• State Not Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 63 subscribers
  • Views 230 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Original question:

UCC28880: UCC28880: 220VAC -> 5VDC (30mA) - UCC blows up now and then

UCC28880: UCC28880 issue

rachappa nandge
Prodigy 30 points
Part Number: UCC28880

hello sir/Madam,

In one of our project am using Texas UCC28880.

 

Simple Block diagram is shown below.

Problem:

 

1.      If we stop supplying input Voltage  250V  bus, 24V output voltage is decreasing Linearly. At certain point  of input voltage(24V) 3.3V off completely.

At same time Voltage at 24V bus is Decreasing  to 14V, because of no load at 24V bus is tying to build the voltage from 14V to some 15.6V, at this input voltage(15.6V) am getting 3.3V.

2.      Due to above issue , we have seen some reboot is at 3.3V bus,

We are using a regulators which will convert 24V to 3.3V and which will have wide input voltage range 4.5V to 42V.

 

Input Bus voltage 250V and we are converting to 24V by using UCC28880

 

Issue Description :

Input bus voltage goes to 32V, at same time 24V Vout is reducing to some 14V even 3.3V output Voltage is low at this voltage Hence there is no load on 24V bus voltage. Because of this 24V bus  is trying to increase slightly  higher voltage like 15.6V, Due to this voltage my 3.3V regulator is turned ON. Need your help to control the 24V Bus And UCC28880 don’t have any control pin to control the 24V bus.

 

Please find the Scope picture with channel description

 

CH1: Main Bus Voltage(250V)

CH2: 24V Main Bus Voltage

CH3:3V3 Main Bus Voltage

CH4: Vcap Voltage

 

  • 0
    TI__Mastermind 31130 points

    Hello Rachappa,

     

    Thank you for your interest in the UCC28880 off-line switcher.

     

    I think I understand the basic problem, however your diagram and scope shots do not appear in the post. Apparently the copy & paste method doesn’t work, if that is what you did.
    Please repost these images by attaching them as files, instead of copy & paste in-line.

     

    Without the diagrams, however, I am making some broad guess-work here:

    The 24V sags to 14V probably due to hitting current limit as the input voltage falls.
    Your 3.3V presents a constant power load and as input voltage falls, 24V Iout rises until it hits the UCC28880 I_limit. At the I-limit, then 24V falls.

    If the 3.3V regulator should work from 42V down to 4.5V input, then at 14V, it too must be hitting some limit of it’s own and turning off. Then with no load, the UCC28880 can boost its output up again from the residual input voltage until the 3.3V regulator restarts.

     

    You are correct, there is no on/off control pin on the UCC28880.

    To prevent 3.3V restart, either the 24V must be rapidly pulled to < 4V before falling to 14V, or the 3.3V regulator must be controlled on/off based on the 24V output voltage, with hysteresis.

    In other words, 3.3V is enabled when 24Vout > 20V and disabled when 24Vout < 16V, for example.

     

    The amount of additional circuitry needed to accomplish either of these possibilities depends on the actual circuit you have now (which I am unable to see).

     

    If neither of these ideas can work for you, please repost the images as files and maybe I can think of another idea based on the additional content.

     

    Regards,

    Ulrich

  • 0 in reply to Ulrich Goerke
    Prodigy 30 points

    Hello Ulrich,

    Thank you for the information,

    I think there is a gap to understand the issue, let me allow once again to explain.

    i am using UCC28880 with high side BUCK regulator, Vin(max) is 265V and output of buck is 24V, this 24V is input for another regulators which is generated 3V3 voltage and this IC is having EN pin to control 3V3, if i change EN pin voltage i am getting some other issue, please find attached file for the same.

    issue: when i gradually decrease the input voltage i have seen some reboot issue at 3V3 voltage this is because of 

    when Vin reaches 14V, 3V3 rail is off, and i have Seen in Vin 14V is increasing to 15V due to this 3V3 is booting up.

    so this issue is getting repeated till 4 to 5 cycle. Find same in the attached file with Scope picture  

    UCC28880 ISSUE.docx

  • 0 in reply to rachappa nandge
    TI__Mastermind 31130 points

    Hello Rachappa,

    Thank you for the additional information in the file. I believe I do understand the issue and my previous comment are still valid.

    If the 3.3V regulator is specified to operate with an input voltage between 4.5V and 42V, then something in the 3.3V regulator block is actively disables the 3.3V regulator when the block’s input falls to 14V, and enables it at 15.6V.

    My suggestion was to disable it at 16V and enable it at 20V. Based on the waveform, my advice is valid. Since the 24V bus falls to ~14V when main bus (250V) falls to ~32V, there is a 18V difference between input and output. When 24Vout loses the 3.3V-regualtor load, the UCC28880 tries to boost its output back up to 24V, but the input is still falling at a -3.5V/s rate. We also see that after the 3.3V stays off, 24Vout does not climb higher than ½ of its input (CH2 is 5V/div, CH1 is 10V/div) and they track down together.   This means the 24V regulator keeps at least a 15-V difference as the input keeps dropping.

    So it appears that when 3.3V stays off, 24Vout will not climb higher than ½*24Vin when 24Vin is < 32V.
    If 24Vin = ~32V when 24Vout = ~14V, then I think 24Vin will be ~34V when 24Vout = 16V. If the 3.3V regulator is actively disabled, and held disabled, when 24Vout falls to 16V, the unloaded 24Vout will only climb back up to about 17V. Then it will continue to track 24Vin down with a 15V difference.

    3.3V will not restart 4-5 times, because it cannot be enabled until 24Vout rise to 20V, and the scope trace shows that it will not do that.  The 4-V hysteresis insures that 3.3V won’t respond to a minor rise in 24Vout. 24Vout will rise above 20V when its input (24Vin = 250V bus) is repowered.
    The 16-V disable threshold is chosen to be above the natural 14V level, to ensure that the 3.3V shuts off due to the hysteresis control and not to the uncontrolled 24V output.

    Regards,
    Ulrich