Feature: Rail electronics


Resonant converters are effi cient 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. T ere are many resonant topologies,


Figure 5: Murata’s DRE range


regulation is not critical. When the input DC varies, ‘semi’ regulation can be used, sensing a primary winding on the transformer as an analogue of the output, but for best accuracy, the output voltage is sensed directly and an error signal passed to the primary, typically via an optocoupler. When isolation is required for safety


reasons, the spacings and insulation arrangements are complex. Creepage, clearance and distance through solid insulation 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, 3kVDC test in production, which might seem adequate for isolation of 230Vac. 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, level tested to and the ‘system voltage’ it refers to. A DC-DC converter isolating a 230Vac-mains-referenced circuit from connections a user can touch in a home/


24 March 2023 www.electronicsworld.co.uk


office environment for example, might show ‘reinforced isolation/250Vac, maximum altitude 5000m’ according to EN 62368-1, the European safety standard Figure 4 is an example of Murata’s


NXJ series of an unregulated buck- derived (push-pull) DC-DC converter, which simply converts 5V to 5V with agency-rated isolation for medical applications. The product features a novel method of embedding the transformer core within the PCB stack, with the winding formed by PCB tracking and vias across many layers.


but a currently favoured one for low to medium power is the LLC (logic-level converter). T e 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 resonance. At high power, the stress on the


LLC switching transistors becomes impractically high and then typically a ‘phase-shiſt full bridge’ topology is used. T is is another resonant circuit in a four- switch bridge arrangement but operates at fi xed frequency, with regulation achieved


Figure 6: Murata’s DRQ module


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