AN 1320 / D 300 - Watt , 100 - kHz Converter Utilizes Economical Bipolar Planar

The continuous growth of SWITCHMODETM Power Supplies (SMPS) worldwide makes this market one of the most important for manufacturers of power semiconductors. It is particularly important for manufacturers of devices in the 3 A to 10 A current range since most of these power supplies are designed to deliver power in the range of 50 to 1000 W. Although MOSFETs are often preferred for the new designs, ON Semiconductor has extended the capabilities of bipolar transistor products to meet the needs of the line operated SMPS with a series of economical bipolar transistors. This new series is in full production as the MJW180xx series which takes advantage of a planar process developed by ON Semiconductor. This Application Note describes the design and performance a 300−W forward converter operating at 100 kHz utilizing the MJW18010 planar bipolar power transistor, and demonstrates the overall performance achievable with these new transistors. LINE-OPERATED CONVERTERS Voltage Requirements The planar process is well established for small signal transistors; however, due to its limited breakdown voltage, it was not used to manufacture the high voltage devices needed to design line-operated SWITCHMODE Power Supplies. For power supplies operating directly from the ac line, it can be seen from the standard equations of either the Flyback [1] or the Forward [2] converter topologies that high voltage devices are required. In single switch configurations, the BVceo of the transistor must be greater than 400 V, in conjunction with a minimum BVces of 800 V for circuits operating from a 220 V line. One can solve this problem by either using a collector-to-emitter clamp or using a dual switch design. The drawback will be higher cost and complexity of the total circuit; therefore, economical high voltage transistors are needed for single switch designs for these applications. Flyback Converter: [1] VCE(peak) Np Ns (Vout Vf) Vspike Forward Converter: [2] VCE(peak) VCC (1 Vclamp VCC ) where: Np = primary winding number of turns Ns = secondary winding number of turns Vout = output voltage VCC = DC supply for the converter, [( 2 )(265) for a high 220 V line] Vspike = uncontrolled voltage spike generated by the converter Power supplies operating from a nominal line voltage of 220 V must be capable of operating from 265 V maximum, therefore, for the standard 5 V output, the VCE(peak) will be, in a first approximation, equal to 750 V, assuming that Np/Ns = Vp/Vs and Vclamp = VCC. PLANAR TRANSISTORS FOR LINE OPERATED CONVERTERS In addition to high voltage capability, transistors for this application must have low conduction loss, fast speed and be economical. In the past, technologies such as triple diffusion and epitaxial collector with moats were used. These processes had sporadic yields and wide distributions of electrical characteristics. A planar process was developed in order to overcome these shortcomings. Since the voltage capability of the planar process was a key issue, considerable design effort was focused on this parameter. The objective was to develop transistors capable of sustaining more than 1000 V in the BVces mode and yet maintain reasonable current gain and fast switching speeds. A junction termination capable of sustaining over 1000 V was developed by putting a resistive field plate around the die. The cross section of a device utilizing this process is shown in Figure 1. APPLICATION NOTE http://onsemi.com