SLR half bridge DC/DC converter in low frequency
Concept of load resonant converters
In these resonant converters ,an LC tank is used that results in oscillating load voltage and current and thus provided zero voltage ad/or zero current switchings.Each circuit in this catagory is analyzed with a load that is most practical for the converter topology being considered .Only the steady state operation is considered.
SLR DC-DC converter
Basic Definition:The series resonant tank is formed by Lr and Cr and current through the resonant tank circuit is full wave rectified at the output and feeds the output stage.Therefore as the name suggests the output load appears in series with the resonant tank.
Modes of SLR DC-DC converter
There are three possible modes of operation based on the ratio of switchingfrequency ws to the resonance frequency wo ,which determines if iL flows or discontinuously
1. Discontinuous condition mode with ws<1/2wo
2. Continuous conduction Mode with 1/2wo<ws<wo
3. Continuous Condition mode with ws>wo
Concept of Steady State Operating Chracteristics and Control of SLR Dc-Dc Converters
concept of steady state operating chracteristics : It is useful to know the relation of the peak and the average values of the circuit volatge and currents to the operating condition(Vd,Vo,Io,wo etc).
A SLR DC- DC converter in the discontinuous conduction mode operates as a current source,that is io stays contant even through the load resistance & hence Vo may change.
Control of SLR DC-DC Converters: In series resonant circuits, the resonant tank impedance depends on the frequency of operation.Therefore,for a given applied inputs voltage Vd and a load resistance Vo can be regulated by controlling the switching frequency fs.
LOAD RESONENT CONVERTERs :
These converters consists of an LC resonant circuit.Oscillating voltage and current ,due to LC resonance in the tank are applied to the load,and the converter switches can be zero voltage and /or zero current.Either a series LC or a parallel LC circuit can be used. In these converter circuits,the power flow to the load is controlled by the resonent tank impedence ,w hich in turn is controlled by the switching frequency fs in comparision to the resonent frequency fo of the tank.
Series loaded resonant (SLR) dc-dc converter :
A half bridge configuration of the SLR converter is shown in the figure below The waveforms and the operating pricniple are the same for the full bridge configuration .A transformer can be included to provide the output voltage of the desired magnitude as well as the electrical isolation between the input and the outputThe series resonent tank is formed by Lr and Cr ,and the current through the resonent tank circuit is full wave rectified at the output and |iL| feed the output stage.Therefore the output load appears in series with the resonent tank.The filter capacitor Cf at the output is usually very large ,and therefore the output voltage across the capacitor can be assumed to be a dc voltage without any ripple.The resistive power loss in the resonent circuit is assumed to be negligible, which greatly simplifies the analysis.The output volatge V o is reflected across the rectifier input as VB'B where VB'B =Vo if iL is positive and V B'B = -Vo if iL is negative
DISCONTINEOUS CONDUCTION MODE WITH ws<wo/2
CONTINEOUS CONDUCTION MODE WITH wo/2<ws<w o
CONTINEOUS CONDUCTION MODE WITH ws>wo
Steady state operating characteristics
Control of SLR dc-dc converters