Feature: Rail electronics
by varying the relative phase of the drive waveforms to each leg of the bridge. Tis technology is utilised in Intermediate Bus Converters such as the DRQ and DRE series (Figures 5 and 6) from Murata.
Switched-capacitor DC-DC converters need no magnetics It is not necessary to use an inductor or transformer in a non-isolated DC- DC converter; switched capacitor arrangements can be used, which charge
Figure 7: Murata’s Psemi switched capacitor technology
capacitors in series or parallel then set them in parallel or series respectively to step down or step up voltages respectively, in discrete multiples. Previously, switch and diode drops have limited the efficiency attainable but, with modern MOSFETs and synchronous rectification, 96%+ can be achieved at 72W like with Murata’s Psemi switched capacitor technology; see Figure 7. Tere is typically no active regulation, and the step up or down is in a fixed ratio (3 or 4), but the technique, without an inductor, lends itself to modern fabrication methods and low- profile products.
Highest-efficiency power conversion Lower-power non-isolated DC-DC converters find a home in many electronic items, including cellphones, where maintaining battery run-time is important and is facilitated by high efficiency of all power conversion stages. Converters that regulate an output with the input higher or lower than the output voltage are particularly valuable, as a battery loses energy and its output voltage drops. Such converters are oſten classified as ‘buck- boost’, although strictly this gives a negative output, which is not always useful. Te SEPIC converter of Figure 8 is
a popular choice to provide a positive voltage with input above or below the output. Q1 in the schematic operates as a synchronous rectifier. L1 and L2 can be separate inductors or wound on the same core.
Figure 8: The SEPIC converter operates with input voltage above or below the output
Figure 9: Murata’s IRH250 ultra-wide DC input range converter
Ultra-wide DC input range converters Having a single DC-DC converter that can operate from a multitude of battery voltages can simplify applications where the OEM is uncertain which battery source will be used by the customer; for example, in railway applications the battery can range from 24V up to 110V depending on the manufacturer of the locomotive and the geographical region. Murata’s IRH250/IRQ150 (Figure 9) meets this challenge with a DC voltage input range of 16V-160V DC.
Automotive requirements Small DC-DC converters in automotive equipment can be subject to harsh environmental and electrical stresses. The automotive AEC-Q qualification test requirements are not generally applicable to power converters, so they are often classified as ‘multi-chip modules’ to AEC-Q104. The manufacturer of the device must also have certification to TS 16949 for their quality management system, above and beyond the ISO 9001 standard. The NXJ series from Murata in Figure 4 is an example of an AEC-Q104 -qualified part.
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