Voltage Undershoot Research Articles

Dynamic response characteristics of proton exchange membrane fuel cell during transient loading: An experiment study

The dynamic performance of Proton Exchange Membrane Fuel Cell (PEMFC) is crucial for safe and efficient operation and remains a major barrier to commercialization. In this paper the dynamic response characteristics of PEMFC under serpentine flow field (SFF) and parallel flow field (PFF) are discussed. The effects of current loading, operating pressures, anode and cathode stoichiometric ratios, as well as operating temperatures on the dynamic response characteristics of PEMFC are systematically studied through experimental methods. Furthermore, the principle and mitigation strategies for voltage undershoot are investigated. It shows that lower current density, higher back pressure and higher temperature are benefit to alleviate voltage undershoot. Compared with the SFF, the Second-stage time delay of PFF is shortened by 9.2 s, the VU is decreased by 17.27 mV, and the energy loss is reduced by 0.71%. Moreover, Pearson correlation coefficient is proposed to evaluate the correlation between index and PEMFC performance.

Insight into the mechanisms and in-plane reaction heterogeneity of the dynamic response of proton exchange membrane fuel cells

The voltage undershoot is one of the main dynamic problems of proton exchange membrane fuel cells (PEMFCs) under dynamic loadings. However, its mechanisms remain poorly understood due to its complex and coupled influencing factors. Besides, PEMFCs with a large active area are more prone to local dynamic failures for their pronounced in-plane (I-P) heterogeneity, while few studies have focused on the I-P heterogeneity of the dynamic response of large-area fuel cells. In this work, the mechanisms and the I-P heterogeneity of the voltage undershoot of a 320 cm2 PEMFC are thoroughly studied based on an in-situ monitoring method and a voltage loss decoupling method. Results show that the voltage overshoot is primarily driven by the ohmic loss (OL) overshoot and the concentration polarization (CP) overshoot. Results also prove that the OL overshoot is mainly caused by the instantaneous water shortage in the proton exchange membrane (PEM) which depends on the initial water content in the PEM. Besides, the CP overshoot is due to the instantaneous reactant shortage at the catalyst, and it is much sensitive to the change in the oxygen diffusion coefficient of the ionomer caused by the change in its water content. Meanwhile, both OL overshoot and CP overshoot demonstrate significant I-P heterogeneities under poor dynamic operating conditions, especially in cases of high load step and low inlet air humidity. Finally, the influences of operating conditions on the voltage undershoot and its I-P heterogeneity are discussed, and suggestions for improving the dynamic performance of PEMFCs are proposed.

Research on power operation typical characteristic state of proton exchange membrane fuel cell based on principal component analysis

The stability, reliability, and lifespan of proton exchange membrane fuel cells (PEMFCs) are closely related to power dynamic response. Therefore, it is necessary to study in depth the relationship between the power operation state and typical characteristics. In this paper, the fuel cell system (FCS) power dynamic response based on constant power change amplitude and various power change amplitude is analyzed. Then, three typical power characteristics of the FCS, namely, low single-cell voltage, drastic voltage uniformity change, voltage undershoot and voltage overshoot, are investigated. Furthermore, to solve the high-dimensional datasets problem, a quick evaluation method based on data dimensionality reduction (DDR) is proposed. The innovation of this method is based on principal component analysis to extract typical characteristic relationships of power operation levels. The results demonstrate that the extracted new one-dimensional (1-D) eigenvectors index exhibits a positive correlation with power levels. Meanwhile, the power operation state empirical model is established through data regression. The Pearson coefficients of the empirical model are all greater than 0.9981. Thus, the new index and proposed method offer novel insights for improving the power adaptive control of fuel cells and quickly assessing their health state.

Experimental investigation on operational stability and its influencing mechanisms for PEMFC with dead-ended anode

Proton exchange membrane fuel cell (PEMFC) with dead-ended anode (DEA) suffers from nitrogen diffusion and water accumulation, leading to unstable performance. The core of this paper is to enhance the stability of DEA operation. By coupling the flow field structure with the DEA mode, the factors affecting the stability are explored, the performance degradation mechanism after DEA operation is revealed, and the purging strategy is formulated by combining the energy management with the voltage stability as a prerequisite. The experimental results show that the serpentine flow field is advantageous for maintaining the most stable DEA operation with a minimum voltage recession of 17.01 mV and a voltage undershoot of 11.63 mV. The local current density distribution exhibited a pattern of highest values at anode inlet and lowest at anode outlet, which could be improved by increasing operating pressure and reducing load current density. In this experiment, PCB was innovatively combined to investigate the degradation of each region of PEMFC after 60h of DEA operation. It is found that the most severe performance degradation occurs in anode outlet with a degradation rate exceeding 50%, while the center area has a degradation rate of no more than 20%.

A 640 nA IQ Output-Capacitor-Less Low Dropout (LDO) Regulator with Sub-Threshold Slew-Rate Enhancement for Narrow Band Internet of Things (NB-IoT) Applications.

An ultra-low quiescent current output-capacitor-less low dropout (OCL-LDO) regulator for power-sensitive applications is proposed in this paper. To improve the gain of the OCL-LDO feedback loop, the error amplifier employs a combination of a cross-coupled input stage for boosting the equivalent input transconductance and a negative resistance technique to improve the gain. Meanwhile, in order to address the issue of transient response of the ultra-low quiescent current OCL-LDO, a sub-threshold slew-rate enhancement circuit is proposed in this paper, which consists of a transient signal input stage and a slew-rate current increase branch. The proposed OCL-LDO is fabricated in a 0.18 μm CMOS process with an effective area of 0.049 mm2. According to the measurement results, the proposed OCL-LDO has a maximum load current of 100 mA and a minimum quiescent current of 640 nA at an input voltage of 1.2 V and an output voltage of 1 V. The overshoot and undershoot voltages are 197 mV and 201 mV, respectively, and the PSR of the OCL-LDO is -72.4 dB at 1 kHz when the load current is 100 μA. In addition, the OCL-LDO has a load regulation of 7.6 μV/mA and a line regulation of 0.87 mV/V.

Mass transfer and round-trip efficiency evaluation of unitized regeneration proton exchange membrane fuel cell during bi-directional mode switching cycle

Bi-directional mode switching is a crucial operation in unitized regenerative proton exchange membrane fuel cells (UR-PEMFC). Mass transfer plays a direct role in influencing the performance of both the fuel cell (FC) mode and the electrolytic cell (EC) mode, consequently impacting the round-trip efficiency (RTE). This study enhanced the three-dimensional two-phase model of UR-PEMFC by incorporating the mass transfer dynamics of the flow field. The round-trip efficiency (RTE) was systematically assessed throughout the bidirectional mode switching cycle. The results indicate that UR-PEMFC with triple-serpentine flow fields (TSF) performed well in FC mode, which was attributed to its good oxygen distribution uniformity and liquid water detachment capacity, in EC mode, its poor effective mass transfer coefficient and higher mass transfer resistance led to higher electrolytic energy consumption. However, UR-PEMFCs with TSF and parallel flow field (PFF) had better dynamic response performance during the bidirectional mode switching, and their voltage undershoot (overshoot) and response time were improved, which were attributed to uniform flow field structures. The UR-PEMFC reached optimum RTE operating at 0.1 A/cm2 in FC mode and 1.1 A/cm2 in EC mode, and the RTEs of the UR-PEMFCs with PFF and TSF were more advantageous, their highest RTEs were 36.62% and 36.30%, respectively. This study proposes two switching strategies to enhance the stability of URFC mode switching, offering a viable new approach for optimizing the flow field and improving overall performance.

Design of High-Reliability Low-Dropout Regulator Combined with Silicon Controlled Rectifier-Based Electrostatic Discharge Protection Circuit Using Dynamic Dual Buffer

Overshoot and undershoot caused by the current load impact the accuracy of the required output voltage and circuit performance. The transient response issue in existing low-dropout (LDO) regulators is a dynamic specification that must be addressed at the design stage. This transient response is influenced by system parameters such as stability and gain. The LDO regulator suggested in this study is designed to minimize the change in output voltage by considerably enhancing the gain using a dynamic dual buffer structure. A dynamic dual buffer is utilized to effectively control undershoot and overshoot. Under the conditions that the input voltage range is from 3.3 to 4.5 V, the maximum load current is 300 mA, the output voltage is 3 V, and the output of the proposed LDO regulator with the dynamic dual buffer structure has undershoot and overshoot voltages of 41 mV and 31 mV, respectively. That is, the output voltage of the proposed LDO regulator effectively provided and discharged an additional current suited for the undershoot/overshoot conditions to enhance the transient response characteristics. Furthermore, the electrostatic discharge (ESD) robustness characteristics of the proposed LDO regulator improved because of the silicon-controlled rectifier underlying the ESD protection device embedded in the output node and power line.

A power-efficient fast-transient OCL-LDO with adaptive super source follower and active capacitor compensation management

This paper presents a flipped voltage follower (FVF) based output-capacitor-less low-dropout regulator (OCL-LDO) with fast transient response, high power supply rejection (PSR), and low quiescent current for noise-sensitive circuits in internet-of-things (IoTs). An adaptive super source follower (ASSF) is proposed to effectively reduce the output impedance of the voltage buffer under heavy-loading conditions while keeping a low quiescent current under light-loading conditions. The active capacitor compensation management (ACCM) is proposed to solve the charge-sharing problem caused by the floating capacitors in the dynamic capacitor compensation circuit. The proposed OCL-LDO has been designed and fabricated in 22-nm CMOS technology. It can stabilize with load current ranging from 0 to 12 mA while consuming only 4.8-μA quiescent current. when the load current steps from 0.1 to 10 mA within 3.8 ns, the measured voltage undershoot is 55 mV and the recovery time is about 60 ns.

A 0.91-[formula omitted]A-quiescent-current capacitor-less LDO with sub-threshold transient enhancement and bulk-driven feed-forward supply ripple cancellation

This paper presents a capacitor-less low-dropout (OCL-LDO) regulator that features a low quiescent current for low power applications. To enhance the transient response of proposed OCL-LDO, a sub-threshold transient enhancement circuit including a transient signal input stage, a current subtractor, and a current amplifier is suggested. Meanwhile, a circuit made up of a bulk-driven ripple feed-forward circuit and a ripple introduction circuit is developed in order to improve PSR of the OCL-LDO. The proposed OCL-LDO is fabricated in 0.18 μm standard CMOS process and has an active area of 0.053 mm2. Based on measurement results, the proposed OCL-LDO has a maximum load current of 100 mA at 1.2 V input and 1 V output, and a minimum quiescent current of 0.91 μA. The overshoot voltage and undershoot voltage are 199 mV and 204 mV, respectively. PSR of the OCL-LDO is −71.8 dB at 1 KHz when the load current is 100 mA. Furthermore, the OCL-LDO exhibits a load regulation of 11.1 μV/mA and a line regulation of 1.05 mV/V.

A transient-enhanced output-capacitorless LDO regulator with non-inverting pull–push gain stage

A novel non-inverting pull–push gain stage output-capacitorless low-dropout regulator (OCL-LDO) is introduced, designed specifically for power management of system-on-chip (SoC). The incorporation of an innovative non-inverting gain stage (NIGS) serves to shift the non-dominant parasitic poles to higher frequencies, thereby enhancing the overall stability of the system. The proposed pull-push configuration markedly improves the slew rate limitation at the gate of the power transistor. Post-layout simulation outcomes, corroborated through a 180-nm CMOS fabrication process, indicate that the proposed LDO regulator maintains stability across a wide range of loading currents, from 0 mA to 100 mA, with a Miller compensation capacitance of only 3.5 pF. The circuit operates with a quiescent current of 143 μA when powered by a 1.5 V single supply. The LDO regulator boasts a dropout voltage of 300 mV, enabling it to deliver up to 100 mA of load current. Simulation results show that the undershoot voltage is only 63.7 mV when the load current jumps from 0 to 100 mA with edge of 100 ns, while employing a 100 pF capacitance load. The recovery time to return to equilibrium post this abrupt change is approximately 0.15 μs. The proposed OCL-LDO regulator exhibits a substantial enhancement in transient response compared to its predecessors, alongside a harmonized balance between line regulation and load regulation performance parameters.

Enhancing PEM fuel cell dynamic performance: Co-optimization of cathode catalyst layer composition and operating conditions using a novel surrogate model

Optimizing the cathode catalyst layer (CCL) composition and operating conditions to enhance the dynamic performance of proton exchange membrane fuel cells garners significant attention. Although machine learning surrogate models are efficient for fuel cell analysis and optimization, the varied voltage dynamic response patterns (e.g., loading failure, voltage undershoot, and voltage hysteresis) challenge regression surrogate models designed for steady-state performance predictions. In response, this study introduces a joint framework combining classification and regression models for dynamic performance prediction. For training, a transient, two-phase, non-isothermal fuel cell model with integrated catalyst agglomerate is developed. The dynamic voltage deviation (σV) is proposed as an index to characterize the dynamic performance of the fuel cell. This joint surrogate model achieves correlation coefficients of 0.9976 and 0.9961 for predicting σV in training and test sets, respectively. Through this model, sensitivity analyses of the CCL composition and operating conditions are conducted to quantify their impact and interactions on the fuel cell's dynamic performance. Besides, the analysis reveals a trade-off between dynamic performance and steady-state output. To balance these, a multi-objective optimization is conducted. The results indicate that, compared to the base case, dynamic and steady-state performance improved by 44 % and 8 %, respectively.

An Output-Capacitorless LDO regulator with fast transient response and high PSRR

Abstract A fast transient response and high supply voltage rejection ratio (PSRR) output-capacitor-less low-dropout (OCL-LDO) regulator with a dual power transistor structure is proposed in this paper. The designed flip voltage follower (FVF) structure and the proposed back-gate control loop improve the transient performance of the system. The proposed PSRR enhancement circuit transmits the power supply voltage fluctuation 1:1 to the gate of the power transistor so that the influence of the power supply voltage fluctuation on the output voltage of the LDO is offset by the difference between the gate and source voltages of the power transistor. The simulated results have shown that the proposed OCL-LDO regulator achieves full range stability from 100 μA to 50 mA. The output voltage undershoot is reduced by more than 100 mV when the load current is stepped from 1 to 50 mA in 500 ns without the load capacitor. The PSRR enhancement circuit increases the PSRR of the LDO from 1 KHz to 10 MHz by more than 20 dB.

Dynamic response characteristics and water-gas-heat synergistic transport mechanism of proton exchange membrane fuel cell during transient loading

During the current transient loading process, the proton exchange membrane fuel cell (PEMFC) is prone to voltage undershoot phenomenon, which result in internal complex chemical reactions, water vapor changes, and even reaction gas starvation. In this paper, the dynamic response characteristics of hydrogen-oxygen PEMFC and the mechanism of water-gas-heat synergistic transport during current transient loading are investigated by combining experimental and simulation methods. The experimental results show that an increase in the current loading amplitude exacerbates gas starvation and membrane dehydration leading to the deterioration of the dynamic response characteristics of PEMFC during transient current loading. To compensate for the lack of experimental methods to study the PEMFC water-gas-heat synergistic transport mechanism, this paper establishes a three-dimensional PEMFC transient model. The minimum values of reactant gas concentration in the anode and cathode catalyst layers are 0.0205 kmol/m3 and 0.0220 kmol/m3 at 65 °C, 0 backpressure and variable load conditions between 800 mA/cm2 and 1400 mA/cm2, respectively, which leads to more gas starvation at the anode. While comparing the energy wastage during current loading, it can be found that smaller current loading amplitude, higher stoichiometric ratio, and higher temperature and backpressure all contribute to the reduction of energy wastage.

A fast-response RBCOT buck converter with second-order differential and integrator compensation based on FVF

A second-order differential and integration circuit based on a flipped voltage follower (SDI-FVF) is proposed in the paper, providing compensation ripple for the buck convert with ripple-based constant on-time (RBCOT). Additionally, an adaptive conduction time module is designed which stabilizes the operating frequency of the converter in continuous conduction mode (CCM) and guarantees stable conduction time in discontinuous conduction mode (DCM). The proposed RBCOT buck convert has been designed using 180 nm process. It operates with an input voltage range of 2.5 V–5 V and an output voltage range of 0.8 V–5 V, accommodating a load current range of 0 A to 3 A. It operates at a frequency of 4 MHz, exhibiting an output ripple of 1.4 mV, achieving a peak efficiency of 91.5 %. The efficiency exceeds 85 % under a load current of 1 mA. The undershoot and overshoot voltage are only 0.13 mV and 7.1 mV with a load current variation rate of 3 A/μs.

Low humidification effect on performance of proton exchange membrane fuel cells under high current density conditions

To enhance water management in proton exchange membrane fuel cells, the influence of drying and flooding on performance in different voltage ranges is investigated by experiment. The current density attenuates as the voltage decreases in the concentration polarization range. Through the analysis of the performance curves under different conditions, the performance attenuation is due to the high current and overpotential at low voltage, which leads to the temperature rising and drying up. At high flow rates, the current attenuates at low voltages, indicating that the current density attenuation is not because of flooding and insufficient reactants. According to calculations, the anode water content is low due to more protons and water being transferred from anode toward cathode at the high current, which causes the current density to attenuate. From the current ratio, the cell can generate an additional current density of 3.2% at 0.1 V voltage, which can evaluate the degree of carbon corrosion and water electrolyzation due to the insufficient supply of reactants. In addition, the voltage undershoot of switching to high current is great, and the fuel cell has a high demand for water. Finally, reducing temperature and increasing humidification can alleviate the attenuation of current density.

Dynamic transport characteristics and performance response of commercial-size polymer electrolyte membrane fuel cell stack: An experimental study

The stability and durability of polymer electrolyte membrane fuel-cell stacks are closely related to their dynamic performance responses and internal transport characteristics, particularly for commercial-size stacks in automotive applications. In this study, the dynamic behaviors of a 10-cell stack (active area: 310 cm2) are investigated under various loading/unloading processes and cathode stoichiometric ratios; additionally, the joint influence of these two factors is analyzed. Voltage hysteresis is observed, and the dominant roles of dynamic oxygen, membrane water, and liquid transport are elucidated. The results indicate that the electro-osmotic effect dominates and results in anode-membrane dehydration as the loading process proceeds (>1.5 A cm−2). A larger airflow velocity enables faster downstream oxygen delivery, causing the oxygen concentration to reduce initially and then increase gradually with time after loading, thus voltage undershoots appear. Voltage hysteresis is enabled during the continuous loading/unloading/loading process owing to the difference in accumulated liquid water drainage ability after step loading and unloading. Although the synergistic loads and cathode stoichiometric ratio variations enable less cell-performance deterioration than that on contrary strategy, the dynamic characteristics are much more unstable and more likely to induce cell-stack failure and durability problems. This study provides insights into the dynamic transport characteristics in a commercial-size fuel cell stack and presents a guidance for control strategy development.

A simplified model‐based nonlinear control with fast response and simple design flow for HB‐LLC resonant converter

SummaryLLC resonant converters have attracted much attention for its characteristics of high efficiency and high‐power density in recent years. Yet, with the diversity of application needs such as photovoltaic power generation and electric vehicles, traditional control solutions have gradually been unable to meet the increasing requirements, such as fast response to parameters perturbations. Besides, LLC converter is a high‐order nonlinear system, and it adds complexity to controller design. In order to solve these issues, taking the HB‐LLC converter as an example, this paper presents a simplified model‐based nonlinear control with fast response and simple design. The proposed controller is based on a second‐order model of LLC circuit, and the reverse design idea of the backstepping method is used to simplify the controller design. An experimental platform is built, and the results show that compared with PID control and linear active disturbance rejection control (LADRC), the proposed method has much smaller voltage undershoot (approximately 53.1% of PID and 68.4% of LADRC) and shorter settling time (approximately 27.8% of PID and 39.3% of LADRC) under input voltage change.

Investigating the transient electrical behaviors in PEM fuel cells under various platinum distributions within cathode catalyst layers

The spatial distribution of platinum (Pt) within the cathode catalyst layer (CCL) is vital for the electrochemical reactions and mass transport in fuel cells. Though important, the transient effects of these distributions are seldom explored. This study examines the impact of Pt distribution on transient electrical behaviors, including voltage and local current distribution (LCD) uniformity, using a transient, two-dimensional, two-phase, non-isothermal fuel cell model that incorporates catalyst agglomerate. Three Pt distribution types are investigated: uniform, MPL-side biased, and PEM-side biased. Results indicate a voltage undershoot occurs during current loading. Compared to the homogeneous CCL, PEM-side bias reduces this undershoot by 12.5% due to shortened proton transfer paths and decreased ohmic loss, while MPL-side bias increases it by 18.8% due to the inverse effect. Additionally, Pt distribution affects both oxygen transport and reaction resistance within the agglomerate, influencing LCD uniformity. Under loading conditions, gradient CCLs show inferior LCD uniformity than homogeneous ones. Peak non-uniformity values of 0.22, 0.59, and 0.71 are observed for homogeneous, MPL-biased, and PEM-biased CCLs, respectively. From the perspective of voltage and LCD uniformity, the MPL-side biased CCL is not found to enhance dynamic characteristics, whereas the PEM-side bias improves voltage undershoot but at the cost of LCD uniformity. This study provides a novel perspective on fuel cell dynamics, emphasizing the transient effects of Pt distribution and their potential for optimizing dynamic performance by adjusting the Pt gradient.

Effect of Operating Conditions on Voltage Transient Response of High‐Temperature Proton Exchange Membrane Fuel Cell Under Load Changing

The transient response of a high‐temperature proton exchange membrane fuel cell (HT‐PEMFC) is critical for control and optimization. The objective of this study is to investigate the effects of operating conditions and control parameters on the transient response of the HT‐PEMFC during load variations to optimize the operating conditions. The main causes of the voltage undershoot are investigated experimentally, and the effects of air stoichiometry, air humidity, operating temperature, and backpressure on the transient response of the HT‐PEMFC are evaluated in terms of internal resistance and voltage response. The results show that during load changes, the local gas starvation of the cell and the change in the hydration state of membrane‐electrode‐assembly may be the main cause of the voltage undershoot. The voltage drop is affected by the combination of ohmic resistance, charge transfer resistance, and mass transfer resistance. In addition, operating temperature and air stoichiometry can affect voltage drop and voltage undershoot during load changes. Proper humidification of the air can alleviate voltage undershoot problems. Increasing the backpressure can improve the transient performance of the cell, but does not alleviate the voltage undershoot problem. The study can provide guidelines for the design and control of HT‐PEMFC operating conditions.

Experimental study of the steady-state and dynamic characteristics of 1 kW water-cooled PEMFC

The Polymer Electrolyte Membrane Fuel Cell (PEMFC) has been widely developed in the shipping sector, prompting the need to accurately understand its characteristics under various operating conditions and load variations. This study presents an experimental investigation into the steady-state and dynamic characteristics of a 1 kW water-cooled PEMFC stack, aiming to optimize reactant consumption and to assess the PEMFC voltage response under varying load conditions. In the steady-state tests, experiments were conducted at three different stoichiometries of hydrogen and air to evaluate the effect of stoichiometry on PEMFC efficiency. The results showed that the electrical efficiency improved by 10% at lower hydrogen stoichiometry (1.2) compared to the manufacturer's recommended values (1.6). In the dynamic tests, the behaviors of undershoot voltage and open circuit voltage (OCV) were examined using load-step and load-ramp conditions. Results showed that the undershoot voltage could be reduced either by narrowing the load step size and ramp rate or by implementing a reactant supply strategy, which increasing the gas flow rates before increasing the current. A consistent 0.55 V recovery was observed at the OCV after 10 s, regardless of the step sizes or ramp rates of load, indicating that the PEMFC was in good state of health after dynamic load conditions. OCV can serve as an effective diagnostic tool for assessing PEMFC health.