Constant On-time Control Research Articles

An on-time control circuit applied to DC-DC interface circuits in power management

Abstract This article proposes an on-time control circuit that improves the overall efficiency of DC-DC in power management. The low maximum power tracking efficiency of the adaptive on-time control technique under low input voltage and the low efficiency of the constant on-time control method DC-DC itself under high input voltage are both resolved by this circuit. It is assumed that this circuit is applied under the condition that the resistance of the environmental source remains unchanged. The circuit enhances the maximum power tracking efficiency under low input voltage, and the DC-DC maximum power tracking efficiency may maintain an efficiency value of more than 95%. At high input voltages, the circuit maintains the DC-DC inductor peak current nearly constant as the input voltage varies. Under DC-DC pre-simulation, when the input voltage exceeds 0.5 V, the overall efficiency will be better than 90%. Under DC-DC post-simulation, when the input voltage reaches 1 V, the overall efficiency will be greater than 90%.

Multiphase Constant On-Time Control With Phase Overlapping–Part II: Stability Analysis

Multiphase Constant On-Time Control With Phase Overlapping–Part II: Stability Analysis

A Comparison of Digital Constant On-Time and Variable On-Time Control in Buck/Buck-Boost PFC Converter

This study compares two frequently used control strategies in the context of completely digitally controlled critical conduction mode (CRM) buck/buck-boost converters, namely constant on-time (COT) and variable on-time (VOT). The aim of this research is to compare and assess the performance of different control systems in terms of power factor (PF) and total harmonic distortion (THD). The COT control technique employs a fixed on-time period for each switching cycle, resulting in a predictable and straightforward control scheme. On the other hand, VOT control dynamically adapts the on-time period based on the input and output voltage conditions, offering potential advantages in terms of load regulation and stability. The analysis results provide valuable insights into the strengths and limitations of each control technique. Additionally, the dynamic behavior of the converters under various operating conditions is investigated, shedding light on the stability and robustness aspects of each control scheme. The study is conducted using a fully digital setup and evaluated in Matlab/Simulink environment. The findings show that VOT control outperforms COT in terms of THD and PF under steady-state conditions. These findings offer valuable guidance for selecting the appropriate control technique for PFC converters, depending on specific requirements of their applications.

Small-Signal Analysis and Design of Constant On-Time Controlled Buck Converters With Duty-Cycle-Independent Quality Factors

This paper presents a frequency-domain analysis that models the behavior of Constant-on-Time (COT)-controlled Buck converters in wide-input-output-range scenarios. Based on this analysis, a design strategy is proposed to enable robust control with a duty-cycle-independent quality factor, also known as a constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula> -value. A constant <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$Q$</tex-math></inline-formula> -value circuit based on the proposed strategy is then constructed and incorporated with a baseline ultra-fast transient constant on-time (UFTCOT) control scheme to experimentally verify the system stability and optimal load transient response under wide input-output range scenarios. In addition, the design strategy is extended to other COT Buck control schemes with simulation and calculation verifications.

Accurate Loop Gain Model of Ripple-Based Constant on-time Controlled Buck Converters

Ripple based constant on-time (RBCOT) control has been widely used in DC-DC buck converters over past years. However, there has been no accurate loop gain model to link the property of RBCOT controlled buck converter to existing knowledge about basic feedback control principles like crossover frequency (Fc) and phase margin (PM) so far. The previously reported stability criterion for the output capacitor's ESR, i.e., Ton/(2C), is insufficient to avoid subharmonic oscillations over all duty cycle values. This paper presents a new accurate loop gain model for the RBCOT control. Without simplification, the proposed model can accurately predict the system response even beyond the switching frequency. The proposed modeling approach has been extended to two types of external ramp compensation schemes (for solving the subharmonic oscillation issue) and optimum ranges of ramp compensation amplitudes have been derived accordingly. The modeling results are verified by the SIMPLIS simulations and experimental results.

An Error Amplifier Scheme to Reduce Compensation Capacitor for Boost PFC Converter

An error amplifier (EA) scheme to reduce type-I compensation capacitor for Boost PFC converter is proposed in this paper. Compared to traditional EA scheme, the compensation capacitance is significantly reduced by applying a switch network to sample the output feedback signal. In this way, the compensation capacitance can be reduced to 10pF and easily fabricated on chip. Therefore, the control circuit combined with the proposed EA scheme can reduce the cost and improve the reliability of the Boost converter. Detailed theoretical analysis has been presented and a boundary conduction mode (BCM) Boost converter with constant on-time (COT) control and proposed EA scheme has been designed. Finally, an 80W experimental Boost PFC prototype has been built up and experimental results show that the converter can maintain good performance with the proposed EA scheme.

A Novel Variable On-Time Control Scheme for Boundary Conduction Mode SEPIC PFC Converter

Power factor correction (PFC) can be achieved by a single-ended primary inductor converter (SEPIC) operating in boundary conduction mode (BCM) with conventional constant on-time (COT) control, but it is challenging to achieve low total harmonic distortion (THD) and high-power factor (PF), particularly at high input voltage. A variable on-time (VOT) control strategy for BCM SEPIC PFC converter without input voltage feedforward and multiplier circuits is proposed to realize unity PF in this paper. By using a variable slope sawtooth generator whose slope is controlled by the duty cycle of the main switch to adjust the conduction time of the main power switch of the converter, the proposed VOT control scheme can use a simple and easy-to-implement circuit to enhance the PF and decrease the THD significantly, especially at high input voltage. The simulation model and 100W experimental prototype are built to verify the feasibility of the suggested control method. Simulation and experiment results demonstrated that the novel VOT control scheme remarkably enhances PF and decreases THD without affecting the efficiency by contrast with the conventional COT control.

A 110/230 V AC and 15–400 V DC 0.3 W Power-Supply IC With Integrated Active Zero-Crossing Buffer

This work presents an offline power supply with fully integrated power stage in 0.18 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> high-voltage (HV) CMOS silicon on insulator (SOI). It supports both ac–dc and dc–dc conversion from 15–400 V input down to 3.3–10 V output and is optimized for applications below 300 mW such as the Internet of Things (IoT), smart home, and e-mobility. An active zero-crossing buffer enables on-chip integration of the HV buffer capacitor. Below 150 V, constant ON-time control is based on voltage intervals, sensed by ten HV threshold-detection circuits. Above 150 V, the converter operates in the resonant mode. It achieves a superior power density of 458 mW/cm3 and 84% peak efficiency.

Parameter design and control of a novel three-phase single-switch power factor correction (PFC) circuit

Three-phase single-switch power factor correction (PFC) circuit has the advantages of simple structure and convenient control, and its current source characteristics is conducive to achieve higher power by parallel interleaving. But theoretical analysis of the circuit was complex and the design theory was not established yet. To solve these problems, a parameters design method based on volt-second balance of the input inductance is proposed. By this method, the key component parameters of a 7.6 kW experimental prototype are designed. At the same time, A constant on-time control method of this circuit based on MATLAB simulation is designed, which can be applied to various converters with constant on-time control. The parameters and control design method of the single-switch PFC circuit are simulated based on the MATLAB. A prototype is built and tested also, the test results verify the correctness of the parameter design theory and constant on-time control simulation method.

Design of DC–DC Buck Converter Based on Improved Voltage-Mode COT Control

In this paper, an improved voltage-mode constant on-time (COT) control DC–DC buck converter is designed, which makes the DC–DC buck converter have good stability. Firstly, an improved compensation scheme is introduced through the analysis and derivation of the traditional voltage-mode COT controlled buck converter model. Then, according to the application requirements of the design, Simplis software modeling is used to verify the correctness of the theoretical analysis. The 0.13[Formula: see text][Formula: see text]m BCD technology is used to tape out the DC–DC converter of the improved voltage mode COT. The input voltage is 3.7–5.5[Formula: see text]V and the output voltage is 3.3[Formula: see text]V. The chip test mainly includes heavy-load condition in CCM operation, light-load condition in DCM operation, and load transient response. The test results show that the chip can remain stable under heavy-load and light-load conditions. The overshoot and undershoot in transient response can be quickly restored to steady state during the recovery process. Finally, compared with chips of the same type, it shows that the DC–DC chip for portable electronic equipment designed in this paper adopts an internal compensation structure, which has good stability, simple design, small main chip area, simple peripheral equipment, and good application value.

A Modified Forward PFC Converter for LED Lighting Applications

A modified forward power factor correction (PFC) converter for light-emitting-diode (LED) lighting applications is proposed in this paper. The proposed circuit topology is an integration of a forward converter and a flyback converter with a multi-windings transformer. The flyback converter is operated as an auxiliary voltage source (AVS) connected in series with the rectified ac grid voltage to achieve low harmonic current distortion and high power factor. The magnetizing inductance energy is recycled to charge the capacitor in the AVS. The commonly used constant on-time control and primary side regulation can be easily adopted to control the proposed LED driver. The operation principle of the proposed converter is presented followed by the mathematical derivation of the parameter design. An 80 W prototype converter was built and tested to verify the performance of the proposed PFC converter. Experimental results show that the proposed PFC converter can comply with the IEC 61000-3-2 standard with high efficiency, low current harmonics, and a nearly unity power factor under universal input voltage range.

A Novel Ultrafast Transient Constant on-Time Buck Converter for Multiphase Operation

Constant on-time control (COT) has been widely adopted in buck converters because of its simplicity, fast transient response, and high light-load efficiency. However, the increasing power demands of CPUs and AI/ML engines is pushing voltage regulators to support higher output currents, which typically requires the use of multiphase operation. However, since most high-performance COT schemes utilize ripple as a control variable, the ripple cancellation point in multiphase designs imposes difficult stability concerns, limiting performance, and robustness. In this article, a novel ultrafast transient COT (UFTCOT) control scheme is proposed to enable fast and robust operation even in multiphase converters. The small-signal models of the proposed control scheme are derived based on a describing function technique, whose theoretical results are compared, along with simulation and measurement results, to contrast the proposed UFTCOT technique to exiting approaches.

Buck Circuit Design With Pseudo-Constant Frequency and Constant On-Time for High Current Point-of-Load Regulation

This paper introduces an accurate frequency regulation method based on a constant on-time (COT) buck regulator designed for point-of-load (POL) regulation with a fast load transient requirement and wide load range. Traditional COT regulators suffer from frequency shift due to load variations. The proposed frequency adjustment circuit helps the COT control engine to reach its target frequency of operation by adjusting the on-time pulse under varying conditions. This circuit includes a phase detector and a voltage-to-current converter block. The phase error between the target clock signal and the PWM is directly used to adjust the width of the on-time pulse. This will fine-tune the bandwidth of the frequency regulation loop, which can help to improve the transient performance of the COT as well. The fabricated chip has been tested under 20A/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> load transient speed with a 10A load step condition. The measured silicon performance shows that the error of the switching frequency is less than 0.16% in the 400kHz to 1MHz range during steady-state operation.

A Detection Circuit for Improving the Unloading Transient Performance of the COT Controller

Fast load transient response and high light-load efficiency are two key features of the constant on-time (COT) control technique that has been widely used in numerous applications, such as for voltage regulators and point-of-load converters. However, when load step-down occurs during an on-time interval, the COT controller cannot respond until the COT interval expires. This delay causes an additional output voltage overshoot, resulting in unloading transient performance limitation. To eliminate the delay and improve the unloading transient response of the COT controller, a load step-down detection circuit is proposed based on capacitor current COT (CC-COT) control. In the detection circuit, the load step-down is monitored by comparing the measured capacitor current with the preset threshold voltage. Once the load step-down is monitored, the on-time is promptly truncated and the switch is turned off. With the proposed detection circuit, the CC-COT-controlled buck converter can monitor the load step-down without any delay and obtain less output voltage overshoot when the load step-down occurs during the on-time interval. PSIM circuit simulations are employed to demonstrate the feasibility of the detection circuit.

High Power Factor CRM Boost PFC Converter With Optimum Switching Frequency Variation Range Control Based on Variable Inductor

Critical conduction-mode boost power factor correction converter with constant <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">on</small> -time control is widely used in low-to-medium power applications. However, the range of the switching frequency varies with different input voltages and load conditions. In this letter, a theoretical all-fixed switching frequency method based on the continuous variable inductance control is proposed first. Considering that the existing variable inductor technology is difficult to achieve a wide variation range, a switching frequency optimization compromise based on the segment variable inductance (SVI) control is put forward. Ensuring unit power factor, the proposed method significantly reduces the switching frequency variation range, which facilitates the electromagnetic interference filter design. A prototype with SVI control has been built and tested in the lab to demonstrate the validity of the proposed method.

Closed-Form Operational Boundaries for Buck Converters With Constant On-Time Control

In this brief, we provide three operational boundaries in closed analytical form for a Constant ON-Time buck converter working in continuous current mode. Two of these boundaries are related to the sudden appearance of pulse-bursting induced by either (i) the hysteresis of the comparator that drives the input of the block implementing the circuit control algorithm ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">hysteresis condition</i> ), or (ii) by a period doubling bifurcation ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bouncing condition</i> ). The third operational boundary corresponds to the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">saturation condition</i> of the circuit controller which does no longer guarantee an OFF-time larger than the minimum allowed one. These stability boundaries are provided both for the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">adaptive</i> and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">fixed</i> ON-time working modes.

A Fast-Response RBAOT-Controlled Buck Converter With Pseudofixed Switching Frequency and Enhanced Output Accuracy

A ripple-based constant on-time (RBCOT) control scheme is excellent in achieving fast transient response during the load current step. However, it suffers from severe electromagnetic interference (EMI) noise and inherent output voltage offset problem. A monolithic ripple-based adaptive on-time (RBAOT)-controlled buck converter is presented to overcome both drawbacks while retaining the advantage of RBCOT. The on-time is adjusted adaptively by detecting the switching node voltage, and thereby ensuring a pseudofixed switching frequency in a steady state, regardless of the input, output voltage, and load current conditions. A correction voltage generated by a coefficient in a virtual inductor current (VIC) circuit is introduced to counteract the offset voltage and enhance the output accuracy. The proposed scheme is simple to implement and suitable for high-conversion ratio applications. Experimental results show that the switching frequency is centered at 1 MHz with less than 6% variation for an output of 5 V and inputs ranging from 8 to 18 V. The output offset voltage is reduced from 16 to 1 mV for an 18 to 1.05-V conversion. For a load current step of 1 A, the output can be settled within <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$6~\mu \text{s}$ </tex-math></inline-formula> and the undershoot and overshoot voltages are controlled to be within 20 mV.

An Adaptive Off-Time Controlled DCM Flyback PFC Converter With Unity Power Factor and High Efficiency

Constant duty cycle controlled discontinuous conduction mode (DCM) flyback power factor correction (PFC) converter has the advantage of high power factor (PF) and the disadvantage of low efficiency. While, constant on-time (COT) controlled critical conduction mode (CRM) flyback PFC converter has the exact opposite features, besides its switching frequency varies in a line cycle, and the variation range is very large, which complicates the electromagnetic interference (EMI) design. In order to obtain both benefits of these two control methods, an adaptive off-time (AOT) control technique for DCM flyback PFC converter is proposed in this paper. By utilizing the output voltage and the amplitude of line voltage to adjust the off-time of the main switch, the magnetizing current of transformer exactly operates in CRM when the rectified input voltage gets the peak. Thus, the root-mean-square (RMS) current of the main switch and the diode, as well as the conduction loss can be effectively reduced, and high efficiency can be obtained. The proposed control technique also can achieve theoretical unity PF over universal input voltage range of 90~264VAC. Moreover, its variation range of switching frequency is greatly reduced compared to that of COT control. A 60W prototype has been fabricated and tested in the laboratory and experimental results are presented to verify the effectiveness of the proposed method.

A GaN BCM AC–DC Converter for Sub-1 V Electromagnetic Energy Harvesting With Enhanced Output Power

A gallium nitride (GaN) boundary conduction mode (BCM) dual-boost ac-dc converter for low-voltage electromagnetic energy harvesting (EH) is presented in this article. The μW-to-mW EH system is very susceptible to any tiny losses from power converters, interconnections, and system parasitic. To have better utilizations of harvested energy, an output power enhanced power conversion design is proposed. First, impedance matching is satisfied between electromagnetic generator and ac-dc converter to achieve maximum delivered power. The ac-dc converter is designed under BCM for constant input impedance. EH's winding inductance is employed as the boost inductor in the ac-dc converter. A codesign principle for converter on-time with electromagnetic generator's winding characteristics is revealed. Second, power transistors in EH low-voltage conversion are optimized to realize high efficiency. Power transistors' low-voltage switching characteristics are significantly different from high-voltage situation with no or little miller plateau region. Low-voltage power transistors are compared from both device characteristics and calculated power loss to provide device selection guidance. A GaN-based dual-boost ac-dc converter under BCM operation with constant on-time controller is designed in this article to provide 3.3 V output for a 500-Ω load resistor. It achieves peak efficiency of 82.4% for open-loop operation at 0.6 V/100 Hz input and peak efficiency of 72.3% for close-loop operation at 0.75 V/100 Hz.

Small signal modelling and stability analysis of internal ramp compensated constant on‐time controller with improved output voltage accuracy for point of load application

This paper investigates a constant on-time controller (COT) with internal ramp compensation and improved output voltage accuracy for point of load applications. The pulse width modulation (PWM) output is generated based on internal ramp: by sensing the regulated output voltage and comparing it to an internal reference target, an error signal is generated. This error signal is used to generate a reference for an added ramp signal, which is compared against an error voltage to generate a PWM trigger signal. An adaptive constant on-time scheme ensures a fairly constant switching frequency over a wide range of input and output voltages. This COT control algorithm is able to achieve high accuracy regulation (±0.5% of target) and fast transient response during load step change. A frequency domain small signal model is proposed, and an open loop gain transfer function is developed for this COT control system. The proposed small signal model and design analyses are verified with SIMPLIS simulations and experimental results.