When designing systems with integrated FET drivers, thermal performance can be a major consideration if users need the BLDC driver to operate with the most efficiency. At the most basic level, driving significant amounts of current from an integrated FET driver can only be possible if the device settings and parameters are optimized. This application note presents lab data showing how several of the customizable parameters on Texas Instruments’ DRV8316 and DRV8317 affect the thermal performance of the devices.
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Some of Texas Instruments' integrated FET motor drivers include thermal calculators that can be found on TI.com. These can be used to predict thermal performance based on system parameters, but lab data can provide additional context for the information presented by the data sheet and calculator. This application note provides real-world data explaining how settings such as PWM frequency, slew rate, and electrical characteristics affect the total power loss during operation. This document also profiles how the DRV8317 heats up over time during operation. Lastly, this application note contains design recommendations to assist in creating products with improved thermal performance.
Throughout this application note, two devices are the subject of the lab tests:
First, the power loss equations that can be used to simulate thermal performance are detailed. There are several parameters which contribute to the thermal dissipation of an integrated FET driver. The most influential factor is the internal Rds(on) of the device, which represents the drain to source resistance of an internal FET. When driving high current, the average power loss due to this resistance (referred to as conduction loss) is the primary contributor to the total power loss. Each of the types of loss is explained below for both trapezoidal and FOC control.
Conduction Loss
Rds(on) is also temperature-variant. For example, Rds(on) increases as the temperature of the driver increases, and contributes to more power loss until a point of saturation. This is why the thermal data shown in the next section was taken several minutes after the driver reached the desired output current. In addition to conduction loss, switching loss and body diode loss can contribute to the total power loss of the device. Switching loss refers to the loss due to slewing as the FET turns on or off, while body diode loss refers to the loss due to current flowing through the FET's body diode during dead time. These are calculated using the following equations:
Switching Loss
Body Diode Loss
These power losses depend on other parameters such as slew rate and PWM frequency, which is discussed later in the lab data. More information on each of these equations can be found in the Thermal considerations for integrated MOSFET drivers video, which explains all of the power loss sources in systems using integrated FET drivers.
Before looking at the data, some predictions can be made about the performance of the DRV8316 and DRV8317. Since the DRV8316 has a lower Rds(on) value, the expectation is that DRV8316 to have an overall cooler temperature at the same output level when compared with the DRV8317. Additionally, based on the previous equations, see the following:
However, there are potential drawbacks to each of these settings, which is explained more in a later section.
Lastly, each device is rated to operate in a range of junction temperatures. Due to the nature of PCB design, the junction temperature is difficult to measure. The lab data presented in this application note refers to the package temperature. These two values can differ due to a variety of factors, such as the thermal resistances, ambient temperature, and available area for the heat to conduct into different media and eventually into the ambient environment. Detailed testing and analysis is required to correlate the two values, but for more information, please refer to the following application note: Semiconductor and IC Package Thermal Metrics. This resource is also listed in some data sheets next to the Thermal Information section.
Based on these predictions and the theory discussed previously, users need to consider real-world lab data produced using the DRV8316 and DRV8317 EVMs.