Technical Information Site of Power Supply Design

- HOME
- Basic Knowledge
- Transfer Function
- Amplifier Transfer Functions
**Slope Transfer Functions: Consideration of Current Mode**

2017.01.12 Transfer Function

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

**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 V_{c} 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 (R_{s})×coil (inductor) current (I_{L}).

Current-mode slope waveform = fixed ramp wave + current sense gain (R_{s})×coil current (I_{L})

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.

Similarly to Fig. 2 in the section on "Voltage mode", when V_{c} > V_{slope}, control to turn on the high-side switch is continued. An offset corresponding to the product of the DC component of the coil current I_{L} and R_{s} 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 I_{L}. In other words, the current-mode slope waveform changes within the range indicated by equation 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 (V_{c}), minus the coil current feedback (R_{s}・I_{L}).

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.