AN4840 datasheet(3/6 Pages) DYNEX

Part #	AN4840

Description	Gate Triggering and Gate

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Manufacturer	DYNEX [Dynex Semiconductor]

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AN 4840

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The limits take into account the temperature

range -40 to +125C. All devices will have

characteristics between these limits.

Gate characteristic information is used in

conjunction with firing circuit output load

lines to pre-determine operating values of

gate current and voltage - see below.

Gate Rating Limits

Thyristors turn on best when Ig and Vg values

are well above Igt and Vgt limits - see below.

However, peak gate current, gate voltage and

power rating limits should not be exceeded.

Vfgm. Peak forward gate voltage.

If the open circuit voltage of the firing circuit

exceeds this rating (usually 30 volts) there is a

danger of internal voltage breakdown. In

practice, 50 or more volts can often be

achieved but is not guaranteed since some

internal device constructions are limited.

Ifgm. Peak forward gate current.

This rating is determined by the

current carrying capability of internal gate

leads, bonds and surface metallisation of the

thyristor, depending on the technology.

Note that the 'X' axis of the graph

only extends to 10 amps so that the Ifgm limit

of large thyristors is not shown.

Pgm. Peak gate power.

The average heating effect of the gate

current is the issue here. Thus, a narrow

rectangular pulse of high Pqm is as

permissible as a wide pulse of low Pgm.

Included on the graph is a table showing

maximum permitted peak gate power for

various pulse widths and repetition rates. Also

on the graph are lines of constant power.

These lines are the power limits

corresponding to the pulse powers given in

the table and are used in conjunction with

gate drive load lines. Note the limit for the

AVERAGE gate power is given by the 50Hz

10,000 µs pulse combination as 10 watts.

Matching the gate drive/firing circuit to

the thyristor triggering requirements

The basic approach is to draw the output load

line of the firing circuit onto the gate

characteristic curve. It is usually assumed that

the gate drive has a purely resistive linear

characterisation - fig 2. The characteristic is

defined by its open circuit source voltage Voc

and its short circuit current Isc. The straight

line between these 2 points represents the

Fig.2 Output load line for gate drive

internal source resistance of the gate drive.

Gate drive output is often defined in terms of

its open circuit voltage and internal source

resistance.

To select the correct gate drive load line it is

useful to draw several possibilities onto the

characteristic curve (Fig 1). Compare the 20V,

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