Abstract
In this paper both synchronous and asynchronous buck-converter were designed to work in continuous conduction mode “CCM” and to deliver small load current. Then the two topologies were tested in terms of efficiency at small load current by use of different values of switching frequencies (range from 150 KHz to 1MHz) and three separated values of duty-cycle (0.4, 0.6 and 0.8). 
 Obtained results turns out that efficiency of both synchronous and asynchronous buck-converter “switching step-down voltage regulator” responds in a negative manner to the increase in the switching frequency. However, this impact is being stronger in synchronous topology because of magnifying effect of losses related to switching frequency compared to those related to conduction when working at small load currents; this behavior makes obtained efficiency of both topologies in convergent levels when they operated to deliver small output current especially when working with higher switching frequencies. Larger duty-cycle can rise up the efficiency of both topologies.
Highlights
In the last years, traditional power supplies were replaced by switching mode power supplies "SMPS" in most electronic and power applications especially in portable devices, computers, and telecommunications
Because of small series ON- resistance “Rdson" of the switch element during the short ON-time duration the power dissipated across the switch element is very small compared with the power dissipated across the series-pass element in the traditional step-down dc to dc converter, and that is the major technique in the modern dc to dc converter which led to increased efficiency and comprises the size of needed heatsink and most of the other improvements in the performance
The simplest construction of this topology consists of switching field effect transistor “FET” with L-C filter and rectifier diode "D", Cin is an input capacitor filter used to control the ripple associated with input DC voltage source Vin, Vout is the DC voltage across the load resistance RL that needed to be regulate by controlling the ON-time duration of a PWM signal feeding at the gate of switch field effect transistor “S1” [4], [5]
Summary
Traditional power supplies were replaced by switching mode power supplies "SMPS" in most electronic and power applications especially in portable devices, computers, and telecommunications. The two designed topologies operated by using a range of switching frequency (150 KHz – 1 MHz) and three different values of duty cycle (0.4, 0.6, 0.8) respectively in order to study the effects of both switching frequency and duty cycle on the performance of the two topologies in term of efficiency Both topologies shown in fig (1) and fig (2) are assumed to work follows: DC input voltage "Vin" is (30 ± 4)V with ripple voltage " Vinr" of 250 mV output load current " Iout" is 750mA required output voltage " Vout" assumed to be 12V at the first test and increased to 16 V, 24 V respectively with typical ripple index "Vr" less or equal to 1% From datasheet [7], MCH5809 composite N-channel MOSFET “MCH3443” with schottky barrier diode SBD SBS006M is suitable to use in both topologies according to suggested considerations in this work. From data sheet [8] schottky diode “1N5820” offers low forward potential voltage "Vf" so that it can be used as rectifier diode in the ASBC topology
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