OPERATING
CONSIDERATONS
BC20 • BC20A
GENERAL
PROTECTION CIRCUITS
Much useful application information for these products can
be obtained from Application Notes 1 (General Operating
Considerations) and 30 (PWM Basics).
There are four protection circuits in the BC20.
1. The coil current sensing circuit, which is programmed by
the value of the current sense resistors placed by the user
between the IGBT emitters and HV return. This circuit is
reset each PWM cycle. If three current sense resistors are
used, as recommended, an analog multiplexer selects
the current sense resistor, which has the same current
as the motor coil. This technique blanks out noise and
provides an excellent sensing of actual coil current.
The programming of this circuit is accomplished by the
folowing formula:
PWM CONSIDERATIONS
The BC20 can be configured with a logic-input (2Q) to
operate either as a 2-quadrant or 4-quadrant controller.
2-quadrant PWM holds one coil terminal at a constant level
and applies PWM at the other. PWM is applied at the positive
terminal when in 2-quadrant mode. 4-quadrant PWM switches
both terminals. 2-quadrant PWM is electrically quieter
and more efficient, but cannot transition through zero. 4-
quadrant PWM has twice the voltage gain of 2-quadrant
PWM. Therefore 4-quadrant PWM is required for applications
such as position servos, phase locked motor control, or
accurately following complex velocity profiles. 2-quadrant
PWMispreferableforunidirectionalspeedcontrolapplications.
The R input may be used to reverse the motor when
using 2-quadrant PWM, but must be at logic “0” when in
4-quadrant mode.
ITRIP = 0.5/RSENSE
Note that for large currents RSENSE becomes very small,
therefore stray resistance in the high current path can have
a large effect. Heavy etch should be used in the current
sensing path, and leads should be very short between the
resistors and the pins of the controller.
2. Thermal Protection
The junction temperature of all power devices is sensed,
and the controller is shut down when too hot. This
circuit is a a latch and can be reset when OE is turned
on, providing the power devices have cooled to a safe
temperature.
COMMUTATION
The BC20 may be configured to operate with either 60°
or 120° Hall sensor patterns by the state of the 120 input.
(Obviously also encoder outputs with the same logic.) When
120 is low the BC20 operates with 60° commutation; when
120 is high it operates with 120° commutation.
3. There is an over-current circuit which shut down the BC20
when the current provided by the HV supply exceeds
about 1.5 times the peak current rating. This circuit latches
and may be reset by cycling the OE input. Although this
is “top rail” protection, a short from output to ground will
probably destroy the BC10.
The relationship between commutation states and motor
drive output is tabulated in the following tables [See Tables
1-4 on previous page]. For the purposes of these tables
PWM that is mostly positive will be designated +; PWM that
is mostly low will be designated −; a constant low state will
be designated by 0; a tri-state condition will be designated
T; REF IN is more positive than FB; and “Forward” rotation
is the only direction tabulated. Position is given in electrical
degrees.
Some motor manufacturers may not use the same
conventions in identifying motor and Hall sense leads as
Apex. In that event you may have to experimentally identify
the corresponding motor and Hall Sense leads. For 3 binary
square waves with equal phase shifts between the square
waves, such as Hall sense outputs, there are only 8 possible
states. 60° commutation fills 6 of the states and 120°
commutation fills the other set of 6 states. Therefore all such
patterns are truly only 60° or 120°. Changing pattern is done in
the Apex controller by inverting HS2 internally.
Once the proper commutation patterns are obtained it is
necessary to determine the motor lead orientation to the
Hall sense. This may be done by turning the motor with a
test fixture and observing the relationship between the
HS patterns and the EMF, or by running the motor at low
voltage and systematically switching motor leads until smooth
running in the desired direction is obtained. The motor can
be expected to run smoothly in the desired direction, run
reverse, run very roughly, not run at all, or vibrate violently
between 2 positions as this is done.
4. The output circuit will shut down if a power supply is
missing. This is not an alarmed fault.
FAULT
The FAULT output is an alarm, a logic 1 indicates the
outputs are disabled. Fault is at 1 when OE is at 0, and it is at
logic 0 when OE is at 1 during normal operation. Outputs will
latch to the disabled state and fault will be at logic 1 when any
IGBT is too hot or when peak IGBT current has exceeded a
safe level for the IGBT. This may be reset by setting OE to
logic 0 and back to logic 1.
When the coil sensing circuit senses that the average
current has exceeded the level set by the selection of current
sense resistors, the output will be disabled and the FAULT
output will go to logic 1. (Even though the output has been
disabled coil current will continue, flowing through the diodes
in anti-parallel with each IGBT.) When coil current has
decayed to below this set level the outputs will be enabled
and FAULT will be at logic 0. Thus when limiting the average
value of coil current the output will cycle between being
disabled and enabled, and FAULT will cycle between logic
1 and 0. This action may cause an audible hiss when driving
low inertia systems.
OPEN LOOP OPERATION
The normal way of operating the controller open loop is
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