AN1362 STMicroelectronics, AN1362 Datasheet - Page 4

• Manufacturer Part Number: AN1362
• Description: SWITCHED RELUCTANCE MOTOR CONTROL BY ST FIVE
• Manufacturer: STMicroelectronics

AN1362
INVERTER TOPOLOGY
The SRM motor is driven by a six switch board in a typical 2N-switch topology, with N indicating the
number of motor phases.
Each motor phase is connected to an asymmetric half bridge consisting of two power switches and two
diodes. The complete DC voltage can be used to energize and de-energize a phase in chopping mode.
When some of the switches are closed, a phase will be energized by the positive DC voltage supply. When
both switches are opened, the current commutates between the switches and the relative freewheeling
diodes. The voltage across the phase is now the negative DC voltage.
This inverter topology is the most used: it presents the main advantage of exploiting the full DC bus and
is therefore suited for driving the motor at high speeds. The drawback disadvantage of this inverter
topology is the high number of power devices: each half bridge needs two power switches and two
freewheeling diodes. Three L6386 high-side/low-side drivers, not shown in Fig.5, are used to control six
IGBTs (12A 600V), with six high speed free-wheeling diodes (5 A).
Figure 5. 2N SWITCHES INVERTER DRIVER BY ST52
If high speed performances are not needed, a simpler and cheaper inverter topology can be used: the
“N+1 switches” topology, where N indicates the number of phases.
This topology uses only one High Side leg to chop each of the three phases, a TD300 three phase driver
and only one L6386, saving two IGBT’s, two Diodes and one L6386 .
The ST52x420 activates the driver by manipulating the encoder signal. A choppered PWM is used
together with an hysteresis control. In Fig. 6 the encoder signal CH2 shows the chopping high side with a
reference current at 2A and implements the Current Mode Control. The scheme is the following:
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+DCbus
-DCbus
High side1
DRIVER
Low side1
Phase A
+
-
High side2
ST52x420
DRIVER
Phase B
Low side2
+
-
High side3
DRIVER
Phase C
Low side3
+
-
-DCBUS
+DCBUS