A silicon-controlled rectifier (or semiconductor-controlled
rectifier) is a four-layer solid state current controlling device. The name
"silicon controlled rectifier" or SCR is General Electric's trade
name for a type of thyristor. The SCR was developed by a team of power
engineers led by Robert N. Hall and commercialized by Frank W. "Bill"
Gutzwiller in 1957.
Some sources define silicon controlled rectifiers and
thyristors as synonymous.
Other sources define silicon controlled rectifiers as a
subset of a larger family of devices with at least four layers of alternating N
and P-type material, this entire family being referred to as thyristors.
This device is generally used in switching applications. In
the normal "off" state, the device restricts current to the leakage
current. When the gate-to-cathode voltage exceeds a certain threshold, the
device turns "on" and conducts current. The device will remain in the
"on" state even after gate current is removed so long as current
through the device remains above the holding current. Once current falls below
the holding current for an appropriate period of time, the device will switch
"off". If the gate is pulsed and the current through the device is
below the holding current, the device will remain in the "off" state.
If the applied voltage increases rapidly enough, capacitive
coupling may induce enough charge into the gate to trigger the device into the
"on" state; this is referred to as "dv/dt triggering." This
is usually prevented by limiting the rate of voltage rise across the device,
perhaps by using a snubber. "dv/dt triggering" may not switch the SCR
into full conduction rapidly, and the partially triggered SCR may dissipate
more power than is usual, possibly harming the device.
SCRs can also be triggered by increasing the forward voltage
beyond their rated breakdown voltage (also called as break over voltage), but again,
this does not rapidly switch the entire device into conduction and so may be
harmful so this mode of operation is also usually avoided. Also, the actual
breakdown voltage may be substantially higher than the rated breakdown voltage,
so the exact trigger point will vary from device to device.
SCR are available with or without reverse blocking
capability. Reverse blocking capability adds to the forward voltage drop
because of the need to have a long, low doped P1 region. Usually, the reverse
blocking voltage rating and forward blocking voltage rating are the same. The
typical application for reverse blocking SCR is in current source inverters.
SCR incapable of blocking reverse voltage are known as
asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown
rating in the 10's of volts. ASCR are used where either a reverse conducting
diode is applied in parallel (for example, in voltage source inverters) or
where reverse voltage would never occur (for example, in switching power supplies
or DC traction choppers).
Asymmetrical SCR can be fabricated with a reverse conducting
diode in the same package. These are known as RCT, for reverse conducting
thyristor.
SCRs are mainly used in devices where the control of high
power, possibly coupled with high voltage, is demanded. Their operation makes
them suitable for use in medium to high-voltage AC power control applications,
such as lamp dimming, regulators and motor control.
SCRs and similar devices are used for rectification of high
power AC in high-voltage direct current power transmission. They are also used
in the control of welding machines, mainly MTAW and GTAW processes. (text by Wikipedia.org)
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