and the voltage at the node of the chip SW-pin and the inductor will always be loaded on the drain of the MOSFET. The PWM switching process will generate an overshoot voltage. The maximum value of the measured overshoot voltage is 8.6V. The overshoot voltage cannot break down the internal MOSFET. If you want to reduce the overshoot voltage, you can add a snubber circuit to suppress it.
The output capacitor generally chooses MLCC, and it is necessary to consider that the ripple current should not exceed the rated value of the MLCC, otherwise the MLCC will heat up. The figure below shows the ripple current of MLCC, about 90mA, and the ESR of MLCC is also relatively small, so it will not generate too much heat.
3. Summary
After completing the above tests, the test values meet the system requirements and customer requirements. The EMC test needs to be carried out later. The EVM board of the LM60440 provided by TI has passed the EMC test and has a large margin. But in the specific product, there is not only this power supply, but also the BOOST circuit, so the EMC data provided by the Datasheet is only for reference. Even if the product’s EMC test fails, it is generally not just a problem with the power module. It is necessary to locate the interference source and radiation source from the perspective of the entire circuit board, and then make corresponding improvements.
After completing the 2PCS test at the R&D level, the circuit board is then handed over to the TQE department for tolerance tests such as high and low temperature, humidity and life.
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