10

OPERATION OF STEP-UP DC/DC CONVERTER

Step-up DC/DC Converter charges energy in the inductor when Lx Transistor (LxTr) is on, and discharges the

energy with the addition of the energy from Input Power Source thereto, so that a higher output voltage than the

input voltage is obtained.

The operation will be explained with reference to the following diagrams :

< Current through L >

< Basic Circuits >

RH5RH

i2

L

SD

CL

Lx Tr

i1

IL

ILmin

ILmax

topen

t

ton

toff

T=1/fosc

Step 1 : LxTr is turned ON and current IL (= i1 ) flows, so that energy is charged in L. At this moment, IL(=i1 ) is

increased from ILmin (= 0) to reach ILmax in proportion to the on-time period (ton) of LxTr.

Step 2 : When LxTr is turned OFF, Schottky diode (SD) is turned ON in order that L maintains IL at ILmax, so

that current IL (= i2) is released.

Step 3 : IL (=i2) is gradually decreased, and in the case of discontinuous mode, IL reaches ILmin (=0) after a time

period of topen, so that SD is turned OFF. However, in the case of a continuous mode which will be mentioned

later,the time period (toff) runs out before IL reaches ILmin (=0), so that LxTr is turned ON in the next

cycle, and SD is turned OFF. In this case, ILmin does not reach zero, and IL (=i1) increases from ILmin (> 0).

In the case of PWM control system, the output voltage is maintained constant by controlling the on-time peri-

od (ton), with the oscillator frequency (fosc) being maintained constant.

•

Discontinuous Conduction Mode and Continuous Conduction Mode

In the above two diagrams, the maximum value (ILmax) and the minimum value (ILmin) of the current which

flows through the inductor are the same as those when LxTr is ON and also when LxTr is OFF.

The difference between ILmax and ILmin, which is represented by

∆I, is :

wherein T=1/fosc=ton+toff

duty (%)=ton/T · 100=ton · fosc · 100

topen

≤toff

change in the current at OFF.