Optimizing efficiency and reducing operating temperatures
     
In motor drive circuits, optimizing the efficiency and the deliverable power to the motor and controlling the maximum junction temperature depend heavily on how the power amplifier IC is mounted. The choices include mounting the IC directly on a printed circuit board, with or without a heatsink, or incorporating air flow with convection or forced air. In addition, the size and orientation of the heatsink, if used, must be selected to manage the average power dissipation of the power IC.

By Dan Leih, Product Marketing Engineer, Cirrus Logic Inc

General Guidelines - Mounting Options

Each motor drive application is unique and there are a number of mounting options to consider. The challenge is to choose the best one for the constraints of the particular application. The following three design examples provide an evaluation of different mounting and cooling options and define the basic capabilities of each. The derivation of the maximum power and thermal system dynamics is discussed in Reference 1.

This first example evaluates surface mount options using a Cirrus Logic SA306-IHZ motor drive IC  married to a Microchip Technology PICtail Plus Adapter Board. This configuration was designed specifically for use with a Microchip 16-Bit dsPIC33 Digital Signal Controller (DSC). This DSC includes a motion control interface that offers the ability to control the SA306-IHZ with both block and sinusoidal excitation waveforms.

Mounting without Heatsink - For Low-Power Applications

The most cost effective way of mounting a surface mount power IC is to solder it in the traditional SMT (surface mount technology) fashion. This involves mounting the IC directly onto a printed circuit board (PCB) without any additional heat sinking components, as depicted in Figure 1.



Mounting without a Heatsink - For low-power applications


By mounting the motor driver IC in this fashion, the heat slug on the bottom of the IC’s conventional power package becomes an effective thermal path for conducting heat away from the device and directing it to a one-ounce or two-ounce copper ground plane on the top of the printed circuit board. Beneath the heat slug, several vias pass through the board for the purpose of carrying heat to the back of the board. It is important with this mounting technique that sufficient solder paste be applied beneath the power IC for a good thermal connection to the ground plane.

As depicted in Figure 2, at an output power of 20 watts, a temperature gradient of +99.6 °C was obtained. This means the junction temperature will be approximately +130°C at an ambient temperature of +30°C. The plots in Figure 2 denote a value of +136°C case temperature at an ambient temperature of +30°C, with a thermal resistance junction-to-air of approx. 5.343 °C /W.

 

Heat Slug Temperature versus Total Electrical Power - Shown for the surface mounted SA306-IHZ when used in low-power motor drive applications

After derating the device to allow for increased ambient temperatures, the maximum continuous deliverable output power is approximately nine watts for an application covering the entire industrial ambient temperature range up to +85°C using this mounting technique.

Thermal Response Time

The values depicted in Figure 2 were generated with motors running at constant speed. However, in applications in which the motor is frequently accelerating and decelerating, it is important to know the thermal response time. Plotted in Figure 3 is the time interval for a case temperature rise of 60°C from +30°C to +90°C. This is the time required to transfer a specific amount of heat from the die to the back of the PCB. It is the temperature characteristic of the ground plane from the ambient T30 to the maximum temperature T90, when the motor is driven continuously at a maximum power of 9 watts.

 
Thermal Response Time - At a maximum output power of 9 watts


Although a maximum power rating of nine watts may seem low, many power devices are nonetheless capable of delivering high peak currents for several seconds. This mounting technique is appropriate for small, low-power or high-speed drives in cost-sensitive applications such as fans, pumps, scanners, surveillance cameras, labeling machines and paper feeders where size and production costs are crucial.

Mounting with SMT Thermal Pad and Heatsink – For Mid-Range Power Applications

For applications where power is required over a longer time interval, but economical production is still a primary design objective, the mounting technique can be modified. In this mounting arrangement, a 100 mm2 cutout in the PCB enables the insertion of a thermal pad.  This pad enhances the heat transfer (lowers the thermal resistance) from the heat slug on the power IC to a heatsink rather than utilizing the vias in the PCB as discussed earlier.


 

Simplified SMT Mounting with Thermal Pad - For mid-range power applications


The thermal pad used in this mounting technique is typically made from a polymer material chosen for low thermal resistance. These pads remain pliable for a number of years and therefore can accommodate shifting that will undoubtedly occur with materials that exhibit different coefficients of expansion and are subjected to variation in temperature. In Figure 4, a 2-mm thermal pad is used to fill the gap between the heat slug along the bottom of the IC’s package and the heatsink. The thermal pad chosen for this example was a Berquist Gap Pad #5000S35. The cutout area is 10 mm x 10 mm. This mounting technique provides three advantages:

•    The device can be mounted as a conventional SMT part.
•    Thermal coupling is virtually constant, even if the thickness from one printed circuit board to the next varies from1.5 mm to 1.7 mm.
•    Accurate placement of the heatsink is not required.

With this mounting technique, combined with the use of a small heatsink (Part #HS33 from Cirrus Logic), the total thermal resistance was reduced and the contact area with the ambient air is increased.
The 100-mm² x 1.5-mm thermal pad exhibits a thermal resistance of 0.174 °C/W. When this value of thermal resistivity is compared to the thermal path through vias in the previous example, the thermal resistance is reduced by a factor of 10.

The use of a thermal pad enables raising the continuous output power to 51 watts at a die temperature of approximately +155 °C and an ambient temperature of +26 °C, with block commutation. This is depicted in Figure 5. The maximum power output for this configuration, up to +85°C, is approximately 20 watts.

With this solution the output power has been doubled at similar system temperatures. What is most important is better performance realized due to the shorter heat path to the heatsink, as well as to the lower thermal resistance of the thermal pad compared with several vias.

 
 Temperature Characteristic Using Thermal Pad and Heatsink - Shown for the surface mounted SA306-IHZ with a HS33 heatsink when used in mid-range power motor drive applications

Furthermore, the thermal pad lends itself to relatively simple assembly along with relatively good thermal performance at moderate power ratings. The short thermal path affords benefits in accelerating heavy mechanical loads for a longer time as shown in Figure 6. This is an appropriate solution for a wide variety of industrial applications in which motors are driving mechanical loads through gear trains and where production runs are in mid-range volumes.
 

Thermal Response Time - At a maximum output power of 20 watts

Additional SMT Mounting Techniques with Heatsink

Taking this discussion one step further, the addition of a heatsink enables the typical power device to begin showing its true capability. For lower power applications, or where heatsink size is limited, two mounting options exist. The first is an innovative, patent-pending mounting technique by Cirrus Logic. In this technique the device is flipped upside down and mounted through a hole in the PCB. Mounting is still from one side, allowing a single pass through the chip placement machine. The heatsink is mounted directly on the heat slug of the Power IC, as depicted in Figure 7.

 Flipped-Over SMT Mounting - By mounting the IC upside-down through the PCB, the integrated copper heat slug on the package provides a very-effective thermal path to the heatsink.

A second approach is to use a power package with a top-side heat slug. For example, devices packaged in an HSOP are available with the heat slug either on top or the bottom of the package. A top-side heat slug device can be mounted to the PCB along with the rest of the SMT devices. A heatsink mounted on the appropriately sized stand-offs is then attached to the PCB as shown in Figure 8. Regardless of the mounting technique used, thermal grease should always be applied between the heatsink and the device to insure good thermal transfer and to minimize the thermal resistance.

 
 SMT Mounting of HSOP Package with Top-Side Heat Slug

Both of these mounting options shorten the thermal path from the motor drive IC to the ambient air and reduce the total thermal resistance of the system. The resulting measurement using the PCB cut-out method resulted in a thermal resistance of 1.674 °C/W. The HSOP example with the same direct contact to the heatsink yields comparable results.


 

Temperature Characteristics with Flipped-Over Mounting - The SA306-IHZ mounted with HS33 heatsink


With this flipped-over mounting technique, the characteristics of the temperature plots in Figure 9 become quite linear across the entire power range. The maximum output power at a die temperature of +135°C is approximately 52 watts at +25°C and approximately 29 watts at +85°C. This solution provides a slightly higher maximum output power than the thermal pad. However, the short thermal path has its biggest advantage in dynamic applications. The fast response time between the die and the heat slug provides benefits in applications in which frequent accelerations occur over short time intervals, as is represented by the temperature rise shown in Figure 10. It is the direct connection to the heat slug on the bottom of the power IC package that enables the wide variety of heat sinking methods and cooling options.



Thermal Response Time - At a maximum output power of 25 watts

Comparing the Low- and Mid-Range Power Results

Benchmark values for the various mounting techniques discussed are compared in Table 1. Note that the columns that denote the maximum output power are rated for ambient temperatures of +85°C and +25°C.

Table 1: Performance Capabilities of the SA306-IHZ Motor Drive IC with Various Mounting and Heat Sinking Options

Motor drive applications vary widely and various thermal techniques are available to match a specific performance requirement. When incorporating a heatsink to aid thermal management, the size and orientation of the heatsink must be selected to manage the average power dissipation of the power device. Standard PCB mounting techniques enable these power devices to dissipate less heat but allow ease of manufacture. The use of a heat pad or small heatsink produces much higher power capability and improves thermal response time. In applications where higher power dissipation or lower junction (case temperatures) is required, a larger heatsink or circulated air can significantly improve performance.
By observing the guidelines covered in this article, power amplifier ICs can provide long-term, reliable performance in motor drive applications.

References

1. Optimizing Power Delivery in PWM Motor Drive ICs, Application Note 50, www.cirruslogic.com
2. SA57 Pulse Width Modulation Amplifier Data Sheet, www.cirruslogic.com
3. SA306 Pulse Width Modulation Amplifier Data Sheet,www.cirruslogic.com
4. 3-Phase Switching Amplifier Application Note SA306, www.cirruslogic.com


Test Procedures

For each mounting configuration three standard tests were performed:
•    Maximum output power for block-commutated motors at approximately 25°- 30°C ambient temperature
•    Maximum output power for sinusoidally-commutated motors at approximately 25°- 30°C ambient temperature
•    Slew rate of the thermal response time at maximum output power
•    Finally, calculating the power derating for maximum ambient temperatures for industrial applications at +85°C.

www.cirrus.com