Fairchild AN-7511 Application Note

An IGT's few input requirements and low On-state resistance
simplify drive circuitry and increase power efficiency in motor-
/Title
control applications. The voltage-controlled, MOSFET-like
AN75
input and transfer characteristics of the insulated-gate transis-
1)
tor (IGT) (see EDN, September 29, 1983, pg 153 for IGT
Sub-
details) simplify power-control circuitry when compared with
bipolar devices. Moreover, the IGT has an input capacitance
ect
mirroring that of a MOSFET that has only one-third the power-
In
handling capability. These attributes allow you to design sim-
ulated
ple, low-power gate-drive circuits using isolated or level-shift-
Gate ing techniques. What's more, the drive circuit can control the
IGT's switching times to suppress EMI, reduce oscillation and
ran
noise, and eliminate the need for snubber networks.
istors
Use Optoisolation To Avoid Ground Loops
im-
The gate-drive techniques described in the following sections
lify
illustrate the economy and flexibility the IGT brings to power
C-
control: economy, because you can drive the device's gate
otor
directly from a preceding collector, via a resistor network, for
peed
example; flexibility, because you can choose the drive circuit's
impedance to yield a desired turn-off time, or you can use a
on-
switchable impedance that causes the IGT to act as a charge-
rol)
controlled device requiring less than 10 nanocoulombs of
Autho
drive charge for full turn-on.
()
Take Some Driving Lessons
Key-
Note the IGT's straightforward drive compatibility with CMOS,
ords
NMOS and open-collector TTL/HTL logic circuits in the
Inter-
common-emitter configuration Figure 1A. R
il
off time, and the sum of R
and R sets the turn-on time. Drive-circuit requirements,
orpo-
2
however, are more complex in the common-collector
ation,
configuration Figure 1B.
emi-
In this floating-gate-supply floating-control drive scheme, R
on-
controls the gate supply's power loss, R
uctor,
time, and the sum of R
1C shows another common-collector configuration employing
va-
a bootstrapped gate supply. In this configuration, R
anche
the turn-off time, while the sum of R
nergy
on time. Note that the gate's very low leakage allows the use
ated,
of low-consumption bootstrap supplies using very low-value
capacitors. Figure 1 shows two of an IGT's strong points. In
witch
the common-emitter Figure 1A, TTL or MOS-logic circuits can
ng
drive the device directly. In the common-collector mode, you'll
ower
need level shifting, using either a second power supply Figure
up-
1B or a bootstrapping scheme Figure 1C.
lie
,
ower
©2002 Fairchild Semiconductor Corporation
Insulated-Gate Transistors Simplify AC-Motor
Application Note
controls the turn-
3
and the parallel combination of R
3
governs the turn-off
2
and R sets the turn-on time. Figure
1 2
and R controls the turn-
2 3
September 1993
ON
OFF
FIGURE 1A
R CONTROLS GATE
1
SUPPLY POWER LOSS
R CONTROLS t
2
R + R CONTROLS t
1 2
1
1
defines
3
OFF
ON
In the common-collector circuits, power-switch current flowing
through the logic circuit's ground can create problems.
Optoisolation can solve this problem (Figure 2A.) Because of
the high common-mode dV/dt possible in this configuration,
you should use an optoisolator with very low isolation capaci-
tance; the H11AV specs 0.5pF maximum.
Speed Control
V
CC
R
1
LOAD
R
3
R
2
.
SIMPLE DRIVING AND TRANSITION-TIME
CONTROL
V
CC
CONTROL
INPUT
R
1
ON
OFF
15V
OFF
ON
FIGURE 1B. A SECOND POWER SUPPLY
R
1
R
R
2 3
LOAD
FIGURE 1C. BOOTSTRAPPING SCHEME
AN-7511
V R
CC 2
≤ ≤
------------------- -
15 25V
R + R
1 2
R CONTROLS t
3 OFF
R
2
LOAD
V R
CC 2
≤ ≤
------------------- -
15 25V
R + R
1 2
R CONTROLS t
3 OFF
R + R CONTROLS t
2 3 ON
τ 5C
« ------------------------------------------------ -
+ +
I I 2I R
CEO GES
Application Note 7511 Rev. A1