SOW Forward Converter With Synchronous Rectification And Secondary Side Control

This article reviews a new technique for secondary side control of a DC-DC converter without using an auxiliary bias supply. A unique primary side start up circuit is used to drive the primary side power transistor. Gate pulse information for the primary transistor is transmitted through a pulse transformer that is shown to be smaller than an opto-isolator. The discussion includes transformer-reset technique, pulse edge transmission circuit design and forward converter design procedure. The converter start-up transient and control hand-off are explained in detail. Experimental results include operating waveforms, start-up waveforms, and efficiency. among paralleled modules or to enable timed gate drives to a synchronous rectifier. INTRODUCTION General Overview This design review will cover the design and verification of a 48V isolated converter to supply lSA at 3.3V. Small size and high efficiency are attained through the use of a forward converter power stage with controlled synchronous rectifiers on the output. A new technique is introduced to provide controlled primary side startup with a control hand-off to a secondary referenced controller. This allows the converter to leverage the benefits of a secondary controller without the expense and complexity of an auxiliary supply. The solution is shown to be both very small and reliable. The market for DC to DC converters has expanded rapidly with the industry shift to distributed power system architectures. Low voltage logic is the driving force that pushes the market in this direction. Conceptually, power is distributed at moderately high voltages, where conduction losses are low, and have localized low voltage-high current supplies for point-ofuse regulation. An additional benefit is realized with distributed power system architectures in modular card based systems; the logic voltage can be easily changed while updating the functional circuitry without changing the main power distribution bus. Secondary Control Bias Techniques Successful implementation of a secondary side PWM controller requires a means to provide a bias voltage to start and run the control circuitry which is galvanically isolated from the input power source. In the past, several methods have been used to provide bias power to the secondary side of a converter, with most solutions requiring some type of isolating transformer. DC-DC converters such as the present supply usually have a primary to secondary isolation voltage of 1500V, while offline supplies generally must have 3750 Vrms isolation. A power distribution bus of -48V has been common in telecommunication applications for many years, and +48V has become one of the most general voltages for distributed power architectures. There is a considerable need for an efficient solution to power low voltage logic for operation in these systems, with both small size and high performance. Secondary side control is used in power converters when the output parameters need tighter control or increased functionality over that possible when using primary side control techniques. Secondary side control can be used to implement current sharing