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Capacitive energy storage for powering pulse loads

Posted: 23 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:pulse loads? energy-storage? capacitor? converter? differential amplifier?

Several applications take advantage from the use of high-voltage capacitive energy-storage to provide power during a loss of input voltage or an unexpected shutdown. These applications include telecom and servers, as well as power supply to repetitive high-power pulse loads, such as a camera flash or a radio transmitter.

Often, the stored energy supplies additional run time for a "last-gasp" write to memory to save important data, allowing for a "clean" uninterrupted system shutdown. Here, the storage capacitors supply a brief, high-power burst of energy and then slowly recharge over a longer time period. This allows the average input current to be low, easing the requirements on the input source and reducing the converter's size.

Let's take a look at a capacitive storage design example.

The boost converter below was designed to charge a 3400 uF capacitor bank to 32V and provide a repetitive output pulse load of 2.5A. The instantaneous output power can be as high as 80 Watts, but the unit only draws a maximum of 12W from the input. To do this, two control loops are implemented: one to regulate the capacitor bank charge current, and a second to regulate its output voltage.

Figure 1: Capacitor charging circuit integrates both a current and a voltage loop.

The current loop limits the charging current to a controlled level, limiting input power. Without the current loop, the input current would rapidly increase as the output voltage increased, causing the FET to hit its programmed current limit. While it may be acceptable to charge the output in current limit, it lacks accuracy that certain timing applications require.

To avoid hitting current limit, the converter's power stage would need to be designed to handle significantly more power, increasing its size and cost. Alternatively, current sense resistor R1 could be moved to the input side rather than the output. This would set the input current to a fixed level, keeping the input power nearly constant; however, the output capacitors would then charge non-linearly. Regardless of the current sense location used, once the output voltage reaches a set level, the voltage loop takes over and halts charging. The boost controller then operates in burst-mode until the load discharges the output capacitors.

The current loop uses a differential amplifier (U2) to scale current sense resistor R1's signal to the 1.229V voltage level required by the controller's feedback (FB) pin. Operational amplifier (op amp) U3 functions as an ideal diode, forcing only one loop to be in control at a time. Initially at start-up, when no output current is flowing and the output voltage is low, the FB pin voltage will be less than 1.229V, forcing the controller to start switching. Since the time it takes to charge the output capacitors is relatively long, the output current will ramp up quickly and the current loop will regulate first. The output of voltage op amp U3B will remain low until the output voltage reaches 32V. At which point U3B output will increase, holding the FB pin at 1.229V.

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