Original question:
Hi team,
I have one question about the current limit function.
current limit=100A
fast trip current=150A
Tflt=100ms
In normal operation, which means that Vin=Vo=12V, if the load wants 130A, can the power supply provide 130A to the load? Or the MOS will regulate the current to 100A?
In other words, I know that current limit function will regulate current <100A in start-up process. But I don't know if it's the same case in normal operation.
Thanks a lot！
Nison
Hi Nison,
Thank you for inquiring about TPS24770.
In normal operation, which means that Vin=Vo=12V, if the load wants 130A, can the power supply provide 130A to the load? Or the MOS will regulate the current to 100A?
Avishek: Power supply can provide 130 A of current to the load only within the specified fault duration (Tflt). After the timer duration (Tflt) is expired, the Hot Swap controller will shutoff.
Or the MOS will regulate the current to 100A?
Avishek: No, MOSFET will not regulate the current to 100 A. Because, after the TPS2477x has gone through start-up it will no longer actively control the gate. Instead it will run the timer when the current is between the current limit and the fast trip threshold. Once the timer has expired the gate will be pulled down.
Please let us know if there are any more queries.
Hi Naizeng,
According to the figure, when the load is 100A, the waveform is right. But when the load is 120A, the waveform is about 115A. Why? I think it should be limited to current limit value 111A. Avishek: After the TPS2477x has gone through start-up it will no longer actively control the gate. Instead it will run the timer when the current is between the current limit and the fast trip threshold. Once the timer has expired the gate will be pulled down. If the current ever exceeds the fast trip threshold, the gate will be pulled down immediately. In other words, after asserting power good, TPS2477x acts as a circuit breaker and runs the fault timer during over current events, but doesn’t current limit. Hence, the waveform is correct. During the timer duration, load current is close to 120 A. There might be a little bit error in calculation of current value from the oscilloscope A/div scale, but ideally it should be 120 A.
In 10.2.2, the Transient load requirement is 130A for 250 ms. But the current limit is 111A according to formula (15), how can the power supply provide 130A？Avishek: Power supply can provide a transient overload of 130 A for 250 ms. Because, the overload current is less than the device fast trip threshold (150 A in this design example) and during this transient overload period of 250 ms the voltage at TFLT pin has not reached to 1.35 V. The value of CFLT (2.2 µF in this design example) has been calculated based on this transient overload requirement. Please refer to Sections 9.3.4 {Dual Timer (TFLT and TINR)} and 10.2.3.5 (Set Fault Timer) for more details. Please note that, if the duration of transient overload is more than 290 ms, the voltage at TFLT pin will reach to 1.35 V and the controller will be shutoff. You may refer to these simulation results.
130 A of Overload for 250 ms:
130 A of Overload for 310 ms:
