Feature: Rail electronics


Figure 2: Murata 78SR series buck converter, rated at 0.5A


AC distribution voltage to DC became necessary. But what DC? Circuitry can require anything from sub-1V for a processor to several kV for the magnetron in a microwave oven, sometimes together. If the voltages required need to be accurate with changes in load and AC line, active regulation circuitry is also needed. Early equipment used 50/60Hz transformers to drop the AC to lower voltages, which could then be rectified, smoothed to DC and regulated down to a lower voltage by a ‘linear’ series transistor; but when load, line and tolerances are taken into account, the power taken from the AC line is around twice the load power, worst case. Te transformer is also large, heavy and expensive, making the arrangement far from ideal. And if a system DC rail needs to be stepped up in voltage, there is no ‘linear’ way to achieve this.


Switched-mode conversion solves the problem Te practical solution to efficient DC down- and up-conversion is the switched- mode technique. When isolation is not needed, buck and boost converters or derived variants are used. Te buck (Figure 1, leſt) effectively ‘chops’ the input DC at high frequency so that its average is lower, and then smooths the resulting waveform with an LC filter. Te chopping transistor is either fully on or off, in both cases dissipating little power; the output voltage is set by the transistor’s switching duty cycle. Te boost converter (Figure 1, right)


operates a little differently – the chopping alternately stores energy in the inductor


Figure 3: Flyback (left) and forward (right) converters


package of only 9mm x 10.5mm x 5.5mm. At high power, ‘multiphase’ bucks spread


the component stresses across duplicated switches and inductors, driven in two or more phases with common input and output capacitors. For best efficiency, bucks also use synchronous rectification, where the rectifier diode with its fixed voltage drop is replaced by a low on- resistance MOSFET.


Figure 4: Murata’s NXJ series of surface- mount DC-DC converter with agency-rated isolation


magnetic field, then releases it. Energy can be released at any chosen voltage, higher than the input. Other circuit arrangements such as the simple buck- boost and Ćuk can produce voltage inversion, while the SEPIC, ZETA and others can produce positive output voltages lower or higher than the input. An example of Murata’s 78SR series


buck is shown in Figure 2. Te module has an input range of 7.5V to 36V for an output of 3.3V at 0.5A. At full load and 12V input it achieves 83% efficiency, dissipating about 0.7W. It is pin-compatible with the popular 78xx series linear regulators, which would dissipate a stressful 4.35W under the same conditions, requiring substantial heatsinking. While this through-hole part can


change out an existing linear regulator for a boost in efficiency, better performance still is achieved by surface mount point-of-load (POL) DC-DC modules with land-grid array footprints. Te MYMGA series from Murata for example, achieves 94% efficiency at its full load current of 4A (5V version), in a


Isolation Simple buck and boost converters do not provide galvanic isolation – their input and output grounds are connected. Oſten, that link needs to be broken to allow the output to ‘float’. Tis could be because the input is referenced to an unsafe voltage, to prevent circulating ground currents, or simply so that the output can be configured as a negative voltage by grounding the positive. Te equivalent isolated topologies to buck and boost are forward and flyback converters (Figure 3), which can be viewed as converting the inductor in each case into a transformer so an isolated winding can provide the DC output. Note the specific phasing of the transformer windings. Isolated DC-DC converters are more


difficult to fully regulate, as the output voltage has to be sensed and an error signal passed back across the isolation barrier to the primary to control duty cycle. Sometimes regulation is not necessary, however; if the input DC is constant, only load variations affect the output, changing only by a few percent, which is oſten acceptable. One of the largest applications for small


isolated DC-DC converters is to provide power for isolated data interfaces where


www.electronicsworld.co.uk March 2023 23


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44