Accumulator and batterypowered applications place extra demand on DC/DC converters than the usual applications that use a mains-powered supply. The additional requirements include extremely high efficiency, a standby function with low residual current, and a very small case size. Galvanic isolation is not necessary because batteries are inherently isolated power supplies. Recom’s R-78 series switching regulators are in demandfor these types of applications. Another important factor is the input voltage range of the switching regulator. The charging voltage of an accumulator is always higher than its nominal output voltage, and a freshly charged battery will exhibit an output over voltage compared to its standard value. This means that even if batteries are not charged in situ, the full charging voltage has to be taken into account when deciding on the input voltage specification of the switching regulator. The battery voltage collapses at the end of the discharging cycle as the cells completely discharge and fail, hence, the output voltage of the accumulator drops rapidly by one cell voltage unit at a time. If the lower limit of the input voltage range is small, the remaining energy of the accumulator-pack can still be utilized accordingly. Since battery and accumulator powered systems are often used in uncontrolled environments, environmental considerations are important. The system must be sealed in a dustproof, watertight case. Any heat generated by the internal components cannot easily escape and heat sinks are inefficient because there is little free movement of air within the case. It is better to avoid generating any heat in the first place than add complexity, cost and weight trying to transport waste heat out of a sealed case. Even very small reduction in conversion efficiency can result in thermal problems and therefore efficiencies of over 95 percent are often called for. Isolated converters should only be used if absolutely necessary for the application, as the efficiency of lower power converters is typically around 80 percent. However, a switching regulator is an inherently low-loss device and efficiencies of up to 97 percent are obtainable. Since battery and accumulator applications also almost always require small-sized components, the SMD version of the switching regulator is particularly suitable. The high efficiency means that a heat sink is not required, and in the standard configuration, no additional external components are necessary. By connecting a single external resistor to the trim input, the nominal output voltage can be adjusted in the range of about ±50 percent. The remote on/off function is especially useful for accumulator and batterypowered applications. Via an external signal, for example from a microcontroller, the converter can be placed in standby mode so that power consumption is drastically reduced. The typical consumption of the switching regulator in standby is 20μA, which approximately corresponds to the self-discharge current of an accumulator. NEGATIVE RAIL In many applications, for example in applications where sensors are used, an additional negative rail is required which usually requires less current than the positive rail. Up until now, the usual solution has been to use an isolated DC/DC converter with dual outputs. Even though isolation is not required, the loads are asymmetric and the relative inefficiency can cause thermal problems, as describedearlier. The subsequent applications show how Recom’s standard switching regulators can be used to generate negative and dual rail outputs. (Figure 1) shows the basic application diagram of how a negative voltage can be generated from a positive voltage. Table 1 shows the pinouts for the R-78xx switching regulators in the standard and negative output modes. The mode of operation is different for the two output polarities and hence, electrical parameters are also different. Even so, efficiencies of up to 91 percent are possible, which compare favorably with isolated converters. The basic circuit arrangement can be used to build several different power supply configurations, but the most common application is the high-efficiency dual rail regulated supply shown in (Figure 2). Based on the desired output voltage, an input voltage range of up to 4:1 is possible. When compared to a standard isolated converter, efficiency savings of approximately 10 percent are achievable. Asymmetric output voltages with different voltages for positive and negative rails can be easily created using this circuit. Naturally, if the SMD versions are used, the additional functions of output voltage trim and on/off control can be utilized and this makes the circuit even more suitable for stored primary power supply applications. CONCLUSION Accumulator and battery powered applications demand a highly efficient voltage regulation circuit in order to utilize the limited stored energy sensibly. The very wide input voltage range of a switching regulator helps the application use the maximum available energy in an accumulator or battery. Furthermore, additional functions like a remote on/off pin, which will put the regulator into ultra low power mode, are very welcome. The R-78 series switching regulators are ideal for these types of application. Datasheets can be found on Recom’s homepage www.recom-power.com. About the author Carl Schramm is a Technical Support Engineer at Recom International Power. He can be reached at schramm@recomdevelopment. |
