This paper proposes closed loop control of transformerless interleaved high step-down conversion ratio dc-dc converter. The proposed converter utilizes two input capacitors which are series charged by the input voltage and parallel discharged by a new two phase interleaved buck converter for providing much higher conversion ratio without adopting an extreme short duty cycle. The proposed converter features uniform current sharing characteristic of the interleaved phases without adding extra circuitry. It also reduces the voltage stress of both switches and diodes. This will allow one to choose MOSFETs with low voltage rating to reduce both conduction and switching losses. Closed loop PI controller is used to control the error and finally achieves the desired output voltage. The operation principles, steady state analysis and comparison with other existing topologies are presented. Simulation results are presented to demonstrate the effectiveness of the converter. KEYWORDS:High step down conversion ratio, Interleaved Buck Converter (IBC), Closed loop, PI controller. I.INTRODUCTIONNowadays, for the increasing high step down ratios with high output current applications such as battery chargers, distributed power systems and VRMs of CPU boards, high performance dc-dc converters have been used. The conventional IBC is required operating at higher switching frequencies for high-input and low-output voltage applications, that will increase both switching and conduction losses. It also experiences an extremely short duty cycle in the case of high-input and low-output voltage applications [1]. Many step-down converters have been proposed to overcome the drawbacks of conventional IBC. II.LITERATURE SURVEYA quadratic buck converter [2] is synthesized by cascading two dc-dc buck converters. It can operate with wider ranges of step-down conversion ratio than conventional IBC but the efficiency is very poor. A three level buck converter [3] possess higher efficiency and better performance than the conventional IBC due to reduced voltage stress across the switch (half of the input voltage). However, so many components are required for this converter. An IBC with single capacitor turn off snubber is introduced in [4].By using this the switching loss associated with turn-off transition can be reduced and the single coupled inductor implements the converter as two output inductors, still the voltage across all the elements is same as input voltage.To reduce the switching losses, an active clamp IBC is introduced in [5], here all the switches are turned on with zero voltage switching (ZVS) whereas it requires additional passive elements and active switches, which increases the cost at low or middle levels of power applications. Recently, an IBC with two coupled inductors are introduced [6,7]. Moreover, a high step-down conversion ratio is obtained and the switching losses can be reduced. However, the leakage energy needs to recycle. A new extended duty ratio multiphase topology has been proposed to deal with a small duty...