One reason I like to write this blog is because it allows me to research new areas of power and analog electronics. Another reason I value the research is I have the opportunity to simplify electronics especially for the hobbyist and beginner. Recently, I found that the area of hysteretic control offered me a change to look in a new area that simplifies control loop design to a degree.
Way back before Pulse-Width Modulators [PWM] were an off-the-shelf part, control was largely performed with discrete components. A favorite control technology at the time was hysteretic control due in part to its ability to operate without a dedicated frequency and therefore clocking circuit. In those ancient times, a bang-bang self-oscillating circuit served the purpose of generating its own frequency while converting power in a switched mode fashion. This offered advantages over the losses and limitations of linear power supplies. Reference 1, Hysteretic-Mode Converters Demystified, Part 1, tells the history of how hysteretic control has again gained popularity.
As technology progressed, fixed frequency designs enabled PWM. This simplified filter design for the input as well as the inductor as it is much easier to design for a fixed frequency. However, PWM also had short comings as it requires complexity in order to compensate. Most of the references refer to a Type 3 compensation network for overcoming the double pole due to the LC filter corner frequency when using voltage mode control. Current mode control does reduce the Bode plot to a single pole compensation puzzle however there is often an additional need for slope compensation in current mode control designs.
In both current mode and voltage mode, a high bandwidth error amplifier and compensation network is beneficial. Hysteretic control in its most basic form doesn’t need an error amplifier. Instead, hysteretic mode control can use a simple comparator. Also, fixed frequency designs can cause issues with slower response speeds whereas the variable frequency of hysteretic control offers a faster response time. This is advantageous for high current transient loads.
Screen capture of output voltage: Undershoot close-up.
One other advantage of hysteretic control is that it uses parasitics as an input signal. Because the control relies on the triangular waveform of the inductor current, control schemes exist for sensing the series resistance of the inductor or the Equivalent Series Resistance [ESR] of the output capacitors. Thus, the use of low ESR ceramic capacitors actually create a disadvantage when using hysteretic control based on the ripple current in the capacitor.
Now that the basic advantages have been introduced, it’s best to focus on the information that provides more detail than a concept introduction in a blog. The basic buck converter design is covered well in the three part series by Masashi Nogawa of Texas Instruments. Part 1 introduces the concept and gives some insight on the advantages. Part 1 also compares the phase contributions for the three modes of control. This is an excellent, basic introduction into control loop theory especially for the beginner. Part 2 compares the performance of various voltage mode, current mode, and hysteretic mode controls in a similar application having a large transient load current. Part 3 focuses on the associated Bode plots for the three modes of control. The three part TI series even compares the tradition of piling on output capacitance at the load adding a twist of reality to the explanation.
Reference 5 adds a new twist by analyzing the stability of current mode control in a hysteretic mode control application.
For topologies beyond the buck converter, Reference 4, Voltage-Mode, Current-Mode (and Hysteretic Control) Technical Note TN-203 Sanjaya Maniktala goes into detail on the plant gain Bode plots and related equations that form the poles and zeros.
Microsemi’s Voltage-Mode, Current-Mode (and Hysteretic Control) Technical Note TN-203 Sanjaya Maniktala application note also illustrates the three types of compensation networks. Graphical representation of the Bode plots for the plant as well as the control loop is the best way to understand stability without having to perform a lot of messy calculations. Also, phase calculations are much easier when one realizes that:
- Poles have a 90 degree phase lag [downslope] starting a decade before and extending a decade after the pole frequency
- Zeros have a 90 degree phase lead [upslope] starting a decade before and extending a decade after the pole frequency
Again, the graphical analysis for phase lag simplifies the math a great deal over computing the trigonometry and imaginary/real number calculations. With a little practice, one can become quite knowledgeable in predicting system stability from a few, simple Bode plots.
In closing, it’s always interesting when technologies come full circle. For hysteretic control, large load transient applications are offering a new way to use an old technology.
- Hysteretic-Mode Converters Demystified, Part 1, Masashi Nogawa, Senior Systems Engineer, Texas Instruments | May 27, 2016
- Hysteretic-Mode Converters Demystified, Part 2: Voltage and Current Mode Control, Masashi Nogawa, Senior Systems Engineer, Texas Instruments | Jun 23, 2016
- Hysteretic-Mode Converters Demystified – Part 3: Regulator Stability, Masashi Nogawa, Senior Systems Engineer, Texas Instruments | Sep 28, 2016
- Voltage-Mode, Current-Mode (and Hysteretic Control) Technical Note TN-203, Sanjaya Maniktala, 2012
- Stability Analysis & Design of Hysteretic Current mode Switched-inductor Buck DC–DC Converters, Carlos J. Solis, Graduate Student Member, IEEE, and Gabriel A. Rincón-Mora, Fellow, IEEE Georgia Institute of Technology, Atlanta, Georgia 30332 U.S.A