EMC & Thermal Management
New technology requires trade-offs between thermal and EMC
By Dr. Min Zhang, consultant to EMC Standards and founder and principal EMC consultant of Mach One Design
The future is here
The past decade witnessed an increasing electrification of the global energy system (with shift to battery-powered cars leading the march) and the trend for a greener future. The change would not have happened without technology breakthrough. A keyword in this development is “efficiency”. Behind the electrification are the emerging wide-band-gap (WBG) material power semiconductors such as SiC (Silicon Carbide) and GaN (Gallium Nitride), which are now increasingly found in fast-charging applications [1]
. Wide-band-gap materials can
exceed the limit of silicon (Si) and are used to pursue ground-breaking high frequencies, high efficiencies, and higher power density power conversions. Today, SiC chips are found in high-end power management applications used in electric vehicles, such as certain newer Tesla models while GaN devices dominate fast chargers for phones and laptop. The application of WBG devices requires thermal management know-how to cope with the heat generated by the increased power level and switching frequency. Thermal management where engineers design a system so that heat is transferred efficiently is the key to improving overall system efficiency. For instance, thermal management is key to an electric vehicle (EV)’s battery pack, it also plays an important role on the EV’s driving range.
These days, semiconductor suppliers (in partnership with their packaging manufacturers) tend to offer thermally enhanced package which often includes a large cooling pad for a low package thermal resistance. This leads to increased thermal performance, hence enabling the design of high-density power electronics products, especially for fully enclosed products with no air flow [2]
.
In order to take full advantage of these thermal benefits, the PCB layout, thermal interfacing, and heatsinking, must all be designed properly. An optimum design is achieved when designers across all functions (that includes product, electric, mechanical, thermal and EMC) can make
24 February 2022
trade-off decisions during the research and development (R&D) stage. But in reality, this is seldom the case as often the EMC performance is compromised. Since EMC is a regulatory compliance, and the product is tested at the final design stage most of the time, fixing the problem is costly and often ends up using filters which are heavy and bulky, defeating the very purpose of achieving a compact design [3]
.
This article presents a case study for engineers to understand the trade-offs that need to be made when designing a compact system such as a fast charger.
Through-hole or surface mounted devices?
The trend from through-hole packages to low- cost surface mounted device (SMD) applications is marked by the improvement of chip technologies. “Silicon instead of heatsink” is, therefore, possible in many cases [4]
. There are
cases in which through-hole packages such as TO-247 devices were selected for high-voltage high-power converters. There are three main reasons for doing so. n A through-hole device is a lot cheaper than
Components in Electronics
an SMD type. For volume manufacturers, cost is always the first thing to consider. n Thermal design could be managed much easier with through-hole devices as large size heatsinks can be mounted to the devices. For surface mounted devices, large size of copper planes might be needed for every layer of the stack up, this complicates the thermal design, it also pushes for more PCB layers, leading to higher cost. n Through-hole devices can result in a smaller size PCB design. While one through-hole device with heatsink might suffice the thermal design, engineers often need to put multiple surface mounted devices in parallel to cope with the heat, this increases the size of the PCB and pushes the cost higher.
The drawbacks of using through-hole devices are:
References
[1] Z. Liu, B. Li, F. C. Lee and Q. Li, “High-Efficiency High-Density Critical Mode Rectifier/Inverter for WBG-Device-Based On-Board Charger,” IEEE Transactions on Industrial Electronics, vol. 64, no. 11, pp. 9114-9123, 2017.
[2] Navitas, “Thermal Management of NV612X GaNFast™ Power ICs”. [3] K. Armstrong, “Design Techniques for EMC,” [Online]. Available: https://www.emcstandards.
co.uk/files/design_techniques_for_emc_1999_part_1_circuit_design.pdf.
[4]
R.Gulino, “Guidelines for using ST’s MOSFET SMD packages,” ST Application Note AN1703.
www.cieonline.co.uk
n It increases the height of the product. n It has much worse EMC performance compared with surface mounted devices. The long lead of the package easily introduces an extra 10 nH inductance. This parasitic inductance, together with the parasitic capacitance of the device, causes over-shoot and ringing when the device is switched at a fast speed. n Depending on the size, heatsinks that are attached to the through-hole devices could also cause EMC problems. Fast switching common-mode currents means that heatsink could act like a quarter-wave antenna. If the board heat sink is not bonded correctly, each device could also increase the radiation emissions.
www.emcstandards.co.uk
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