Eric Persson, Toshio Takahashi Advanced Development Group Internationa

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Eric Persson, Toshio Takahashi Advanced Development Group Internationa

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Eliminate Ripple Current Error from Motor Current Measurement
Eric Persson, Toshio Takahashi Advanced Development Group International Rectifier Corp. U.S.A.
INTRODUCTION Most motor drives today measured current control variable. Simple drives only current limit control, motor phase current used primary feedback variable more advanced drives. Although motor current interest relatively bandwidth hundred Hz), traditional pulse-width modulation (PWM) techniques ripple current frequency associated harmonics. motors with large inductance values, like typical induction motors, magnitude ripple current small compared current interest. higher performance permanent magnet motors with inductance, magnitude ripple current quite large. extract useful motor current information feedback purposes, system must either filter ripple current (which adds phase delay), sample proper instant effectively cancel-out ripple. This requires separate sample hold circuits addition current measurement transducers. short circuit protection, additional fast comparators required each motor phase well. integrated circuit (IC) presented, that simplifies this function directly sampling motor phase current (synchronized PWM), providing digital output whose duty cycle directly proportional current, along with fast overcurrent shutdown logic.
TRADITIONAL MEASUREMENT METHODS Current typically measured with methods: voltage drop across resistor (resistive shunt), magnetic transducer. Resistive shunt sensing advantage relatively low-cost sensor. disadvantage that there trade-off between sensitivity power dissipated resistor. higher currents, therefore lower shunt resistance values, inductive component impedance begins dominate. transmission zero formed series combination therefore falls lower frequency. This compensated somewhat cancelling with suitably matched pole filter. Even with compensation, however, useable upper frequency limit reduced resistance value drops. second issue with resistive current sensing where measure current. Since actual motor current desired value, obvious choice sense resistor series with motor phase. complication that choice that signal interest millivolt differential value across sense resistor, common-mode voltage motor phase typically hundreds volts switching high frequency with rapid dv/dt. There isn't practical differential amplifier capable rejecting that much common-mode voltage with required bandwidth, alternative approach required. common approach optically isolated amplifier which differential signal modulated communicated across isolation barrier infrared signal. optical isolation effectively eliminates common-mode voltage problem, floating side isolator needs isolated power supply. further problem that performance typical optocouplers changes with temperature degrades over time. often overlooked issue that optocoupled amplifier output differential signal which requires differential amplifier scaling, level-shifting stage work with input most analogue digital (A/D) converters. This external amplifier easily become limiting factor overall system performance. example, achieve 12-bit resolution, high performance op-amp necessity. cost adding high performance op-amps each current measurement channel rapidly increases cost this solution.
Gate Driver
MOTOR
Microcontroller
Overcurrent Comparators
Sample Hold Bipolar Unipolar
Current Channel
Figure Traditional Current Sensing Method Motor Drives Another common alternative measure current through resistors each low-side transistor emitter three-phase inverter shown Figure long measurement circuit referenced common, this approach eliminates common-mode voltage problem. However, second problem arises: measured current longer motor phase current, half-bridge current. side switch conducting (through either transistor freewheeling diode) then current equal that motor phase current. This certainly occurs periodically throughout cycle, reconstruction circuit including sample hold amplifier required. Moreover, order sample three motor phase currents simultaneously (for zero phase-shift), three side switches must conducting. This only happens zero vector state which three side switches (corresponding negative peak triangle wave Figure Even conventional modulation methods like sinusoidal space vector modulation, width pulses measured become very narrow, placing increasing performance burden sample hold circuit. When modulation index meets exceeds unity does overmodulation methods), however, current pulses disappear altogether. Overmodulation methods require different more complex strategy motor current reconstruction this method sensing used. Magnetic sensors, other hand, isolated their very nature. This means that motor phase current directly measured without common mode voltage reconstruction circuit problems just discussed. Traditional current transformers cannot used because very frequency current components present motor drives. Hall-effect current transducers solve that problem. They ring-type magnetic core with Hall-effect semiconductor element placed air-gap measure magnetic flux resulting from current through centre core. These so-called "open-loop" Hall-effect sensors suffer from several serious limitations, however. magnetic flux core depends upon magnetic properties core material, which non-linear temperature dependent. Moreover, Hall element itself temperature dependence, does exhibit wide bandwidth. Overall, accuracy bandwidth openloop Hall effect current sensors suitable high performance drives. clever solution aforementioned limitations found "closed-loop" Hall-effect current sensors. these devices, cancelling coil e.g. 1,000 turns wound upon magnetic core described above. builtin feedback amplifier drives current through cancelling coil such that flux (measured Hall-effect sensor) always driven zero thus cancelling ampere-turns generated current through centre hole. output current transducer then that cancelling current, which equal measured
current scaled-down turns ratio. majority temperature dependent gain core nonlinearities also cancelled using this method, although some offset issues remain. overall bandwidth accuracy these transducers proven very good motor drive applications. However, complex construction large magnetic cores required make these transducers larger more costly than alternatives. These transducers have built-in overcurrent sensing, additional reference threshold circuits comparators (for must added, further increasing size, cost complexity.
MEASUREMENT ERRORS RIPPLE CURRENT far, discussion error sources focused transducers themselves, motor current reconstruction techniques. mentioned introduction, strategies introduce ripple current multiple carrier frequency. desired motor phase current without ripple (which objective closing current loop) possible simply filter ripple using analogue pass filter. However, unless switching frequency very high (>16 kHz), current loop bandwidth very (<200 Hz), this method impractical because pass filter adds much phase shift (delay) current loop. alternative sample current peaks (positive, negative, both) carrier frequency centre-aligned strategy, thereby sampling average current that moment [1]. Figure shows example carrier frequency triangle wave, three motor phase output voltages motor phase current phase some additional timing waveforms explained later.
Carrier Frequency
Iu-v Motor Phase Current SYNC
PWSYNC
Output Sawtooth Waveform
Figure Timing Diagram sample motor current accurate, must align closely time peaks carrier, errors move sample point down ramp ripple. Note that consistently late sample will result fixed offset, general distortion waveform, slope ramp varies with overall motor phase current. same true consistently early sample timing. second concern that
measurement transducer, optically magnetically coupled previous examples, required faithfully reproduce motor current with full bandwidth rippl order obtain best accuracy. phase delay transducer bandwidth limitations will result inaccurate current sample acquisition discussed above. were possible move sample hold circuit motor side transducer, then bandwidth limit would lesser problem.
SYNCHRONOUS SAMPLING That effect, exactly what integrated circuit shown block diagram Figure does. Motor phase current measured across external sense resistor (200 peak) applied input differential amplifier, between Vin+ Vin-. bipolar signal passes through gain stage (x4.25) level-shifted unipolar (positive only) signal. This full-bandwidth representation motor current then passes through track hold amplifier that synchronized low-side referenced sync signal. measured sample converted pulse width synchronized motor frequency. period generator measurement external capacitor, selected slightly shorter than motor period. This allows synchronization with small set-up time. Motor current then proportional duty cycle output waveform seen Figure Temperature dependencies affect both "on" time period waveform, duty cycle extremely temperature dependence ppm/°C).
High Voltage "Up" Level Shifter Sawtooth Generator
SYNC
Pulse Gen.
Ramp Comparator Pulse Gen.
PEAK
Comparator Track/ Hold Invert Level-shift
Diff.
High Voltage "Down" Level Shifters
PEAK
Pulse Gen. Latch Overcurrent Comparators
Figure Block Diagram Integrated Circuit
also integrated fast overcurrent detection circuit (for both polarities measured current). detect overcurrent condition send signal low-side output less than This twice fast optocoupler methods with built-in overcurrent detection This truly isolated optical magnetic transducers are, merely level shifts signals between "high-side" (motor phase) "low-side" (negative usually common drive circuits). fabricated using junction isolation process with added features negative voltage transient immunity. level shifters communicate accurate digital timing signals presence commonmode voltage high 1,200V dv/dt rates V/ns. high side circuits amplifier require very power (typically only 1mA) enough that powered simple bootstrap power supply. overall accuracy equivalent 12-bit performance without expensive external op-amps. that required timer input microcontroller with resolution better (for PWM). equivalent -3dB bandwidth carrier. CONCLUSIONS current measurement integrates functions sample hold, level shifting, fast overcurrent detection into monolithic piece Silicon featuring high accuracy temperature dependence. Sampling synchronized motor carrier frequency, average motor current measured directly, without influence ripple current. Output from another signal that interface timer input microcontroller achieve overall level performanc consistent with 12-bit systems. REFERENCES Richardson, "Implementation Regular Sampling Strategy Drives," IEEE Power Electronics Specialist Conference Record, 1989,

Eric Persson, Toshio Takahashi Advanced Development Group Internationa

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