Page 88 - Power Electronics Handbook

Isolating components 81

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A.C. supply Triac
1 Mac

3.3.4 siuum rtllfl.tarrl .d #hacrrldtClmJ
The silicon unilntcral switch (SUS) is primsrily an integrated circuit in
whicb the -r point of the two tranridors is deterrmnsd by azcner,
'
¬ed by Z1 in Fii 3.7(a). Ibe device is ncxmdy OtT until the
breakdown voltrye of the zener diode is rcILchcd, whcn the p-n-p
tmmhox turns on ptovidiru bare drive to the n-p-n tradstor and so
turning the two deviceson. lhirtrigprpoint isdetermraed bythevalucof
*
the zener voltage and is typicrlEy 5-1OV. Two SUS components can be
ummetcd in a reverse amnpnent within the same caac, to provide a
bi-dkdmal trisgcr device called a siliaon bihttral switch (SBS). Fii
3.7 shows the symbols for the SUS and SBS trigger devices.

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3.4 Isolating components

This section describes two classes of components which have been used to
isolate the power semiconductor from the low-power circuit which is
controlling it. Often the power semiconductor regulates megawatts of
power, whilst the low-power circuit is handling a few watts and is