POWER ELECTRONIC


ONICS


FEAT RE FEA ATURE


OPTIMISING A SWITCHING DC/DC CONVERTER DESIGN


OPTIMISING A SWITCHING DC/DC CONVERTER DESIGN


Rohmhas dev


rve. For instance, there are significant differences between the synchronous and nonsynchronous rectificationmethods for DC/DC conversion systems. Because each method has its own pros and cons, it is important to use themappropriately to suit the specific requirements for a given power supply.


he on-board implementation, which is a technique of incorporating switching regulators onto a circuit board through the use of a switching regulator IC, is no longer a curiosity. However, because the switching regulator is an analogue feedback circuit that performs high-speed switching operations, there are some key points to observ


T Rohm has devel ped a synchronous boost DC/DC conver eveloped a synchronous boostDC/DC conve ter IC for sin le verter IC for single-cell b tte ll battery applica ioications


enough VGS, that is, a voltage higher than the drain voltage. Normally because the drain voltage is Vin (input voltage), representing the highest voltage in the circuit, any higher voltagemust be


,


In terms of a circuit configuration, the question is whether Switch 2 is a diode or a transistor. Nonsynchronous rectification is also referred to as diode rectification, where depending on whether the upper side transistor is on or off, the diode becomes conducting or non-conducting, or the current flows or ceases to flow. Known for simplicity of op eration and rugged construction, the nonsynchronous method is widely used in industrial equipment and devices.


In the synchronous rectificationmethod, by contrast, which shares the same basic operation, the control circuit also controls the on/off operations of the low-side switch. Because if both switches turn on simultaneously, a current flows directly


destroys the transistors. T o fromVin to GND, the event


prevent this potentially


possibility, what is called “dead time”, in which both switches remain off,must be provided, adding complexity to the control operation. Nevertheless, because of its high efficiency when compared with the nonsynchronousmethod, the


synchronous rectificationmethod – like in Rohm’s new BD1865GWL contributed substantially to


increases in - has


runtime for battery-driven devices. The synchronous rectificationmethod offers a higher efficiency d ue to th e replacement of a diode in the


resulting fromthe output-stage switch. Although the diode VF changes as a function of current, it amounts to 0.3 to 0.5V even in low-VF Schot tky diodes. By


contrast, the on-resistance of an Nch- MOSFET is exceedingly low,measuring 50mΩ. In terms of a voltage drop, it is significantly lower than the VF for diodes. In sum, the synchronous rectification method delivers high efficiencies but usually requires some increases in the number of components and entails amore complex circuitry when compared with the nonsynchronousmethod which – less efficient than the synchronousmethod – consists of a relatively simpler circuit. Due to the low on-resistance, the Nch


nonsynchronous rectificationmethod with MOSFET can be used as a power switch to a transistor, thusminimising loss


improve efficiency. Given the same on- resistance, the NchMOSFET can be a lower cost option compared with the Pch MOSFET. Turning NchMOSFET completely on as a high-side switch requires a high


/ ELECTRONIC S ELECTRONICS


Figure 1: Synchronous rectification


Synchronou Figure 1:


Figure 2: Figure 2:


Nonsychronous (diode) rectification


Nonsychronous (diode) rectificatio n


s rectification


supplied externally. To generate that high voltage, a bootstrap circuit is used. Simple in structure, a bootstrap circuit is a step- up charge pump composed of a switch, a capacitor, and a diode, where a voltage equal to the switch voltage (Vin) plus the internal supply voltage is used as the gate drive for the high-side NchMOSFET . Nowadays a number o comes with a bootstra Lately, NchMOSFETs used for the output sw


p circuitmounted. f power supply ICs


itching transistors in have beenmainly


medium- or larger-power circuits, fuelled by a trend that gives priority to efficiency at the expense of amodest increase in the number of components required.More recently, there has been an increase in the use of circuits incorporating external diodes as a way of cutting the number o f required components.


Incidentally, for the same reasons, bootstrap circuits are also used in nonsynchronous rectifying circuits. Beyond the NchMOSFET, bootstrap circuits are also applied to reduce the saturation voltage in circuits that use bipolar NPN transistors.


Just recently, Rohmhas announced the availability of a compact, high-efficiency, high power synchronous boost DC/DC converter IC optimised for single-cel l battery applications. Many of today’s portable devices are driven by single-cell li-ion batteries that deliver an output of approximately 4.2V at full charge.


However, when connecting to interfaces requiring 5V output, such as USB, HDMI, and audio amps for speakers, a DC/DC converter is needed to boost the voltage. In response, the BD1865GWL features a lower operating limit of 2.5V, prolonging battery life in devices by extending operation down fromthe conventiona l 2.7V. When the remaining battery level is into the system e) decreases.


(battery output voltag low, the input voltage


Rohm Semiconductor www.rohm.com


www.rohm.com 01908 272400


ELEC RO ELECTRONICS CS | JUNE 201 JUNE 2016 17 17


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