www.ti.com NETWORKING DESIGN 25 www.power-mag.com Issue 3 2023 Power Electronics Europe boost converter to charge the high voltage capacitor to 60V and switch in the capacitor when the holdup circuitry detects a power interruption. Figure 4 shows this proposed high voltage holdup solution. The boost converter is not in the main power path, and therefore does not affect system efficiency. The converter size is small given the low power level, which is just enough to charge the high voltage capacitor. The diode in Figure 4 could be a hot swap device or an ORing device, which is commonly available for telecommunication applications. The energy transfer switch also needs special attention. It has to be fast acting; otherwise, the design needs a large amount of fixed input capacitance. It also has to limit power. During energy transfer, the flyback converter may drop to minimal operation levels while the holdup capacitor is fully charged, creating a large differential voltage across the switch. At the same time, a large amount of current is injected into the flyback input, generating tremendous electrical stress on the switch. Figure 5 illustrates a scalable current source with on/off control. This energy transfer switch has a fast acting delay of less than 2.5µs. It also has an adjustable current limit set by the current-sense resistor. Connecting multiple current sources in parallel extends the power level. When the control FET (field effect transistor) gate is high, it pulls the main FET gate down, turning off the main transfer switch. Figure 6 illustrates the verification of this concept in an IoT application. The flyback converter has a wide input range from 9V to 60V and the output is 12V/5A. There is only one holdup capacitor. The boost converter is small and three current sources are connected in parallel, placed on the back of the board, to relieve the device stress. The worst-case test condition is when the input voltage is 9V. The small boost converter charges the holdup capacitor up to 60V. The power interruption detection circuitry sets the threshold at 8V. When the input voltage drops below 8V after a power interruption, the energy transfer switch turns on, thus transferring the energy from the holdup capacitor to the main flyback input capacitor. The result is that the Figure 3: The traditional two-stage holdup solution. Figure 4: A proposed single-stage holdup solution maintains high efficiency. Figure 5. A scalable current source with on/off control. Figure 6. An IoT system example using a 60W flyback converter, holdup capacitor and a small boost converter.
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