Optimized thermal design for three-phase motor drives in power tools – Part 1

Other Parts Discussed in Post: CSD18540Q5B

Everyone loves power tools, whether cordless or corded. Cordless tools can use brushed or brushless DC (BLDC) motors. But, brushless motors are more efficient and have less maintenance, low noise and a longer life. In this two-part blog series, we will first discuss the basics of thermal design for three-phase motor drives used in these power tools, then the options available for your design.

Power tools have stringent requirements for form factor and thermal performance. Therefore, highly-efficient power stages that are small in size are required to drive the BLDC motors used in power tools. High efficiency provides maximum battery duration and reduces cooling efforts. Small form factor enables design flexibility for optimal cooling and optimum placement close to the battery pack to minimize impedance on connections carrying high current.

In cordless power tools, the battery voltage can vary from 12-V to 72-V, depending on the battery connections. The operating currents will be high. For example, a 1-kW power tool operating from a 36-V battery will take 30-A current. MOSFETs are a good choice in low-voltage, high-current applications. The inverter for the three-phase BLDC motor consists of six MOSFETs. The small form factor of the power stage demands small form factor MOSFETs, and high efficiency demands MOSFETs with low RDS_ON.

For example, the NexFET™ power MOSFET CSD18540Q5B is a 60-V N-channel MOSFET. It has a low RDS_ON of 1.8-mΩ and is available in a very small SON 5×6 mm package with a package-limited continuous current rating of 100-A and a peak current rating of 400-A. With proper thermal design for these small packages, we can enable the power section to carry high currents safely. The CSD18540Q5B datasheet specifies that these devices have a very low junction-to-case thermal resistance (RϴJC) of 0.8 oC/W, which is determined with the device mounted on a one-inch2 (6.45-cm2), two-oz. (0.071-mm thick) Cu pad on a 1.5-inch × 1.5-inch (3.81-cm × 3.81-cm), 0.06-inch (1.52-mm) thick FR4 PCB. The junction-to-ambient thermal resistance (RϴJA) for the above test conditions is 50 oC/W. In order to achieve the best thermal design, it is important to understand the heat dissipation paths of the package in calculating the junction to ambient thermal resistance.

  (a)       (b)

                                                                          

Fig 1. (a). Heat Dissipation paths of SMD packages with exposed pads; (b). Thermal equivalent circuit

As shown in Fig 1(a), there are two paths for the heat dissipation from MOSFET junction to the ambient. The first one is from the junction of the device to the ambient through the exposed thermal pad of the device and PCB. The second path goes from the junction of the MOSFET to the ambient through the top plastic surface of the packaging.

The first path consists of two thermal resistance components, (1) the thermal resistance from the junction of the device to the bottom exposed pad (RϴJC) and (2) the thermal resistance from the exposed pad to the ambient through the PCB (RϴCA). Therefore, thermal resistance from junction to ambient through the PCB,

RϴJCA = RϴJC+ RϴCA.                                                                                       

The second path consists of two thermal resistance components, (1) the thermal resistance from the junction of the device to the plastic molding at the package top (RϴJT) and (2) from the package top to the ambient air (RϴTA). Hence, thermal resistance from junction to ambient through the device top surface, RϴJTA = RϴJT+ RϴTA. .                                                                                                                             

The parameters RϴJC and RϴJT are properties of the device package. RϴJC is a very good indicator of a package’s thermal performance. A package with a low RϴJC will have good heat transfer to the exposed pad. The parameter RϴJT may not be mentioned for most of the devices as its value is generally high. For the CSD18540Q5B, RϴJC is 0.8 oC/W and RϴJT is typically around 12 -15 oC/W.

So, for better system level thermal design, the designer needs to reduce the parameters RϴCA and/or RϴTA. As RϴJC is much smaller compared to RϴJT, most of the heat travels from the exposed copper pad of the device to the PCB. If we assume, there is no heat sink on the top of the device, 95 percent or more of the heat transfers to the PCB, and hence the most critical part of thermal design will be taking away the heat through the PCB and to the ambient.

For more information, check out our TI Designs reference design for a 1-kW/36-V power stage for brushless motor in battery-powered garden and power tools.

 

Stay tuned for part 2 to learn the general guidelines and heat sink options for thermal design.

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