Adding the depletion-mode FET QD, operating as a source follower, enables the circuit to scale up in voltage from the 45-V rating of the LM74610-Q1 to 100 V or more. To add this bypass switch to an optimizer in Figure 1, connect the ANODE and CATHODE pins to the VO- and VO+ lines, respectively.
Power Tips for Converter Designs and More
Power supplies are critical to almost all analog designs. Even circuits that run on battery power may need buck or boost dc-dc converters to develop the proper voltages for the various components used in the design. TI’s Best of Power Tips eBook includes a variety of designs for efficient low-noise converters.
For example, one chapter describes flyback converters that operate either in continuous-conduction mode (CCM) or discontinuous-conduction mode (DCM), with the latter mode often proving to be the compact, lower-cost option for low-power applications. Figure 3 shows a simplified schematic of a converter that can operate in either mode, depending on the timing of the switching transistor Q1.
With DCM, the current through diode D1 returns to zero before each switching cycle, which minimizes losses in the transistor and diode as well as permits the use of a smaller, lower-cost transformer (T1). In general, DCM supports lower transformer primary inductance than CCM, with the inductance determining the maximum duty cycle. In addition, DCM eliminates rectifier reverse-recovery losses and minimizes FET turn-on losses.
DCM’s drawbacks include higher peak primary and rectifier currents, higher input and output capacitances, and potentially higher levels of electromagnetic interference.
The eBook also presents a comparison of two circuit configurations for an electric-vehicle onboard charger (OBC): the capacitor-inductor-inductor-inductor-capacitor (CLLLC) and the dual-active-bridge (DAB) configurations, shown at the top and bottom, respectively, of Figure 4. Each has advantages and disadvantages. The DAB converter can provide higher levels of power over the entire output-voltage range, but it requires two commutation inductors (highlighted in yellow in Figure 4), whereas the CLLLC approach requires only one, thereby reducing weight.