Chapter 10.1.2 - DC-DC Switching Converters (continued)

Filtering components must be added to smooth the waveforms. To
maintain high efficiency, only inductors and capacitors are added. To
help stiffen the bus voltage, a capacitor is added in parallel with the left
terminals. An inductor is used to isolate the 150-volt terminal pair on
the side with the shunt switch. Of course, the inductor will pass a
continuous dc current, but will tend to block ac current. A revised version
of the simple two-port interface is shown in Fig. 10.7.

This minimal topology “interface” is in fact the skeleton of four basic
types of high-frequency dc-dc switching converters. This simple form,
shown in Fig. 10.7, is so fundamental that it has been called the canonical
switching cell⁵. It is often redrawn as shown in Fig. 10.8.^5,6

As long as the switches are ideal (so that currents and voltage may
flow or be blocked in any direction), power may flow bidirectionally
through the circuit. By varying the interconnection of the designated

input and output terminals and the implementation of the ideal switches
with real devices (which may or may not limit the flow of current or
voltage in certain directions), all four basic high-frequency topologies
may be derived. When the canonical cell is connected so as to allow
power to flow directly from one terminal to another when one of the
switches is closed, the buck or boost converters are formed. The type of
converter formed depends on the direction of power flow; in the case of
the buck converter, or chopper, power flows from a high-voltage source
to a lower-voltage load when the controllable switch is closed. In the
case of the boost converter, power flows from a low-voltage source to a
load operating at a higher voltage when the diode is conducting. The
classic buck and boost implementations of the canonical cell are shown
in Figs. 10.9 and 10.10.

The braces in Figs. 10.9 and 10.10 indicate the canonical cell portions
of the buck and boost converters. Note that actual implementations
invariably make modifications to the canonical cell. For example, an
extra output capacitor was added to make the buck topology shown in
Fig. 10.9. For simplified discussions, the input capacitor is usually
discarded from the buck configuration of the canonical cell, i.e., the
voltage source is presumed to be “stiff,” or to exhibit little internal
impedance.

The buck-boost and boost-buck converters are implemented by
configuring the canonical cell so that power never flows directly from
one port terminal to another; energy is accumulated in one of the storage
elements during part of the cycle and then is removed from the element
to power the load on another part of the cycle. In the buck-boost topology,

the most common of the “indirect” power supplies, the inductor is the
intermediary energy-storage element. Also, the capacitance in the
canonical cell is typically split between the input and output terminals.
A simple buck-boost converter is shown in Fig. 10.11. A discussion of
the less-common boost-buck converter is deferred to References 14, 16,
and 22.

For now, all of the switching-power supply circuits have been drawn
with a resistive load. Assuming that the switches, inductors, and
capacitors in the power supplies are ideal, the resistive load is the only
element in the circuits that can dissipate time-average power. This
means that, in theory, any power taken from the input source is
ultimately delivered to the load. In other words, the circuits are
theoretically 100% efficient. Of course, the components would not be
ideal in practice, but it is commonplace to achieve efficiencies in excess
of 90% in actual implementations. The output voltage or current provided
to the load can be actively controlled to a particular value or reference
waveform by appropriately varying the duty cycle of the controllable
switch. The next section examines a basic scheme for controlling the
output voltage of a buck converter.

Filtering components must be added to smooth the waveforms. To
maintain high efficiency, only inductors and capacitors are added. To
help stiffen the bus voltage, a capacitor is added in parallel with the left
terminals. An inductor is used to isolate the 150-volt terminal pair on
the side with the shunt switch. Of course, the inductor will pass a
continuous dc current, but will tend to block ac current. A revised version
of the simple two-port interface is shown in Fig. 10.7.

This minimal topology “interface” is in fact the skeleton of four basic
types of high-frequency dc-dc switching converters. This simple form,
shown in Fig. 10.7, is so fundamental that it has been called the canonical
switching cell⁵. It is often redrawn as shown in Fig. 10.8.^5,6

As long as the switches are ideal (so that currents and voltage may
flow or be blocked in any direction), power may flow bidirectionally
through the circuit. By varying the interconnection of the designated

input and output terminals and the implementation of the ideal switches