Pnpn DEVICES

1
Pnpn DEVICES

• silicon-controlled rectifier
• By
• Franco A. Catajoy

2
Introduction

• The two- layer semiconductor diode has to three,
  four, and even five-layer devices. A family of
  four layer pnpn devices will first be
  considered.
• SCR (Silicon-controlled rectifier)
• SCS (Silicon-controlled switch )
• GTO (Gate turn-off switch)
• LASCR (Light-activated SCR)
• UJT (Unijunction transistor)

3
Silicon-controlled rectifier
Within the pnpn devices, the silicon-controlled
rectifier(SCR) is unquestionable of the greatest
interest today. It was first introduced in 1956
by Bell Telephone Laboratories.

• Common areas of application for SCRs
• Time-delay circuits, regulated power
  suppliers, static switches,motor controls
  choppers, battery charger and heater controls.
  Etch..

4
In recent years, SCRs have been designed to
control powers high as 10 Mw with individual
ratings as high as 2000 A AT 1800 V.

• Basically, an SCR power controller consists of
  the following
• semiconductor power devices (SCR's and Diodes)
• a control circuit normally referred to as the
  firing circuit
• a means to dissipate the heat generated from the
  semiconductor devices
• protective circuits (fuses and transient
  suppressors).

5
The SCRs

•    The heart of the SCR power controller is the
  SCR (silicon controlled rectifier, also sometimes
  referred to as a thyristor).    The SCR has two
  states, ON or OFF, and it allows current to flow
  in only one direction.
• Is constructed of silicon material with third
  terminal for control

Why Silicon?

• high temperature
• high power capabilities

6
basic scr operation
- is different from the fundamental two-layer
semiconductor diode in that terminal, called a
gate, determines when the rectifier switches from
the open circuit state. it is not enough to
simply forward-bias the anode to cathode region
of the device.
-In the conducting region, the dynamic resistance
of the SCR is 0.01 to 0.1ohms and the reverse
resistance is 100 kohms or more.
7
Graphic symbol of SCR
Anode must be positive with respect to the
cathode.
I gt
Gate
I A
A pulse of sufficient magnitude must be also be
applied to the gate to establish a turn-on gate
current, represented symbol by I gt
anode
cathode
FIG. 21.1 A
anode

p
n
n
p
Basic operation of an SCR
gate
cathode
FIG.21.1B
8
Splitting is the best operation for the
four-layer pnpn structure FIG.21.1B into
three-layer transistor structure as shown in this
figure 21.2a then considering the resultant
circuit of FIG.21.2B
anode
pnp
I gt
anode
gate
npn

p
n
n
p
- cathode
cathode
gate
FIG.21.2A
9
In FIG.21.2 is an npn devices while the other is
an pnp transistor.

• SCR has two states
• off states
• - SCR's can remain in the off state
  even though the applied potential may be several
  thousand volts.
• on states
• - they can pass several thousand
  amperes. When a small signal is applied between
  the gate and cathode terminals, the SCR will
  begin conducting within 3 microseconds. Once
  turned on, it will remain on until the current
  through it is reduced to a very low value, called
  the holding current.

10
Because the SCR allows current to flow in only
one direction, two SCR's are connected in an
inverse parallel configuration to control AC
current.
AC 'BACK TO BACK' SWITCH
11

• If we ground both the cathode and the gate, and
  apply a positive voltage to the anode, no current
  will flow through this device. This is in keeping
  with the basic four-layer diode. In this case,
  however, we will not allow the applied anode
  voltage to exceed the SCR breakover voltage.
  Thus, if nothing happens, the SCR will remain
  turned off indefinitely.
• However, if we now apply a small positive
  voltage to the gate lead sufficiently to forward
  bias the cathode junction, the device will
  immediately turn fully on. Again, this is in
  keeping with the behavior of the basic four-layer
  diode. The difference is that we can accurately
  control the timing and the applied gate voltage,
  if necessary. Thus, we can determine the
  conditions under which the SCR will fire more
  accurately than we can for the basic four-layer
  diode.

12

• Three SCR construction styles are available
• the disc (a.k.a. hockey puck)
• - The hockey puck style is an assembly that has
  essentially the same physical shape as a hockey
  puck. The construction provides excellent cooling
  of the semiconductor material and is generally
  used in higher current applications.
• the module
• - SCR modules are assemblies in which the SCR's
  are contained in a plastic enclosure with an
  electrically isolated mounting plate. SCR modules
  are becoming increasingly popular and modules
  containing a variety of SCR configurations are
  available. They are easy and inexpensive to mount
  to a heat sink, they typically have large surge
  current ratings and they provide electrical
  isolation, allowing multiple modules to be
  mounted on a common heat sink.

13

• stud mount
• Modern SCR controllers generally use hockey puck
  or module construction. HEATSINK
  REQUIREMENTSSCR's emit about 1.5 watts of
  energy in the form of heat, per ampere conducted.
  Failure to dissipate this energy is perhaps one
  of the main sources of SCR failure. The
  reliability of SCR's decreases about 50 for
  every 10C increase of semiconductor temperature.
  Other critical parameters such as the dv/dt (See
  glossary) rating and the blocking voltage rating
  also decrease rapidly with temperature.  

14

• The heat generated by the SCR must be dissipated,
  thus, all controllers have some means to cool the
  SCR's. Typically an aluminum heatsink, with fins
  to increase the surface area, is used to
  dissipate this energy to air. Controllers with
  relatively small current capacities rely on
  natural convection. Higher current capacity
  controllers use a fan to force air past the fins
  in order to increase heat dissipation.