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Issue 2 January 1996 Bipolar Transistor Considerations for Battery Powered Equipment Leading to Efficiency and Competitive

 

Application Note 21 Issue 2 January 1996 Bipolar Transistor Considerations for Battery Powered Equipment Leading to Efficiency and Competitive Advantages in Portable Systems Neil Chadderton Introduction The last few years has witnessed an increasing trend towards portability, this no doubt being due to a waiting market, and the advances in the enabling technology within the digital domain. This in turn has produced impetus to the advancement of the analogue technologies, as the customer requirements dictate a move to integrated, lower cost, energy efficient products. These new technologies include higher capacity battery systems and a re-assessment of power management techniques. The new philosophy includes careful charge control to ensure maximum battery capacity and lifetime, and consideration of voltage and current ratings - leading to the design and characterisation of components specific to the application. It is easy to assume that within say, a laptop computer or a mobile phone, that th e c i r c u i t bo a r d s w i l l b e w h ol l y populated with digital ICs, with little or no analogue circuitry. This is in fact far from the truth; as the control systems tend to incorporate more features, and microprocessors and microcontrollers move to higher speeds and lower operating voltages, the demands placed on the system power supplies, battery charge schemes, and circuit block power switches become more exacting. Portability implies smaller and lighter components, which is usually considered to be in opposition with higher power requirements. The analogue sections and consequently the s w i t c h i ng d e v i c e s us e d for the s e systems, must then be considered and chos en c a re fully to m e et product objectives. Zetex Semiconductors has developed a bipola r transistor technology that enables a range of devices ideally suited to many of the high current, efficiency conscious circuit concerns of todays battery powered and portable products. This technology, the Matrix Geometry, was initially introduced to effect small DC motor drivers for cameras, and has been greatly enhanced to provide a range of transistors un-matched by any other manufacturer. This range includes a 5A continuous device in the TO-92 style E-Line package; SOT223 rated up to 7A; the SuperSOT SOT23 series which includes a 3A continuous part; and the c a p a b i l i t y o f p r o d u c i n g S u p e r - transistors, thereby allowing cost-effective replacement of larger Darlington and MOSFET transistors. AN21- AN21- 1 Application Note 21 Issue 2 January 1996 Bipolar Vs MOSFET Bipolar technology has perhaps been somewhat overshadowed in recent years, particularly since the birth of the MOSFET. This is to be expected due to two main reasons. Firstly; each major new technological advancement brings a wealth of publicity, promotion and a vast exposure of new design methods and circuitry. Unfortunately, this same PR drive comes at some cost: it is by its nature very selective, and has led to a commonly held view that bipolar can always be replaced with a MOS based product, particularly when speed, cost effectiveness and on-state efficiency are of concern. This view is true only some of the time. If adopted in too general a fashion, this approach can lead to non-optimised products, with the usual market disadvantages in performance and cost. There is no single device, or single technology solution to every application. The second factor is a general stagnation of bipolar device research as semiconductor manufacturers move to the latest technologies. Zetex recognises that opti mised bipolar products offer the best fit design option in many cases, and has pushed the technology to higher performance standards than any of it's competitors. This section shows that modern bipolar technology, can provide a credible alternative to MOS based designs, and in many cases is the preferred choice. This is not to decry the use of MOSFETs, but rather to demonstrate that each device technology has it's advantages and disadvantages, and that each new application should be judged individually, not on a wholesale basis. The information shown in Table 1 provides a basic technology overview of important Bipolar and MOSFET characteristics. To do this, a comparison has been effected between a Zetex 3rd generation bipolar product such as the geometry/process used for the SuperSOT series, and a typical latest generation MOSFET device. 1. Bipolar still claims the highest silicon utilisation of any transistor technology. This is due to the pattern of current flow within the geometry. Optimised bipolar geometries force the majority of the current flow vertically through the structure. MOSFETs however, still need to channel the current initially in a lateral manner before conducting through the bulk of the device. This fact allows an optimised bipolar device to use a smaller area to exhibit a given level of on-state loss, or, put another way, a bipolar device can conduct higher levels of current for the same area of silicon. This smaller silicon area leads to smaller packaging options required to encapsulate that silicon (which is a main contributer to the final product cost) and of course smaller products. Figure 1 illustrates the Zetex pioneered Matrix geometry on which many of the leading edge products are based. Another point worth considering is how the on-state loss varies with temperature. While the components of AN21- AN21- 2 Table 1 Performance Feature Bipolar and MOSFET Technology 1. Silicon utilisation Overview. 2. Drive voltage 5. pure resistance will increase with increasing temperature, this may be compensated for (within a given drive condition) by decreasing threshold voltage (for the MOSFET) or increasing hFE (for the bipolar). MOSFET datasheets typically show RDS(on) increasing by a factor of x1.7 to x2 over the operating temperature range. Bipolar devices, particularly low voltage variants as designed for battery powered applications, show a high degree of Base Contacts Very low Very fast Rugged Price