9.2. Six-channel PWM Interface
The PWM interface operates with a PWM carrier clock of 300 MHz for high resolution control of the MOSFET switching times.
The PWM interface ensures a dead time between switching to ensure both outputs are not high at the same time; the dead time prevents short circuit “shoot-through” in the power transistors. The input clock and a PWM counter set the PWM frequency. The counter alternately ramps up from zero to a maximum value and ramps down from the maximum value to zero. The sequence is as follows:
0, 1, 2, …, max - 1, max, max - 1, …, 2, 1, 0, …
The maximum value of the counter ramp, max, is software configurable. The PWM frequency is fPWM = fCLK/(2 x max)
The design generates high- and low-side drive signals for the MOSFET module by comparing the ramp counter value with the values you set in the PWM threshold configuration registers. The design inserts a dead period between the switching of the upper and lower drive signals according to the value set in the PWM blocking time configuration register.
The design sets carrier_latch output signal high for one clock cycle when the PWM counter is at 0 or max. This signal triggers a position encoder to take a position reading.
Based on the PWM counter value, the PWM component generates configurable timing output strobes for triggering ADC conversion for feedback-current readings. Configure the ADC start pulse to perform the conversion during the quietest period of the PWM cycle away from PWM switching events (around the min and max values of the PWM counter).
The start output signal is a trigger for the ADC IP to start conversion. The trigger_up configuration register sets the PWM count value and the start signal is high for one clock cycle while the PWM is counting up. The trigger_down configuration register sets the PWM count value and the start signal is high for one clock cycle while the PWM is counting down. Set the trigger_up and trigger_down registers symmetrically to ensure a regular ADC sample position offset before the reversal point of the counter. In other words, trigger_up = MAX - offset, and trigger_down = offset.
The design calculates the PWM blocking time configuration register as pwm_block = dead time x fCLK. Dead time refers to the time when the design turns off both upper and lower transistors, to prevent short circuits. You must obtain specific dead time values for the specific MOSFET module you are using. For example, with a dead time requirement of 2μs and a PWM module clock of 300 MHz, the pwm_block value is 600 (=2μs x 300 MHz. Figure 5 shows PWM output generation (including dead time).