TPS259827LEVM: When the output terminal is connected to a load, it will fail to start.

Hi!

guanming ji said:

When we tested, we used the public version for testing, and the backend was connected to the Electronic Load.

Do you mean public version of your end equipment. Can you tell me you end application?

guanming ji said:

Electronic Load is used to simulate the inrush part, so before the TPS259827LEVM is powered on, the Electronic Load will be kept on. At this time, after the TPS259827 is powered on, it will fail to start.

CdVdt is the only part that can control the inrush slew rate according to specifications. Even if I adjust it to a high value or open, I still can't solve the problem

increasing dvdt should help. Making it open will make it worse. If the inrush current is high, we want to make the inrush slow so that thermal shutdown doesn't happen. We have a design calculator in our website which can help in finding the dvdt value. SLVC779 Calculation tool | TI.com

guanming ji said:

If you change the order of opening, turn on Power first and then Electronic Load, the working status will be normal.

Yes, in case you have load connected before power on the device, you will have inrush current. If the inrush is large, device will go into current limiting and can go into thermal shutdown due to excess power dissipation. 

guanming ji said:

.Electronic Load CC Mode, current 9A.

Please use CR mode for testing. When you use CC mode for over current testing, it may give you wrong results. During current limit mode, our device is a current source and you are connecting another constant current source at different current level.

----------------------------------------------------------------

guanming ji said:

When observing the waveform, assuming I set 9A, the protection will trip immediately after the current rises to 9A.

Immediate trip is due to CC load or due to high Power dissipation.

guanming ji said:

Therefore, my preliminary analysis is that this architecture is a purely capacitive circuit. It does not start the dvdt at the beginning, but enters the current limit.Then the device is triggered going into thermal shutdown during startup.

Can you tell me operating conditions. This architecture should handle resistive load as long as it is not taking very high current.

guanming ji said:

So I would like to ask if it can be solved by adjusting the resistor at the output end? If it can be solved, what role does RlStart play in this system? What value does RlStart usually take? Will there be any impact if the size is too large?

Very small load resistance will take very high current. That can cause issue. Again share some details like schematic and operating conditions for my inputs on how to correct this. Also without these inputs, I don't fully understand what might be going wrong.

guanming ji said:

Also, I would like to ask, thermal shutdown is the protection component of the IC. The protection should be triggered regardless of output and input. The output can be adjusted using Cdvdt. What about input?

TSD only depends on the power dissipation at the IC. This device doesn't provide reverse current blocking so in general output can be lower than input and that will cause some power dissipation and which result in TSD. Yes output startup voltage slew rate can be controlled using dvdt. No input cannot be controlled. Input depends on the voltage source. Device will provide undervoltage protection and overvoltage protection.

If there is any issue in sharing schematic and operating conditions, you can direct message also in E2E forum.

Best Regards,
Arush

Hello, thank you for your help.
In subsequent tests, I adjusted the electronic load mode to CR mode.

But still encountered some problems.

case 1:Vin 12v, electronic load mode: CR, value: 1.2R, Cdvdt: Open.
In this case, everything works fine, but if I adjust R again, i.e. when the current increases, the protection is triggered.

case 2:
Same test adjustments as step 1. Later, I connected a 10K resistor in parallel to the output end, and it started normally.

--

Below is my question

1. After adjusting the output resistance, the startup will not reach the upper limit of protection in this case. So what does the resistor here do? It seems that this resistor is necessary.

2. If we want to simulate the surge part, how should we set the environment? What modifications do we need to make to TPS259827LEVM?

3.
in this case 2, if the electronic load is switched to CC mode and then the device is turned on, the startup will still fail, and the waveform is as shown in the response below.
So can I think of CC mode as a large capacitor? When the capacitance is large and there is no RL, is it easy to trigger the thermal protection?
But I also adjusted Rlstart, but it didn't achieve the effect I wanted. What could be the problem? Has the value of Cout reached the upper limit? The maximum value of Cout is in SPEC. Is there any place I can refer to?

4.Is there any Inrush test data for TPS25982 that I can refer to?

--

Thanks again for your help and have a nice day.