Latest PCB technology boosts power electronics performance

Paul Boughton

Stephan Ruhnau explains how to transmit loads of 150A@1000V with wires up to 50mm2 straight to high current PCBs.

Technology trends - such as energy efficiency, increasing power density or the overall integration of power, signals and data on the same system platform - are both, a challenge for engineering, but also a chance on diversification and innovation. Thus, trends drive growth and attempt to be a decisive factor for an application's success.

Obviously, one of these trends is the increasing performance of power electronics, in correlation with the rapidly growing demand for applications based on power electronics, such as frequency converters, servo drives or photovoltaic devices.

From this point of view, it is recommended to consistently align the component specifications with each phase in the application's life cycle - including conception, design, construction, production, installation and maintenance. This guarantees the optimal level of integration into the system environment. The function and form of the application's connection method plays a key role that should not be underestimated, as it influences several critical technical and economic factors, such as:

- Electrical, thermal and mechanical design flexibility.

- Costs of production, installation and maintenance.

- The user satisfaction level as influenced by the ease of use.

- The facility availability as a link on the transmission path.

- The long-term reliability, especially under threshold limits, and last but not least,

- The safety provided to man and machine.

Referring to power applications, definitely two of the challenges that electronics engineers have to solve, is first the question of thermal management, and second the design of the current carrying systems. Mixed up with EMC requirements, plus the continuous demand for smaller devices, plus production efficiency and last but not least, the cost reduction, you gain an exciting play on balancing technical and economical issues.

One does not need to mention that each of the topics is a science on its own. Looking at the conducting elements of a power electronics device, we may mainly identify the wires, the PCB, usually some bus bars and the junction points.

For example, due to an ongoing improvement of the manufacturing process, particulary the performance of the PCB has been increased along the recent years. Moving from 35µm to 400µm thick-copper paths, extended by iceberg technology and supplemented by wire laid technology, a state-of-the-art board with copper bar inlays is able to carry currents of 800Amps and even more (Fig. 1).

It is one question to get copper bars with a size of 3 x 15mm integrated to a PCB, and another one to get such a power board connected to the peripherals. It is also one thing to stick on the 'good old' bus-bar for the distribution of high currents, and another thing to minimise the dimensions of the device - while maintaining the same level of performance and protection against electrical shock. Finally, it will be one solution, to connect ring lugs to stud bolts, and the other problem to replace a device immediately because every minute of downtime burns loads of money due to production stop.

The significant differences between bus bar based design and PCB based design additionally emphasise the above mentioned advantages, as simply visualised by the contrast between the original application and it's PCB-based Redesign.

Providing the new LXXX 15.00 high-power PCB terminal (Fig. 2), Weidmüller enables the engineer to replace extensive and expensive bus bar constructions up to 150Amps by a compact single component for easy wave soldering, simple integration and fast und safe connection. The features in detail:

- Unique clamping capacity: With its unmatched clamping range, the LXXX 15.0 connects wires up to 50mm2/AWG1 securely to the PCB. Weidmüller's self-securing steel clamping yoke has proven in millions of cases that it is truly 100 per cent maintenance free.

- Protection against incorrect use = functional security: The integrated Wire Guard provides protection against miswiring and eliminates the risk of wires being inserted under the clamping yoke, fusing contact.

Weidmüller's high-performance WEMID insulating material contains neither halogen nor phosphor and is compliant with the strictest environmental standards. The so-called RTI (relative temperature index) of 120°C, the LXXX 15.0 exceeds the maximum continous operating temperature of standard PA (100°C) by 20°K. The benefits: additional performance reserves for the highest level of security due to temperature cycling and overloads.

- Application-Based Design: This transborder perspective of the component directives makes the application approval process quicker and easier. The LXXX 15.0 can be used without any additional covering because of its extended clearance and creepage distances (in accordance with the IEC 61800-5-1 device directive).

- Multiple Marking Options: Labelling options and advantages: individualised (direct printing), flexible (Dekafix marker strips) or cost-efficient (adhesive strips).

Particularly when dealing with power, the top priority is usage and maintenance that is reliable and secure. The test plug (available as an accessory) enables measurement over an extended period of time.

In total, high applicability enables the PCB to also be used in high-current applications for integrated and sustainable systems platforms.

In order to handle this current and voltage level and even the related wire sizes, there is more expertise and competence required than meeting the terminal specifications and component standards.

It is essentially, to avoid mechanical stress to the solder points and the entire circuit board, if the cables connected are finger-thick, but it is even more important, to avoid temperature overload - not only to prevent the device from heat collapse. It is even a question of the reliability and durability (the maximum operating life cycle) due to the so called term of Arrhenius, which proves a reduction of 50 per cent per each temperature rise of 10°C. To put it in another way: An application with a maximum operating temperature rise of 30°C provides double the life time and reliability, compared to a device which indicates 40°C temperature rise under full load.

According to Weidmüller's investigations and experiences in the field of power connectivity, there are several factors influencing the temperature rise of an electrical system: Besides the power loss of the active components and the constructive layout of heat sinks, it is also simple things to consider, such as surface size, cross connection and distance of the wires resp. the power conducting paths in the board or even pin layout, molding and current bar design resp. material of the terminal - the higher the current, the higher the impact.

Last but not least, due to the huge mass of high power components and boards, the solder quality depends strongly on the pin design and the soldering heat profile. The different temperature shapes of a system can easily be determined by thermal imaging (Fig. 3).

It is also important to be aware of the physical phenomenon, that the spot of highest temperature dispense is NOT necessarily the point of heat origin, because the mass and the thermal resistance between the surface of the heat conducting material and the environment determines the relative temperature loss resp. heat sink capability of each spot of the system.

So, if one experiences that a terminal shows the highest share of temperature on the thermal image, the problem is regularly not the terminal, it is only the best heat sink due to the mass and surface of the clamping yoke and the connected wire, which draws the heat out of points of heat origin such as undersized conductor paths or low quality solder connections.

To put it in a nutshell: The latest power PCB technology in combination with high performance connection technology PLUS the experience and expertise concerning special requirements on high currents - this is the key success factor on replacing bulky bus bar constructions in order to save space, production efforts and finally money while maintaining smaller devices at the same or a higher performance level.

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Stephan Ruhnau is Product Manager PCB-Components, Weidmüller Interface GmbH & Co. KG, Detmold, Germany.