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[FAQ] What precision amplifiers are right for an Inverter & Motor Control system (IMC)?

Other Parts Discussed in Thread: LMV841-Q1, TLV197-Q1, OPA197-Q1, ALM2402F-Q1, ALM2403-Q1, OPA192-Q1, OPA322-Q1, OPA388-Q1

Inverter and Motor Control systems are used in electric vehicles to manipulate an AC signal with pulse width modulation that ultimately controls the vehicle’s DC motor in such a way that steering and acceleration feel smooth to the driver. Other functions of the system include internal sensing and protection features as well as the circuitry behind regenerative braking.


Position sensing:

 

Key functions of acceleration and steering require the vehicle to “know” the position of moving parts so a circuit must be developed to sense this. Due to the importance of this function many original equipment manufacturers (OEMs) require that this sensing be done in redundancy, two unique solutions preforming the same task but with different components to minimize the chance of component failure at the same time.

 

Discrete solution:

 

Position sensing amplifiers require a high current output because of the low impedance found in resolver excitation coils as well as a wide bandwidth and high slew rate to minimize distortion. The majority of amplifiers will not have the current output necessary to fulfill the necessary output current and thus require sinking and sourcing transistors at the output to act as a current pump. There are exceptions to this rule with amplifiers such as the OPA564-Q1, which has current capabilities even greater than what is required for position sensing.

Table 1Amplifier recommendations for discrete position sensing LMV841-q1 opa197-q1 tlv197-q1

Number of
Channels

Vs min
 (V)

Vs max
(V)

Output current
typ (mA)

GBW typ
(MHz)

Slew Rate
typ (V/µs)

Vos max
@25°C (mV)

LMV84x-Q1

1,2,4

2.7

12

33

4.5

2.5

0.5

OPAx197-Q1

1,2,4

4.5

36

65

10

20

0.25

TLVx197-Q1
Cost Sensitive

1,2,4

4.5

36

65

10

20

0.5

Integrated Solution:

 

To fulfill the needs for redundancy TI has created a family of resolver drivers designed to fit perfectly into IMC circuits. The ALM family features wide bandwidths and high output currents resulting in a well-balanced output stage that does not require any additional current boosting circuits. In addition to integrating the additional transistors required in the discrete solution, these devices also include protection features including shutdown, current limit, thermal protection, and are functional safety capable. All this functionality fits into an HTSSOP|14 package potentially reducing board space by as much as 300% compared to a discrete solution.

Table 2: Amplifier recommendations for integrated position sensing alm2402f-q1 alm2403-q1

Number of 
Channels

Vs min
 (V)

Vs max
(V)

Output current
typ (mA)

GBW typ
(MHz)

Slew Rate
typ (V/µs)

Vos max
@25°C (mV)

ALM2402F-Q1

2

4.5

16

400

2

3.4

7

ALM2403-Q1

2

5

24

450

21

50

25

Signal-Processing:

 

Having moved beyond the previous stage the signal chain continues with post processing. Here, we are amplifying and attenuating the signal, the degree of accuracy required will vary between individual systems but regardless of precision required the key specifications for maintaining signal integrity are offset voltage and offset voltage drift. When choosing amplifiers for HEV/EV functions devices that can be powered by a 12-V rail are usually more desirable as this is the most ubiquitous supply seen in automotive end equipment.

Table 3: Amplifier recommendations for position sensing post-processing Opa192-q1

Number of 
Channels

Vs min
 (V)

Vs max
(V)

Vos max
@25°C (mV)

Drift typ
(µV/°C)

Vn typ
(nV/√Hz)

OPAx192-Q1

1,2

4.5

36

0.025

0.1

5.5

OPAx197-Q1

1,2,4

4.5

36

0.25

0.5

5.5

TLVx197-Q1
Cost Sensitive

1,2,4

4.5

36

0.5

1

5.5

Current/Voltage Sensing:

It’s important for the current sensing amplifier to be able to monitor current quickly and detect leakage/overcurrent as it occurs. To accomplish current monitoring tasks in a timely manner,  a faster transient response and quick settling of the output is required from amplifiers with high slew rates and wide bandwidth.

Table 4: Amplifier recommendations for current/voltage sensing opa388-q1 oap320-q1 opa322-q1

Number of
Channels

Vs min
 (V)

Vs max
(V)

GBW typ
(MHz)

Slew Rate
typ (V/µs)

Vos max
@25°C (mV)

OPAx388-Q1
Ultimate Precision

1,2

1.8

5.5

10

5

0.005

OPAx320-Q1

1,2

1.8

5.5

20

10

0.15

OPA322-Q1
Cost Sensitive

1,2,4

1.8

5.5

20

10

2

Temperature Sensing:

Temperature sensing has looser requirements than voltage or current as minor temperature variation has significantly less impact on a system than variation in current or voltage. When designing for this function a higher voltage amplifier will enable wider temperature measurements, particularly important as the circuit heats up towards its upper limit. Furthermore, choosing an amplifier with low offset voltage drift will prevent the need for additional calibrations saving time and effort using complex algorithms.

Table 5: Amplifier recommendations for temperature sensing

Number of
Channels

Vs min
 (V)

Vs max
(V)

Vos max
@25°C (mV)

Drift typ (µV/°C)

Vn typ
(nV/√Hz)

OPAx192-Q1

1,2

4.5

36

0.025

0.1

5.5

OPAx197-Q1

1,2,4

4.5

36

0.25

0.5

5.5

TLVx197-Q1
Cost Sensitive

1,2,4

4.5

36

0.5

1

5.5

For useful resources and additional information, please make sure to check out the following content:

Inverter & motor control integrated circuits and reference designs (Reference Design)

Voltage and current sensing in HEV/EV applications (Technical Article)

Understanding current sensing in HEV/EV batteries (Technical Article)

IMC EE Slides (End Equipment Slides)

Applying the ALM2403-Q1 Single-Chip Resolver Solution to Reduce System Costs, and Improve the Reliability and Performance of Automotive and Industrial Applications (Application Note)

Overvoltage Protection of Resolver-Based Circuits (Application Note)