Jonathan Harper, ON Semiconductor
Exacting demands are currently being placed onto the manufacturers of white goods, in what is an increasingly competitive market. Pressures are intensifying to implement design concepts which enable major energy savings while also supporting more streamlined form factors that will attract consumers and offering reliable, low noise operation. In such situations specifying the most suitable supporting technologies is crucial to a product’s success.
There are a series of key criteria that define contemporary white good design and OEMs serving this sector must address each of them in turn if they are to keep consumers happy. These criteria are:
1. Energy efficiency - The latest European Union Energy-Related Products (ErP) program builds on existing legislative measures, broadening out its EcoDesign directive to include everything from white goods, vacuum cleaners and TVs to air conditioning units and water heaters. The objective is that by 2020 a 20% reduction in the energy consumption of European Union member states can be achieved. In tandem with international legislation, another aspect that should not be overlooked is that, in these austere times, consumers are keen to keep household utility bills as low as possible.
2. Reliable operation - Consumers expect longevity in this area. Though items of portable electronics might be replaced every 18 month to 2 years, for white goods they want several years of trouble-free operation. Many of the leading brands have built their reputation on manufacturing products that exhibit higher degrees of reliability than those of the competition. They cannot afford to have their good reputation damaged by poor quality units that are not dependable.
3. Compactness - Keeping the unit lightweight and aesthetically pleasing will all be vital ways by which to provide product differentiation.
4. Low noise operation - Consumers are irritated by noisy equipment in their homes. Furthermore, electrical noise can worsen home wireless LAN performance, which will be an inconvenience for the residents.
5. Overall cost - With a highly competitive market to serve, everything must be done to keep system costs low.
Uptake of BLDCs & PMSMs
In order to improve energy efficiency, save space and prolong working lifespan, there has been a mass migration away from conventional universal AC motors, which are bulky, expensive and relatively unreliable, towards more advanced 3-phase motors. By employing brushless direct current (BLDC) motors or permanent magnet synchronous (PMSM) motors, white good OEMs are able to manufacture products that support reliable variable speed operation.
PMSM and BLDC motors rely extensively on power inverters rather than outdated triac drives. This helps ensure reliability and also curbs the overall cost due to reductions in thermal management. The tactic is thus very much in line with several of the key criteria defining white good design that have already been outlined.
An inverter power stage used in modern white goods will consist of the following devices.
-Motor drivers ICs
-Other passive components (IGBT gate resistors, shunt resistors, etc.)
Engineers can, if they choose, create their own discrete solution - taking all the necessary component parts and mounting them onto a PCB. This takes time and effort. Individual parts have to be sourced and subsequently soldered on, then heat-sinking and extensive electromagnetic interference (EMI) testing will need to be undertaken. The costs in terms of engineering resources and bill of materials can be restrictive. Also this method is likely to be inefficient in terms of space utilization.
A module-based strategy, using high density hybrids, is now proving a lot more popular. Through the utilization of dedicated intelligent power modules (IPM) it is now possible to drive the various motors, compressors and fans that are incorporated into white goods in a much more efficient manner. It also means that the necessary circuit protection mechanisms are markedly easier to include within the motor drive system (see Figure 1). However, knowing which of these modules will deliver the best overall performance is not always clear. There are numerous shortfalls that are inherent with conventional IPM solutions.
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Circuit protection mechanisms within the motor drive system
On an IPM inverter, components can either be:
-Mounted onto a pair of separate leadframes, with the power devices onto one leadframe and the drivers onto a second leadframe. This adds to the production complexity and can be difficult to carry for certain passive components.
-Mounted all together onto a single direct bonded copper (DBC) substrate. This option is becoming universally recognized to be the best course of action. This allows the thermal path to be shortened without any compromise being made on isolation. It makes the IPM simple to construct and maximizes reliability levels.
Innovative new IPM technology is helping OEMs to implement more operational effective inverter systems within their latest models. Increasingly IPMs are being based on an arrangement where active ICs and discrete components are mounted onto a DBC substrate. This results in significant performance improvements: better thermal conductivity figures, greater reliability and smaller size.
Figure 2 describes a DBC-based compact IPM module from ON Semiconductor. It has voltage rating of 600V and is capable of supporting currents up to 15A and. Requiring far fewer components than conventional IPMs, it has a footprint of just 450mm2. This means that is around 40% smaller than the other DIP modules with corresponding specifications. Its thermal resistance is 2.6K/W, which is considerably less than such modules, despite the smaller size. Assuming that the power losses in the modules being compared were identical, for example by using the same drivers and power components, the temperature difference between the junction and ambient would be 5% lower.
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Figure 2 describes a DBC-based compact IPM module
The use of latest generation of highly integrated IPM modules based on advanced thermally conductive metal substrates has major implications for future developments within the white good sector. They provide the means by which to efficiently drive the 3-phase motors in modern white goods in order to provide energy efficient variable speed operation.
By utilizing a transfer-molded IPMs for the inverter power stage, as opposed to either a discrete arrangement or a conventional IPM (which both have the disadvantage of resulting in larger footprints), enables a downsizing of white goods, supporting power electronics. The strong thermal performance that is offered means that reliability is assured. In addition, distributed capacitance helps mitigate the effects of EMI.