Power Tips: How to control multiple loops in a power supply


Power Tips: how to control multiple loops in a power supply

Usually the output voltage of a power supply is regulated and provided to a load. In some cases, the output current can be regulated (such as in a LED application). Some applications do require more complex control, however. A battery charger uses two loops: one to control the output voltage and another to control the output current. If the battery voltage is too low, the power supply will try to increase the output voltage by pumping current into the battery. This current needs to be regulated or at least controlled to a maximum level. At the same time, the battery voltage will start to rise; generating the need for this level to be clamped. Figure 1 shows a block diagram of how to implement this.


Figure 1: Block Diagram of a Flyback Battery Charger

A practical example of how this system would work can be seen in TIDA-00200. This is a 200W interleaved flyback design using the LM5032 controller and TLC272 dual-channel amplifier. The design is set up to deliver a constant-current/constant-voltage profile. The max charge current is 9.5A and the max voltage is 21V. Figure 2 shows the output voltage and current behavior as measured on the board.

Figure 2: CC/CV Profile of the Charger

At current levels below 9.5A, the voltage is clamped to 21V. As the current is increased to above 9.5A, the voltage decreases. During this operation, only one of the loops (current or voltage) is active at a time. Take care when transitioning between the two loops. In some cases there could be an overvoltage or overcurrent condition. But because the battery is connected to the output, the issue is mitigated.

The multiple-loop architecture is very useful in a number of other applications, including:

  • Power supplies that use USB input source and need to limit the input current.
  • Charging super caps
  • Inrush current limiting
  • Power limiting (input and/or output)
  • Maximum power point tracking (MPPT)

The level of information I covered here is just the beginning. To learn more about controlling multiple loops in a power supply, see my latest Power Tips post on EETimes.

For more information on TIDA-00200, our 120V AC input 200W interleaved flyback for battery charging applications please ti.com/tool/TIDA-00200.

  • Robert,

    I spent a couple hours looking over the TIDA-00200 design. Its very close to what I need. I have 2 questions

    1. The flyback transformers, G135067LF, look like custom parts from GCI. Do you have the design files for this part? I'd like to compare the design to the equations provided.

    2. You mention MPPT above,  do you have a write up  that discusses the electrical control mechanism, does it control a LM5032 type part, or is there a different mosfet driver.

    Thanks,

    Craig

  • Robert,

    Is it possible to use multiple loops (having +15V & -15V as their supply voltages) with PWM IC UC1825?

    Thanking You

    kamlesh