Powerex SCR, GTO, Diode, POW-R-BLOK Application Information

2.0 POW-R-BLOK™ Module
Mounting

When mounting POW-R-BLOK™
modules to a heatsink, care
should be taken to avoid applying
uneven torque to the baseplate
due to one sided tightening. It is
recommended that the mounting
screws be tightened in the fashion
shown in Figure 2.1.The device
data sheet lists the maximum
torque rating for both the mounting
screws and, where applicable, the
terminal screws.

The use of thermal compounds
when mounting POW-R-BLOK™
modules to heatsinks is highly
recommended to prevent hot spots
due to voids between the package
and the heatsink surface.It is
important to select a thermal
compound which has a stable
characteristic over the operating
temperature range and the lifetime
of the equipment.

The compound should be applied
in a very thin layer, applying a thin
coating with a spatula or lintless
brush and wiping lightly to remove
excess material. Another method
is to place a predetermined
minimal amount at or along the
center of the contact area.Then
in mounting, rotation and
pressure will force the compound
over the contact area and
experience will indicate whether
the quantity is sufficient as excess
will appear around the edges of
the contact area. Excess
compound may be wiped away
using a cloth wetted with acetone
or alcohol.The use of thick
consistency thermal compounds
should be avoided, particularly
with larger modules, since it may

not compress evenly when the
module is torqued.

A number of manufacturers supply
a wide variety of thermal grease
and fluid type compounds. Among
these are Wakefield, Dow-Corning,
Alcoa, and Thermalloy.In addtion,
some manufacturers offer
alternative thermal interface pads
which avoid the application
problems of greases.These
materials, such as
THERMSTRATE™, are often

available in pre-cut shapes that
accommodate many
POW-R-BLOK™, module
packages.

It is recommended that heatsink
surfaces be flat within ±1 mil/inch
over the mounting area and have
a surface finish of less than 64
microinches. It is also impor tant to
properly prepare the heatsink
mounting surface just prior to
module mounting.The heatsink
surface should be thoroughly
cleaned to remove any foreign
material, oxides, or films.A
satisfactory cleaning technique is
to polish the mounting area with

No.000 fine steel wool, followed
by an alcohol or acetone rinse.

2.1 Gate Drive
Recommendations

SCRs have extraordinarily high
power gain. For example, a
90 Ampere, 1000 Volt SCR is
guaranteed to turn on if a 100 mA,
3 Volt gate drive is applied.This is
a power gain of 3 x 105.The
power gain is further magnified as
the required gate controlled signal
is a pulse only a few
microseconds wide.

To achieve reliable performance of
the SCR, a gate drive signal
greater than the minimum
specified IGTand VGTvalues is
required. Because of the diverse
range of SCR applications, a DC
gate test condition with a resistive
load was established for the basic
gate parameters, IGTand VGT,
found on a typical data sheet.

These DC gate trigger parameters
are not intended to reflect
operational application
requirements.

Powerex,Inc., 200 Hillis Street,Youngwood, Pennsylvania 15697-1800 [phone]

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SCR/GTO/Diode
POW-R-BLOK™ Modules
Application Information

Figure 2.1 Mounting Screw Fastening Pattern

➀

➀

➁

➁

➂

➃

Two-point mounting type
temporary tightening ➀ → ➁
final tightening ➁ → ➀

Four-point mounting type
temporary tightening ➀ → ➁ → ➂ → ➃
final tightening ➃ → ➂ → ➁ → ➀