How do you select and design the feedback loop for your dc to dc converter?

1 Types of feedback loops

There are two main types of feedback loops for dc to dc converters: voltage mode and current mode. In voltage mode, the feedback signal is derived from the output voltage and compared with a reference voltage to generate an error signal. This error signal controls the duty cycle of the switch that drives the converter. In current mode, the feedback signal is derived from both the output voltage and the inductor current, and compared with a ramp signal to generate a pulse width modulation (PWM) signal. This PWM signal controls the switch directly. Both types have advantages and disadvantages, depending on the converter topology, load characteristics, and design objectives.

2 Criteria for feedback loop design

To ensure proper operation and performance of the dc to dc converter, the feedback loop design should meet certain criteria. Stability can be achieved by selecting the appropriate loop gain, phase margin, and crossover frequency. The bandwidth should be high enough to reject noise and ripple, but low enough to avoid excessive switching losses or overshoot. Accuracy is accomplished by selecting the appropriate reference voltage, feedback divider, and compensation network. To ensure robustness, the feedback loop should use components with low tolerances, temperature coefficients, and drift.

3 Steps of feedback loop design

The feedback loop design process consists of four main steps: selecting the type of feedback loop based on the converter topology, load characteristics, and design objectives; calculating the open-loop transfer function of the converter; designing a compensation network to modify the open-loop transfer function; and verifying the feedback loop design. For example, voltage mode is suitable for buck converters with constant loads, while current mode is suitable for boost converters with variable loads. The compensation network can consist of passive or active components, such as resistors, capacitors, op-amps, or integrators. The verification should confirm that the feedback loop meets the design specifications and requirements.