Three Options for Optimizing Power Converter Control Loops

Almost every power supply has a control loop to ensure that the output voltage is a constant value. The power supply is designed to optimize the control loop to minimize the deviation of the control output voltage from the setpoint when there are fluctuations in the input voltage or load transients. An important relationship here is the relationship between the size of the output capacitor and the response speed of the switching regulator IC. If the loop response is particularly fast, a smaller output capacitor can be used while keeping the output voltage within the allowable range.

By Frederik Dostal, Field Applications Engineer, Analog Devices

question: Is there an easy way to select external components for a power converter?

Three Options for Optimizing Power Converter Control Loops

Answer: There are three methods.

Almost every power supply has a control loop to ensure that the output voltage is a constant value. The power supply is designed to optimize the control loop to minimize the deviation of the control output voltage from the setpoint when there are fluctuations in the input voltage or load transients. An important relationship here is the relationship between the size of the output capacitor and the response speed of the switching regulator IC. If the loop response is particularly fast, a smaller output capacitor can be used while keeping the output voltage within the allowable range. Therefore, optimizing the response speed of a switching regulator can reduce system cost and circuit space requirements because smaller output capacitors can be used.

Most switching regulator ICs have a compensation pin, usually called ITH or VC, for control loop adjustment. By clever selection of capacitors and resistors, poles and zeros can be added to the switching function of the control loop to ensure optimal dynamic performance and high control loop stability. But how to choose these compensation components?

There are three methods to use for this.

1) Manual calculation using data from data sheet:

The first method is to use the calculation formula in the data sheet of the switching regulator IC. Considering a selected power stage, the stabilization concept is presented. Figure 1 shows the LTC3311 IC with the corresponding ITH pin and appropriate compensation components.

Three Options for Optimizing Power Converter Control Loops
Figure 1. The LTC3311 switching regulator IC has an ITH pin for adjusting control loop speed and stability

Three Options for Optimizing Power Converter Control Loops
Figure 2. Selecting Compensation Components and Optimizing Control Loops Using LTPowerCAD

2) Using design tools:

The second way to find a suitable transfer function setting is to use LTPowerCAD®and other design tools to calculate external components. This method provides additional insight into the control loop response. Figure 2 shows the LTPowerCAD user interface with the control loop represented as a Bode plot and additionally showing the output voltage response to load transients in the time domain. The ITH setting value can be easily changed and the optimum setting can be found using this method.

Goethe said, “All theory is gray.” In practice, parasitic elements should also be considered and checked before moving from development to mass production. The selected compensation component is connected to the ITH pin and a load transient test is performed to check VOUTWhether the voltage change is within the allowable range, and whether the voltage converter works stably.

This hardware test checks only one setup option for compensation. However, parameter fine-tuning can be used to optimize this setting. To do this, all soldering must be done on hardware, as external components must be changed to new values ​​to find the optimal compensation component combination.

Three Options for Optimizing Power Converter Control Loops
Figure 3. Optimizing Compensation Components Using Analog Devices’ LB013A Board

3) The elegant approach – use a preconfigured RC network:

Figure 3 shows a third solution to the problem – using a preconfigured RC network, which is elegant. Analog Devices’ LB013A board is a small circuit board on which a simple switchable and adjustable RC network is implemented. The total capacitance and resistance values ​​can be changed by actuating small switches and rotating potentiometers. Without laboriously soldering compensation modules, compensation settings can be optimized in real-time during load transient testing. Boards like the LB013A are easy to manufacture, but can also be purchased from Analog Devices.

With these three methods of optimizing switching regulator compensation, any power supply can be compensated.

About the Author

Frederik Dostal studied Microelectronics at the University of Erlangen, Germany. He started his career in 2001 in the power management business and has held various applications engineer positions and spent 4 years in Phoenix, Arizona working on switch mode power supplies. He joined Analog Devices in 2009 and worked as a power management field applications engineer at Analog Devices in Munich. Contact information:[email protected]

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