B ASIC C ONVERTER T OPOLOGIES

A major decision that must be considered at the beginning of a SMPS design is which basic topology to use. The term topology refers to the arrangement of the power components within the SMPS design. There are more than ten different topologies can be used in DC-DC conversion [1], [12], [39]. Here we limit our introduction to three basic non-isolated topologies of DC-DC SMPS: buck, boost and buck-boost.

2.2.1 Buck

A more detailed discussion of the buck regulator as opposed to the other topologies is presented due to popularity of the buck regulator.

Fig. 2.3 Buck converter topology and related waveforms.

For the buck converter of Fig. 2.3, the output voltage VOUT is less than the input voltage VIN, hence the name buck. When the switch is closed, input current flows through the filter inductor, the filter capacitor, and the load. When the switch is opened, the voltage across the inductor reverses since VL becomes a voltage source (VL = L × di/dt), and the energy stored in the inductor is delivered to the load. Since the current in the inductor cannot change instantaneously, the current flowing through the switch at the time the switch is opened now flows through the inductor, the capacitor, the load, and the diode. When the switch is again closed, the current, which

The average output voltage is

V_OUT = V_IN × D (2.1)

Where D is the duty cycle. The duty cycle is the ratio of the switch on-time to the period T. Since

VIN × IIN = VOUT × IOUT (2.2)

The average input current is

IIN,AVG = IOUT × D (2.3)

We can see that when D = 100 %, VOUT = VIN and IOUT = IIN. Conversely, when the duty cycle approaches 0 %, the output voltage becomes very small and the peak input current becomes very large.

2.2.2 Boost

For the boost converter of Fig. 2.4, the output voltage is greater than the input voltage, hence the name boost. With input voltage applied, the current flows through the inductor, the diode, the capacitor and the load. When the switch is closed, the current flows through the inductor and switch and in effect, the voltage across the inductor is the input voltage. When the switch is opened, the induced reverse voltage in the inductor is then in series-adding with the input voltage to increase the output voltage, and the current which was flowing through the switch now flows through the inductor, the diode, the capacitor, and the load. The energy stored in the inductor is transferred to the load. When the switch is again closed, the diode becomes reverse

biased, the energy in the capacitor supplies the load voltage, and the cycle repeats.

Fig. 2.4 Boost converter topology and related waveforms.

The average output voltage is

VOUT = VIN / (1-D) (2.4)

Where D is the duty cycle. The duty cycle is the ratio of the switch on-time to the period T. Since

× I × I (2.5)

The average input current is

IIN,AVG = IOUT / (1-D) (2.6)

We can see that when D = 0 %, VOUT = V_IN and IOUT = I_IN. Conversely, when the duty cycle approaches 100 %, the output voltage does not necessarily approach infinity because the conducting operation of the semiconductor switch produces a peak current which will quickly exceed the safe operating area (SOA) limit. In other words, we can say that the conduction loss will limits the output voltage when D approaches 100 %.

2.2.3 Buck-Boost

The buck-boost converter shown in Fig. 2.5 is referred to by many names. The buck-boost terminology will be used since the output voltage may be less than, or greater than, the input voltage. The converter is sometimes referred to as a flyback converter. The flyback designation is appropriate due to the inherent action of the inductor. This action in itself is sometimes referred to as a ringing-choke regulator.

Also, the topology is sometimes referred to as an inverting regulator, since the output voltage polarity is opposite the input voltage polarity.

When the switch is closed, the current flows through the inductor since the diode is reverse biased. When the switch is opened, the current, which was flowing in the switch, now flows through the inductor, the diode, the capacitor and the load. The energy stored in the inductor is transferred to the load. When the switch is again closed, the current, which was flowing through the diode, now flows through the switch, and the diode becomes reverse biased.

Fig. 2.5 Buck-boost converter topology and related waveforms.

The average output voltage is

-VOUT = VIN × D / (1-D) (2.7)

Where D is the duty cycle. The duty cycle is the ratio of the switch on-time to the period T. Since

V_IN × IIN = V_OUT × IOUT (2.8)

The average input current is

IIN,AVG = IOUT × D / (1-D) (2.9)

We can see that when D equals 50%, -VOUT = VIN and –IOUT = IIN.

2.2.4 Synchronous Rectification

In the above topologies, we can see a master switch and a diode served as a slave switch. We can use a controlled switch (synchronous switch) instead of a diode in the above topologies, i.e. synchronous rectification. Synchronous rectification is used in DC-DC converters when low output voltage and high current is needed. Synchronous rectification utilizes power MOSFETs instead of rectifying diodes. These MOSFETs are synchronized to the converter frequency and perform more efficiently the rectification of the output voltage than rectifying diodes due to the low I × R drop through the channel. [40]-[41]