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SA8282 Datasheet(PDF) 6 Page - Mitel Networks Corporation |
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SA8282 Datasheet(HTML) 6 Page - Mitel Networks Corporation |
6 / 14 page SA828 6 pdy fCARR x 512 Counter reset When the CR bit is active (i.e., Iow) the internal power frequency phase counter is set to 0 degrees for the red phase. It will remain at 0 degrees until the CR bit is released (i.e., high). Control Register Function This 24-bit register contains the parameters that would normally be modified during PWM cycles in order to control the operation of the motor. The parameters set in the control register are as follows: Power frequency (speed) Allows the power frequency of the PWM outputs to be adjusted within the range specified in the initialisation register Forward/reverse Allows the direction of rotation of the AC motor to be changed by changing the phase sequence of the PWM outputs. Power frequency amplitude By altering the widths of the PWM output pulses while maintaining their relative widths, the amplitude of the power waveform is effectively altered whilst maintaining the same power frequency. Overmodulation Allows the output waveform amplitude to be doubled so that a quasi-squarewave is produced. A combination of overmodulation and a lower power frequency can be used to achieve rapid braking in AC motors. Output inhibit Allows the outputs to be set to the low state while the PWM generation continues internally. Useful for temporarily inhibiting the outputs without having to to change other register contents. The power frequency range is a function of the carrier waveform frequency ( fCARR) and a multiplication factor m, determined by the 3-bit FRS word. The value of m is determined as shown in Table 5. FRS word Value of m 000 1 001 2 010 4 011 8 100 16 101 32 Table 5 Values of carrier frequency multiplicaion factor m 110 64 The power frequency range, fRANGE, is then given by: where fCARR = carrier frequency and m = 1, 2, 4, 8, 16, 32 or 64 (as set by FRS). fCARR 384 fRANGE = x m X X PDY5 PDY4 PDY3 PDY2 PDY1 PDY0 DON’T CARE PULSE DELAY SELECT WORD PDY5 = MSB PDY0 = LSB Fig. 7 Temporary register R2 PDY word Value of pdy 000000 64 ...etc... ...etc... 111110 2 Table 6 Values of pdy 111111 1 The pulse delay time, tpdy, is then given by: where pdy = 1- 64 (as set by PDY) and fCARR = carrier frequency. Fig 8 shows the eftect of the pulse delay circuit. It should be noted that as the pulse delay circuit follows the pulse deletion circuit (see Fig. 2), the minimum pulse width seen at the PWM outputs will be shorter than the pulse deletion time set in the initialisation register. The actual shortest pulse generated is given by tpd – tpdy. Pulse delay time The pulse delay time affects all six PWM outputs by delaying the rising edges of each of the outputs by an equal amount. The pulse delay time is a function of the carrier waveform frequency and pdy, defined by the 6-bit pulse delay time select word (PDY). The value of pdy is selected as shown in Table 6. Fig. 8 Effect of pulse delay on PWM pulse train PWM SIGNAL REQUIRED AT INVERTER OUTPUT tpdy tpdy tpdy tpdy OUTPUT SIGNAL TO DRIVE TOP SWITCH INVERTER ARM OUTPUT SIGNAL TO DRIVE BOTTOM SWITCH INVERTER ARM tpdy = PULSE DELAY TIME tpdy = CR PDT6 PDT5 PDT4 PDT3 PDT2 PDT1 PDT0 COUNTER RESET PULSE DELETION TIME SELECT WORD PDT6 = MSB PDT0 = LSB Fig. 9 Temporary register R0 Pulse deletion time To eliminate short pulses the true PWM pulse train is passed through a pulse deletion circuit. The pulse deletion circuit compares pulse widths with the pulse deletion time set in the initialisation register. lf a pulse (either positive or negative) is greater than or equal in duration to the pulse deletion time, it is passed through unaltered, otherwise the pulse is deleted. The pulse deletion time, tpd , is a function of the carrier wave frequency and pdt, defined by the 7-bit pulse deletion time word (PDT). The value of pdt is selected as shown in Table 7. PDT word Value of pdt 0000000 128 ...etc... ...etc... 1111110 2 Table 7 Values of pdt 1111111 1 The pulse deletion time, tpd, is then given by: where pdt = 1-128 (as set by PDT) and fCARR = carrier frequency. Fig. 10 shows the effect of pulse deletion on a pure PWM waveform. tpd = pdt fCARR x 512 |
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