2017.01.12

Points of this article

・In this section, we first consider transfer functions for voltage-mode control.

Voltage-mode control in a DC-DC converter is the most basic system. Only the output voltage is fed back through a feedback loop. By using the voltage difference obtained by comparison with a reference voltage using an error amplifier, and further comparing with a slope wave (triangular wave), the pulse width of a PWM signal is determined and the output voltage is controlled.

Fig. 1 shows a block in which the output V_{c} from the error amplifier and the slope voltage are input to the PWM generator to generate the PWM output.

Here, by comparing a fixed slope wave with V_{c}, the transfer function of the block F_{m}, which is PWM controlling the duty D = T_{on}/T, is derived.

Fig. 2 shows the input waveform and the PWM output waveform. The slope wave rises with a constant slope by V_{ramp} over one period. Through comparison with V_{c}, the on and off times of the driver output V_{Dout}, that is, the duty cycle, is determined. The control conditions are as follows.

When V_{c} ＞ V_{slope} , V_{Dout} = ON

When V_{c} ＜ V_{slope} , V_{Dout} = OFF

Moreover, in Fig. 2:

When V_{slope} = V_{c} = ０ , D = 0

When V_{slope} = V_{c} = V_{ramp} , D = 1

Equation 3-1 is obtained from similarity of the triangle formed from V_{slope} and V_{c}, and by modifying this equation using D = T_{on}/T, we obtain equation 3-2.

Moreover, from perturbation of equation 3-2 （ D → D + ?D , V_{c} → V_{c} + ?_{VC}） equation 3-3 is obtained.

Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.

- What are transfer functions?
- What is the Transfer Function of an Amplifier?
- What is the Transfer Function of a Slope?
- What is the switching transfer function?
- Transfer function of each converter
- Example of Derivation for a Step-Down Converter
- Example of Derivation for a Step-Up/Step-Down Converter – 1
- Example of Derivation for a Step-Up/Step-Down Converter – 2
- Switching Transfer Functions: Derivation of Step-down Mode Transfer Functions Serving as a Foundation