Voltage Control Mode and Current Control Mode Comparison

With the current development of electronic power design, switching power supplies are deeply involved in the design of various devices. Engineers are always looking for a control structure that maximizes the efficiency of a switching power supply, but in reality, there is no single control structure that is optimal for a switching power supply. The optimal structure for a switching power supply can only be said to be selected for a particular application. Therefore, it is essential to familiarize yourself with the advantages and disadvantages of each method. The following discussion will illustrate the strengths and weaknesses of voltage control mode and current control mode!

Voltage Mode Control

Let's start with voltage mode control, which was the method used in the earliest switching regulator designs and has served the needs of power supplies well for many years.

General fixed frequency PWM frequency will not change with the feedback changes, only the pulse width will change, is usually called the duty cycle, the voltage mode VR for a fixed reference point, the current mode can be viewed as the signal of the current check as a reference point, when the power supply enters the steady state according to the law of volts second, VIN * TON * VOUT * TOFF. power supply enters the steady state will not appear signal instability.

Advantages of voltage mode control:

1. A single feedback loop is used, so it is easier to design and analyze;.

2. A large ramp waveform provides sufficient noise margin to achieve a stable modulation process; and

3. A low impedance power output provides better cross-regulation for multiple output supplies.

There are advantages and disadvantages, so the disadvantages are:

1. Any change in voltage or load must be detected as an output transformer and then corrected by feedback. This implies a slow response time; and

2. The output filter adds two poles to the control loop, thus requiring the dominant pole to be attenuated at low frequency when compensating for the design of the error amplifier, or a zero added to the compensation to cancel out the poles; and

3. The loop gain varies with the input voltage, thus further complicating compensation.

The above drawbacks are relatively prominent, so current mode control mitigates all these drawbacks, and has therefore been withdrawn with great interest from engineers who have studied this control structure!

Current Mode

Current mode control is shown below

Advantages of current mode control:

1. Since the inductor current rises at a slope determined by Vin-Vo, the waveform will respond immediately to changes in the input voltage, thus eliminating the delayed response and the gain changes that occur with changes in the input.

2. Since the error amplifier now controls the current, the influence of the inductor is minimized, and the filter now supplies only a single pole to the feedback loop, simplifying compensation and yielding a higher gain bandwidth than similar voltage modes.

3. Inherent pulse-by-pulse current limiting, only the control signal from the error amplifier can be embedded (such as the commonly used 3843 chip is embedded to 1V), in the power supply in parallel, easy to achieve load equalization. (e.g. to mitigate the bias magnetization of push-pull topologies).

Despite the many benefits of the improvements offered by current mode, there are also problems specific to it that must be taken into account in the design. The following is a brief description of its disadvantages:

1. There are two feedback loops, increasing the difficulty of circuit analysis; and

2. When the duty cycle is greater than 50%, the control loop will become unstable and require additional slope compensation; and

3. Since the control modulation is based on a signal derived from the output current, resonance in the power stage introduces noise into the control loop; and

4. Current spikes caused by transformer winding capacitance and secondary rectifier reverse reversal currents; and

5. Poor load regulation due to the use of a control loop for current drive; and

6. Multiple outputs require coupled inductors for acceptable voltage regulation.

While current-mode control will relax many of the limitations of voltage-mode control, it also presents many design challenges. So voltage mode now has a new improvement, then the two main improvements are voltage feed-forward, which is used to eliminate the effects of voltage variations, and high-frequency capability, which puts the poles of the output filter outside the bandwidth of the standard control loop.

Voltage feedforward is accomplished by making the slope of the ramp waveform proportional to the input voltage. This provides a corresponding and corrected duty cycle modulation without any action by the feedback loop. A constant control loop gain and transient response to changes in input voltage are obtained.

The high frequency is achieved by using the BiCOMS process for the IC, which produces smaller parasitic capacitances and lower circuit delays. So many of the problems of voltage controlled mode are mitigated.

Selection between voltage control mode and current control mode

Both voltage and current modes can be selected, it's just that for each specific application, certain designs that control mode is more appropriate.

Current mode can be considered for the following applications:

1. The power supply output will be a current source or a very high output voltage.

2. For a given switching frequency, the fastest dynamic response is required.

3. The application is for a DC-DC converter with relatively limited input variation.

4. Modular applications requiring parallelizability and load equalization.

5. In push-pull circuits where transformer flux is important.

6. In low-cost applications where very few components are required.

Voltage modes with feed-forward capability can be considered for the following applications:

1. Where a wide range of input voltage and/or output load variations is possible; and

2. Especially under low voltage-light load conditions, when the current ramp slope is too gentle for stable PWM operation.

3. High power applications and/or noisy applications (here, noise on the current waveform will be difficult to control).

4. Multiple output voltages are required as well as better cross-tuning performance; and

5. Saturable reactor controllers will be used as auxiliary secondary side regulators.

6. Applications that need to avoid the complexity of dual feedback loops and/or slope compensation.

This article is mainly on the switching power supply in the current mode and voltage mode characteristics and differences between the introduction, and the latest voltage mode characteristics are summarized. We hope that you can learn from this article useful knowledge, or through the knowledge of this article to find some new knowledge.