Voltage Mode vs. Current Mode-EEWORLD

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Voltage Mode Control

This is the approach taken by the earliest switching regulator designs and has served the industry well for many years. The basic voltage mode control configuration is shown in Figure 1.

The main characteristics of this design are that there is only one voltage feedback path, and pulse width modulation is accomplished by comparing the voltage error signal with a constant ramp waveform. Current limiting must be performed separately.

The advantages of voltage analog control are:

1. A single feedback loop is used, so it is easier to design and analyze.
2. A large amplitude ramp waveform provides sufficient noise margin for a stable modulation process.
3. A low impedance power output provides better cross-modulation performance for multi-output power supplies.

The disadvantages of voltage mode control can be listed as follows:
1. Any change in voltage or load must first be detected as an output change and then corrected by the feedback loop. This often means slow response speed.
2. The output filter adds two poles to the control loop, so when compensating the error amplifier, it is necessary to attenuate the dominant pole at low frequency or add a zero in the compensation.
3. Since the loop gain changes with changes in input voltage, the compensation is further complicated. The above disadvantages of

current mode control

are more prominent, and since current mode control can alleviate all these disadvantages, it has aroused great interest among designers as soon as it was introduced, and they have studied this topology. As can be seen from the schematic diagram given in Figure 2, the basic current mode control only uses the oscillator as a fixed frequency clock and replaces the ramp waveform with a signal obtained from the output inductor current.

The advantages of this control method are as follows:

1. Since the inductor current rises at a slope determined by Vin - Vo, the waveform will respond immediately to changes in input voltage, eliminating delayed response and gain changes that occur with changes in input voltage.
2. Since the error amplifier is now used to control the output current rather than voltage, the effects of the output inductor are minimized, and the filter now only provides a single pole to the feedback loop (at least in the normal region of interest). This simplifies
compensation .
3. Additional benefits of using current mode circuits include inherent pulse-by-pulse current limiting (simply clamping the control signal from the error amplifier) and easy load balancing when multiple power supply units are connected in parallel.

Although the improvements provided by current mode are impressive, this technology also has its own unique problems that must be addressed during the design process.

The following is a brief list of some of its disadvantages:

1. There are now two feedback loops, which makes circuit analysis more difficult.
2. When the duty cycle is greater than 50%, the control loop will become unstable unless additional slope compensation is used.
3. Since the control modulation is based on a signal derived from the output current, resonances in the power stage introduce noise into the control loop.
4. A particularly annoying source of noise is the leading-edge current spike, which is usually caused by the transformer winding capacitance and the output rectifier recovery current.
5. Since the control loop is current driven, load regulation is poor, and coupled inductors are required to obtain acceptable cross-modulation performance when multiple outputs are used.

So, we can conclude that while current-mode control will relax many of the limitations of voltage-mode control, it will also introduce many new challenges to designers. However, using the knowledge gained from more recent power control technology developments, people have re-evaluated voltage-mode control and found that there are some other correction methods for its major shortcomings, and the UCC3570 is the result of the industry's research and development.
Re-examining voltage-mode control
The two major improvements that the UCC3570 makes to voltage-mode control are voltage feedforward and higher frequency capability, the former is used to eliminate the effects of input voltage changes, and
the latter allows the output filter pole to be placed above the standard control loop bandwidth range. Voltage feed-forward is accomplished by making the slope of the ramp waveform proportional to the input voltage. This provides a corresponding and corrective duty cycle modulation without any action by the feedback loop. The result is a constant control loop gain and instantaneous response to input voltage changes. Higher frequency capability is achieved by using a BiCMOS process for the IC, which results in smaller parasitic capacitances and lower circuit delays. Thus, many of the problems of voltage-mode control are alleviated without incurring
the troubles .

Choosing Circuit Topologies

None of the above discussion should leave you with the impression that "current-mode control no longer has a place" - only that "both current-
mode ." For each specific application, certain design considerations may indicate that one or
the other topology is more appropriate. Some of the design considerations are outlined below:

Consider using current mode when:

1. The power supply output will be a current source or very high output voltage.
2. The fastest dynamic response is required for a given switching frequency.
3. The application is for a DC/DC converter with relatively limited input voltage variation.
4. Modular applications that require parallelability and load sharing
5. In push-pull circuits where transformer flux balance is important.
6. In low-cost applications where very few components are required.

Voltage mode with feedforward can be considered in the following situations:

1. There may be a wide range of input voltage and/or output load variations.
2. Especially under low voltage-light load conditions, when the current ramp slope is too shallow to achieve stable PWM operation.
3. High power applications and/or noisy applications (here, the noise on the current waveform will be difficult to control).
4. Multiple output voltages and good cross-modulation performance are required.
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.

Based on these design considerations, the UCC3750 is optimized for low- to medium-power, isolated, primary-side control applications (with isolated feedforward). In addition to the control characteristics mentioned above, the device also achieves many performance improvements for this type of operation. However, since this is not the topic of this article, interested readers can refer to the product's data sheet for more relevant information.

Reference address：Voltage Mode vs. Current Mode

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