LM22678 Unstable output

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP












3












$begingroup$


I am building a voltage regulator with the LM22678-5.



I used TI WE-Bench designer to get the component values needed for the regulator:
regulator values



The regulator outputs 5V when there is no load however it drops to around 2-3V when a load is connected. Ceramic Capacitors were used for Cin, Cinx and Cbst. Cout is an electrolytic capacitor (35SVPF120M). Below is the PCB layout I used.



PCB Layout



Note: Although the PCB has the enable pin tied to VIn I just don't connect that pin to the IC and leave it floating as per the datasheet.



This is the output on my DSO:
Voltage Output



Does anyone know what I am doing wrong?










share|improve this question









$endgroup$











  • $begingroup$
    What is your input voltage doing while the output is doing the fandango?
    $endgroup$
    – TimWescott
    Mar 4 at 3:24










  • $begingroup$
    The input voltage does the fandango too
    $endgroup$
    – jdm
    Mar 4 at 3:58










  • $begingroup$
    serious layout violation with Vfb, measurement probe errors with ground lead... and poor source impedance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:00











  • $begingroup$
    Try using ac coupled to coax with 50 Ohm terminator on DSO . Your probe noise may be > 10MHz and show dV/dt next time If V=100mV=LdI/dt and dI is 1A and dt is 50ns what is L?.. Changes are good it is just measurement error from probe ground resonance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:02











  • $begingroup$
    What are you using for an input supply, and how long are the leads? While the switcher itself could be oscillating like mad, if you've got it hanging off of a high-impedance (or inductive) supply that would also impact stability, particularly without any bulk capacitance at the input.
    $endgroup$
    – TimWescott
    Mar 4 at 4:45















3












$begingroup$


I am building a voltage regulator with the LM22678-5.



I used TI WE-Bench designer to get the component values needed for the regulator:
regulator values



The regulator outputs 5V when there is no load however it drops to around 2-3V when a load is connected. Ceramic Capacitors were used for Cin, Cinx and Cbst. Cout is an electrolytic capacitor (35SVPF120M). Below is the PCB layout I used.



PCB Layout



Note: Although the PCB has the enable pin tied to VIn I just don't connect that pin to the IC and leave it floating as per the datasheet.



This is the output on my DSO:
Voltage Output



Does anyone know what I am doing wrong?










share|improve this question









$endgroup$











  • $begingroup$
    What is your input voltage doing while the output is doing the fandango?
    $endgroup$
    – TimWescott
    Mar 4 at 3:24










  • $begingroup$
    The input voltage does the fandango too
    $endgroup$
    – jdm
    Mar 4 at 3:58










  • $begingroup$
    serious layout violation with Vfb, measurement probe errors with ground lead... and poor source impedance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:00











  • $begingroup$
    Try using ac coupled to coax with 50 Ohm terminator on DSO . Your probe noise may be > 10MHz and show dV/dt next time If V=100mV=LdI/dt and dI is 1A and dt is 50ns what is L?.. Changes are good it is just measurement error from probe ground resonance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:02











  • $begingroup$
    What are you using for an input supply, and how long are the leads? While the switcher itself could be oscillating like mad, if you've got it hanging off of a high-impedance (or inductive) supply that would also impact stability, particularly without any bulk capacitance at the input.
    $endgroup$
    – TimWescott
    Mar 4 at 4:45













3












3








3





$begingroup$


I am building a voltage regulator with the LM22678-5.



I used TI WE-Bench designer to get the component values needed for the regulator:
regulator values



The regulator outputs 5V when there is no load however it drops to around 2-3V when a load is connected. Ceramic Capacitors were used for Cin, Cinx and Cbst. Cout is an electrolytic capacitor (35SVPF120M). Below is the PCB layout I used.



PCB Layout



Note: Although the PCB has the enable pin tied to VIn I just don't connect that pin to the IC and leave it floating as per the datasheet.



This is the output on my DSO:
Voltage Output



Does anyone know what I am doing wrong?










share|improve this question









$endgroup$




I am building a voltage regulator with the LM22678-5.



I used TI WE-Bench designer to get the component values needed for the regulator:
regulator values



The regulator outputs 5V when there is no load however it drops to around 2-3V when a load is connected. Ceramic Capacitors were used for Cin, Cinx and Cbst. Cout is an electrolytic capacitor (35SVPF120M). Below is the PCB layout I used.



PCB Layout



Note: Although the PCB has the enable pin tied to VIn I just don't connect that pin to the IC and leave it floating as per the datasheet.



This is the output on my DSO:
Voltage Output



Does anyone know what I am doing wrong?







voltage voltage-regulator switch-mode-power-supply buck






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked Mar 4 at 3:19









jdmjdm

312




312











  • $begingroup$
    What is your input voltage doing while the output is doing the fandango?
    $endgroup$
    – TimWescott
    Mar 4 at 3:24










  • $begingroup$
    The input voltage does the fandango too
    $endgroup$
    – jdm
    Mar 4 at 3:58










  • $begingroup$
    serious layout violation with Vfb, measurement probe errors with ground lead... and poor source impedance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:00











  • $begingroup$
    Try using ac coupled to coax with 50 Ohm terminator on DSO . Your probe noise may be > 10MHz and show dV/dt next time If V=100mV=LdI/dt and dI is 1A and dt is 50ns what is L?.. Changes are good it is just measurement error from probe ground resonance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:02











  • $begingroup$
    What are you using for an input supply, and how long are the leads? While the switcher itself could be oscillating like mad, if you've got it hanging off of a high-impedance (or inductive) supply that would also impact stability, particularly without any bulk capacitance at the input.
    $endgroup$
    – TimWescott
    Mar 4 at 4:45
















  • $begingroup$
    What is your input voltage doing while the output is doing the fandango?
    $endgroup$
    – TimWescott
    Mar 4 at 3:24










  • $begingroup$
    The input voltage does the fandango too
    $endgroup$
    – jdm
    Mar 4 at 3:58










  • $begingroup$
    serious layout violation with Vfb, measurement probe errors with ground lead... and poor source impedance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:00











  • $begingroup$
    Try using ac coupled to coax with 50 Ohm terminator on DSO . Your probe noise may be > 10MHz and show dV/dt next time If V=100mV=LdI/dt and dI is 1A and dt is 50ns what is L?.. Changes are good it is just measurement error from probe ground resonance
    $endgroup$
    – Sunnyskyguy EE75
    Mar 4 at 4:02











  • $begingroup$
    What are you using for an input supply, and how long are the leads? While the switcher itself could be oscillating like mad, if you've got it hanging off of a high-impedance (or inductive) supply that would also impact stability, particularly without any bulk capacitance at the input.
    $endgroup$
    – TimWescott
    Mar 4 at 4:45















$begingroup$
What is your input voltage doing while the output is doing the fandango?
$endgroup$
– TimWescott
Mar 4 at 3:24




$begingroup$
What is your input voltage doing while the output is doing the fandango?
$endgroup$
– TimWescott
Mar 4 at 3:24












$begingroup$
The input voltage does the fandango too
$endgroup$
– jdm
Mar 4 at 3:58




$begingroup$
The input voltage does the fandango too
$endgroup$
– jdm
Mar 4 at 3:58












$begingroup$
serious layout violation with Vfb, measurement probe errors with ground lead... and poor source impedance
$endgroup$
– Sunnyskyguy EE75
Mar 4 at 4:00





$begingroup$
serious layout violation with Vfb, measurement probe errors with ground lead... and poor source impedance
$endgroup$
– Sunnyskyguy EE75
Mar 4 at 4:00













$begingroup$
Try using ac coupled to coax with 50 Ohm terminator on DSO . Your probe noise may be > 10MHz and show dV/dt next time If V=100mV=LdI/dt and dI is 1A and dt is 50ns what is L?.. Changes are good it is just measurement error from probe ground resonance
$endgroup$
– Sunnyskyguy EE75
Mar 4 at 4:02





$begingroup$
Try using ac coupled to coax with 50 Ohm terminator on DSO . Your probe noise may be > 10MHz and show dV/dt next time If V=100mV=LdI/dt and dI is 1A and dt is 50ns what is L?.. Changes are good it is just measurement error from probe ground resonance
$endgroup$
– Sunnyskyguy EE75
Mar 4 at 4:02













$begingroup$
What are you using for an input supply, and how long are the leads? While the switcher itself could be oscillating like mad, if you've got it hanging off of a high-impedance (or inductive) supply that would also impact stability, particularly without any bulk capacitance at the input.
$endgroup$
– TimWescott
Mar 4 at 4:45




$begingroup$
What are you using for an input supply, and how long are the leads? While the switcher itself could be oscillating like mad, if you've got it hanging off of a high-impedance (or inductive) supply that would also impact stability, particularly without any bulk capacitance at the input.
$endgroup$
– TimWescott
Mar 4 at 4:45










2 Answers
2






active

oldest

votes


















3












$begingroup$

"what I am doing wrong"? - it is likely a sum of small deviations from recommendations of manufacturer. You are not following the recommended layout and details of their component suggestions. You are using non-SMD D1 (inductive leads), the output cap looks too small, input caps are recommended to be at least 2 X 6.8 uF ceramic (webbench comes out with 3 x 22 uF caps, you have only one), and you didn't mention anything about your inductor (pads are too small for a proper inductor).



You need to examine carefully the fundamental current-flow diagram presented in the LM22678 datasheet:



enter image description here



As the datasheet says:




Board layout is critical for the proper operation of switching power
supplies The most important layout rule is to keep the ac current
loops as small as possible




Please trace the current loops as they develop on your PCB, you will find that they are far from optimal.



Addition: here is the WB suggested layout:



enter image description here



While the BOM lists DO-201 package for diode, the layout shows SMD (funny, pictured in opposite direction). Also, there is no need for massive ground trace coming around the inductor, the capacitance of this fast switching node must be minimized. You also should try to characterise your design not just for "no-load" and "full-load", but try to load it gradually to see, where it starts barfing.



Best thing is to get their reference design, and copy exactly everything.






share|improve this answer











$endgroup$












  • $begingroup$
    The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
    $endgroup$
    – jdm
    Mar 4 at 4:04










  • $begingroup$
    Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
    $endgroup$
    – jdm
    Mar 4 at 4:10










  • $begingroup$
    @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
    $endgroup$
    – Ale..chenski
    Mar 4 at 4:10










  • $begingroup$
    @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
    $endgroup$
    – Ale..chenski
    Mar 4 at 4:12










  • $begingroup$
    my target current is 5A
    $endgroup$
    – jdm
    Mar 4 at 4:15


















1












$begingroup$

While waiting for you to make better measurements to measure spike risetime and 50% pulse width (PW50) and spike resonant freq. ....I suspect your biggest issues are;



a) Wrong L. Shielded Inductor capacitance



This part may have an SRF in the range of the 5th harmonic of 500kHz f_sw and passthru switch spikes from shield shunt capacitance which is in series with output.



Instead, a non-shielded part SRR1208-4R5ML with 30MHz SRF would be better. or next size up 6.5 uH



b) Scope probe measurements



If not done with calibrated 10:1 probe which gives a flatline on ground test pin. then the probe ground loop antenna effect is giving false measurements. But since the design cannot handle a load, we do not know the threshold of load regulation error.



c) test methods



-Measure with 50 Ohm terminated DSO ( AC coupled !!) to ground, for noise-free results then input and output for actual noise with hi-res spurious measurements.

-Measure slow ramped active load ( NPN with triangle wave current) for the cutoff threshold.

-Measure with various step loads ( using NPN active load) to get load regulation error.

-Measure input voltage regulation error ( ensure input supply is adequate)

-Compare with WebBench test results for startup, steady state and BODE plots.



enter image description here






share|improve this answer









$endgroup$













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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    3












    $begingroup$

    "what I am doing wrong"? - it is likely a sum of small deviations from recommendations of manufacturer. You are not following the recommended layout and details of their component suggestions. You are using non-SMD D1 (inductive leads), the output cap looks too small, input caps are recommended to be at least 2 X 6.8 uF ceramic (webbench comes out with 3 x 22 uF caps, you have only one), and you didn't mention anything about your inductor (pads are too small for a proper inductor).



    You need to examine carefully the fundamental current-flow diagram presented in the LM22678 datasheet:



    enter image description here



    As the datasheet says:




    Board layout is critical for the proper operation of switching power
    supplies The most important layout rule is to keep the ac current
    loops as small as possible




    Please trace the current loops as they develop on your PCB, you will find that they are far from optimal.



    Addition: here is the WB suggested layout:



    enter image description here



    While the BOM lists DO-201 package for diode, the layout shows SMD (funny, pictured in opposite direction). Also, there is no need for massive ground trace coming around the inductor, the capacitance of this fast switching node must be minimized. You also should try to characterise your design not just for "no-load" and "full-load", but try to load it gradually to see, where it starts barfing.



    Best thing is to get their reference design, and copy exactly everything.






    share|improve this answer











    $endgroup$












    • $begingroup$
      The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
      $endgroup$
      – jdm
      Mar 4 at 4:04










    • $begingroup$
      Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
      $endgroup$
      – jdm
      Mar 4 at 4:10










    • $begingroup$
      @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:10










    • $begingroup$
      @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:12










    • $begingroup$
      my target current is 5A
      $endgroup$
      – jdm
      Mar 4 at 4:15















    3












    $begingroup$

    "what I am doing wrong"? - it is likely a sum of small deviations from recommendations of manufacturer. You are not following the recommended layout and details of their component suggestions. You are using non-SMD D1 (inductive leads), the output cap looks too small, input caps are recommended to be at least 2 X 6.8 uF ceramic (webbench comes out with 3 x 22 uF caps, you have only one), and you didn't mention anything about your inductor (pads are too small for a proper inductor).



    You need to examine carefully the fundamental current-flow diagram presented in the LM22678 datasheet:



    enter image description here



    As the datasheet says:




    Board layout is critical for the proper operation of switching power
    supplies The most important layout rule is to keep the ac current
    loops as small as possible




    Please trace the current loops as they develop on your PCB, you will find that they are far from optimal.



    Addition: here is the WB suggested layout:



    enter image description here



    While the BOM lists DO-201 package for diode, the layout shows SMD (funny, pictured in opposite direction). Also, there is no need for massive ground trace coming around the inductor, the capacitance of this fast switching node must be minimized. You also should try to characterise your design not just for "no-load" and "full-load", but try to load it gradually to see, where it starts barfing.



    Best thing is to get their reference design, and copy exactly everything.






    share|improve this answer











    $endgroup$












    • $begingroup$
      The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
      $endgroup$
      – jdm
      Mar 4 at 4:04










    • $begingroup$
      Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
      $endgroup$
      – jdm
      Mar 4 at 4:10










    • $begingroup$
      @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:10










    • $begingroup$
      @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:12










    • $begingroup$
      my target current is 5A
      $endgroup$
      – jdm
      Mar 4 at 4:15













    3












    3








    3





    $begingroup$

    "what I am doing wrong"? - it is likely a sum of small deviations from recommendations of manufacturer. You are not following the recommended layout and details of their component suggestions. You are using non-SMD D1 (inductive leads), the output cap looks too small, input caps are recommended to be at least 2 X 6.8 uF ceramic (webbench comes out with 3 x 22 uF caps, you have only one), and you didn't mention anything about your inductor (pads are too small for a proper inductor).



    You need to examine carefully the fundamental current-flow diagram presented in the LM22678 datasheet:



    enter image description here



    As the datasheet says:




    Board layout is critical for the proper operation of switching power
    supplies The most important layout rule is to keep the ac current
    loops as small as possible




    Please trace the current loops as they develop on your PCB, you will find that they are far from optimal.



    Addition: here is the WB suggested layout:



    enter image description here



    While the BOM lists DO-201 package for diode, the layout shows SMD (funny, pictured in opposite direction). Also, there is no need for massive ground trace coming around the inductor, the capacitance of this fast switching node must be minimized. You also should try to characterise your design not just for "no-load" and "full-load", but try to load it gradually to see, where it starts barfing.



    Best thing is to get their reference design, and copy exactly everything.






    share|improve this answer











    $endgroup$



    "what I am doing wrong"? - it is likely a sum of small deviations from recommendations of manufacturer. You are not following the recommended layout and details of their component suggestions. You are using non-SMD D1 (inductive leads), the output cap looks too small, input caps are recommended to be at least 2 X 6.8 uF ceramic (webbench comes out with 3 x 22 uF caps, you have only one), and you didn't mention anything about your inductor (pads are too small for a proper inductor).



    You need to examine carefully the fundamental current-flow diagram presented in the LM22678 datasheet:



    enter image description here



    As the datasheet says:




    Board layout is critical for the proper operation of switching power
    supplies The most important layout rule is to keep the ac current
    loops as small as possible




    Please trace the current loops as they develop on your PCB, you will find that they are far from optimal.



    Addition: here is the WB suggested layout:



    enter image description here



    While the BOM lists DO-201 package for diode, the layout shows SMD (funny, pictured in opposite direction). Also, there is no need for massive ground trace coming around the inductor, the capacitance of this fast switching node must be minimized. You also should try to characterise your design not just for "no-load" and "full-load", but try to load it gradually to see, where it starts barfing.



    Best thing is to get their reference design, and copy exactly everything.







    share|improve this answer














    share|improve this answer



    share|improve this answer








    edited Mar 4 at 4:31

























    answered Mar 4 at 3:55









    Ale..chenskiAle..chenski

    28.9k11866




    28.9k11866











    • $begingroup$
      The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
      $endgroup$
      – jdm
      Mar 4 at 4:04










    • $begingroup$
      Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
      $endgroup$
      – jdm
      Mar 4 at 4:10










    • $begingroup$
      @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:10










    • $begingroup$
      @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:12










    • $begingroup$
      my target current is 5A
      $endgroup$
      – jdm
      Mar 4 at 4:15
















    • $begingroup$
      The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
      $endgroup$
      – jdm
      Mar 4 at 4:04










    • $begingroup$
      Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
      $endgroup$
      – jdm
      Mar 4 at 4:10










    • $begingroup$
      @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:10










    • $begingroup$
      @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
      $endgroup$
      – Ale..chenski
      Mar 4 at 4:12










    • $begingroup$
      my target current is 5A
      $endgroup$
      – jdm
      Mar 4 at 4:15















    $begingroup$
    The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
    $endgroup$
    – jdm
    Mar 4 at 4:04




    $begingroup$
    The inductor is the Vishay IHLP4040DZER4R7M01. Can you explain what is wrong with this inductor?
    $endgroup$
    – jdm
    Mar 4 at 4:04












    $begingroup$
    Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
    $endgroup$
    – jdm
    Mar 4 at 4:10




    $begingroup$
    Also webench does not tell me 3x22uF it lists the quantity as 1. And the Through hole diode is the SB10150 which was the part shown on webench
    $endgroup$
    – jdm
    Mar 4 at 4:10












    $begingroup$
    @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
    $endgroup$
    – Ale..chenski
    Mar 4 at 4:10




    $begingroup$
    @jos, the inductor might be OK, but the diode (1.05V) doesn't look like a good Schottky. As I said, a sum of details...
    $endgroup$
    – Ale..chenski
    Mar 4 at 4:10












    $begingroup$
    @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
    $endgroup$
    – Ale..chenski
    Mar 4 at 4:12




    $begingroup$
    @jos, my WB shows "Qty.3" on schematics. And diode is B340A. What is your target current?
    $endgroup$
    – Ale..chenski
    Mar 4 at 4:12












    $begingroup$
    my target current is 5A
    $endgroup$
    – jdm
    Mar 4 at 4:15




    $begingroup$
    my target current is 5A
    $endgroup$
    – jdm
    Mar 4 at 4:15













    1












    $begingroup$

    While waiting for you to make better measurements to measure spike risetime and 50% pulse width (PW50) and spike resonant freq. ....I suspect your biggest issues are;



    a) Wrong L. Shielded Inductor capacitance



    This part may have an SRF in the range of the 5th harmonic of 500kHz f_sw and passthru switch spikes from shield shunt capacitance which is in series with output.



    Instead, a non-shielded part SRR1208-4R5ML with 30MHz SRF would be better. or next size up 6.5 uH



    b) Scope probe measurements



    If not done with calibrated 10:1 probe which gives a flatline on ground test pin. then the probe ground loop antenna effect is giving false measurements. But since the design cannot handle a load, we do not know the threshold of load regulation error.



    c) test methods



    -Measure with 50 Ohm terminated DSO ( AC coupled !!) to ground, for noise-free results then input and output for actual noise with hi-res spurious measurements.

    -Measure slow ramped active load ( NPN with triangle wave current) for the cutoff threshold.

    -Measure with various step loads ( using NPN active load) to get load regulation error.

    -Measure input voltage regulation error ( ensure input supply is adequate)

    -Compare with WebBench test results for startup, steady state and BODE plots.



    enter image description here






    share|improve this answer









    $endgroup$

















      1












      $begingroup$

      While waiting for you to make better measurements to measure spike risetime and 50% pulse width (PW50) and spike resonant freq. ....I suspect your biggest issues are;



      a) Wrong L. Shielded Inductor capacitance



      This part may have an SRF in the range of the 5th harmonic of 500kHz f_sw and passthru switch spikes from shield shunt capacitance which is in series with output.



      Instead, a non-shielded part SRR1208-4R5ML with 30MHz SRF would be better. or next size up 6.5 uH



      b) Scope probe measurements



      If not done with calibrated 10:1 probe which gives a flatline on ground test pin. then the probe ground loop antenna effect is giving false measurements. But since the design cannot handle a load, we do not know the threshold of load regulation error.



      c) test methods



      -Measure with 50 Ohm terminated DSO ( AC coupled !!) to ground, for noise-free results then input and output for actual noise with hi-res spurious measurements.

      -Measure slow ramped active load ( NPN with triangle wave current) for the cutoff threshold.

      -Measure with various step loads ( using NPN active load) to get load regulation error.

      -Measure input voltage regulation error ( ensure input supply is adequate)

      -Compare with WebBench test results for startup, steady state and BODE plots.



      enter image description here






      share|improve this answer









      $endgroup$















        1












        1








        1





        $begingroup$

        While waiting for you to make better measurements to measure spike risetime and 50% pulse width (PW50) and spike resonant freq. ....I suspect your biggest issues are;



        a) Wrong L. Shielded Inductor capacitance



        This part may have an SRF in the range of the 5th harmonic of 500kHz f_sw and passthru switch spikes from shield shunt capacitance which is in series with output.



        Instead, a non-shielded part SRR1208-4R5ML with 30MHz SRF would be better. or next size up 6.5 uH



        b) Scope probe measurements



        If not done with calibrated 10:1 probe which gives a flatline on ground test pin. then the probe ground loop antenna effect is giving false measurements. But since the design cannot handle a load, we do not know the threshold of load regulation error.



        c) test methods



        -Measure with 50 Ohm terminated DSO ( AC coupled !!) to ground, for noise-free results then input and output for actual noise with hi-res spurious measurements.

        -Measure slow ramped active load ( NPN with triangle wave current) for the cutoff threshold.

        -Measure with various step loads ( using NPN active load) to get load regulation error.

        -Measure input voltage regulation error ( ensure input supply is adequate)

        -Compare with WebBench test results for startup, steady state and BODE plots.



        enter image description here






        share|improve this answer









        $endgroup$



        While waiting for you to make better measurements to measure spike risetime and 50% pulse width (PW50) and spike resonant freq. ....I suspect your biggest issues are;



        a) Wrong L. Shielded Inductor capacitance



        This part may have an SRF in the range of the 5th harmonic of 500kHz f_sw and passthru switch spikes from shield shunt capacitance which is in series with output.



        Instead, a non-shielded part SRR1208-4R5ML with 30MHz SRF would be better. or next size up 6.5 uH



        b) Scope probe measurements



        If not done with calibrated 10:1 probe which gives a flatline on ground test pin. then the probe ground loop antenna effect is giving false measurements. But since the design cannot handle a load, we do not know the threshold of load regulation error.



        c) test methods



        -Measure with 50 Ohm terminated DSO ( AC coupled !!) to ground, for noise-free results then input and output for actual noise with hi-res spurious measurements.

        -Measure slow ramped active load ( NPN with triangle wave current) for the cutoff threshold.

        -Measure with various step loads ( using NPN active load) to get load regulation error.

        -Measure input voltage regulation error ( ensure input supply is adequate)

        -Compare with WebBench test results for startup, steady state and BODE plots.



        enter image description here







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered Mar 4 at 5:41









        Sunnyskyguy EE75Sunnyskyguy EE75

        70k225101




        70k225101



























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