Handbook for Sound Engineers

324 Chapter 12

neously in the same n 1 and p 2 regions. Therefore, at the
high injection levels that exist in thyristors, the
mobile-carrier concentration of minority carriers far
exceeds that from the background-doping density.
Accordingly, the space charge is practically neutralized
so that the forward drop becomes almost independent of
the current density to high current levels. The major
resistance to current comes from the ohmic contacts of
the unit and load resistance.
The price paid for this low-impedance capability in a
standard thyristor is a long turnoff time relative to
turn-on time necessary to allow the high level of
minority current carriers to dissipate. This long turnoff
time limits the speed of a thyristor. Fortunately, this
long turnoff time does not add significantly to switching
power losses the way that a slow turnon time would.

Turnoff time is the minimum time between the
forward anode current ceasing and the device being able
to block reapplied forward voltage without turning on
again.

Reverse-recovery time is the minimum time after
forward conduction ceases that is needed to block
reverse-voltage with ac applied to the anode-cathode
circuit.
A third specification, turnon time, is the time a
thyristor takes from the instant of triggering to when
conduction is fully on.

These timing specifications limit the operating
frequency of a thyristor. Two additional important spec-
ifications, the derivative of voltage with respect to time
(dv/d t) and the derivative of current with respect to time

(di/d t) limit the rates of change of voltage and current

application to thyristor terminals.

A rapidly varying anode voltage can cause a

thyristor to turn on even though the voltage level never

exceeds the forward breakdown voltage. Because of

capacitance between the layers, a current large enough

to cause firing can be generated in the gated layer.

Current through a capacitor is directly proportional to

the rate of change of the applied voltage; therefore, the

dv/d t of the anode voltage is an important thyristor

specification.

Turnon by the dv/d t can be accomplished with as

little as a few volts per microsecond in some units, espe-

cially in older designs. Newer designs are often rated in

tens to hundreds of volts per microsecond.

The other important rate effect is the anode-current

di/dt rating. This rating is particularly important in

circuits that have low inductance in the anode-cathode

path. Adequate inductance would limit the rate of

current rise when the device fires.

When a thyristor fires, the region near the gate

conducts first; then the current spreads to the rest of the

semiconductor material of the gate-controlled layer over

a period of time. If the current flow through the device

increases too rapidly during this period because the

input-current d i/d t is too high, the high concentration of

current near the gate could damage the device do to

localized overheating. Specially designed gate struc-

tures can speed up the turnon time of a thyristor, and

thus its operational frequency, as well as alleviate this

hot-spot problem.

Silicon-Controlled Rectifiers. The SCR thyristor can

be considered a solid-state latching relay if dc is used as

the supply voltage for the load. The gate current turns

on the SCR, which is equivalent to closing the contacts

in the load circuit.

If ac is used as the supply voltage, the SCR load

current will reduce to zero as the positive ac wave shape

crosses through zero and reverses its polarity to a nega-

tive voltage. This will shut off the SCR. If the positive

gate voltage is also removed it will not turn on during

the next positive half cycle of applied ac voltage unless

positive gate voltage is applied.

The SCR is suitable for controlling large amounts of

rectifier power by means of small gate currents. The

ratio of the load current to the control current can be

several thousand to one. For example, a 10 A load

current might be triggered on by a 5 mA control current.

The major time-related specification associated with

SCRs is the dv/dt rating. This characteristic reveals how

fast a transient spike on the power line can be before it

Figure 12-17. Bilateral arrangement to create a triac or ac
operating device.

Ohmic

contacts

Off state

On state

On state

Firing

point

A

G

A

B

G

G

A (pV)

B ( V)

VA VA+

IA

n 1

p 2 n 2

n 4

n 3 p 1

p