AN1931D Freescale Semiconductor / Motorola, AN1931D Datasheet - Page 24

• Manufacturer Part Number: AN1931D
• Description: 3-Phase PM Synchronous Motor Vector Control
• Manufacturer: Freescale Semiconductor / Motorola

System Concept
4.3.2 Current Sensing
24
Phase currents are measured by a shunt resistor in each phase. A voltage drop on the shunt resistor is
amplified by an operational amplifier, and shifted up by 1.65V. The resultant voltage is converted by
an A/D converter (see
As shown in
flows through Phase A (and shunt resistor R1) only if transistor Q2 is switched on. Likewise, the
current in Phase B can be measured if transistor Q4 is switched on, and the current in Phase C can be
measured if transistor Q6 is switched on. To get a moment of current sensing, a voltage shape analysis
must be done.
The voltage shapes of two different PWM periods are shown in
correspond to center-aligned PWM sinewave modulation. As shown, the best moment of current
sampling is in the middle of the PWM period, where all bottom transistors are switched on.
To set the exact moment of sampling, the DSP56F80x family offers the ability to synchronize ADC
and PWM modules via the SYNC signal. This exceptional hardware feature, patented by Motorola, is
used for current sensing. The PWM outputs a synchronization pulse, which is connected as an input to
+3.3V_A
I_sense_C1
I_sense_C2
C307
100nF
Source_AB
I_sense_A1
I_sense_A2
GNDA
Gate_AT
Phase_A
Gate_AB
Figure
+
C306
3.3uF/10V
R323 390
sense
sense
4-7, the currents cannot be measured at any moment. For example, the current
Figure 4-7
LM285M
U304
Freescale Semiconductor, Inc.
3-Phase PM Synchronous Motor Vector Control
R320 10k-1%
R321 10k-1%
For More Information On This Product,
Figure 4-7. Current Shunt Resistors
GNDA
R1
0.1 1%
Q1
SKB04N60
Q2
SKB04N60
Figure 4-8. Current Amplifier
1.65V ref
Source_BB
I_sense_B1
I_sense_B2
and
5
Gate_BT
Phase_B
Gate_BB
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Figure
R324
100k-1%
R325
33k-1%
sense
sense
4-8).
R322
75k-1%
R2
0.1 1%
Q3
SKB04N60
Q4
SKB04N60
6
5
R318 75k-1%
-
+
Source_CB
I_sense_C1
I_sense_C2
Gate_CT
Phase_C
Gate_CB
U301B
MC33502D
7
Figure
sense
sense
1.65V +/- 1.65V
4-11. The voltage shapes
I_sense_C
R3
0.1 1%
Q5
SKB04N60
Q6
SKB04N60
@ +/- Imax
MOTOROLA