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Regulating multiple loops in power supply

Posted: 10 Apr 2015 ?? ?Print Version ?Bookmark and Share

Keywords:power supply? LED drivers? voltage? battery chargers? boost converter?

During my career as a power supply designer, the majority of the supplies I have designed have been single input, single output with a fixed regulated voltage. However, there are many situations where it is necessary to regulate something other than the output voltage.

For example, with LED drivers, it is often desirable to control the LED current. In battery chargers, the charging current is usually limited until the battery reaches a predefined voltage threshold; then the voltage is regulated. USB ports can only source a certain amount of current, which varies based on the application. In these cases, it is useful to be able to limit the input current. There are many different ways to accomplish these different functions. Let's go through a few examples and consider their associated tradeoffs.

For the first example, consider a battery charger where the output current is regulated to a certain level during fast-charging. Then a voltage loop takes over to regulate the battery voltage. Figure 1 shows the block diagram of an isolated flyback battery charger.

Figure 1: Isolated flyback for charging batteries.

During operation, only one of the control loops is active because the two are diode "ORed" together by D1. There are a couple of advantages to this approach. Both loops can provide very precise regulation. The compensation for each loop is separate, which makes it easier to properly stabilize both loops. During voltage regulation, there is an extra pole in the power stage that is not present during current regulation. One of the major drawbacks of this method occurs during rapid state changes.

When the power supply is operating in the current-regulation-mode, the voltage amplifier output is railed high. If the battery is removed, the current drops abruptly and the voltage loop needs to take over control. Because of the response time of the amplifier and compensation, this often leads to a condition where the power supply output overshoots. If compensation is adjusted to increase the mid-band loop gain, overshoot can be reduced. Another option is to add an extra diode (figure 1, D2) from the amplifier's output to the reference. This can help to clamp the amplifier's output to a lower voltage, preventing saturation and speeding up the response time. Another drawback of the multiple external amplifier method is that a separate reference and soft circuit need to be added. This increases the system's complexity and cost.

The second example comes from a boost converter designed to draw power from a USB port. The USB port, depending on type, can source 500 mA, 1A, 1.5A or even up to 3A. If an accessory tries to draw too much current, it can overload the bus and cause the port to overheat or shutdown the host device. One crude method is to monitor the input voltage and watch for it to fall below a certain threshold before reducing the load current. This method is okay, but not ideal. If the port contains a USB switch, this method could cause the port to continuously reset. If the current is not limited, it could cause more power to be drawn than the host can support. Another method for solving this issue is very similar to our battery charger example. However, this time we will regulate the input current and output voltage. Figure 2 shows the block diagram of an input current limited boost.

Figure 2: USB boost converter that regulates input current.

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