SLVAFT0 July   2024 DRV8706-Q1 , DRV8714-Q1 , DRV8718-Q1

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Background and Theory
    1. 2.1 Factors Affecting Bulk Capacitor Sizing
    2. 2.2 Pulse Width Modulation
    3. 2.3 Estimating Motor Current Variation
    4. 2.4 General Guideline Calculations Assuming Ideal Capacitors
  6. 3Real-World Results
    1. 3.1 Example Measurements
    2. 3.2 Revised Practical General Guidelines
    3. 3.3 Other Considerations
  7. 4Summary
  8. 5References

3.2 Revised Practical General Guidelines

Based on these real-world observations, we modify the ideal general guideline given previously to account for the non-ideal characteristics of the capacitor:

Equation 6. C B U L K > k   ×   ∆ I M O T O R × T P W M / ∆ V S U P P L Y

Where:

CBULK is the bulk capacitance

k is a scale factor to account for the ESR for typical capacitors in this type of application; based on the lab measurements above with DRV8718 EVM, k of about 3 is practical for these cases.

ΔIMOTOR is the expected variation in motor current, imax – imin

TPWM is the PWM period which is the reciprocal of the PWM frequency

ΔVSUPPLY is the allowable variation in the motor supply voltage

Figure 3-7 plots several data points and applies this general guideline, showing relatively good agreement.

Measured Results and 3x General Guideline, Accounting for Real-World Non-Zero ESR Values of Electrolytic CapacitorsFigure 3-7 Measured Results and 3x General Guideline, Accounting for Real-World Non-Zero ESR Values of Electrolytic Capacitors