Tài liệu Power Electronic Handbook P4 pptx

© 2002 by CRC Press LLC

4

Rectifiers

4.1 Uncontrolled Single-Phase Rectifiers

Single-Phase Half-Wave Rectifiers • Single-Phase Full-Wave

Rectifiers

4.2 Uncontrolled and Controlled Rectifiers

Uncontrolled Rectifiers • Controlled Rectifiers • Conclusion

4.3 Three-Phase Pulse-Width-Modulated Boost-Type

Rectifiers

Introduction • Indirect Current Control of a Unity Power

Factor Sinusoidal Current Boost-Type Rectifier • Appendix

4.1 Uncontrolled Single-Phase Rectifiers

Sam Guccione

Single-Phase Half-Wave Rectifiers

Operation

A single-phase half-wave rectifier consists of a single diode connected as shown in Fig. 4.1. This is the

simplest of the rectifier circuits. It produces an output waveform that is half of the incoming AC voltage

waveform. The positive pulse output waveform shown in Fig. 4.1 occurs because of the forward-bias

condition of the diode. A diode experiences a forward-bias condition when its anode is at a higher

potential than its cathode. Reverse bias occurs when its anode is lower than its cathode.

During the positive portion of the input waveform, the diode becomes forward biased, which allows

current to pass through the diode from anode to cathode, such that it flows through the load to produce

a positive output pulse waveform. Over the negative portion of the input waveform, the diode is reverse￾biased ideally so no current flows. Thus, the output waveform is zero or nearly zero during this portion

of the input waveform.

Because real diodes have real internal electrical characteristics, the peak output voltage in volts of a

real diode operating in a half-wave rectifier circuit is

(4.1)

where VP(in) is the peak value of the input voltage waveform and VF is the forward-bias voltage drop across

the diode. This output voltage is used to determine one of the specification values in the selection of a

diode for use in a half-wave rectifier.

Other voltage and current values are important to the operation and selection of diodes in rectifier

circuits.

VP(out) = VP(in) – VF

Sam Guccione

Eastern Illinois University

Mahesh M. Swamy

Yaskawa Electric America

Ana Stankovic

Cleveland State University

© 2002 by CRC Press LLC

Important Diode Current Characteristics

Peak Forward Current

The peak forward or rectified forward current, IFM, in amperes is the current that flows through the diode

as a result of the current demand of the load resistor. It is determined from the peak output voltage Eq.

(4.1) as

(4.2)

where RL is the load resistance in ohms. IFM is also a specification value used to select a diode for use in

a rectifier. Choose a diode with an IFM that is equal to or greater than the IFM calculated in Eq. (4.2).

rms Forward Current

Since rms values are useful, the rms value of forward current in amperes is determined from

(4.3)

This value is sometimes called the maximum rms forward current.

Mean Forward Current

To find the continuous forward current that the diode in a half-wave rectifier circuit is subjected to, the

mean or average rectified current, IFAV, can be found from

(4.4)

Because this average current is a continuous value, it is sometimes suggested that a diode be selected that

has an IFAV value of 1.25 times that determined from Eq. (4.4).

Single Cycle Surge Current

One additional current is important in rectifier circuits. That current is the single cycle surge current,

IFSM. This is the peak forward surge current that exists for one cycle or one half cycle for nonrepetitive

conditions. This could be due to a power-on transient or other situations.

Important Diode Voltage Characteristics

Average Output Voltage

The average output voltage of a half-wave rectifier is determined from

(4.5)

Repetitive Peak Reverse Voltage

Another characteristic that is important to the operation of rectifier circuits is the voltage that the diode

experiences during reverse bias. When the diode is reversed, it experiences a voltage that is equal to the

value of the negative peak input voltage. For example, if the negative peak input voltage is 300 V, then

the peak reverse voltage (prv) rating of the diode must be at least 300 V or higher. The prv rating is for

FIGURE 4.1 Single-phase half-wave rectifier.

IFM VP(out) = /RL

IFRMS I = FM × 0.707

IFAV I = / FM π

VAVG (out) = / VP(in) π

© 2002 by CRC Press LLC

a repetitive input waveform, thus producing a repetitive peak reverse voltage value. A nonrepetitive prv

is also an important specification value, as will be described below.

The repetitive peak reverse voltage is given different names. It is called variously the peak reverse

voltage, peak inverse voltage, maximum reverse voltage (VRM), and maximum working peak reverse

voltage (VRWM). The most common name is the repetitive peak reverse voltage, VRRM. The repetitive peak

reverse voltage is one of the critical specification values that are important when selecting a diode for

operation in half-wave rectifier circuits.

Forward Voltage Drop

The value of the maximum forward voltage, VF , is the voltage value that occurs across a diode when it

becomes forward biased. It is a small value usually in the range of 0.5 V to several volts. VF is sometimes

identified as the maximum forward voltage drop, VFM. The threshold value of the forward voltage is

sometimes listed in specifications as VF(TO).

Nonrepetitive Peak Reverse Voltage

Diodes used in rectifiers are also specified in terms of their characteristics to nonrepetitive conditions.

This is usually identified as the voltage rating for a single transient wave. The symbol, VRSM, is used. VRSM

is a specification value. This voltage is sometimes identified as the nonrepetitive transient peak reverse

voltage.

Single-Phase Full-Wave Rectifiers

Operation

A single-phase full-wave rectifier consists of four diodes arranged as shown in Fig. 4.2 in what is called

a bridge. This rectifier circuit produces an output waveform that is the positive half of the incoming AC

voltage waveform and the inverted negative half. The bias path for the positive output pulse is through

diode D1, then the load, then D4, and back to the other side of the power supply. The current flow through

the load is in the down direction for the figure shown. Diodes D2 and D3 are reverse-biased during this part.

The bias path for the negative cycle of the input waveform is through diode D3, then the load, then

D2, and back to the opposite side of the power supply. The current flow through the load resistor is once

again down. That is, it is flowing through the load in the same direction as during the positive cycle of

the input waveform. Diodes D1 and D4 are reverse-biased during this part. The resulting output waveform

is a series of positive pulses without the “gaps” of the half-wave rectifier output.

As in the half-wave rectifier circuit description, real diodes have real characteristics, which affect the

circuit voltages and currents. The peak output voltage in volts of a full-wave bridge rectifier with real diodes is

(4.6)

FIGURE 4.2 Single-phase full-wave bridge rectifier.

VP(out) = VP(in) – 2 × VF

© 2002 by CRC Press LLC

where VF is the forward-bias voltage drop across one diode. Because there are two forward-biased diodes

in the current path, the total drop would be twice the drop of one diode.

As in the half-wave rectifier, there are other voltages and currents that are important to the operation

and selection of diodes in a full-wave rectifier. Only those values that are different from the half-wave

circuit will be identified here. The other values are the same between a half-wave and a full-wave rectifier.

Important Diode Current Characteristics

Peak Rectified Forward Current

The peak rectified forward current, IFM, in amperes has the same equation (4.1) as for the half-wave

rectifier. The difference is that the value VP(out) is as shown in Eq. (4.6).

rms Forward Current

The rms value is computed using the same Eq. (4.2).

Average Forward Current

The mean or average forward current for a full-wave rectifier is twice the value for a half-wave rectifier.

The equation is

(4.7)

Single-Cycle Surge Current

This current is the same for either type of rectifier.

Important Diode Voltage Characteristics

Average Output Voltage

The average output voltage of a full-wave rectifier is twice that of a half-wave rectifier. It is determined

from

(4.8)

Repetitive Peak Reverse Voltage

The repetitive peak reverse voltage, VRRM, is slightly different for a full-wave bridge rectifier. It is deter￾mined by

(4.9)

where VP(out) and VF have been defined before in Eq. (4.1).

Forward Voltage Drop

This voltage is the same for either type of rectifier.

Nonrepetitive Peak Reverse Voltage

This voltage is the same for either type of rectifier.

4.2 Uncontrolled and Controlled Rectifiers

Mahesh M. Swamy

Rectifiers are electronic circuits that convert bidirectional voltage to unidirectional voltage. This process

can be accomplished either by mechanical means like in the case of DC machines employing commutators

or by static means employing semiconductor devices. Static rectification is more efficient and reliable

compared to rotating commutators. This section covers rectification of electric power for industrial and

commercial use. In other words, we will not be discussing small signal rectification that generally involves

IFAV 2 I = × FM/π

VAVG (out) = 2 × / VP (in) π

VRRM = VP (out) – VF