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A guide to the fundamentals of DC-DC converter design

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By Ann-Marie Bayliss, Senior Product Marketing Manager, Murata and John Quinlan, Strategic Technical Marketing Manager, Murata

Most electronic equipment involves the use of DC-DC conversion. Switched-mode technology is an efficient solution for step-up and step-down voltages and insulation with small magnetic components. This article will examine this technology in detail and give some examples of commercially available products.

DC-DC conversion has always been a problem for product and system designers, and this became apparent in a significant way towards the end of the 19th century, when Edison was defeated in the "war of currents" against Westinghouse. The alternating current distribution of ever-increasing power required even higher voltages to keep current values within acceptable limits and to allow the use of reasonably sized cables. However, this was easily achieved with transformers. In those days, direct current (DC) could only be increased by means of bulky dynamo generators, and the rest is history: Westinghouse met the challenge and alternating current became the universally adopted standard for electricity distribution in the 20th century.

The last century has also seen the advent of the electronics age, whose components operate on direct current: hence the need to convert the AC alternating voltage of the distribution network into DC voltage. At this point it is useful to ask what kind of DC voltage is required. Circuits may require voltages of less than 1 V (in the case of a processor) or several kV (in the case of the magnetron of a microwave oven). In some cases, it is also necessary to provide such different values simultaneously. If it is necessary to guarantee the accuracy of the values of the rails (i.e. the terminals supplying the power) as the AC line voltage and the load change, an active regulation circuit is required. Early equipment used 50/60 Hz transformers to reduce the AC voltage to lower voltages which could then be rectified, DC smoothed and regulated to a lower voltage by means of a 'linear' series transistor. However, when the load, line and tolerances are taken into account, the power drawn from the AC line is approximately twice the power of the load (in the worst case). A large, heavy and expensive transformer is also not an ideal solution. If it is necessary to increase the voltage of a DC rail of a system, there is no linear way to achieve this.

Switched-mode conversion: solving the problem

A practical solution for performing a DC up-down conversion is the use of switching. When isolation is not required, buck and boost converters (or variants derived from these two topologies) are used. The buck converter (Figure 1, left) basically 'slices' (hence the name chopper) the high-frequency DC input voltage so that the average voltage has a lower value, and then levels the resulting waveform with an LC filter. The transistor of the chopper circuit is either completely on (on) or off (off) and, in both cases, dissipates little power, while the output voltage is set by the switching duty cycle of the transistor. The boost converter (Figure 1, right) operates in a slightly different way: the chopper alternately stores energy in the magnetic field of the inductor and then releases it. The energy can be released at any chosen voltage higher than the input voltage. Other types of circuit configurations, such as buck-boost and Ćuk topologies, can invert the voltage, while SEPIC, ZETA, and other topologies can produce positive output voltages at values lower or higher than the input.

Figure 1: Circuit diagrams of DC-DC buck and boost converters.

An example of a 78SR series converter from Murata [1], distributed by Darton, is shown in Figure 2. The input voltage range of the module is between 7.5V and 36V for an output of 3.3V at 0.5A. At full load with a 12 V input, the module provides an efficiency of 83%, while dissipation is approximately 0.7 W. It is pin-compatible with the linear regulators of the "78xx" series, which, being characterised by a much higher dissipation (4.35 W under the same conditions), require the presence of heat sinks.

Figure 2: Murata 78SR series buck converter with a rated current of 0.5 A

While this through-hole component can be used to replace an existing linear regulator for significantly higher efficiency, even better performance can be achieved using surface-mount PoL (Point of Load) type DC-DC modules housed in LGA (Land Grid Array) packages. Murata's MYMGA series of converters [2], for example, achieve efficiency levels of 94% with a full-load current of 4 A (5 V version) and come in a package size of 9×10.5 mm with a height of only 5.5 mm.

For high power ratings, "multiphase" buck converters are able to distribute component stresses among several switches and inductors, driven in two or more phases with common input and output capacitors. To ensure better efficiency, buck converters also use synchronous rectification, where the rectifier diode with a fixed voltage drop is replaced by a MOSFET with a reduced on-resistance value.

The need for isolation

Simple buck and boost converters do not provide galvanic isolation, their input and output grounds are connected. Often this connection must be broken in order to have a floating output. This can be attributed to several factors: the output is referenced to an unsafe voltage, preventing the circulation of ground currents, or simply being able to configure the output as a negative voltage by grounding the positive voltage. The equivalent isolated topologies of buck and boost converters are forward and flyback converters (fig. 3). As can be seen from the same figure, the inductors have been replaced by transformers so that an isolated winding can provide the DC output. Note the specific phasing of the transformer windings.

Figure 3: Diagram of flyback (left) and forward (right) converters

Complete regulation of isolated DC-DC converters is very difficult to achieve since it is necessary to detect the output voltage and transfer an error signal through the isolation barrier to the primary for duty cycle control. However, sometimes isolation is not necessary: if the DC input is constant, the output is only affected by load variations, which would result in a small percentage variation, a situation that is most often acceptable. One of the most common applications of low-power isolated DC-DC converters is to supply power to isolated data interfaces, where regulation is not critical. When the DC input varies, partial regulation can be used by carrying out a voltage sensing on the primary winding of the transformer assuming that the sensed value is similar to that of the output but, to ensure greater accuracy, the output voltage is sensed directly and an error signal is transferred to the primary usually via an optocoupler.

When insulation is required for safety reasons, the insulation and clearance configurations are complex. The creepage and clearance values and the distance across the solid insulation required depend on the degree of protection required (e.g. basic, double or reinforced) and other parameters such as the level of environmental pollution, the over-voltage category of the input and even the altitude. The standards that are adopted depend on the application: in the case of a medical application with connections to the patient, the separation distances must be greater than for an industrial application. Confusion can sometimes arise with regard to the declared insulation rating: some components are often declared to be tested in production at a voltage of 3 kVDC, which would seem adequate to guarantee insulation of a 230 VAC voltage. However, it must be remembered that this is a test voltage that has only been applied once, so there is no guarantee that these components will be able to withstand high voltages continuously.

Users should examine the actual certification issued by the safety agency, the level tested and the 'system voltage' referred to. A DC-DC converter that isolates a mains-powered circuit (230 VAC) from connections that a user might touch in a home or office environment should have a reinforced isolation of 250 VAC and can be used at a maximum altitude of 5,000 m as specified in EN 62368-1, the safety standard in force in Europe.

Figure 4 shows an example of an unregulated DC-DC converter derived from Murata's NXJ series buck topology (actually a push pull) that simply converts a 5 V voltage to a similar voltage in accordance with the isolation specifications of the Medical Application Standards Board. This product uses an innovative method of integrating the transformer core within the PCB stack, with the windings formed from the PCB's tracks and vias through several layers.

Figure 4: Murata's NXJ Series surface mount DC-DC converters feature insulation that complies with the specifications defined by the standards bodies.

The efficiency of resonant converters

The forward converter is available in several versions, each with its own advantages and disadvantages, often dictated by the inevitable compromises, in terms of efficiency, cost and size, required for the particular application considered in relation to the set power and voltage conversion values. In order to achieve optimum efficiency, resonant converters are often used, which, thanks to the "soft switch", change state when the current or voltage value is zero. This avoids the occurrence of a momentary spike in power dissipation caused by high voltage and current values at the same time. Although there are a number of resonant topologies, the most common for low to medium power applications is the LLC topology. The circuit applies pulses to an LC tank at a frequency usually slightly higher than its resonant frequency, and the pulses are transferred as sine waves to a secondary load winding on the inductor of the tank circuit by the action of the transformer. Regulation is achieved by varying the frequency of the pulse, which transfers more or less energy through the transformer as a result of the LC circuit inductor's impedance increasing with frequency, above resonance.

At high power levels, the stress on the switching transistors of the LLC converter increases to an almost unbearable degree, so a Phase Shift Full Bridge (FSFB) topology is typically used. This is another resonant circuit with a four-switch bridge configuration that operates at a fixed frequency, and regulation is achieved by varying the relative phase of the driving waveforms of each copper (leg) of the bridge. This technology is used in Intermediate Bus Converters (IBAs) such as Murata's DRQ series (Fig. 5). [5]

Figure 5: Murata DRE series converter

Switched-capacity DC-DC converters do not require magnetic components

In a non-isolated DC-DC converter it is not necessary to use an inductor or transformer: switched capacitor configurations can be used in this case. Capacitors are charged in series or parallel and arranged in parallel or series, respectively, to increase or decrease voltage values in discrete multiples.

Previously, voltage drops across the diode and losses in the switch limited the achievable efficiency but, with modern MOSFETs and synchronous rectification, it is now possible to achieve efficiencies of over 96% at 72 W, as is the case with Murata's innovative Psemi switched-capacitor technology (Fig. 6). [3]. In general, there is no active regulation and the upward or downward conversion takes place according to a fixed ratio of 3 or 4 [3]. In any case, this inductor-free technique is compatible with modern manufacturing methods and suitable for the manufacture of low-profile products.

Figure 6: Murata's Psemi switched-capacity technology

Mobile phones require the highest efficiency in power conversion

Low-power, non-isolated DC-DC converters are used in many commonly used electronic products such as mobile phones, where extending battery life is important and requires high efficiency at all stages of power conversion. Converters capable of regulating an output with a higher or lower input than output voltage are particularly important as a battery discharges and its output voltage decreases. These converters, often classified as "buck-boost", can also provide a negative output, which is not always useful. The SEPIC converter mentioned above (Fig. 7) is a fairly common choice that can provide a positive voltage with inputs of higher or lower value than the output. In the proposed circuit diagram, Q1 operates as a synchronous rectifier, while L1 and L2 can be separate inductors or a coil wound on the same core.

Figure 7: The SEPIC converter operates with an input voltage higher or lower than the output voltage.

Converters with extremely wide input range

The availability of a single DC-DC converter capable of operating at a wide range of battery voltages can undoubtedly simplify the development of applications for which the equipment manufacturer does not know exactly the type of battery that will be used by the user: a classic example is the railway sector, where battery voltages can vary from 24 to 110 V depending on the locomotive manufacturer and geographical region. Murata's IRH250 / IRQ150 series of converters (Fig. 8) are ideal for such applications due to their input voltage range of 16 to 160 VDC.

Figure 8: Murata's IRH250 extremely wide DC input range converters

The stringent requirements of the automotive sector

Small DC-DC converters used in the automotive industry typically operate in harsh environments and can be subjected to significant electrical stress. The qualification test requirements of the automotive AEC-Q standard are generally not applicable to power converters, which are therefore usually classified as AEC-Q104 qualified multi-chip modules. The device manufacturer must also be certified for compliance with TS 16949 quality control systems, which includes additional requirements on top of all those of the well-known ISO9001 standard. Murata's NXJ series converters mentioned above are an example of AEC-Q104 qualified components.

Although the overview given of DC-DC conversion has necessarily been concise, this article has sought to highlight some important aspects relating to the design types, performance and applications of DC-DC converters.

Bibliography:

For information [1] click here

For information [2] click here

For information [3] click here

For information [4] click here

For information [5] click here

 

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