The output characteristics can be modified by means of the control of duty ratio. As a matter of fact, the feedback path will control duty ratio to maintain constant output voltage no matter what conditions of input and output. Generally, there are two major options to control the duty ratio in DC-DC converter, voltage mode and current mode. The following sections are therefore focused on how the duty ratio is well elaborated. [4]
1.4.1 Voltage Mode Control
For voltage mode, as its name implies the output voltage is the parameter feedback loop mainly controls. As illustrated in Figure 7 that is very typical buck converter with voltage mode controller, and it is not difficult to know that voltage mode is a single loop control. In voltage-mode control, a control signal is compared to a ramp wave to control the duty cycle of the power switch. The higher the error voltage, the longer the switch is on. And the error voltage is derived in the feedback loop from the error amplifier that amplifies the difference between the output voltage and the reference voltage. It is applying pulse width modulation (PWM) to regulate the output voltage as shown in Figure 8 how the switching signal is modulated by the control signal which is generated from error amplifier and then compared to a ramp wave.
Voltage mode is a direct duty cycle control as the control signal or error voltage directly drives the duty ratio, and consequently it has a wide range of duty ratio; however the bandwidth of voltage mode control would be limited by the compensation of LC resonant frequency, and then the performance of the transient characteristics is dramatically poor if the bandwidth is not wide enough. In the next chapter, the solution to this problem will be further studied with conventional off-chip and on-chip compensation.
Figure 7 Voltage mode control buck converter
Figure 8 Voltage mode control duty ratio waveforms
Voltage Loop
Compensator Control
Signal
Ramp Control Signal
Switching
signal
1.4.2 Current Mode Control
Current mode control was formally introduced to the power electronics world in 1978 [10]. It was quickly accepted as the common approach to control power supplies. A standard implementation of current-mode control is shown in Figure 9 A general implementation of current mode control Figure 9. Compared to single loop in voltage mode, current mode control features a current loop or the inner loop in Figure 9 and a voltage loop or outer loop.
The error voltage is used to directly control the peak of the switch current. (A sawtooth ramp is often used in most cases and is referred to as slope compensation in current mode control.) This dramatically changes the behavior of the DC-DC converter. From the mathematical point of view, the transfer function of voltage mode is the second order which is corresponding to LC double poles, and the control to output with current mode is the first order. The major pros and cons of current mode control are briefly summarized in Table 2.
Figure 9 A general implementation of current mode control
Inner Loop
Outer Loop
Table 2 Pros and cons of current mode control [10]
Control to output T.F. of Voltage mode % Control to output T.F. of current mode
%
It takes additional circuits to accurately sense either switch current or inductor current. The current enough signals to control the converter smoothly over the full range of operation. Solution includes filtering the current wave form in different places for example in buck it would be the output side or leading edge blanking where the most noise in the initial part of the signal is ignored. Even with these approaches, which all increase circuitry complications, current mode control could be not working.
1.Easy Compensation:
With voltage mode, the sharp phase drop requires a type III compensator to stabilize the system. Current mode control looks like a single pole system, and a type II compensator is adequate.
2.RHP Zero Converters:
In a converter where the crossover frequency is restricted by the presence of an RHP zero.
Although current mode control could not eliminate RHP zero, it makes the compensation easier.
3 CCM and DCM Operation
When moving from CCM mode to DCM mode, the characteristics with voltage mode control are very different. It is difficult to design a compensator with voltage mode that provides good performance in the two regions.
4 Line Rejection
Closing the current loop gives a lot of attenuation of noise. For buck, it can even be nulled under some specific conditions with proper compensation ramp.
As the pros mentioned in Table 2, the technique how to accurately sense the inductor current or switch current, and the proper slope compensation to prevent sub-harmonic oscillation instability are two major disadvantages for current mode control. The followings are to take a look at the techniques of current sensing and slope compensation.
1.4.3 Slope Compensation
Sub-harmonic oscillation is a well-known problem for current-mode switching converters when the duty ratio larger than 0.5 as shown in Figure 10. To avoid sub-harmonic oscillation, the slope compensation ramp is required to suppress the issue on as illustrated in Figure 10.[3]
Figure 10 Sub-harmonic oscillation and slope compensation
1.4.4 Current Sensing
In Table 2, it also reveals that current sensing scheme is also playing a very key role for the converter with current mode control to precisely reconstruct the current signal of inductor.
As shown in Table 3, it provides the reference of current sensing techniques for their advantages and the main concerns.
Table 3 Overview of current sensing techniques [13]
# Technique Advantage Main Concern
1 External sensing
Complexity of design with different topologies, and The exact value of the inductor should also be known.
Moreover, the accuracy of this current sensing scheme is influenced by different factors such as the time-constant value tolerance of the filter and manufacturing tolerances on the inductor.
3 On-resistance of
power switch Lose-less
Low accuracy due to that it is also sensed by a resistor, and the on-resistance of switch varies with temperature and different processes and thus affects accuracy.
4 Internal current
sensing Integrated on-chip
Increased the complication of circuit design for accuracy and variation from temperature, process and switch frequency.