SystemDesigner includes a high-level PWM module based on the TI Piccolo™ ePWM. One of the powerful features of the ePWM is the ability to generate start-of-conversion (SOC) signals, SOCA and SOCB, at any point in the PWM period. The SOC signals are then routed to the analog-to-digital converters in the design and are used to sample voltages or currents at the SOC time instants.
The SystemDesigner architecture automatically routes the SOCA and SOCB signals from the ePWM blocks to the ADCs without any visible wiring. You can download a circuit example that demonstrates this from here.
The table below describes the design:
|V1||A 1V peak ( -1V to +1V ) sine wave source|
|ADC1||An ADC which quantizes the input voltage with 2mV LSB weight. The output is limited to 0|
|CONSTANT_1||A 2 LSB constant offset|
|ADDER_1||Adds the 2 LSB offset to the ADC output and applies this value to the ePWM CMPA input|
|SHIFT_1||Divides the CMPA by 2, applying the result to the ePWM CMPB input|
|ePWM_1||Generates trailing edge PWM patterns based on the CMPA bus and also generates SOCB pulses when the count value reaches the CMPB input bus|
Starting at the left side of the schematic, the sine wave voltage source V1 is input to the ADC. This sine wave has a maximum value of 1V and a minimum value of -1V. The ADC then quantizes this voltage into a 2mV LSB. Ordinarily, when the analog input is negative, the ADC output would become negative, but in this case the ADC limits its output to unsigned integers with 31 bits of resolution, which means that the output of the ADC is never less than zero.
The constant and the adder ensure that the output of the shift block, the CMPB input, is never zero. This also means the ePWM has a minimum pulse width of 2/600 or approximately 0.33%.
The ePWM has two inputs: CMPA and CMPB with CMPB=0.5*CMPA. The CMPA input sets the pulse width, and the CMPB input sets the SOCB generation. Since the CMPB input is always half the CMPA input, the SOCB signal is generated midway through the PWM ON time. In many switch-mode power converters, sampling midway through the ON time captures the average value of the signal. Capturing that average value often helps to eliminate any peak-to-average error in the samples.
The integer-sampled data simulation results are shown below.
* Piccolo is a trademark of Texas Instruments Incorporated.
Along with the TI Piccolo™ ePWM, an SOC signal can be generated by a node within the schematic. The SystemDesigner architecture automatically routes the input signal from the SystemDesigner Start of Conversion Clock Breakin to the rest of the SystemDesigner components without any visible wiring. You can download a circuit example that demonstrates this from here.
A brief description of the example circuit is as follows:
The double precision floating point-sampled data simulation results are shown below.