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IXMS150PSI 数据表(PDF) 9 Page - IXYS Corporation |
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IXMS150PSI 数据表(HTML) 9 Page - IXYS Corporation |
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9 / 10 page  I - 43 © 1998 IXYS All rights reserved IXMS 150 Fig. 10b Input Offset Adjust Circuit L m = motor inductance R m = motor winding resistance R sw = power switch resistance R s = sense resistor It is very important that the motor induc- tance value used in the analysis is not the value on the manufacturers data sheet but rather the value observed in actual operation. The PWM action causes high frequency effects that can change the apparent small signal inductance significantly. These effects are dependent upon voltage as well as current and frequency. It is best to measure the observed current ripple at the motor supply voltage and switching frequency you expect to use and calculate the actual motor inductance using: L m = VHV/((2 Fosc)(Imax-Imin)) (19) It is also important to note that both R m and R sw are temperature dependent. The motor winding resistance can increase by as much as 30 % at high temperatures, and if FETs are used as power devices, R sw can increase to 2.2 times its value at room temperature. Substituting equations 15 through 18 into equation 14 gives the expanded loop gain equation (eq. 20): (1+sRC) 2Vhv 1 2Rs G loop(s) = sR 2C VA (sLm+Rm+Rs+Rsw) which can be written as (eq.21): 4 V HV Rs G loop(s) = V A(Rm+Rs Rsw) (1+RC) (sR 2 C) [1+sLm/Rm+Rs+Rsw)] Therefore the poles and zeros of the system are: pole at DC, with a 0dB intercept of: 4V HVRs/[VAR2C(Rm + Rs + Rsw)] zero at 1/(R C) pole at (R m + Rs + Rsw) /Lm A simple Bode analysis can be per- formed to provide the necessary infor- mation to guarantee the stability of the loop. A stable system will result when the gain crossover occurs at a point where the loop phase shift is less than - 180 degrees. The gain crossover point is defined as the frequency where the magnitude of G loop(s) = 1 (0dB). The Bode plot will show two figures of merit that give an indication of the behavior of the closed loop system, gain margin and phase margin. Gain margin is the amount of loop signal at- tenuation at the point where the loop phase has reached -180 degrees. It is a qualitative measure of how susceptible the loop is to noise outside its band- width. Phase margin is the amount of Fig. 10c Gain Adjust Circuit G e/a(s) = (1 + sRC)/(sR2C) (15) K pwm = 2 VHV/VA (16) G m(S) = 1/(sLm + Rm +Rsw +Rs) (17) G i(s) = 2 Rs (ignoring sampling effects) (18) where: R, C = external compensation components R 2 = internal input resistor, typically 20 kΩ V HV = motor high voltage power supply V A = oscillator amplitude, typically 7 V Fig. 11a Loop Compensation Block Diagram |
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