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

2 Background and Theory

Experienced engineers often use general guidelines about bulk capacitance to select the capacitor values. One such guideline says to use at least 1 to 4μF of capacitance for each Watt of motor power. For example, a motor which draws 10 Amps from a 12V supply has a power of 120 Watts, leading to bulk capacitance of 120 to 480μF, using this general guideline. We dig a little deeper and see what further discussion supports those estimates.

To improve a motor drive design with regards to bulk capacitance, we first refresh our understanding of the fundamentals behind the relevant current and voltage relationships. Before moving on to specific practical cases, we look at the mathematics that describe a simplified designed for situation.