Need for Variable Frequency Control in DC-DC Switching Converters– Challenges and Opportunities using Digital Implementation

  • Santanu Kapat Department of Electrical Engineering, IIT Kharagpur, West Bengal - 721302, India
  • K Hariharan
Keywords: Engineering


DC-DC converters play a vital role in efficient power management in various portable devices. It offers fast dynamic performance and high energy efficiency over a wide operating range. In the context of fixed-frequency pulse-width modulation (PWM), multi-loop current-mode control (CMC) offers higher bandwidth and superior line regulation compared to single-loop voltage-mode control (VMC). However, CMC suffers from sub-harmonic oscillations with the duty ratio ‘d’>0.5, and a compensating ramp is needed for stabilization. This degrades the closed-loop bandwidth, which further degrades as the ramp slope increases with ‘d’. On the other hand, variable frequency control methods, such as hysteresis control and constant on/off-time control techniques offer improved stability, transient response, and light-load efficiency. However, undesirable steady-state frequency variations complicate the input filter design and eventually lead to EMI problems. While a digital platform is helpful for fast and precise frequency regulation using an all-digital-PLL, the choice of sampling remains an important concern for implementing variable frequency digital control. This paper highlights the above challenges and scopes. Then it presents event-driven digital control solutions and design insights for next generation power management solutions.

Author Biography

Santanu Kapat, Department of Electrical Engineering, IIT Kharagpur, West Bengal - 721302, India

Santanu Kapat received his M. Tech and Ph.D. degrees in electrical engineering from the IIT Kharagpur, in 2006 and 2010, respectively. From 2009 to 2010, he was a visiting scholar in the ECE Department, UIUC. From 2010 to 2011, he was a research engineer at GE Global Research, India. Since 2011, he has been with the Electrical Engineering Department, IIT Kharagpur, where he is an Assistant Professor. His research interests include analysis and design of digital and nonlinear control in DC-DC converters, and applications to dynamic voltage scaling, RF power amplifier, LED drivers, and DC nano-grid.

Dr. Kapat is the recipient of the INSA Young Scientist Medal (by Indian National Science Academy) and the INAE Young Engineers Award (by Indian National Academy of Engineer) in 2016. He has been serving as an Associate Editor in IEEE Transactions on Power Electronics since 2015.


Redl W R and Jian S (2009) Ripple-based control of switching regulators–An overview in IEEE Trans. Power Electron., 24(12) 2669–2680.
Yan Y, Lee F C, and Mattavelli P (2012) Unified three-terminal switch model for current mode controls IEEE Trans. Power Electron., 27(9) 1965–1972.
Yan Y, Lee F C, and Mattavelli P (2013) Comparison of small signal characteristics in current mode control schemes for point-of-load buck converter applications IEEE Trans. Power Electron., 28(7) 3405–3414.
Li J, Qiu Y, Sun Y, Huang B, Xu M, Ha D S, and Lee F C (2007) High resolution digital duty cycle modulation schemes for voltage regulators in Proc. IEEE APEC 871–876.
Fang C C and Redl R (2014) Subharmonic stability limits for the buck converter with ripple-based constant on-time control and feedback filter IEEE Trans. Power Electron., 29(4) 2135–2142.
Fang C C (2012) Closed-form critical conditions of instabilities for constant on-time controlled buck converters,” IEEE Trans. Cir. Syst.-I 59(12) 3090–3097.
Qian T (2013) Subharmonic analysis for buck converters with constant on-time control and ramp compensation IEEE Trans. Ind. Electron., 60(5) 1780–1786.
Sahu B and Rincon-Mora G A (2007) An accurate, low-voltage, CMOS switching power supply with adaptive on-time pulse-frequency modulation (PFM) control IEEE Trans. Cir. Syst.-I, 54(2) 312–321.
Lin H C, Fung B C, and Chang T Y (2008) A current mode adaptive on-time control scheme for fast transient DC-DC converters in Proc. IEEE ISCAS 2602–2605.
Fu W, Tan S T, Radhakrishnan M, Byrd R, and Fayed A A (2015) A DCM-only buck regulator with hysteretic assisted adaptive minimum-on-time control for low-power microcontrollers IEEE Trans. Power Electron., 31(1) 418 – 429.
LM5017 (2015) 100-V, 600-mA Constant on-time synchronous buck regulator Texas Instruments.
Jing X and Mok P K T (2013) A fast fixed-frequency adaptive-on-time boost converter with light load efficiency enhancement and predictable noise spectrum IEEE J. Solid-State Cir., 48(10) 2442–2456.
Li P, Bhatia D, Xue L, and Bashirullah R (2011) A 90–240 MHz hysteretic controlled DC-DC buck converter with digital phase locked loop synchronization IEEE J. Solid-State Cir., 46(9) 2108–2119.
Maksimovic D, Zane R, and Erickson R (2004) Impact of digital control in power electronics in Proc. IEEE ISPSD 13–22.
Liu Y F, Meyer E, and Liu X D (2009) Recent developments in digital control strategies for DC/DC switching power converters IEEE Trans. Power Electron., 24(11) 2567–2577.
Saggini S, Ghioni M, and Geraci A (2004) An innovative digital control architecture for low-voltage high-current DC–DC converters with tight load regulation IEEE Trans. Power Electron., 19(1) 210–218.
Trescases O, Prodic A, and Ng W T (2011) Digitally controlled current-mode DC-DC converter IC IEEE Trans. Cir. Syst.-I 58(1) 219–231.
Krein P T and Bass R M (1989) Multiple limit cycle phenomena in switching power converters in Proc. APEC 143–148.
Hariharan K, Kapat S, and Mukhopadhyay S (2016) Unified constant on/off-time hybrid compensation for fast recovery in digitally current-mode controlled point-of-load converters in Proc. IEEE APEC 315–321.
Verghese G C, Malik M E, and Kassakian J G (1986) A general approach to sampled-data modeling for power electronic circuits IEEE Trans. Power Electron., PE-1 76–89.
Kapat S (2017) Parameter-insensitive mixed-signal hysteresis-band current control for ooint-of-load converters with fixed frequency and robust stability IEEE Trans. Power Electron., 32(7) 5760–5770.
Research Papers