Power Electronics Europe April/May Issue 2022

www.murata.com CONVERTER DESIGN 33 www.power-mag.com Issue 2 2022 Power Electronics Europe The MYMGA series [2] for example, achieves 94 % efficiency at its full load current of 4 A (5 V version). This is all in a package 9 mm x 10.5 mm x 5.5 mm high. 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. Isolation is often required Simple buck and boost converters do not provide galvanic isolation – their input and output grounds are connected. Often, that link needs to be broken to allow the output to ‘float’. This 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. The 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, which might only change by a few percent, which is often acceptable. One of the largest applications for small isolated DC/DC converters is to provide power for isolated data interfaces where regulation is not critical. When the input DC varies, ‘semi’ regulation can be used, sensing a primary winding on the transformer as an analog of the output, but for best accuracy, the output voltage is sensed directly and an error signal passed to the primary, typically through an optocoupler. When isolation is required for safety reasons, the spacings and insulation arrangements are complex. Creepage, clearance and distance through solid insulation necessary depends on the level of protection required (basic, double or reinforced, for example) and other parameters such as environmental pollution degree, over-voltage category of the input and even altitude. The application sets the standards applied, with patient- connect medical for example, requiring larger separation distances than industrial. There can be confusion about stated isolation rating; parts are often promoted with say, 3 kVDC test in production, which might seem adequate for isolation of 230 VAC. However, this is only a one-off test voltage and does not guarantee that the part continuously withstands a high voltage. Users should look for an actual safety agency certification, the level tested to and the ‘system voltage’ it refers to. A DC/DC converter isolating a 230 VAC mains referenced circuit from connections a user can touch in a home/office environment for example, might show ‘reinforced isolation/250 VAC, maximum altitude 5000 m’ according to EN 62368- 1, the relevant safety standard in Europe. Figure 4 is an example of NXJ series of an unregulated buck-derived (actually push-pull) DC/DC converter which simply converts 5 V to 5 V with agency-rated isolation for medical applications. The product features a novel method of em bedding the transformer core within the PCB stack, with the winding formed by PCB tracking and vias across many layers. Resonant converters are more efficient The forward converter appears in many varieties with different pros and cons, often dictated by the application trade-offs of efficiency, cost and size for given power and voltage conversion ratings. For optimum efficiency, ‘resonant’ converters are often used, which ‘soft switch’, that is, change switch state while current or voltage is zero. This avoids the momentary spike in power dissipation if high voltage and high current occur together. There are many resonant topologies but a current favored one for low to medium power is the ‘LLC’. The circuit applies pulses to an LC ‘tank’, typically just above its resonant frequency, with the pulses then passed as sine waves to a secondary load winding on the tank inductor, by transformer action. Regulation is achieved by varying the pulse frequency, which passes more or less energy through the transformer as a result of the increasing inductive impedance of the LC circuit with frequency, above Figure 3: Flyback (left) and forward (right) converter outlines Figure 4: NXJ series of surface mount DC/DC converter with agency-rated isolation