May 11, 2015 | By:

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Fairchild’s Motion SPM® Smart Power Modules integrate drive and protection circuitry for all sorts of industrial motion and motor control applications into a single package.

We recently introduce the SPM 7 Module for compact motor control applications of up to 50W. Seldom do these applications have room for heatsinks, which means estimating junction temperature (TJ) to ensure system reliability is critical. What follows are a few methods that can be used for calculating junction temperate of the SPM 7 series.

Motion SPM 7 Series Overview

The high-performance SPM 7 modules series combine a number of devices – including a high-performance FRFET® MOSFET and half-bridge gate driver IC’s – to provide a fully-featured, high-performance inverter within a single PQFN package (Fig. 1).

The key design objective of the Motion SPM 7 series was of course to provide a solution for compact and reliable inverter design when assembly space is constrained. Think of systems such as small pedestal and ceiling fan motors, air conditioner indoor/outdoor fans, and water pumps.

FIg. 1 Package Outline Structure

FIg. 1 Package Outline Structure

Estimating Junction Temperature

There are four methods used to measure the junction temperature. Each method is presented and the results that are obtained from each are compared. The methods are as follows:

A. Direct Measurement

(TJ sensing by IR Camera)

B. TC_TOP measurement

(Measuring Case Temperature TC_TOP Sensing by IR Camera)

C. TP Measurement

(PCB Temperature TP Sensing by Thermocoupler)

D. VTS Measurement

(Junction Temperature of the High Voltage IC THVIC Sensing by VTS)

A. Direct Measurement ( TJ Sensing by IR Camera)

You can measure the junction temperature directly by using an infrared camera. While this may seem straightforward, it requires that the module be de-capped. The process is laborious, complicated, expensive, and alters the final operating arrangement of the final assembly. However, this can be worth the effort and cost in order to provide the ability to “look inside” the case and “see” the actual components. For the purposes of our measurement, the de-capping process was performed by a professional facility.

Fig. 2 shows the de-cap position and captured image during operation. Among the 6 MOSFETs included in the package, the W-phase H/S MOSFET is the hottest point. This hot point is mainly an effect of the leadframe structure where there is overlapping temperature caused by adjacent MOSFETs.

Fig. 2 De-Cap Position

[Fig. 2]

B. TC_TOP measurement (TC_TOP Sensing by IR Camera)

ΨJT of the SPM 7 series is about 0.77 ℃/W, which is acquired thru extensive modeling. The variation between TJ and TC_TOP turns out to be negligible because it is less than 5℃, given that the maximum power dissipation (Pd) is less than 6W and ΨJT of the SPM 7 series is about 0.77 ℃/W .

Fig. 3 and Fig. 4 show the thermal equivalent circuit and structure of the SPM 7 series focused on top-side. Note that even though a heat sink is absent, the PCB plays a heat-sinking role. In this case, TJ can be calculated as:



Fig 3 Thermal Equipment Circuit

[Fig. 3] Thermal Equipment Circuit regarding Top-side

[Fig. 4] SPM 7 Package Thermal

[Fig. 4] SPM 7 Package Thermal

C. TP Measurement (TP Sensing by Thermocoupler)

The temperature of PCB layer P, TP, represents a similar TJ value due to low thermal resistance from junction to PCB . When TP is compared with real TJ with test results shown in Fig. 5, measured TP value is closed to real TJ.

[Fig. 5] Graph for Temperature Measurement

[Fig. 5] Graph for Temperature Measurement

During the process of acquiring the data we discovered that the attachment position as shown in Fig. 6 and Fig. 7 is very important for reducing the variations between TJ and TP. By minimizing the gap between thermocouple, and device, you can minimize the variation between TJ and TP.

[Fig. 6] Examples of Good & Bad Attachment for Reducing the Variations Between TJ and TP Measurements

[Fig. 6] Examples of Good & Bad Attachment for Reducing the Variations Between TJ and TP Measurements

[Fig. 7] Proper Thermal Couple Placement Greatly Reduces the Variation Between TC_TOP and TP

[Fig. 7] Proper Thermal Couple Placement Greatly Reduces the Variation Between TC_TOP and TP







D. VTS Measurement (Junction Temperature of the High Voltage IC THVIC Sensing by VTS)

The TSU analog voltage output, VTS, reflects the temperature of the HVIC in the SPM 7 series modules. The equation below shows the relationship between VTS voltage and V-phase HVIC temperature. It can be expressed as:

VTS = 0.019 x THVIC + 0.2 [V]

Even when properly placing the thermocouple as per Fig. 6 and 7, the variation between TJ and THVIC gradually increases corresponding to load current. Judging from the experiment, the MOSFET junction temperature can be estimated by calculating HVIC temperature. However, system conditions, such as heat dissipation, can change the curve. Therefore, it is necessary to make a profile according to set application conditions.


As you can see, there are pros and cons to each method that are weighed against cost, simplicity and tolerance to variations. The table below summarizes the different aspects of each method. For much deeper technical details, you can follow the links below the table.

Direct Measurement
De-cap Facility,
IR Camera
Measuring Junction Temperature TJ
TC_TOP Measurement
IR Camera
Easy Measurement
Measurement for Case Temperature TC_TOP
TP Measurement
Thermal Recorder
Large variation dependent upon attaching point
Measurement for PCB Temperature TP
VTS Measurement
Voltage Meter
Easy & Low-cost Measurement
Correlation is required between THVIC and TJ
Above methods can be used for the TJ correlation

To find out more about the SPM® 7 series power rating lineup: go to:

For a list of SPM products, go to:

To analyze and simulate smart power modules in motor drive applications go to:

To view application notes:

“Motion SPM®7 Series User’s Guide”

“Surface mounting Guidance for Motion SPM®5 Series”

This post is also available in: Chinese (Simplified)