Reduce the impact of noise on the TPS54202 in white goods applications
作宇：South China FAE Skylar Li
In the field of white appliances, buck converters are widely used, and different power rails are required to achieve different functions. The TPS54202 device is a 28V, 2A synchronous buck converter with two integrated N-channel MOSFETs that is very mature in the field of home appliances. It is usually used in 12V to 5V or 3.3V circuits to power MCUs or operational amplifiers and other chips. . The chip's optimized internal compensation network minimizes the number of external components and simplifies control loop design.
Before mass production of the final product, various tests are usually performed on the power supply chip, such as sticking a thermocouple on the plastic case of the power supply chip for temperature rise testing, or using an antenna close to the power supply chip to detect radio interference on the chip. The noise introduced by these tests sometimes affects the normal output of the chip, so reducing the noise is critical to the normal operation of the chip and the entire system. Next, we will introduce two specific cases of the impact of noise on TPS54202 applications and how to deal with them.
Case 1: Use thermocouple cloth to conduct temperature rise experiment on the plastic case of TPS54202
In the application of home appliances with motors, there has been an abnormal phenomenon where the TPS54202 EVM board circuit design is used. The thermocouple is clothed on the TPS54202 plastic case. When the motor rotates, the back-end MCU is reset. By capturing the waveform, at the moment of power outage, the voltage of the EN pin drops below the threshold, as shown in Figure 1.
Figure 1: Waveform diagram of EN and Vout of TPS54202 (C1: VIN C2: Vout）
By observing the captured waveform, it is inferred that the thermocouple cloth introduces motor noise to the EN pin on the 54202 plastic case. So I checked the schematic and layout of TPS54202. R4 and R5 are set larger because it can improve the overall efficiency. Because high resistance will reduce the power loss of the resistor, if you do not use a thermocouple for testing (thermocouples easily introduce noise), then these large resistors are more suitable; regarding the degree of interference of the EN pin, it is because the signal generated by the large resistor is relatively small , so it is more likely to be overwhelmed by noise, which means that the signal-to-noise ratio is not high. To improve the signal-to-noise ratio, the signal must be strengthened or the noise must be weakened. Reducing the resistor will enhance the signal, and adding a capacitor in parallel will reduce the noise.
Figure 2: Schematic diagram of TPS54202 EVM
For this, if you need to test the temperature with a thermocouple on our EVM board, reduce R4/R5 to 51k/10.5k ohms and also add a 1nF X7R capacitor in parallel with R5. After testing, it was shown that the backend MCU would not reset again. The R&D engineer changed the output voltage to 3.3V, and the measured waveform is as follows. It can be seen that the output voltage Vout is stable at 3.3V and will not affect the back-end MCU.
Figure 3: Waveform diagram of EN, SW and Vout of TPS54202
Case 2: Use an antenna close to TPS54202 to detect radio interference on the chip
Products such as wire controllers usually have WiFi modules. The radio frequency energy of the antenna of the WiFi module can easily cause interference to the chip or even the system. Therefore, in the process of developing wired controller products, R&D engineers often use 2.4Ghz antennas to test the anti-interference ability of the power chip. During the detection process, TPS54202 had a problem that when the antenna was close, the output voltage increased, causing damage to the back-end MCU.
Figure 4: Waveform diagram of TPS54202 SW, VFB and Vout
By observing Figure 4, we can find that the voltage of the FB pin of the TPS54202 is pulled to 0 due to the continuous huge radio frequency energy, which causes the chip to continuously send pulses and increase the output voltage. Analyzing the characteristics of the FB pin and the schematic diagram of the EVM board in Figure 2, the internal structure of the FB pin is a receiver, which is easily interfered by coupled noise with a large amount of energy. C8 in Figure 2 is Cff (before Feed capacitor), Cff, as a low-impedance path, can introduce a pair of zero poles into the loop, and the bandwidth will increase. The increase in bandwidth will easily introduce more noise (Figure 5).
Figure 5: Characteristics of FB pin
Therefore, considering that removing Cff will reduce the loop bandwidth, the loop will be less disturbed by external noise due to the narrower bandwidth. The waveform measured by removing the feedforward capacitor is shown in Figure 6.
Figure 6: Vout waveform after removing the feedforward capacitor
It can be seen that after removing the feedforward capacitor, it can be seen that the output voltage will not increase, and it also shows that the back-end MCU will not be damaged.
To sum up, the FB pin and EN pin of TPS54202 are easy to introduce external noise. During the actual application and testing process, you need to pay more attention to avoid interference with these pins, and use the above methods to reduce noise interference. .
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