Environmental Engineering Reference
In-Depth Information
4.5.7 Converter Internal Control Systems
Very simple converters may employ analogue electronics to provide the transistor gate
or base control signals. In all other cases, the internal control of a modern converter
is provided by a dedicated microprocessor or digital signal processor (DSP). By con-
trolling the exact timing of transistor gate or base control signals, the microprocessor
has control over the currents, voltages and power fl ow through the converter. The
microprocessor will often be used to provide a number of other functions. In a grid-
connected PV inverter, for example, the microprocessor will typically perform the
maximum power point tracking (MPPT), anti-islanding protection and perhaps some
diagnostics and communications.
Depending on the application, the microprocessor based control system in a modern con-
verter may have a variety of control objectives. For example, in a standalone inverter, the
objectives would be to keep the output at constant voltage and constant frequency; in a grid-
connected PV inverter, the objective would be to achieve maximum power extraction as the
solar radiation varies, a task that would require control of the inverter output current; and in
a motor drive inverter, the objective might be to control the torque. The power electronic
circuits used in all three of these examples could be very similar, but the implemented control
systems make them behave very differently.
4.5.8 DC - DC Converters
An AC voltage can be easily stepped up or down through the use of the ubiquitous trans-
former. Unfortunately, this facility was not as easily available for direct current until the
recent explosion in power electronic developments. As will be shown later, a DC transformer
widens considerably the application of power electronic converters.
An ideal DC-DC transformer will convert power at one voltage to power at a different
voltage. If the voltage is increased, the current will be decreased by the same factor, and vice
versa, such that:
VI
=
V I
(4.29)
in
in
out
out
DC transformers are known as choppers . or DC - DC converters . In such choppers a power
transistor is switched at high frequency and the duty cycle (the on-to-off time ratio known
as the mark-space ratio) controls the output voltage.
Step - down DC - DC Converter
Provided that the current in the inductor is continuous, the steady state operation of the circuit
in Figure 4.36 can be described as follows. The transistor is switched on for time T on , during
which v l = v in
v out . Then it is switched off for a time T off , during which v l =
v out . In the
steady state, the volt-seconds across the inductor must be zero, so
T
TT
on
V
=
V
where
D
is the duty cycle
(4.30)
out
in
+
on
off
 
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