Mohammad Reza Amini

Novel Family of PWM Soft-Single-Switched DC–DC Converters With Coupled Inductors

In this paper, a novel family of pulsewidth-modulation soft-single-switched DC-DC converters without high voltage and current stresses is described. These converters do not require any extra switch to achieve soft switching, which considerably simplifies the control circuit. In all converter family members, the switch is turned on under zero-current condition and is turned off at almost zero-voltage condition. From the proposed converter family, the boost topology is analyzed, and its operating modes are explained. The presented experimental results of a prototype boost converter confirm the theoretical analysis.

Three-Phase Soft-Switching Inverter With Minimum Components

In this paper, a novel three-phase soft-switching inverter is presented. The inverter-switch turn on and turn off are performed under zero-voltage switching condition. This inverter has only one auxiliary switch, which is also soft switched. Having one auxiliary switch simplifies the control circuit considerably. The proposed inverter is analyzed, and its operating modes are explained in details. The design considerations of the proposed inverter are presented. The experimental results of the prototype inverter confirm the theoretical analysis.

Quasi Resonant DC Link Inverter with a Simple Auxiliary Circuit

In this paper, a new soft switching three phase inverter with a quasi-resonant dc-link is presented. The proposed inverter has a dc-link switch and an auxiliary switch. The inverter switches are turned on and off under zero voltage switching condition and all auxiliary circuit switches and diodes are also soft switched. The control utilizes PWM and the auxiliary switch does not require an isolated gate drive circuit. In this paper, the operation analysis and design considerations of the proposed soft switching inverter are discussed. The presented experimental results of a realized prototype confirm the theoretical analysis.

Analysis of a multi-user cognitive radio network considering primary users return

A complete distributed single-channel multi-user cognitive radio network with a common control channel is analyzed. To derive the necessary metrics, we exploit renewal reward theory by which we are able to carry out a continuous time analysis of the whole network. This approach can be used as a framework for some other scenarios with a few changes in the modeling of the renewal process.The imperfect behavior of secondary sensor as well as changes in the occupancy state of PU traffic during SU transmission is considered in deriving the probability of collision and it is introduced as a constraint for the problem of maximizing the network throughput. Consequently, the interference due to both primary user re-occupancy and sensing error in the detection of primary user are considered.We derive mathematical closed form expressions for the expected amount of interference and transmission time in a renewal cycle and hence for the collision and transmission rates as well as other performance measures, those which are important for network design and network planning.The analysis used in this paper can be easily extended to some other scenarios such as slotted transmission protocols. Furthermore, additional constraints such as energy can be easily included in the analysis since the complete continuous-time behavior of secondary and primary users in the spectrum access can be modeled. Multi-user cognitive radio networks have been considered in the literature recently. However, there is no analytical framework which can provide a model in order to derive the system performance metrics. This paper presents an analytical continuous time framework for a multi-user cognitive radio in which secondary users (SUs) communicate on a single common control channel, the most common protocol discussed in the multi-user category. The proposed analysis method is based on the renewal theory. We prove that the spectrum access of SUs with respect to the primary user (PU) traffic behavior forms a renewal process. Furthermore, the corresponding renewal cycle is derived and metrics such as collision probability and interference time due to both sensing error and PU re-occupancy are formulated in the renewal cycle. Thanks to the proposed continuous time analyses, the transmission efficiency for the secondary network is formulated. Finally, some numerical analyses are provided on the derived equations to discuss the optimum transmission time for SUs in order to have the maximum efficiency under the prescribed collision constraint. As an example, for the sensor, probabilities of false alarm and miss-detection are taken to be equal to 0.1 and 0.01, respectively, with a primary network in which the idle and busy cycle rates are 0.1 and 0.3; also, the transmission time must be bounded to 500 ms in order for the collision probability to remain under 0.01. At the end, we show that simulation results are consistent with the numerical analyses. Display Omitted

Introducing Self-Oscillating Technique for a Soft-Switched LED Driver

In this paper, a self-oscillating soft-switched converter for light-emitting-diode (LED) driver application is presented. In the proposed converter, the self-oscillation conditions are provided with a low number of additional components, and this converter provides a zero-current switching (ZCS) condition for instant switch turn-OFF. This converter does not need additional output current feedback because the output current changes only 10% when the output voltage changes by 33%. The self-oscillating part of this converter applies three low-power BJT and guarantees a soft-switching condition. The proposed self-oscillating control circuit can be implemented with a low cost since it does not need any IC and power supply for the control circuit. The proposed self-oscillating circuit detects the proper moment to turn the switch OFF under the ZCS condition. In order to justify the validity of theoretical analysis, the results of the implemented prototype for driving an LED string are presented. The measured power factor of the prototype shows that, although the input current of the converter is not sinusoidal, this converter can meet the IEC61000-3-2 standard at an operating power of less than 25 W, in addition to having a simple structure and achieving the robust ZCS condition.

Analysis of frost visualization over a fin and tube heat exchanger by natural convection

ABSTRACT In this article, studies are made on frost formation and flow over a fin and tube heat exchanger due to natural convection for various conditions of relative humidity, air ambient temperature, and mean refrigerant temperature. The results include frost deposition, steps of frost formation, and its effect on heat transfer rate for different conditions. The results show that frost is formed only on the tip of the fins with higher thickness from top to bottom due to small distance between the fins. Frost causes to trap the air which increases the thermal resistance and reduces heat transfer in the system.

Coexisting with the dynamic PU, the effect of PU‐returns on a secondary network

Int J Commun Syst. 2017;e3316. https://doi.org/10.1002/dac.3316 Summary Coexisting with a dynamic primary user network is a challenge in cognitive radio networks, since the rate at which primary user reoccupies the spectrum is high. Therefore, it is important to investigate deeply the effect of primary traffic dynamics on secondary network performance metrics. To do that, interference due to both sensing error and primary user re‐occupancy must be formulated and considered in throughput and collision probability. Besides them, secondary packet retransmissions due to such interference should be considered in metrics such as average energy consumption and average packet delay. In this study, we formulate the mentioned secondary network metrics regarding to such kinds of interference and secondary packet retransmission. Numerical results along with discussions are given to clarify derived expressions. Simulations are also performed to justify the theoretical results, and optimum operating point for secondary network settings is derived by genetic algorithm.

Energy Efficiency Optimization of Secondary Network Considering Primary User Return With Alternating-Phase-Type Traffic

Much work has been done in the area of cognitive radio networks to formulate the inherent tradeoff among different metrics, such as sensing and throughput. Primary user (PU) traffic behavior can affect the mentioned tradeoff and must be considered in analyses, especially when cognitive users coexist with the dynamic-traffic PU. In this case, PU-returns to the spectrum during secondary users’ (SUs) transmission are more likely and, hence, such kinds of interference are inevitable. Furthermore, how to model the PU traffic is important. Note that there are two kinds of interference between SU and PU in cognitive radio networks, one due to sensing error and the other due to primary user reoccupancy. In this paper, a new metric called energy per successful transmission time is defined to address both kinds of inference by formulating SUs’ successful transmission time as well as energy consumption. Furthermore, collision probability and average packet delay are formulated considering these interference metrics and the primary user traffic is modeled as an alternating phase type renewal process by which many traffic behaviors, such as long range dependency and self-similarity can be modeled. Finally, some numerical examples are given and the corresponding curves for different metrics are discussed.