Synchronous Buck Converter TPS54339 Failure Case Analysis

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Synchronous Buck Converter TPS54339 Failure Case Analysis [Copy link]

Sveinn Jia

TPS54339 is a synchronous rectifier buck converter based on D-CAP control mode, input voltage 4.5V-23V, 3A, launched by TI in 2013. It is widely used in low-voltage systems. This article mainly introduces a failure case. Through the analysis of this case, it gives the risks, experimental test results, and methods to avoid the risks of FCCM mode devices under D-CAP control mode when the backup power supply voltage is higher than the preset buck output voltage.

Background

The customer uses the TPS54339DDAR device in FCCM mode for 12V to 5V conversion, and also needs a backup power supply, which is connected to the output bus of the BUCK via the switch circuit S1. When the TPS54339 input voltage is detected to be lower than 10V, the switch circuit S1 is closed and the system is powered by the backup battery.

Figure 1: System framework diagram

Remark:

The output voltage set by the BUCK is called V_target

The actual output voltage of the BUCK terminal is called V_out;

The backup battery voltage is called V_backup;

Fault description

According to the TPS54339 manual, the chip will not shut down until the EN voltage is lower than 0.6V and the UVLO voltage is lower than 3.45V on the falling edge. When the backup power supply is turned on, the EN and UVLO of the chip are both enabled, so the chip is in normal working condition. The rated voltage of the backup battery is 5.4V, and the battery voltage can be charged up to 5.6V. After passing through the switch circuit S1, the voltage reaching the BUCK bus may be as high as 5.3V. This causes V_out > V_target and the TPS54339 is in the enabled state. In the FCCM mode of the D-CAP, this will cause the input side voltage to increase, resulting in an overvoltage breakdown short circuit on the chip input side. The fault is manifested as a short circuit between the 1/2/3/6 pins of the TPS54339 and the ground.

Figure 2: TPS54339 block diagram

Root Cause Analysis

Assuming that VIN can establish a stable voltage, the entire BUCK converter is in stable operation, and the volt-second balance and inductor current balance must be met.

Since V_out > V_target, the lower tube of the BUCK will continue to be turned on until the NOC (negative current) protection of the chip is triggered.

Due to stable operation, the following equation can be obtained by combining the above formula:

According to Faraday's law of electromagnetic induction, under working conditions, will be a very small value, so there is a risk of high voltage at the VIN end.

Experimental Results

Take TPS54339EVM-056 as the test board, and only change the R1 resistor from 8.25KΩ to 120KΩ. Add a certain voltage to the output end and test the input voltage. When the input is 12V, the rated no-load output voltage is 5.25V.

Input and output no-load test

Adjust the output voltage to 5.26V-5.32V. It can be seen that the voltage at the input end is as high as 33V, which has exceeded the maximum withstand voltage of 25V of the TPS54339 VIN pin and may cause permanent damage to the device.

Table 1: Buck input and output no-load test results

Figure 3: TPS54339 different pin waveforms when V_backup voltage is 5.32V

Input no-load test, output 0-3A load test

The test results are consistent with the no-load input and output. When doing the actual experiment, you need to pay attention to the line loss voltage, because a voltage drop of 0.01V will have a great impact on the voltage value of the input end.

Input end 30mA load test, output no-load test

Considering that in the actual system, the input end of TPS54339 may also have a load. Here we take the case of a 30mA load at the input end of TPS54339 as an example. The test results are as follows:

Table 2: Buck input end 30mA load, output end no-load test results

in conclusion

When the FCCM device in D_CAP control mode is used in a system that requires a backup power supply, it should be noted that the voltage of the backup power supply cannot be higher than the preset voltage of the BUCK, otherwise a high voltage may appear on the input side.

For systems where the backup power supply voltage is higher than the preset voltage of the BUCK, it is recommended to select a device with light load frequency modulation mode, such as TPS54339EDDAR, or control the EN pin so that when the input power is off, the EN is quickly turned off to stop the chip from working. If you are not very concerned about system efficiency, you can also connect a diode in series after the BUCK output to prevent current backflow.