SLVAF66 June   2021 DRV3255-Q1 , DRV8300 , DRV8301 , DRV8302 , DRV8303 , DRV8304 , DRV8305 , DRV8305-Q1 , DRV8306 , DRV8307 , DRV8308 , DRV8320 , DRV8320R , DRV8323 , DRV8323R , DRV8340-Q1 , DRV8343-Q1 , DRV8350 , DRV8350F , DRV8350R , DRV8353 , DRV8353F , DRV8353R

 

  1. 1 Introduction to High-Power Motor Applications
    1. 1.1 Effects of a Poorly-Designed High-Power Motor Driver System
    2. 1.2 Example of the High-Power Design Process
  2. 2 Examining a High-Power Motor Drive System at a High Level
    1. 2.1 Anatomy of the Motor Drive Power Stage and How to Troubleshoot
    2. 2.2 Troubleshooting a High-Power System
  3. 3 High-Power Design Through MOSFETs and MOSFET Gate Current (IDRIVE)
    1. 3.1 MOSFET Gate Current
      1. 3.1.1 How Gate Current Causes Damage
      2. 3.1.2 Gate Resistors and Smart Gate Drive Technology
        1. 3.1.2.1 Gate Resistors
        2. 3.1.2.2 Smart Gate Drive and Internally-Controlled Sink and Source Gate Currents
        3. 3.1.2.3 Summary for Gate Resistors and Smart Gate Drive Technology
      3. 3.1.3 Example Gate Current Calculation for a Given FET
  4. 4 High-Power Design Through External Components
    1. 4.1 Bulk and Decoupling Capacitors
      1. 4.1.1 Note on Capacitor Voltage Ratings
    2. 4.2 RC Snubber Circuits
    3. 4.3 High-Side Drain to Low-Side Source Capacitor
    4. 4.4 Gate-to-GND Diodes
  5. 5 High-Power Design Through a Parallel MOSFET Power Stage
  6. 6 High-Power Design Through Protection
    1. 6.1 VDS and VGS Monitoring
      1. 6.1.1 Turning Off the FETs During an Overcurrent, Shoot-Through, or FET Shorting Event
    2. 6.2 Passive Gate-to-Source Pulldown Resistors
    3. 6.3 Power Supply Reverse Polarity or Power Supply Cutoff Protection
  7. 7 High-Power Design Through Motor Control Methods
    1. 7.1 Brake versus Coast
      1. 7.1.1 Algorithm-Based Solutions
      2. 7.1.2 External Circuit Solutions
      3. 7.1.3 Summary of Brake versus Coast
  8. 8 High-Power Design Through Layout
    1. 8.1 What is a Kelvin Connection?
    2. 8.2 General Layout Advice
  9. 9 Conclusion
  10. 10Acknowledgments

4.3 High-Side Drain to Low-Side Source Capacitor

Figure 4-4 Example High-Side Drain to Low-Side Source Capacitor Location

At first glance in Figure 4-4, high-side drain to low-side source capacitors seem to be self-explanatory and are often confused with the decoupling or bulk capacitors. However, most motor driver applications do not have the low-side source connected to GND. Instead, the low-side source is usually connected to a shunt resistor which is used for current sensing, and is then is then connected to GND.

This is important because decoupling capacitors need stable references to reliably provide charge. GND instability can be present in a system as a result of inductance introduced by the sense resistor layout, motor current flowing through the low-side FET, or bad grounding techniques. If GND bounces along with the switch node then the decoupling capacitors cannot do their job to provide charge from a stable reference and low inductive path. For reference, a 0.2512 component package size, which is a common package for sense resistors, introduces 1–5 nH of parasitic inductance.

The HS drain to low-side source capacitor can circumvent these problems because it is connected to VDRAIN, which is assumed to be stable, and can dump charge directly onto the node, instead of through the path of a sense resistor. This is the concept of an AC GND and is the reason why the RC snubber can also be connected to the HS drain, as well as the LS source.

As a result:

  • This method does a great job suppressing negative bouncing on the low-side source and GND.
  • Selecting a value of around 0.01 µF–1 µF and placing them as close to the FET as possible ensures they work correctly
    • Specifically, the value should be low enough to not impact the non-parasitic ripple of the current sense waveform, which reflects the real behavior of the motor

A lot of engineers underestimate this mitigation technique and fail to use footprints as they have already prioritized RC snubbers and bulk capacitors at this point. If GND or the sense resistor is ringing negative, or below GND, the HS drain to LS source capacitor provides charge in a low impedance path. Waveforms showing the GND and LS source voltage are helpful to determine if negative ringing is occurring and whether to update the design to add HS drain to LS source capacitors to the half bridges.