Summary.
The TPS54339 was introduced by TI in 2013 based on the D-CAP control mode with an input voltage of 45V-23V, 3A synchronous rectification buck converter, widely used in low voltage systems. This article mainly introduces a fault case, through the analysis of this case, the risk of the FCCM mode device in D-CAP control mode, when the standby power supply voltage is higher than the buck preset output voltage, the experimental test results, and the method to avoid the risk.
Background. The customer uses the TPS54339DDAR device in FCCM mode for 12V to 5V with the need for a backup power supply that 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 switching circuit S1 is closed and the system is powered by a backup battery.
Figure 1: System framework diagram.
Note: The output voltage set by buck is called V target
The actual output voltage on the buck side is called V out;
The backup battery voltage is called V backup;
Description of the fault. As we know from the TPS54339 manual, only the EN voltage is lower than 0 on the falling edge6V will turn off, and UVLO will be lower than 345V, the chip will stop working. When the standby power is turned on, both EN and UVLO of the chip are enabled, so the chip is in normal working condition. And the rated voltage of the backup battery is 54V, the battery voltage can be charged up to 56V, after passing through the switching circuit S1, the voltage reaching the buck bus can be as high as 53v。This causes V out > V target and TPS54339 is enabled. In the FCCM mode of the D-CAP, this causes the voltage on the input side to rise, resulting in an overvoltage breakdown short circuit on the input side of the chip. The fault manifests as a short-to-ground at 1 2 3 6 pins of the TPS54339.
Figure 2: TPS54339 block diagram
Root cause analysis. Assuming that VIN can establish a stable voltage, then the entire buck converter is working stably, and the volt-second balance and inductor current balance must be satisfied.
Since V out > V target, the buck's down tube continues to turn on until the chip's NOC (negative current) protection is triggered. So.
Since it works stably, the following equation can be obtained by combining the above equation:
From Faraday's law of electromagnetic induction, it can be seen that in the working state,
It will be a very small value, so there is a risk of high voltage on the vin side.
Experimental results. Using TPS54339EVM-056 as the test board, only the R1 resistor is changed from 825k modified to 120k. Add a certain voltage to the output and test the voltage at the input terminal. At 12V input, the rated no-load output voltage is 525v。
No-load experiment at input and output.
Adjust the output voltage to 526v-5.32V, as you can see, the voltage at the input is up to 33V, which has exceeded the TPS54339 VIN pin's maximum withstand voltage of 25V, which may cause permanent damage to the device.
Table 1: No-load test results at buck input and output.
Figure 3: V backup voltage is 5At 32V, the TPS54339 has a different pin waveform.
No-load experiment at the input end, 0-3a on-load experiment at the output.
The test results are consistent with the no-load at the input and output terminals. When actually doing experiments, you need to pay attention to the line loss voltage, because 0The voltage drop of 01V will have a great impact on the voltage value of the input terminal.
The input terminal is 30mA with load test, and the output is no-load test.
Considering that in a real system, the input of the TPS54339 may also have a load. In this example, the TPS54339 has a 30mA load on the input. The test results are as follows:
Table 2: 30mA load at the input and no-load test results at the output.
Conclusion. When using a DCCM device in the D CAP control mode for a system that requires a backup power supply, it should be noted that the voltage of the backup power supply should not be higher than the voltage preset by the buck, otherwise a high voltage may occur on the input side.
For systems where the backup supply voltage is higher than the buck preset voltage, it is recommended to choose a device with a light load FM mode, such as TPS54339EDDAR or a control EN pin, so that when the input power is off, the EN will be quickly shut down, so that the chip will stop working. If you are not concerned about system efficiency, you can also connect another diode in series after the buck output to prevent current backflow.
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