One of the fundamental decisions required at the outset of a power supply design is whether to use voltage mode control (VMC) or current mode control (CMC). For any design, a trade-off exists between the two implementation methods. However, these tried and trusted trade-offs are facing a serious challenge due to the introduction of digital control methods. This article will examine the main line drawn and how digital control has become an unstoppable re-perception method in the battle between whether to use VMC or CMC.

A great deal of detailed material describes both current mode control and voltage mode control, so this article will only briefly review them. VMC consists of a single control loop that uses the output voltage as the feedback signal. After the output voltage changes, the loop is reactivated to produce an instantaneous response to the change. Since the goal is to maintain a constant output voltage, a slow response is an obvious drawback. CMC adds an external feedback loop. In addition to the voltage control loop, it is also sensitive to the direction of current flow and the turn-off action of the power switch when the peak current is reached. The current signal serves as the primary control loop. One might think that VMC would have a slower response than CMC, but this view is incorrect because the loop response is ultimately determined by the system bandwidth. Both approaches can be designed to have the same loop bandwidth, so VMC is not necessarily slower than CMC.

Overcoming VMC Issues

So let's examine the traditional view of why VMC or CMC are the preferred solutions. In a push-pull or full-bridge circuit topology, the current flowing to the two legs of the switch circuit may not always be equal. This will cause FET imbalance or timing errors. The network effect is that DC buildup on the transformer will quickly cause the transformer core to saturate. The traditional method is to use a capacitor in series with the main transformer.

This DC blocking capacitor couples the AC signal, which prevents the transformer core from saturating. The disadvantage of this is that it is connected in series with the main power path, so it must be a reliable device. The rating of the blocking capacitor of the 10uF level required for 1kW power supply must exceed the full voltage fluctuation. In addition, transformers are usually designed with redundancy to suppress saturation. Since the control loop of the CMC turns off the power switch when the same current limit of both pins is exceeded, the CMC will not be damaged by this appearance. This means that no DC current is established in the transformer.

Figure 1 illustrates how digital control solves this problem. The controller measures the current in each half-bridge circuit and calculates the difference. The difference signal drives the main PWM signal to compensate for the imbalance. Conceptually, this method is similar to CMC, which can be viewed as average CMC rather than peak CMC.

Figure 1: Digital controller with integrated voltage secondary balancing circuit.

Loop Stability and Filtering

It is often considered easier to stabilize a CMC loop than a VMC loop because the control output transfer function of a CMC loop approximates a unipolar roller (-20°dB per decade). A VMC loop approximates a bipolar roller (-40°dB per decade) transfer control, which makes the compensation network more complex. In another area, digital control has brought a new face to the battle. The advent of user-friendly graphical user interfaces (GUIs) means that loop compensation has become a simple task. The designer enters the desired filter response, and the GUI calculates the filter coefficients needed to achieve the response. Not only does a well-designed GUI remove external complexity, it is also a major step forward because it makes the entire compensation process much simpler than before.

Figure 2: Filter compensation graphical user interface.

VMC has more complex requirements for loop stability. The transfer functions are different for discontinuous control mode (DCM) and continuous control mode (CCM). The complex compensation required for both DCM and CCM makes it difficult to stabilize the system. CMC circuits have similar transfer functions in CCM and DCM modes, which makes CMC easier to compensate than VMC circuits. The flexibility of digital control provides a perfect solution for this. A programmable digital controller can adjust its filter response and can determine when it is in DCM or CCM mode and use an appropriate filter. This is not a practical option for the analog control world because analog controllers require different filters.

From a loop stability perspective, CMC can suppress input voltage fluctuations. This is another reason why power supply designers recognize that CMC is a more preferred approach than VMC. However, a digital controller that can compensate for input voltage fluctuations can compensate the digital filter accordingly. Moreover, for many systems such as switch mode power supplies (SMPS), the input voltage is regulated by power factor correction. This means that the controller can treat it as a constant input voltage. VMC is generally considered to have a lower bandwidth than CMC. The reason for this is that the additional factors are built into the filter design to ensure stability.

Overcurrent protection

Since the CMC turns off the power FET when the peak current is reached, the CMC has an inherent peak overcurrent protection (OCP) function. The VMC uses only the output voltage for its control loop, so a dedicated circuit is required to perform the OCP function. However, the transformer balancing circuit described above uses average current mode control. Therefore, if the digital controller uses a simple analog comparator to implement the peak OCP function, it will have the same level of protection as the CMC.

Figure 3: Dedicated fast OCP comparator.

in conclusion

The advent of digital control is irresistibly challenging the traditional approach to power supply design. Many of the disadvantages of voltage mode control can now be effectively overcome by using intelligent digital solutions. This factor can be the culmination of bringing digital control into the mainstream of current design.