SNVAA20 July   2021 DRV8833 , DRV8833 , LMR33630 , LMR33630

 

  1.   Trademarks
  2. 1Introduction
  3. 2Motorized Resistive Load Architecture
    1. 2.1 Controller Board
    2. 2.2 Resistor Plate
  4. 3Motorized Resistive Load Design
    1. 3.1 Controller Board Design
      1. 3.1.1 Power Management
      2. 3.1.2 Power Converter Selection
      3. 3.1.3 Interface and ADC Selection
    2. 3.2 Resistor Plate Design
      1. 3.2.1 Motor and Motor Driver Selection
      2. 3.2.2 Resistor Track
      3. 3.2.3 Mechanical Arm Assembly
      4. 3.2.4 Feedback Control
  5. 4Thermal Considerations
  6. 5Performance and Results
  7. 6Summary
  8. 7Appendix
    1. 7.1 Controller Board Main Schematic
    2. 7.2 Controller Board Sub-Schematics
    3. 7.3 Resistor Plate Schematics
    4. 7.4 Python Code

1 Introduction

A motorized resistive load is a programmable load that can be used to help test or characterize a system or DUT. This type of load is useful in evaluating DUT behavior under steady-state operation or during start-up, shutdown, or other dynamic conditions. To use this load, the user connects the DUT to the load terminals and sets a target resistance. After confirming the selection, the motorized resistive load forms the target resistance between its input terminals. At this point, the DUT may be enabled, tested, and its behavior under load can be observed. The motorized resistive load design described herein offers advantages in utility, size, and ease-of-use compared to alternative solutions.

Electronic loads are typically utilized when static or steady-state performance of the system is of interest, and these loads can exhibit undesirable behavior when used during transient or dynamic operation. Electronic loads utilize active circuitry to implement feedback that could potentially interact with the DUT and as a result exhibit load profiles that are different than the profile generated by a resistive load. For example, during the initial start-up of a buck converter, its output voltage will increase linearly and if the applied load is resistive, then the load will also increase linearly until the output voltage reaches its nominal value. Applying a nonresistive load via an electronic load could result in an abnormal load profile or oscillatory behavior under the same conditions, even if the load is set for constant-resistance mode. Given these unpredictable load profiles and behaviors, system or DUT behavior can be difficult to characterize, making unexpected DUT behavior difficult to debug unless a resistive load is employed.

Electronic loads typically have minimum voltage requirements to ensure expected operation, whereas the described motorized resistive load does not have this limitation. Depending on the electronic load used, operation is not guaranteed under a set voltage threshold. This disqualifies electronic loads from being utilized for low-voltage applications or when the input voltage to the system is in dropout (when the input voltage is near the set output voltage). Conversely, a resistive load can be used for extremely low voltages and reliably produce accurate results. Figure 1-1 shows the load profile when using the motorized resistive load compared to a standard electronic load when loading a buck converter during its start-up.

Figure 1-1 Load Profile Comparison

A motorized resistive load is a more compact and easy-to-use alternative to an electronic load. Commercial electronic loads can be relatively large, while a motorized resistive load can be manufactured with roughly the same load range at a fraction of the size. The design described in this paper is circular and has a diameter of approximately 10 in, and is approximately 3 in tall, whereas electronic loads are typically rectangular and can be nearly twice as large. The only setting required for the motorized resistive load is the target resistance, making it an intuitive solution. Additionally, the programmability of the motorized resistive load can help reduce test time and is more convenient than manually changing individual load resistors.