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IXMS150PSI 数据表(PDF) 8 Page - IXYS Corporation |
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IXMS150PSI 数据表(HTML) 8 Page - IXYS Corporation |
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8 / 10 page  I - 42 © 1998 IXYS All rights reserved IXMS 150 mode, either SW1 and SW4 are on (V m = V HV) or SW2 + SW3 are on (Vm = - V HV). By appropriately controlling the duty cycle of SW1//4 vs. SW2/3, the averagemotor voltage can be controlled such that: V m(avg) = 2 VHV (0.5-DUTY) Note: DUTY is defined as the duty cycle of V OUTA. The IXMS150 can now regulate the motor coil current by commanding the voltage level and polarity required. enhance the MOSFETs,with the top two transistors (Q2, Q4) being destroyed due to excessive power dissipation. Therefore one has to limit the duty cycle excursions. The solution selected by IXYS limits the minimum output pulse-width to 0.5 ms, which translates to a duty cycle range of 5 % to 95 % when operating at 100 kHz, or wider at lower frequencies. There is a penalty of slightly limiting the maximum slew rate to (1-2 Min Duty) of the unrestricted case, which translates to 90 % of the ration with a particular motor. The basic elements involved in the current loop are illustrated in Fig. 11a. Referring to Fig. 11b, the loop gain for this system (the product of the forward and feed- back gain terms) can be expressed as: G loop(s) = Ge/a(s) Kpwm Gm(s) Gi(s) (14) where G e/a(s) = error amplifier gain K pwm = cascade of pwm and output H- bridge gain Fig. 8 Simplified H-Bridge Diagram SW1 SW3 SW2 SW4 D1 D2 Vm D3 D4 The Power Stage: An AC Coupled H-Bridge Fig. 9 shows the power driver selected for this application. Two of these are required to drive the two phase step motor. This circuit uses two N-channel and two P-channel power MOSFETs as opposed to an all N-channel architec- ture. The drawback of using P-channel transistors is that they are larger and therefore more expensive than similarly rated N-channel devices. But the advantages are much simplified drive and level shifting circuitry. This results in a lower component count and therefore higher reliability. It also lends itself easily to hybridization. Other advantages of this topology are: a) the high efficiency associated with level shifting by AC coupling since no power is dissipated in the capacitors, and b) the same circuit can be used for motor applications ranging from 12 V to several hundred volts, the only modification being appropriately rated power tran- sistors and coupling capacitors. A limitation of this circuit is that it cannot be used at duty cycle extremes. This would require one input to be continu- ously low while the other is continu- ously high. Eventually the coupling capacitors (C1, C2) would charge up to a voltage that would no longer fully Fig. 9 AC Coupled H-Bridge Diagram Fig. 10a Simplified Microstepping System unrestricted maximum slew rate for 100 kHz operation. Loop Compensation Information When used with the appropriate power stage, each channel of the IXMS150 acts as a closed loop transconductance amplifier. As such, it must be properly compensated to guarantee stable ope- G m(s) = cascade of motor winding impedance and H-bridge parasitic resistance G i(s) = current sense resistor and sampling amplifier gain The value of each of these terms can be determined from the Laplace transform diagram in Fig. 11b: |
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