Page 65 - Power Electronics Handbook

58 Power semiconductor devices

rating of the thyristor is not exceeded. If required, a small reactor can be
connected in series with each device, to compensate for unequal tum-on
times.
If the latching currents of a parallel set of thyristors differ, it could mean
that for pulse firing some thyristors will reach their latching currents before
the end of the pulse whereas others will not. This will mean that only these
devices will conduct and support the total load current, and clearly the
firing signal must be long enough to prevent this from occurring. Similarly,
if the thyristor currents decrease momentarily, devices with holding
currents below this value will turn off. When the load current again
increases to its full load value the off thyristors will not turn on unless
refired, and some devices may be overloaded. It is therefore essential to
ensure that the load current does not fall to too low a value.
Although series connection is common, paralleling power devices is
inefficient, due to the loss across the series impedance, and is not often
used. In the instances where it is necessary, specially matched pairs, with
equal voltage drops, can normally be obtained from the semiconductor
manufacturers.

1.15 Power semiconductor packaging
The package used with power semiconductor devices has to accomplish
several functions, some of which are as follows:
(i) Provide a convenient method for electrical current to flow from the
device.
(ii) Enable the heat generated in the silicon to be conducted away to the
ambient, usually via a heatsink.
(iii) Give mechanical support to the semiconductor dice.
(iv) Protect the semiconductor dice from the chemical effects of the
environment.
(v) Give adequate insulation between the external terminals of the device
such as gate, cathode and anode.

Many different packages have been used for power semiconductors,
from the small metal can TO18, T039, TO3 through to plastic TOE,
T0220, T0218, and the large stud-mounted and ‘hockey puck’ devices. A
few of the larger packages are illustrated in Figure 1.36. The relevant
surface of the silicon dice, which can be the cathode or anode for a power
diode, the collector for a power transistor, and the anode for the thyristor,
is coated with a multilayer structure of titanium, nickel and silver. Gold
was used in the past but is becoming less popular. This dice is then attached
to the semiconductor package header, made from a copper-nickel alloy,
using soft or hard solders.
Soft solders, such as lead, silver, indium, antimony, or a mixture of
these, are easier to use, but are not able to withstand repeated thermal
cycles without fatigue. Hard solders, such as molybdenum, and eutectics
like Au-Ge, Au-Sn, Au-Si, are able to withstand many more thermal
cycles. It has been shown that the number of cycles to failure (N) for a