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Inductor current delay in BOOSTXL-BUCKCONV eval kit

Part Number: BOOSTXL-BUCKCONV
Other Parts Discussed in Thread: OPA353

I have this BOOSTXL-BUCKCONV with Lanchpad F28379D as my setup.

HW setup:
- 200KHz PWM frequency
- ILFB (Inductor current feedback, unfiltered) pin for sensing current - : H2[9]
- VoutFB  (Output voltage feedback) pin no: H2[7]
- Using constant 20% PWM Duty cycle with updown counter mode and triggering ADC start of conversion at "Mid of PWM" (Counter is equal to period)
- ADC End of Conversion is used to trigger controller algorithm
- Only static load (RL = 7.5 ohm, active load is off)

Issues: At 20% PWM constant duty
With only Single MOSFET: (Only switch 1, Switch 2 is turned off)

- We expect inductor current (triangular wave) to be in sync with PWM ON time. But we see 616ns delay (between PWM ON transition and inductor current starting to rise)

With both MOSFET is used:
- Above delay is 856ns delay (between PWM ON transition and inductor current starting to rise)

The above delay reduces as we go on increasing PWM duty cycles. For ex: For 30% duty, delay is around 312ns with single MOSFET.


Questions:

  1. Why we see this significant delay. From the datasheet PWM gate driver and MOSFET can contribute around 82ns.
  2. Why this delay varies with respect to PWM ON time?
  3. Is that ok to use single MOSFET or do we need to use both MOSFET always (2nd is complimentary)
  4. Also, we see this delay varies with active load enabled and disabled. With active load (effective RL = 1.5789 ohm) enabled, above delay reduces.
  5. Why inductor current is not increasing linearly, this is important to understand to sense the correct average current.
  6. Also, as we see from image PWM ON time is 1us and we expect inductor current to rise for 1us and this is not happening in HW. Inductor current rises for a smaller time compared to PWM ON time.

These questions are important for us, since we try to rely on reading the average current using only with PWM-ADC synchronization and not with any CMPSS (analog sensing).

  • Arun Kumar M S said:
    Why we see this significant delay. From the datasheet PWM gate driver and MOSFET can contribute around 82ns.

    I recommend capturing Vout (or VoutFB-1) when observing ILFB.  The current flow across the inductor is also dependent on the voltage potential across the inductor, which is an unknown in your observations.

    Arun Kumar M S said:
    Why this delay varies with respect to PWM ON time?

    The PWM duty-cycle has an influence on the voltage potential across the inductor.

    Arun Kumar M S said:
    Is that ok to use single MOSFET or do we need to use both MOSFET always (2nd is complimentary)

    A controlled path to GND is customary for buck converters.  A diode has traditionally been used instead of a MOSFET.

    Arun Kumar M S said:
    Also, we see this delay varies with active load enabled and disabled. With active load (effective RL = 1.5789 ohm) enabled, above delay reduces.

    The RL load an influence on the voltage potential across the inductor.

    Arun Kumar M S said:
    Why inductor current is not increasing linearly, this is important to understand to sense the correct average current.

    Also, as we see from image PWM ON time is 1us and we expect inductor current to rise for 1us and this is not happening in HW. Inductor current rises for a smaller time compared to PWM ON time.

    I suspect that the voltage may be a factor.

    Arun Kumar M S said:
    These questions are important for us, since we try to rely on reading the average current using only with PWM-ADC synchronization and not with any CMPSS (analog sensing).

    My personal approach would be to customize the RC filter on the ILFB_AVG path to remove the current ripples.

  •  1, 2 and 4 here is the reply:

    I am attaching the image of ILFB and Vout for open loop fixed 20% and 30% duty when both MOSFETS are used.

    Vout for 20% - 1.76V

    Vout for 30% - 2.60 V

     

    I couldn’t get voltage drop across inductor since am using DSO. But I guess Vinductor = Vin -Vout, since other drops across current sensor resistor is tiny and I neglect that.

    Q1: Why we see delay in inductor current rise time?

    Q2: Why delay varies for 20% and 30% duty? Could you elaborate more why voltage drop across inductor affects the delay between PWM ON and rise in the inductor current?

    Q3: We expect for On period voltage drop across inductor is also for 1us.Current across the inductor also should increase for 1 us, but from the figure it doesn’t. Why is this behavior?

     

    3: I will use both MOSFET

    5: I am using unfiltered ILFB instantaneous current, not the ILFB_AVG, since ILFB_AVG is heavily filtered and delay is even more.

  • Arun Kumar M S said:
    I am attaching the image of ILFB and Vout for open loop fixed 20% and 30% duty when both MOSFETS are used.

    When observing Vout vs ILFB, I recommend AC-coupling the Vout signal and zooming in to focus on only the AC ripple.

    Arun Kumar M S said:
    I couldn’t get voltage drop across inductor since am using DSO. But I guess Vinductor = Vin -Vout, since other drops across current sensor resistor is tiny and I neglect that.

    You can probe the two terminals of the inductor separately to see what voltages are present on the component relative to the PWM control signals. On my setup, there is negligible delay between PWM-HI asserting high vs 9V0 appearing on the inductor terminal.

    Arun Kumar M S said:
    Q1: Why we see delay in inductor current rise time?

    Note that the OPA353 used for the current sense amplifier is unipolar on this board -- meaning that it can only represent positive current flow from 9V0 to Vout.  So negative inductor currents will appear as zero on ILFB.

    Your described "delay" time is most likely related to the converter operating in discontinuous mode, where the inductor current can reverse to a negative flow during the Off time.  During the On duty time, the inductor current will transition from negative to positive as expected, but the negative portion of the transition will not be observable using the ILFB sense amplifier.

    Arun Kumar M S said:
    Q2: Why delay varies for 20% and 30% duty? Could you elaborate more why voltage drop across inductor affects the delay between PWM ON and rise in the inductor current?

    Different duty cycles will charge and discharge different amounts of energy for different amounts of time.  I assume that you would be able to reproduce your observations in SPICE simulation if you have the desire to do so.

    Arun Kumar M S said:
    Q3: We expect for On period voltage drop across inductor is also for 1us.Current across the inductor also should increase for 1 us, but from the figure it doesn’t. Why is this behavior?

    The most likely explanation is that the inductor current is trending positive for the entire On time, but you are not able to see the negative currents in discontinuous mode with ILFB.

  • Thanks for the detail help.

    When observing Vout vs ILFB, I recommend AC-coupling the Vout signal and zooming in to focus on only the AC ripple.

    >> We can see negligible ripples on Vout while doing AC coupling at 20% duty. Any suggestions here?

    You can probe the two terminals of the inductor separately to see what voltages are present on the component relative to the PWM control signals. On my setup, there is negligible delay between PWM-HI asserting high vs 9V0 appearing on the inductor terminal.

    >> We already measure delay between PWMH from controller to the input terminal of inductor, which is 57 ns.Can you provide exact steps to check voltage across inductor? as you speculated this drop influences the delay seen in inductor current rise (600 -800ns)

    >>Update on checking on SPICE:

    We are trying to use LT Spice and create schematic. If you have any schematic for this eval board, please share. Because we have difficulty in picking the right parts, as we are new users of this tool. Mainly exact Gate-driver and Op-amp is difficult to find.

    New question:

    For 5V output we see almost negligible currents in ILFB. Expected behavior is to have a increase in ILFB current when Vout is increased. Observe the results in image.

  • I am attaching the images here, since they were not uploaded properly in my last reply.

    >> We can see negligible ripples on Vout while doing AC coupling at 20% duty. Any suggestions here?

    New question:

    For 5V output we see almost negligible currents in ILFB. Expected behavior is to have a increase in ILFB current when Vout is increased. Observe the results in image.

  • Arun Kumar M S said:
    When observing Vout vs ILFB, I recommend AC-coupling the Vout signal and zooming in to focus on only the AC ripple.

    >> We can see negligible ripples on Vout while doing AC coupling at 20% duty. Any suggestions here?

    The intent of recommending AC-coupling on Vout was to help deduce the inductor current flow by observing the Vout ripple.  They should roughly coincide.  This is meant to provide some additional information.  There is no conclusive observation to make.  AC-coupling on ILFB is probably not as useful.

    Arun Kumar M S said:
    You can probe the two terminals of the inductor separately to see what voltages are present on the component relative to the PWM control signals. On my setup, there is negligible delay between PWM-HI asserting high vs 9V0 appearing on the inductor terminal.

    >> We already measure delay between PWMH from controller to the input terminal of inductor, which is 57 ns.Can you provide exact steps to check voltage across inductor? as you speculated this drop influences the delay seen in inductor current rise (600 -800ns)

    The inductor terminals indicated by the yellow arrows can be probed directly to measure their voltage.  I would again emphasize that negative inductor currents are not represented on ILFB.

    Arun Kumar M S said:
    >>Update on checking on SPICE:

    We are trying to use LT Spice and create schematic. If you have any schematic for this eval board, please share. Because we have difficulty in picking the right parts, as we are new users of this tool. Mainly exact Gate-driver and Op-amp is difficult to find.

    There is no Spice schematic to share for this board.

    Arun Kumar M S said:
    For 5V output we see almost negligible currents in ILFB. Expected behavior is to have a increase in ILFB current when Vout is increased. Observe the results in image.

    I suppose that the negative inductor current ripple in discontinuous conduction mode is worse for 5V vs 3V.  I recommend maintaining the converter in continuous conduction mode where possible.

  • For some added clarity, here is an illustration of what negative inductor currents might look like: