How to Run an HDD Spindle Motor Quietly and Efficiently Bert White Corporate Fellow Silicon Systems Storage Products Group S T O R A G E P R O D U C T S 1 Outline • Spindle Motor Basics – Electrical equivalent of a spindle motor – PWM – Transconductance loop • Acoustic Noise – How does a motor driver create it? – How can a motor driver minimize it? • 32H6910 SPM Driver Section • 32H6910 Measured Data • Summary Electrical Equivalent of Spindle Motor ia PHA R + - bemfa = nbemfa nrpm VRUN + - L ib PHB R bemfb = nbemfb nrpm VRUN CT L ic + PHC R torque = K T (ianbemfa - bemfc = nbemfc nrpm VRUN L + ionbemfb + icnbemfc) Efficient Motor Driver • Motor current must be in phase with bemf • Peak motor current should occur when bemf is peakŁ • Pulse Width Modulation (PWM) should be used to minimize driver power dissipation 6-State Winding Current and Torque 4.00 Torque, Current, bemf bemf total torque winding torque 0.00 winding current -4.00 0 60 120 180 240 300 360 Angle Note: These ideal curves neglect the effect of winding inductance – a significant omission! PWM Drivers are Required for Maximum Power Efficiency VP Ton = B T OUT Vout = B VP Toff = (1 - B )T GND • • Typical PWM frequency (1/T) is 20-30 kHz PWM Drivers are LOW IMPEDANCE! Transconductance Loop Current Command Voltage Shaping and PWM + A1 - VIMAG VP PHA SPM Motor PHB PHC Current Sense Resistor The transconductance loop permits current control with voltage mode drivers Transconductance Loop Advantages • Permits supply current control during motor start • Simplifies the speed control equations • Reduces torque ripple caused by phase to phase resistance variations Jerk Causes Acoustic Noise • • • • Jerk is defined as d/dt of acceleration Jerk excites noise-generating mechanical resonances Jerk is minimized by minimizing torque ripple Per-phase torque ripple is essential — but its slope should be minimized 6-State Torque Ripple Not a Pretty Sight! Trapezoidal Torque Ripple Only a Little Bit Better! Sinusoidal Torque Ripple A h h h! Torque Ripple Summary Waveform Type 6-State Trapezoidal Sinusoidal Torque Ripple 37% 32% 9% Peak d/dt of phase A torque 6 va/ms 3.9 va/ms 1.4 va/ms • Sinusoidal drive reduces torque ripple 4x • Per-Phase Torque has 4x less d/dt slope 32H6910 SPM Driver Section 2.5 A DMOS PHASE DRIVERS SINEWAVE GENERATOR VMAG V12 VA PWM Modulator A sin (60 Fvco t) VB PWM Modulator A sin (60 Fvco t - 120) PGADJ UPA DRIVER V12 UPB VC PWM Modulator V12 UPC DRIVER These sinusoids are 60x lower Frequency than VCO Modulation Frequency RSNS 3 SIGN(IA) PHASE DETECTOR VCO IB PHB PHA PHB IC PHC PHC CT PLLSTATE VCO IA PHA DRIVER 3 A sin (60 Fvco t - 240) SPINDLE MOTOR SIGN(IB) Adjust VCO phase and frequency so IA, IB, and IC change polarity in the center of a PLLSTATE SIGN(IC) CT VMAG TRANSCONDUCTANCE AMPLIFIER PLL LOOP FILTER VMAG VMAG LOOP FILTER Gm + SPMDAC 8 SPM_DAC[7:0] (From Serial Port) 8 bit DAC ISNS Phase Detector Operation • The Phase Detector compares the phase current’s zero crossing with the center of its corresponding PLLSTATE window. Phase Detector Waveforms Phase Voltage, referenced to Center Tap These bemf voltages . . . 6.00 Drive Voltage PLLSTATE 0.00 bemf Voltages 5 degrees lag properly phased 5 degrees lead -6.00 0 60 120 Angle 180 Phase Detector Waveforms (cont.) . . . generate these phase currents . . . 1.00 Phase Current PLLSTATE bemf leads by 5 degrees bemf properly phased bemf lags by 5 degrees 0.00 -1.00 0 60 120 Angle 180 Phase Detector Waveforms (cont.) . . . and result in these charge pump currents. 100 PLLSTATE Speed up VCO Pump Current, uA bemf leads by 5 degrees bemf properly phased bemf lags by 5 degrees 0 Slow down VCO -100 0 60 120 Angle 180 32H6910 Lab Data Phase Current 0.5A 32H6910 Lab Data (cont.) Acoustic Noise (A-weighted) 6-State • • 3.8 dB rms reduction, 30 kHz to 20 kHz 10-20 dB pure-tone reduction Sinusoidal Conclusion • Driving an HDD motor quietly and efficiently requires: – PWM drivers – Sinusoidal Phase Currents • The 32H6910 implementation of PWM sinusoidal drive: – Reduces both measures of torque ripple by 4x – Reduces pure-tone acoustic noise by 10 to 20 dB – Reduces broad-band acoustic noise 3.8 dB