24 NETWORKING DESIGN www.ti.com Issue 3 2023 Power Electronics Europe www.power-mag.com IoT calls for a new approach to backup circuitry design Two backup options are compared and new backup circuitry is proposed to meet a 15ms holdup time for a 12V/60W flyback converter with a 9V to 60V wide input range. By Tiger Zhou, Applications Engineer Battery Charging Products, Texas Instruments In telecommunication applications, network devices often need input status data so that they can send out dying lastgasp messages to users in the event of a power interruption. These network devices rely on temporary energy storage such as capacitor banks, which enables graceful shutdowns and the generation of these messages. The backup (holdup) circuity is designed to last from 10 to 20ms in order to perform these tasks. This extended period is called the holdup time. Power supply designers are likely to have two questions about the holdup circuitry, especially for a wide input DC/DC converter. The first is should the holdup capacitor be placed on the input side or output side. Traditionally, the power supply has a bulky output capacitor bank. The output capacitor holds up the output voltage and slowly decays, thus extending operation time before total system shutdown. The holdup energy, Ecap is quadratically proportional to the capacitor voltage, V as shown in the equation: where, Ccap is the capacitance. Since the output voltage is slowly decaying, it requires a downstream system with a wide input voltage tolerance. If the input range is limited, the energy utilisation is poor. In the following equation, the energy utilisation rate (EU%), is defined as a percentage of energy used over the energy stored: The second question is: for a wide input range is a two-stage or single-stage approach preferrable? Where to place the holdup capacitor Consider a 60V input, 12V/60W flyback converter as an example, with a design holdup time of 30ms. In a typical 12V system operating with a minimum 8V input, the utilisation on the capacitor bank would be 55%. For sensitive equipment with a tight voltage tolerance, such as 10%, the utilisation rate would be just 19%. It is also possible to use high voltage capacitors on the input side. If the input voltage is allowed to discharge from 60V to 9V, the energy utilisation rate improves to 97.8%. A high voltage capacitor has higher energy density than a low voltage capacitor. For example, a 1,200µF, 80V aluminium capacitor is the same size as a 6,800µF, 16V aluminium capacitor, but its energy density is 4.4 times higher than the low voltage capacitor. There are two designs to consider. The first design uses a simple and straightforward approach, with holdup capacitors on the output side. This requires seven 6,800µF, 16V, 16 x 40mm output capacitors, which occupy more than half the available board space. The holdup time is an estimated 32ms with a full 60W load. The second design uses one high voltage, 1,200µF, 80V, 16 x 40mm input capacitor as the energy source. This single input capacitor provides a 32ms holdup time at a full load, assuming 90% system efficiency. The system efficiency reduces the available holdup time, since the flyback converter processes the input-side energy. The first design (shown in Figure 1) measures 116.84 x 93.98mm (4.6 x 3.7 inches) which is twice as big as the second design, shown in Figure 2. The holdup time is 32ms for both designs. This comparison shows that placing the high voltage capacitor on the input side results in the use of fewer capacitors and that the input-side holdup design halves holdup capacitor bank size - and cost. Comparing approaches If the converter has a wide input range, such as 9V to 60V, the stored energy and energy utilisation rate will drop significantly as the input voltage level drops. At the minimum 9V input, the high voltage input capacitor offers virtually zero holdup capability. One quick remedy is to add a boost converter in the front end (Figure 3). The boost converter steps up the wide input to 60V or higher. There are drawbacks to this two-stage approach, however. It lowers system efficiency and adds extra cost. An alternative is to use an auxiliary Figure 1: The holdup solution with the capacitor located on the output side. Figure 2: The input-side holdup design option measures 116.84 x 47mm. Ecap = 1 CcapV 2
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