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LM5176: LM5176: On the problem of large standby current when lm5176 is no load

Part Number: LM5176
Other Parts Discussed in Thread: CSD18534Q5A

ALL HI!

     Currently, LM5176 is used to build a BUCK-BOOST circuit, input 8V~36V output 12V/3A. The first version of the PCB debugging function is normal, input 24V input current 20mA when the output is no-load. The second version of the PCB uses exactly the same components as the first version, but the size of the PCB has been modified, and the layout is different from the first version. When the output is no-load, the input voltage is 24V and the input current is 40mA .The no-load current becomes larger. Similar problems occurred in the first version of the PCB, but the switching frequency was changed from 200KHZ to 400KHZ, and the output switching ripple was reduced and improved. But the second version of the PCB switching frequency is already 400KHZ. The output ripple is obviously reduced by increasing the output capacitance, but the no-load current is not reduced. At present, there is no good way to optimize as desired. Can the no-load current be optimized by modifying the parameters of COMP (see the figure below)? Or there are other ways to optimize this no-load current. thank you very much. 

  • Hi Sheng,

    Thank you for designing with the LM5176.  The no load power consumption basically go to (1) the MOSFET driver because it continues switching, and (2) the losses in the power circuit due to the circulating energy: your inductor current is ramping between a negative peak and positive peak, with averaging current to be zero but there are currents flow through the MOSFETs and inductor to cause conduction losses.  If you use the same components, but the layout is quite different, the power dissipations can be different.  Usually a smaller board with poorer thermal management would consume more due to increased conduction losses in the MOSFETs and inductor.   However, before we can review your circuit, we cannot draw a conclusion.

    Could you share your schematic, and show some key waveforms like SW1 and SW2 under the said operating condition?

    Thanks,

    Youhao Xi, Applications Engineering

  • thank you!Youhao Xi!

    I use inductors from different manufacturers (with the same inductance) to test, and the effect will be different. A good inductor will have an input current loss of 40mA, and a poor inductor will have an input current loss of 60mA. The desired waveform pictures will be provided later. At present, there is no idea to solve the problem. Can you provide a thinking direction?

  • Hello Sheng,

    As the device is permanently operating an input current of 40mA is expected. You could optimize the no load current by turning the device off when there is no load at the output.

    Best regards,
    Brigitte

  • Hello Brigitte!

    I measured the waveform of the sampling resistance of the first and second PCB versions (R9 in Figure 1 below). The resistance of the first PCB version does not have a high peak (as shown in Figure 2 below), but the resistance of the second PCB version has a high peak (as shown in Figure 3 below). I suspect that this peak causes the inductance to heat.

    The following spike is the waveform at no-load. Under the same load, the power consumption of the second version power supply is greater than that of the first version power supply. The components of the two are exactly the same, but the PCB is different. I'm confused about what causes the differentiation. I hope you can help me analyze it. thank you!

                                         Figure 1

                                           Figure 2

                                             Figure 3

  • Hello Sheng,

    The spikes are caused by parasitic effects at the moment when the switches turn on and off.

    If both boards have exactly the same external components, the difference in layout causes these spikes.

    I am confused, because above you mentioned that the inductance is different. Can you please clarify what exactly is the difference between the 2 boards?

    Please share the full schematic of both boards.

    Best regards,
    Brigitte

  • Hello Brigitte!

    At present, the latest circuit board and template use exactly the same components except PCB.

    I did a test today. The switch tube and synchronous tube of the buck circuit of the new circuit board were replaced with the field effect tube used in my template, and the circuit fault was eliminated. I'm confused because the names and specifications of the two field tubes are the same. It's just that the purchase time is different.

    Figure 1 below shows the driving waveform of the problem chip, yellow is the switch tube driving, and blue is the synchronization tube driving. Fig. 2 is a normal chip driving waveform. When the switch tube is closed and the synchronous tube is on, the difference is obvious. The parameters of the two oscilloscopes are consistent.

    Fig. 3 is the screen print of the problem chip, and Fig. 4 is the screen print of the normal chip.

    Fig. 5 is the waveform of the inductance output terminal of the problem circuit, and Fig. 6 is the waveform of the inductance output terminal of the normal circuit. The faulty circuit will have large switching spikes. Light load or heavy load have large switching spikes. This will increase the power consumption of the inductor.

    I modified the field tube of four circuit boards. After modification, all faults can be eliminated.What causes the difference of field tubes. Can you analyze it for me? thank you!

                                               Figure 1

                                              Figure 2

                                Figure 3

           Figure 4

                                              Figure 5

                                          Figure 6

  • Hello Sheng,

    Before going into detailed analysis of the components, could you please check if the problem is consistent and take a now working board and solder back the transistors you suspect causing issues? Will the issue come back?

    Best regards,
    Brigitte

  • Hello Brigitte!

    yes. I began to suspect that the SMT problem caused the exception. I did patch weld the faulty field tube again, and the fault still exists.

    As shown in the figure below, the field pipe on the left is normal, and the origin is the Philippines. The field pipe on the right is abnormal, and the origin is Malaysia. Is it the problem caused by the difference of factory production process? thank you!

  • Hi Sheng,

    Thanks for the inquiry. The switching spikes on the abnormal device is higher and will result in higher losses. I'm checking into the device markings and will get back to you as soon as I have more information.

    Best Regards,

    John Wallace

    TI FET Applications

  • Hi  John Wallace!

    Is the chip on the right of the above picture a very old version, and the latest version on the left. Has the chip been upgraded? I won't buy fake goods.

  • Sheng,

    I believe the device on the right is a counterfeit device. The device on the left is a TI device.

    The device on the right does have a legitimate lot trace code for the CSD18534Q5A but the package is incorrect.

    We have only ever manufactured the CSD18534Q5A with the package style on the left in the photo you sent.

    I will send you a private email with some follow up questions.

    I will close this thread out.

    Many Thanks

    Chris Bull

    TI FET Marketing

  • thank you! Chris Bull!