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2017.01.12 Transfer Function

Slope Transfer Functions: Consideration of Current Mode

DC/DC Converters: Sharing of Transfer Functions among Control Systems

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In succession to the voltage mode discussed last time, we consider the second type of slope transfer functions, which are current mode transfer functions.

Fig. 3

Fig. 3

Consideration of current mode

Current-mode control in a DC/DC converter is, together with voltage mode, a basic technique. In the voltage mode, the output voltage Vc of an error amplifier is compared with a fixed slope wave (triangular wave/ramp wave) to determine the PWM signal duty cycle.

On the other hand, in the current mode, control is executed using a slope waveform resulting by adding a fixed ramp wave to the current sense gain (Rs)×coil (inductor) current (IL).

Current-mode slope waveform = fixed ramp wave + current sense gain (Rs)×coil current (IL)

The current sense gain is a constant that indicates by how many times the error amplifier output has changed compared with a change in the coil current.

As shown in Fig. 3, generally a method is employed in which the coil current is detected when the high-side switch (transistor) is turned on and the information is used. Fig. 4 shows the slope waveform when using this technique.

Fig. 4

Fig. 4

Similarly to Fig. 2 in the section on "Voltage mode", when Vc > Vslope, control to turn on the high-side switch is continued. An offset corresponding to the product of the DC component of the coil current IL and Rs occurs with the timing of turning-on the high-side switch, and from then until the switch is turned off, a slope is generated by the ripple current in IL. In other words, the current-mode slope waveform changes within the range indicated by equation 3-4.

式3-4

In the current mode, the slope waveform shown in Fig. 4 and described by equation 3-4 is used to control the duty cycle, and so the duty cycle is determined by the output voltage of the error amplifier (Vc), minus the coil current feedback (Rs・IL).

Next time, a current-mode transfer function will be derived drawing on what has been discussed up to this point, and including a comparison with the voltage mode.

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