dsPIC33CK Low-Voltage Motor Control Board User’s Guide
DS50002927A-page 34 2020 Microchip Technology Inc.
3.2.4.2 CURRENT AMPLIFIERS
Field-Oriented Control (FOC) of the PMSM/BLDC motor requires the motor phase
current information for implementation. In the Motor Control Board, shunt resistors,
Rsh1, Rsh2 and Rsh3, are provided in each inverter leg to measure the amount of
current flowing through the motor phases. An additional shunt resistor, Rsh4, is
provided for sensing the total bus current as this information is necessary for over-
current protection and current control of BLDC motors. The DC bus current information
can also be used for reconstruction of motor phase currents by appropriately sampling
currents during the PWM switching period, which is called a single-shunt reconstruction
algorithm.
Noninverting differential amplifier configuration is used for amplifying the voltage drop
across the shunt resistors proportional to the currents flowing through three-phase
Inverter Phases A, B and C, and bus current, respectively. The output voltage of the
amplifiers is shifted by Voltage Reference (V
REF) +1.65V to allow positive and negative
current swings. The Common-mode and Differential-mode filters are added between
the input pins of all the amplifiers for noise filtering. It is also possible to add filters at
the output of the external amplifiers, U5-A, U5-B, U5-C
and U15.
The block diagram in Figure 3-4 illustrates the interconnections between the external
amplifiers and the dsPIC DSC analog peripherals, including internal amplifiers,
comparator, ADC, etc. The Motor Control Board enables phase and bus current ampli-
fication through external amplifiers, U5 and U15, and dsPIC DSC internal amplifiers,
Op Amp 1 (OA1), Op Amp 2 (OA2) and Op Amp 3 (OA3). The op amps, OA1, OA2 and
OA3, that are internal to dsPIC33CK256MP508, are used for Phase A, Phase B and
bus current amplification. Three out of four amplifiers (U5-A, U5-B and U5-C) in the
quad amplifier, MCP6024, are configured to amplify current flowing through Inverter
Phases A, B and C. Amplified Phase C current (IC_EXT) is connected directly to an
analog input of the dsPIC DSC. The selection between internal and external amplifier
outputs is done via resistor jumpers (see Table 3-2) for Phase A, Phase B and the bus
currents when they are used as current feedbacks.
The op amp, MCP651S (U15), is added for DC bus current amplification. This amplifier
is configured to sense bus current. The U15 amplifier output is further filtered
(IBUS_FILT_EXT) and is connected to the internal Comparator 1 positive input
(CMP1C). The Comparator 1 negative input is configured to use the internal DAC out-
put to set the overcurrent threshold. The Comparator 1 output (CMP1) generates an
active-high output when overcurrent is detected. This comparator output is available to
the PWM Generators of the high-resolution PWM module as a Fault input. If the Fault
is enabled in the PWM Generators, and CMP1 is selected as an active-high Fault
source during an overcurrent Fault condition, the motor control PWMs will be disabled,
thus protecting the MOSFETs.
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