semiconductor rectifier
semiconductor rectifier
[¦sem·i·kən¦dək·tər ′rek·tə‚fī·ər]Semiconductor rectifier
A semiconductor diode that is used in rectification and power control. The semiconductor diode conducts current preferentially in one direction and inhibits the flow of current in the opposite direction by utilizing the properties of a junction formed from two differently doped semiconductor materials. Doped silicon is by far the most widely used semiconductor. Semiconductor diodes are intrinsic to integrated circuits and discrete device technology and are used to perform a wide variety of isolation, switching, signal processing, level shifting, biasing, control, and alternating-current (ac) to direct-current (dc) conversion (rectification) functions. See Rectifier, Semiconductor
Either as a key element of an integrated circuit or as a discrete packaged part, the silicon rectifier diode is used in a plethora of applications from small power supplies for consumer electronics to very large power-rectification industrial installations. Many semiconductor diodes are used in non-power-conversion applications in signal processing and communications. These include avalanche or Zener diodes; diodes used for amplitude-modulation radio detection, mixing, and frequency translation; IMPATT, PIN, and step-recovery diodes, used at microwave frequencies; diodes fabricated from gallium arsenide and related compounds, used in optoelectronics; and light-emitting diodes (LEDs) and solid-state lasers. See Amplitude-modulation detector, Laser, Light-emitting diode, Microwave solid-state devices, Mixer, Zener diode
Silicon rectifier diodes
The electrical heart of the semiconductor diode is the junction between p-type and n-type doped silicon regions. Discrete silicon diodes are commercially available with forward-current specifications from under 1 A to several thousands of amperes. Diodes may be connected in parallel for greater current capability as long as the design provides for the current being uniformly distributed between the parallel diodes. This is usually done with a ballast resistor in series with each diode. See Ballast resistor
Ideally, the current through a reverse-biased diode, called the saturation current (IS) or reverse current (IR), approaches zero. Practically speaking, this current is several orders smaller than the forward current (IF). The maximum value of the reverse blocking voltage is limited primarily by the structure and doping of the semiconductor layers. This maximum voltage is referred to as the avalanche breakdown voltage, or the peak reverse voltage (PRV) or peak inverse voltage (PIV). It is a very important parameter for power supply and power conversion designs. Exceeding the peak inverse voltage is usually destructive unless the circuit design provides for limiting the avalanche current and resultant heating. In summary, at positive voltages and currents (quadrant I of the voltage-current characteristic), the silicon rectifier diode shows the on-state conducting characteristic, with high current and low forward voltage drop; at negative voltages and currents (quadrant III), it shows the reverse-blocking or reverse-bias, off-state characteristic, with high blocking voltage and low (ideally zero) reverse blocking current.
Integrated-circuit diode-junction avalanche breakdown voltages are of the order of several tens of volts. Single silicon rectifier diodes designed for power conversion applications are available with ratings from a few hundred to a few thousand volts. Several diodes can be connected in series for greater voltage capability. Prepackaged series diode strings can be rated to tens of thousands of volts at several amperes. This series connection must ensure equal voltage division across each diode to guard against catastrophic failure of the entire series. Typically this is done by including a high-value equal-value resistor in parallel with each diode to obtain equal voltages, and a parallel capacitor to provide a low-impedance path for high-voltage transients that are often present in industrial environments.
Schottky diodes
Unlike a silicon diode formed from a pn junction, the Schottky diode makes use of the rectification effect of a metal-to-silicon interface and the resultant barrier potential. The Schottky diode, sometimes called the Schottky-barrier diode, overcomes the major limitation of the pn junction diode; being a majority carrier device, it has a lower forward voltage drop (0.2–0.3 V, compared to 0.7–1.0 V) and faster switching speed than its minority-carrier pn junction counterpart. However, other factors confine its use to low-voltage power applications, chiefly the relatively small breakdown voltage, typically 45 V. Secondary shortcomings include a high reverse current and restricted temperature of operation, with commercial devices providing a maximum of 175°C (347°F) compared with 200°C (392°F) for pn junction diodes.
Integrated circuits used in computer and instrument systems commonly require voltages less than 15 V and as low as 3.3 V. Thus the advantage of low forward-voltage drop and faster switching favors the Schottky diode. This is particularly true for high-frequency switching voltage regulator power supply applications where voltages at 20–50 kHz must be rectified. The higher reverse current can be tolerated. However, cooling or heat sinking is more critical because of the higher reverse-current temperature coefficient and lower maximum operating temperature. See Schottky barrier diode
Rectifier circuits
The greatest usage of rectifier diodes is the conversion of ac to dc. The single diode of a half-wave rectifier for a single-phase ac voltage conducts only on the positive half-cycle. Because of this, the output voltage across the load resistance is unidirectional and has a nonzero average value. This output waveform is called a pulsating dc. Therefore the input ac voltage has been rectified to a dc voltage. For most applications, a filter, usually consisting of large electrolytic capacitors, must be employed at the output to smooth the ripple present on the pulsating dc voltage to come close to a constant dc voltage value. See Capacitor, Electronic power supply, Ripple voltage
In lower-power applications from a few watts to a few hundred watts, such as used in computers, television receivers, and laboratory instruments, a switching voltage regulator is commonly used to generate a 10-kHz–50-kHz ac signal from the high-ripple ac power supply voltage. The advantage is the ease and lower cost in filtering the ripple resulting from rectifying high-frequency ac as opposed to filtering low-frequency ac.
Thyristors
Whereas the basic semiconductor rectifier has two terminals, an anode and cathode, a silicon controlled rectifier (SCR) has three terminals: an anode, cathode, and control electrode called the gate. The silicon controlled rectifier is a four-layer device modeled as two interconnected pnp and npn transistors.
Normally, there is no current flow from the anode to cathode. Both transistors are off; that is, they are blocking any current flow. By applying a relatively small trigger pulse control signal to the gate electrode, the npn transistor is switched on. When the npn transistor is switched on, the pnp transistor is also switched on. Consequently the silicon controlled rectifier is turned on and a current flows through the silicon controlled rectifier and external circuit. The resultant internal voltages keep both the npn and pnp transistors on even when the gate voltage is removed. The device is said to exhibit regenerative, positive-feedback, or latching-type switching action. There is a voltage drop of about 1 V across the on-state silicon controlled rectifier. The power dissipation rating required in specifying a silicon controlled rectifier is given by this 1-V drop multiplied by the peak current flowing through the device. See Transistor
Current continues to flow even when the gate signal is reduced to zero. To reset the silicon controlled rectifier, the external current must be reduced below a certain value. Thus, the thyristor can be switched into the on state (conducting condition) by applying a signal to the gate, but must be restored to the off state by circuit action. If the anode current momentarily drops below some holding current or if the anode voltage is reversed, the silicon controlled rectifier reverts to its blocking state and the gate terminal regains control. Typical silicon controlled rectifiers turn on in 1–5 microseconds and require 10–100 μs of momentary reverse voltage on the anode to regain their forward-blocking ability.
Other semiconductor diode topologies are also used for power control. A generic term for these power-control devices is the thyristor.
Thyristor applications fall into two general categories. The devices can be used from an ac supply, much like silicon rectifier diodes. However, unlike the rectifier diode, which conducts load current as soon as the anode voltage exceeds about 0.7 V, the thyristor will not conduct load current until it is triggered into conduction. Therefore, the power delivered to the load can be controlled. This mode of operation is called ac phase control. It is extensively used in applications requiring conversion from ac to variable-voltage dc output, such as adjustable-speed dc motor drives, and in lighting and heating control. See Direct-current motor
The other category of applications is operation in dc circuits. This allows power conversion from a battery or rectified ac line to a load requiring either an alternating supply (dc-to-ac conversion) or a variable-voltage dc supply (dc-to-dc conversion). Since the rate of switching the thyristors in dc circuits can be varied by the control circuit, a thyristor inverter circuit can supply an ac load with a variable frequency. The fundamental approach in both cases is to convert a dc voltage to a chopped voltage of controllable duty cycle. Changing the duty cycle either at a variable rate (frequency power modulation) or by varying the pulse width at a fixed frequency (pulse-width power modulation) effectively controls the power delivered to the load.
Important applications for dc-to-dc conversion, dc-to-ac power conversion at variable frequency, and dc-to-ac power conversion at fixed frequency are, respectively, control of battery-powered industrial vehicles such as forklift trucks and mining locomotives, adjustable-speed operation of ac synchronous and induction motors in industrial processing, and power transmission conversion. See Alternating-current motor, Converter