4.8. DIODE APPLICATIONS: AN OVERVIEW 191
4.8.14 that the second-order nonlinearity also produces a second harmonic and a DC term.The
secondharmonicgenerationisthepropertyusedinfrequencymultipliers. Also, the DC term
amplitude is proportional to the square of the input voltage, hence input power. Thisisthe
principleofoperationofdiodepowerdetectors.
4.8.4 PowerDevices ...............................
A DC-to-DC converter is a module that accepts a DC input voltage and produces a DC output
voltage typically at a different voltage level or of different polarity. These modules have become
ubiquitous in modern electronic systems. For example, laptops use them to convert the mains
power supply voltage to the battery voltage (18 V), which in turn is converted to the supply
voltage for the computing electronics (1.5-3.5 V) and the voltage for the display (voltage variable
depending on type of display). All are different! In addition, DC-to-DC converters are used to
provide bus isolation, power bus regulation, etc. There are several topologies to achieve the
desired conversion and we will briefly discuss a Buck or Step- Down Converter to appreciate the
functional requirements of the transistor switch and diode that this employed. As in most power
conversion circuits it is imperative to not have current flow with a large voltage across dissipative
elements such as transistor switches. This will cause power dissipation and excessive heating in
the circuit. To reduce the voltage across a switch while it is conducting, an inductor is typically
employed in circuits. Furthermore a capacitor is used at the output to stabilize the output voltage
through the switching cycle. In the Buck/Step-Down circuit (figure 4.28), an input transistor is
turned on causing the input voltage Vin(which has to be stepped-down) to appear at one end of
the inductor while the other remains at the output. This voltage will cause the inductor current to
rise, storing energy as magnetic flux. During this process the diode is reverse biased and turned
off and the current flows through the transistor and the inductor to the output capacitor and load.
When the transistor is turned off, the current through the inductor will continue flowing but now
be forced through the diode causing the diode to turn on. This process is called free-wheeling.
The voltagesVxandVowill follow standardLandCcharging/discharging relationships as
shown in figure 4.28.
Figure 4.28 shows schematically the change in the current and voltage across the inductor over
a switching cycle of the transistor. From the relation
Vx−Vo=Ldi
dt(4.8.15)
the change of current satisfiesif−ii=∫
ON(Vx−Vo)dt+∫
OFF(Vx−Vo)dt (4.8.16)where iiand ifare the currents through the inductor at the beginning and the end of a cycle.
For steady state operation it is required that the current at the start and end of a periodTbe the
same. To get a ’simple relation’ between voltages we assume ’no voltage drop across transistor
or diode’ while ON and a perfect switch change. Thus during the ON timeVx=Vinand in the