Advantages and disadvantages of three power control topologies

Every power engineer knows and learns about the traditional control topologies of overvoltage mode and current mode control, but less well-known are hysteretic-based topologies and their advantages. While pure hysteretic control may not be practical for certain applications such as medical or industrial automation, many of the newer power topologies are hysteretic-based and have additional features designed to overcome the shortcomings of pure hysteretic control. Such topologies are used in a wide range of applications, from processor core power supplies to automotive systems.

Almost all power supplies are designed to provide a regulated output voltage or current. Providing this output regulation requires a closed loop system and feedback of the output voltage or current to be regulated. Although there are many different control topologies used to compensate for the available feedback loops, they can generally be categorized into two categories: pulse width modulation (PWM) or hysteresis. From these two basic topologies has evolved a third topology that is a fusion of the two: hysteresis-based topologies. Each of these control topologies has advantages and disadvantages for different applications.

Voltage Mode Control

Pulse width modulation (PWM) control is classified into two basic types: voltage mode and current mode. For simplicity, this article only discusses voltage mode control with input voltage feedforward. For a more detailed comparison of voltage mode and current mode, Figure 1 shows the basic block diagram of voltage mode control in a buck converter.

Figure 1: Voltage-mode control includes the error amplifier, clock, and internal reference voltage (VREF)

When voltage mode control is used to regulate the output voltage, it senses a scaled version of the output voltage through a resistive divider connected to its feedback (FB) input. An error amplifier with high gain then compares this FB signal to a high accuracy internal reference voltage. The loop compensation circuit surrounding the error amplifier is responsible for maintaining system stability.

Voltage mode control has many advantages. By regulating only the output voltage and other well-controlled internal signals such as the clock and internal reference voltage, the topology is very noise-immune. It is also fairly straightforward. Simplicity is maintained by using input voltage feed-forward to maintain a constant loop gain under varying input voltage conditions. In addition, input voltage feed-forward can significantly improve the response to line voltage transients. Finally, the clock enables control of the switching frequency, including the possibility of synchronizing the circuit to an external clock source.

The main disadvantage of voltage mode control is the required loop compensation and the corresponding loop bandwidth limitation. By its nature, voltage mode control introduces a double pole in the power stage, which is located at the corner frequency of the output filter, thus requiring two correctly positioned zeros around the error amplifier. Since the frequency of this double pole is usually very low, the loop bandwidth is limited to a low level. Generally, it is limited to no more than 1/10 of the switching frequency. This has a significant negative impact on the transient response of the power supply. Therefore, designers must increase the output capacitance to achieve better transient results, which leads to higher system costs.

Considering the above trade-offs, voltage-mode control is still valuable, especially in noise-sensitive applications. The high noise tolerance of voltage-mode control and its ability to synchronize to a system clock make it suitable for the most noise-sensitive applications, such as medical and instrumentation equipment.

Hysteresis control

In its pure and basic form, hysteretic control is extremely simple, the simplest of all control topologies (Figure 2). A comparator with some small hysteresis between its terminals compares the output voltage directly to a highly accurate internal reference voltage, VREF, via the FB input.

Figure 2: Simple hysteretic control topology requires only one comparator and internal VREF

The advantage of this direct control of the output voltage is the speed of the control loop. When the output voltage changes due to a transient, the time required for the control loop to begin reacting is limited only by the propagation delays in the comparator and gate driver. The error signal does not have to pass through a low-bandwidth error amplifier. Therefore, the hysteretic topology is the fastest control topology.

In addition, the simplicity of its operating principle makes it inherently stable without any loop compensation. And this simplicity also makes it a low-cost topology. There are no oscillators or error amplifiers in the power supply that need to be designed, built, and tested. Only a basic comparator is required to control the switching action.

The main drawback of the hysteretic topology is its switching frequency variation. There is no clock or synchronization signal responsible for setting the switching frequency. Instead, the switching frequency is set by the amount of hysteresis as well as external components and operating conditions.

When using a pure hysteretic converter, large frequency variations are expected over the input voltage and load range. Also, the DC set point of the output voltage achieved may not be as accurate as when using voltage-mode control if a high-gain error amplifier is not used. Finally, hysteretic control requires the use of the equivalent series resistance (ESR) in the output capacitor. Therefore, ceramic output capacitors with very low ESR are generally not suitable when using a pure hysteretic topology.

However, in some low-power, very low-cost applications (such as toys), hysteretic converters may be acceptable due to the very low price point of such end equipment and the low electromagnetic interference (EMI) levels generated by its low power over the wide switching frequency range of the hysteretic power supply. In addition, systems with very harsh transients require hysteretic or hysteretic-based topologies to maintain acceptable output voltage regulation. If the input voltage, output voltage and other operating conditions of these systems are well controlled, the switching frequency is maintained within an acceptable range. This makes hysteretic control an effective choice for applications that operate from a fixed input voltage and produce a fixed output voltage.

Hysteresis-based control

Many control topologies are fundamentally hysteretic, but contain additional circuitry designed to overcome frequency variations and other limitations of pure hysteretic topologies. Examples include the D-CAP, D-CAP2, COT, COT with ERM, and DCS-Control topologies. This article only analyzes and compares DCS-Control 4 and similar devices.

Basically, DCS-Control (direct control with seamless transition to power-saving mode) is a hysteretic topology, but it combines some characteristics of voltage mode and current mode (Figure 3). As in voltage-mode control, the hysteresis comparator compares the output of an error amplifier with a sawtooth waveform.

Figure 3: In the hysteresis-based DCS-Control topology, the error amplifier and internal VREF are the same as in the voltage-mode control, while the hysteresis comparator is taken from the hysteresis topology. The on timer is unique to the hysteresis-based topology.

The sawtooth wave is not generated from a clock, but is generated at the VOS input pin by a special circuit that is directly connected to the output voltage. Essentially, the hysteresis comparator still has a direct connection to the output voltage through the VOS pin, coupled with a high gain error amplifier to provide very good output voltage set point accuracy.

In addition to combining a hysteresis comparator and error amplifier from hysteresis and voltage mode topologies, DCS-Control also implements an on-time circuit to control the switching frequency. Finally, the necessary loop compensation function is built in to achieve stability.

The main advantage of DCS-Control is that it maintains the very fast transient response of a hysteretic converter and the output voltage accuracy of a voltage-mode converter, while overcoming the other key drawbacks of these two topologies: slow response time, limited control loop bandwidth and frequency variation.

Since the VOS pin provides direct control of the output voltage, any changes in the output voltage will propagate directly through the control loop without being limited by the error amplifier bandwidth. This will greatly speed up transient response.

The main drawback of current DCS-Control implementations is the inability to synchronize to a clock. As a hysteresis-based topology, it does not provide a clock input signal, but rather a controlled switching frequency that varies very little under various operating conditions. In some cases, this variation is less than the clock frequency tolerance of the voltage-mode converter.

Hysteresis-based topologies such as DCS-Control are best used in applications that experience large transients and require very high output voltage accuracy. Such applications include powering processor cores in embedded or computing systems, as well as industrial automation and automotive infotainment systems.

in conclusion

The three main power control topologies, "voltage mode," "hysteretic," and "hysteresis-based," each have advantages and disadvantages for different applications. While most power engineers are accustomed to and comfortable using voltage mode control, the hysteretic and hysteresis-based topologies offer best-in-class transient response and should be explored for applications that require this fast response speed, such as processor core power. The sheer number of devices in use for each control topology means that there is likely an optimal power solution for almost any application.