INTRODUCTION:-
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 coupling. 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
asnubber. "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 break down
voltage (also called as break ver 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.
SCRs are made with voltage ratings of up to 7,500 V, and with current ratings up to 3,000 RMS
amperes per device. Some of the larger ones
can take over 50 kA in single-pulse operation. SCRs are used in power switching, phase control,
chopper, battery charger, and inverter
circuits. Industrially they are applied to produce variable DC voltages for moters (from a few to
several thousand HP) from AC line voltage.
They control the bulk of the dimmers used in stage lighted, and can also be used in some electric
vehicles to modulate the working
voltage in a jacabson circuit. Another common application is phase control circuits used with
inductive loads. SCRs can also be found in
welding power suplies where they are used to maintain a constant output current or voltage.
Large silicon-controlled rectifier assemblies
with many individual devices connected in series are used in high voltage DC converter stations.
Two SCRs in "inverse parallel" are often used in place of a TRIAC for switching inductive loads
on AC circuits.
Because each SCR only conducts for half of the power cycle and is reverse-biased for the
other half-cycle, turn-off of the SCRs is assured.
By comparison, the TRIAC is capable of conducting current in both directions and assuring that
it switches "off" during the brief
zero-crossing of current can be difficult.
Typical electrostatic discharge (ESD) protection structures in integrated circuits produce a
parasitic SCR. This SCR is undesired;
if it is triggered by accident, the IC can go into latch up and potentially be destroyed.
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