Fairchild AN-7511 User Manual

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Insulated-Gate Transistors Simplify AC-Motor

Application Note September 1993 AN-7511

An IGT’s few input requirements and low On-state resistance
simplify drive circuitry and increase power efficiency in motor­control applications. The voltage-controlled, MOSFET-like
input and transfer characteristics of the insulated-gate transis­tor (IGT) (see EDN, September 29, 1983, pg 153 for IGT
details) simplify power-control circuitry when compared with
bipolar devices. Moreover, the IGT has an input capacitance
mirroring that of a MOSFET that has only one-third the power­handling capability. These attributes allow you to design sim­ple, low-power gate-drive circuits using isolated or level-shift­ing techniques. What’s more, the drive circuit can control the
IGT’s switching times to suppress EMI, reduce oscillation and
noise, and eliminate the need for snubber networks.

Use Optoisolation T o Avoid Ground Loops

The gate-drive techniques described in the following sections
illustrate the economy and flexibility the IGT brings to power
control: economy, because you can drive the device’s gate
directly from a preceding collector, via a resistor network, for
example; flexibil ity, because you can choose the drive circuit’s
impedance to yield a desired turn-off time, or you can use a
switchable im pedance that causes the IGT to act as a charge­controlled device requiring less than 10 nanocoulombs of
drive charge for full turn-on.

Take Some Driving Lessons

Note the IGT’s straightforward drive compatibility with CMOS,
NMOS and open-collector TTL/HTL logic circuits in the
common-emitter configuration Figure 1A. R
off time, and the sum of R

and the parallel combination of R

3

and R2 sets the turn-on time. Drive-circuit requirements,
however, are more complex in the common-collector
configuration Figure 1B.

In this floating-gate-supply floating-control drive scheme, R
controls the gate supply’s power loss, R2 governs the turn-off
time, and the sum of R

and R2 sets the turn-on time. Figure

1

1C shows another common-collector configuration employing
a bootstrapped gate supply. In this configuration, R3 defines
the turn-off time, while the sum of R

2

on time. Note that the gate’s very low leakage allows the use
of low-consumption bootstrap supplies using very low-value
capacitors. Figure 1 shows two of an IGT’s strong points. In
the common-emitter Figure 1A, TTL or MOS-logic circuits can
drive the device directly. In the common-collector mode, you’ll
need lev el s hifting, using either a second power suppl y Figure
1B or a bootstrapping scheme Figure 1C.

controls the turn-

3

and R3 controls the turn-

Speed Control

V

CC

R

1

R

3

ON

OFF

FIGURE 1A.SIMPLE DRIVING AND TRANSITION-TIME

R1 CONTROLS GATE
SUPPLY POWER LOSS

CONTROLS t

R

2

R1 + R2 CONTROLS t

1

1

CONTROL

FIGURE 1B. A

OFF
ON

FIGURE 1C. BOOTSTRAPPING SCHEME

R

2

V

CC

CONTROL

ON

OFF

R

1

R

INPUT

OFF

ON

SECOND POWER SUPPLY

2

R

LOAD

VCCR

2

------------------- -

15

≤≤

R1R

R3 CONTROLS t

R

15V

3

LOAD

R

1

2

VCCR

------------------- -

15

≤≤

R1R

R3 CONTROLS t
R2 + R3 CONTROLS t

-------------------------------------------------«

τ

I

CEOIGES

25V

+

2

OFF

LOAD

2

25V

+

2

OFF

O

5C

++

2I

R

er

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 ver y low isolation capaci­tance; the H11AV specs 0.5pF maximum.

©2002 Fairchild Semiconductor Corpo ration Application Note 7511 Rev. A1

In the common-collector circuits, power-switch current flowing

Application Note 7511

For optically isolated “relay-action” switching, it makes sense
to replace the phototransistor optocoupler with an H11L1
Schmitt-trigger optocoupler (Figure 2B).) For applications
requiring extremely high isolation, you can use an optical f iber
to provide the signal to the gate-control photodetector. These
circuit examples use a gate-discharge resistor to control the
IGT’s turn-off time. To exploit fully the IGT’s safe operating
area (SOA), this resistor allows time for the device’s minority
carriers to recombine. Furthermore, the recombination occurs
without any current crowding that could cause hot-spot forma­tion or latch-up pnpn action. For very fast turn-off, you can use
a minimal snubber network, which allows the saf e use of lower
value gate resistors and higher collector currents.

V

CC

R

1

R

C

CONTROL

INPUT

OFF
ON

FIGURE 2A. AVOID GROUND-LOOP PROBLEMS BY USING AN

OPTOISOLATOR. THE ISOLATOR IGNORES SYS­TEM GROUND CURRENTS AND ALSO PRO­VIDES HIGH COMMON-MODE RANGE.

2

H11AV2

R

3

LOAD

directly from TTL levels, thanks to its 1.2V, 20mA input
parameters.

Available photovoltaic couplers have an output-current
capability of approximately 100µA. Combined with
approximately 100kΩ equivalent shunt impedance and the
IGT’s input capacitance, this current level yields very long
switching times. These transition times (typically ranging to 1
msec) vary with the photovoltaic coupler’s drive current and the
IGT’s Miller-effect equivalent capacitance.

Figure 3 illustrates a typical photovoltaic-coupler drive along
with its transient response. In some applications, the
photovoltaic element can charge a storage capacitor that’s
subsequently switched with a phototransistor isolator. This
isolator technique - similar to that used in bootstrap circuits
pro v id e s rapid tu rn- o n and tu rn - o f f w hi l e maintai n in g s m a l l s i ze,
good isolation and low cost.

In common-collector applications involving high-voltage, reac­tive-load switching, capacitive currents in the low-level logic cir­cuits can flow through the isolation capacitance of the control
element (eg, a pulse transformer, optoisolator, piezoelectric
coupler or level-shift transistor). These currents can cause
undesirable effects in the logic circuitry, especially in high­impedance, low-signal-level CMOS circuits.

+

I

ON

OFF

CONTROL

INPUT

DIG22

IGT

-

VCC = 300V

43k

1N5061

CONTROL

INPUT

OFF
ON

FIGURE 2B. A SCHMITT-TRIGGER OPTOIS OLATOR YIELDS

10µF

35V

H11L1

“SNAP-ACTION” TRIGGERING SIMILAR TO
THAT OF A RELAY.

5.6k 5.6k

5.6k

LOAD

Pulse-Transformer Drive Is Cheap And Efficient

Photovoltaic couplers provide yet another means of driving the
IGT. Typically, these devices contain an array of small silicon
photovoltaic cells, illuminated by an infrared diode through a
transparent dielectric. The photovoltaic coupler provides an
isolated, controlled, remote dc supply without the need for
oscillators, rectifiers or filters. What’s more, you can drive it

OUTPUT

CURRENT

INPUT

CURRENT

FIGURE 3. AS ANOTHER OPTICAL-DRIVE OPTION, A PHOTO-

VOLTAIC COUPLER PROVIDES AN ISOLATED,
REMOTE DC SU PPIY TO THE IGT’S INPUT. ITS
LOW 100µA OUTPUT, HOWEVER, YIELDS LONG
IGT TURN-ON AND TURN-OFF TIMES.

012ms

The solution? Use fiber-optic components Figure 4 to elimi­nate the problems completely. As an added feature, this low­cost technique provides physical separation between the
power and logic circuitry, thereby eliminating the effects of
radiated EMI and high-flux magnetic fields typically found
near power-switching circuits. You could use this method
with a bootstrap-supply circuit, although the fiber-optic sys­tem’s reduced transmission efficiency could require a
gain/speed trade-off. The added bipolar signal transistor
minimizes the pot enti al for compromise.

©2002 Fairchild Semiconductor Corpo ration Application Note 7511 Rev. A1

CONTROL

)

-

E

-

P

INPUT

ON

OFF

EMITTER

(DISCONNECTED)

Application Note 7511

R

1

1N914

GFOE1A1

R

2

2N5354

Q

1

C

10M (30FT)

QSF2000C

(W/CONNECTORS)

R

3

GFOD1A1

+

IGT

-

DETECTOR

(CONNECTED

FIGURE 4. ELIMINATE EMI IN HIGH-FLUX OR NOISE ENVI-

RONMENTS BY USING FIBER-OPTIC COMPO
NENTS. THESE PARTS ALSO ALLEVIAT
PROBLEMS ARISING FROM CAPACITIVE COU
PLING IN ISOLATION ELEMENTS.

iezos Pare Prices

ACOUSTIC WAVE

OUTPUT
VOLTAGE

OSCILLATOR

FIGURE 5A. YIELDING 4-kV ISOLATION, A PIEZOELECTRIC COUPLER PROVIDES TRANSFORMER-LIKE PERFORMANCE AND AN

ISOLATED POWER SUPPLY.

2.5k

3.3k

2.7k

0.001
µF

18V

NE555

0.001
µF

PZ61343

1k

D33D21

1N914

IGT

4.7k

1N914

FIGURE 5B. THIS CIRCUIT PROVIDES THE DRIVE FOR THIS ARTICLE’S MOTOR-CONTROL CIRCUIT.

©2002 Fairchild Semiconductor Corpo ration Application Note 7511 Rev. A1

Application Note 7511

A piezoelectric coup ler operationally similar to a pulse-train
drive transformer, but potentially less costly in high volume is
a small, ef ficient device with iso lation capability ranging to
4kV. What’s more, unlike optocouplers, they require no
auxiliary power supply. The piezo element is a ceramic
component in which electrical energy is converted to
mechanical energy, transmitted as an acoustic wave, and
then reconver ted to electrical energy at the output terminals
Figure 5A.

The piezo element’s maximum coupling efficiency occurs at
its resonant frequency, so the control oscillator must operate
at that freq uen cy. For example, the PZT61343 piezo c oup ler
in Figure 5B’s driver circuit requires a 108kHz, ±1%-accurate
astable multivibrator to maximize mechanical oscillations in
the ceramic material. This piezo element has a 1W max
power handling capability and a 30mA p-p max secondar y
current rating. The 555 timer shown provides compatible
waveforms while the R C ne twork sets the frequency.

Isolate With Galvanic Impunity

Do you require tried and true isolation? Then use
transformers; the IGT’s low gate requirements simplify the
design of independent, transformer-coupled gate-drive
supplies. The supplies can directly drive the gate and its
discharge resistor Figure 6, or they can simply replace the
level-shifting supplies of Figure 2. It’s good practice to use
pulse transformers in drive circuitry, both for IGT’s and
MOSFETs, because these components are economical,
rugged and hi ghly reliable.

+

ON

OFF

TRANSFORMER

CONTROL

INPUT

PULSE

1N914

1N914
2N5232

1k

IGT

C

1

-

FIGURE 6A. PROVIDING HIGH ISOLA TION A T LO W COST , PUL SE

TRANSFORMERS ARE IDEAL FOR DRIVING THE
IGT. AT SUFFICIENTLY HIGH FREQUENCIES, C
CAN BE THE IGT’S GATE-EMITTER CAPA CITANCE
ALONE.

+

ON OFF

CONTROL

INPUT

1N914

IGT

CR

-

1N914 RC = 3µSEC

FIGURE 6B. A HIGH-FREQUEN CY OSCI LL ATOR IN THE TRANS -

FORMER’S PRIMARY YIELDS UNLIMITED ON­TIME CAP ABILITY.

In the pulse-on, pulse-off method Figure 6A, C1 stores a
positive pulse, holding the IGT on. At moderate frequencies
(several hundred Hertz and above), the gate-emitter
capacitance alone can store enough energy to keep the IGT
on; lower frequencies require an additional external capacitor.
Use of the common-base n-p-n bipolar transistor to discharge
the capacitance minimizes circuit loading on the capacitor.
This action extends continuous on-time capability without
capacitor refreshing; it also controls the gate-discharge time
via the 1kΩ emitter resistor.

1

VARIABLE

220V AC

3φ 60Hz

THREE-PHASE

BRIDGE

RECTIFIER

LOW VOLTAGE

TRANSFORMER

RECTIFIER

FILTER

SIGNAL PATH ISOLATOR

I

EG: OPTOCOUPLIER PIEZO COUPLER

24V DC

SWITCHING

REGULATOR

POWER SUPPLY

FOR CONTROL

CIRCUITS

VOLTAGE
ENABLE

ADJUST VOLTAGE

5V

VOLTAGE

CONTROLLED

OSCILATOR

24V

III

MOTOR

CONTROL

LOGIC

DC VOLTAGE

TIMING

AND DRIVE

I

SHUT DOWN
DRIVE
OSCILLATOR

THREE-PHASE

INVERTER

ENABLE

LOWER

LEGS

OVERLOAD

PROTECTION

IGT

3φ

INDUCTION

MOTOR

CURRENT
SENSE
SIGNAL

TACHO­METER

FEEDBACK

FIGURE 8. THIS 6-STEP 3-PHASE-MOTOR DRIVE USES THE IGT-DRIVE T ECHNIQUES DESCRIBED IN THE TEXT. THE REGULATOR AD-

JUSTS THE OUTPUT DEVICES’ INPUT LEVELS; THE VOLTAGE-CONTROLLED OSCILLATOR VARIES THE SWITCHING
FREQUENCY AND ALSO PROVIDES THE CLOCK FOR THE 3-PHASE TIMING LOGIC. THE V/F RATIO STAYS CONSTANT
TO MAINTAIN CONSTANT TORQUE REGARDLESS OF SPEED.

©2002 Fairchild Semiconductor Corpo ration Application Note 7511 Rev. A1