Low power voltage supplies

In the past, linear regulators for the stabilization of low power voltage supplies were, as a rule, always used. As long as the difference between input and output voltage was not too high, their relative inefficiency was acceptable. However, if the input voltage is variable, then the voltage drop across the regulator could become significant, leading to a further degraded efficiency, higher internal dissipation and higher running temperatures. A radically new solution is offered with the R-78xx-series from RECOM, a non-isolated DC/DC converter family which offers many advantages over traditional linear regulators for many applications.



The individual members of the R-78xx series are characterised by many different useful characteristics.



Thus the R-78xx-0.5 family in the 3-Pin SIP housing, whose pin layout and spacing are identical to 78xx-linear regulators, offers an extremely high component density and the highest efficiency (97% max.) for a maximum output current of 500mA. Output voltages covering the usual standard voltages from 1.5 to 15V are available. The converter dimensions are only 11.5 x 7.6 x 10.2mm, therefore they can be used as direct drop-in replacement for 78xx linear regulators, but without the need for a heat sink and the consequent requirement for board space.

SAME INPUT AND OUTPUT

The SMT version offers the same input and output voltages as the SIP version, but with the additional feature of a remote on/off pin, with which the converter can be switched into a standby mode. The standby current is typically only 20ìA (35ìA max), which offers many advantages for battery-powered applications where power conservation is critical. Additionally, the output voltages are adjustable, which simplifies the design of many high specification power supplies. The SMD package is a 9mm high, 10- pin DIL housing.



Both SIL and SMD families are specified for an ambient temperature of up to 85°C and an input voltage range of 4.5 to 34V. Additionally to the two families mentioned above, the members of the R-78xx-1.0 series can supply an output current of 1000mA. The physical dimensions match the R-78xx-0.5 SIP and SMD series, but the input voltage is reduced to 18V maximum, and the output voltage limited to 5V maximum. The R-78xx-1.0 SIL and SMD families are also specified for an ambient temperature of up to 85°C.



Examples of highly variable input voltages are when the energy source is derived from one of the many types of accumulators available, from car batteries, supplied by UltraCaps or in systems where the supply is switched from one source to another. As a result of the very wide input voltage range of the R-78xx families, many innovative new applications arise, as is shown in the following examples:



Figure 1 shows a circuit which produces an output voltage with very low ripple and a high efficiency. The complimentary characteristics of the two types of regulator are used to good effect.







Firstly, the adjustable output facility of the R-78A5.0-0.5SMD is used to efficiently reduce the input voltage, Ue, to just above the minimum voltage that the LDO (low drop out) linear regulator needs in order to deliver the required stable output voltage, Ua. LDO’s that deliver an output current of 0.5A need an input/output voltage difference of typically 150 to 300mV. The required intermediary output voltage is set by the resistor combination R1 and R2. The ability of the LDO to react quickly to regulate out the residual 20mVp-p ripple from the R-78A5.0-0.5SMD creates an extremely low noise output. This circuit is best suited for sensitive sensor technology or DAC/ADC applications. Even including the ‘lost’ current that flows down through the linear regulator’s ground pin, this circuit still has an overall efficiency of 86%.



Figure 2 shows a similar circuit that is well-known in high power supplies under the name “intermediate bus architecture” or IBA for short. IBA topology uses an isolated converter to reduce the main supply voltage to an intermediate bus voltage, which then feeds non-isolated point-ofload (POL) converters distributed over the circuit board near to their respective loads. Thus the desired end supply voltage is locally produced. This principle is used particularly to optimize the power losses and to reduce overall costs.







With the application presented here, apart from the cost reduction, different technical requirements are also at the centre of attention. Figure 2 shows a circuit that is ideal for lower power supplies and offers an optimal solution. In this circuit, the non-isolated DC/DC converter R-785.0-yy converts the widely varying input voltage into a stable and precise 5V “bus” voltage.



The isolated DC/DC converters downstream see a tightly regulated input voltage which means that their output voltages are likewise delivered with closer tolerances, even though they are unregulated. Concerning the overall efficiency, this serial connection of DC/DC converters still delivers an efficiency of 75%, which compares favourably with more expensive all-in-one solutions, which additionally need more board place on the printed circuit board. The inverse arrangement of the galvanic separation is chosen mainly for cost reasons.





OVERALL EFFICIENCY

With the original IBA layout, the cost saving is obtained by the employment of only one isolator in the system, whereby additionally an improvement of the overall efficiency is obtained. With the “inverse” IBA, the cost advantage lies, however, in the availability, wide choice and low prices of low power isolated DC/DC converters.



A further advantage over a modular solution is that the user can arrange arbitrary output voltages in arbitrary combinations to suit their own supply requirements. The wide input voltage specification is particularly useful in supply systems with multiple energy sources (e.g. mains/battery) as well as applications that need battery backups to eliminate power interruptions.



Figure 2 can be consulted again when considering battery powered systems or similar energy storage systems. When an energy store discharges, the output voltage always drops. The upstream converter makes as much use of the declining input voltage as possible, since it can regulate the output voltage over a wide input voltage range from fully charged to almost fully discharged. The lower the intermediate bus voltage, the more deeply the supply can be discharged before the supply fails. It must be noted, however, that if a very low bus voltage is chosen that the efficiency of the following converters will also be lower.



With applications where temporary supply interruptions (“brown-outs”) need to be taken into account, supplementary energy stores — “backup capacitors” are often used. Here a similar viewpoint to the battery supply application above applies. The more deeply the backup capacitor can be discharged, the smaller its capacity must be. Thus a substantial reduction in the board space requirement can be obtained, particularly if higher supply voltages are used.



Figure 3 shows a circuit, which prevents primary supply interruptions affecting the output supply. The diode D prevents the reverse discharging of the backup capacitor Cs if the input voltage drops below the capacitor voltage. Figure 4 shows the voltage interruption over time. UE is the voltage on the storage capacitor, UV the supply voltage, UEmin1 the minimum permissible output voltage and UEmin2 the minimum permissible intermediate bus voltage. If the output voltage is produced from the intermediate bus voltage via a step-up converter, then the backup capacitor can be more deeply discharged. Thus a larger interruption can be bridged with smaller backup capacitor.









SUMMARY

The RECOM converters of the R78 series offer, apart from advantages like high component density - 8,5 W/cm3, small volume - 0,9cm3 and a favourable price, further technical advantages as shown above. However, the total costs and the total space requirement of the overall system must be always compared when comparing different solutions.

With the applications shown in this article, decoupling capacitors have been omitted in order to represent the circuits more clearly.