Part Number: TIDA-00637
Hello, I understand that the duty cycle of the pilot signals communicates the limit of current the EVSE is capable of supplying to the vehicle, however, I am just a bit confused about what seems to be a "fixed" duty cycle used in the example firmware and the reasoning for it. I am just hoping to get some clarity on this...
In section 6.3.1 of Level 1 and Level 3 EVSE Reference (https://www.ti.com/lit/ug/tidub87/tidub87.pdf?ts=1629281282344&ref_url=https%253A%252F%252Fwww.google.com%252F) --> Pilot Signal Setup; it is stated that "The PWM duty cycle can be set once and left alone because it is static through the operation of the EVSE (because it is based on the service connection and electromechanical design).". However, I am still not clear on what the duty cycle is set to, even with the comments provided in the code (emeter-setup.c file) - perhaps I am missing something obvious:
/* Set Initial PWM Period */
TA2CCR0 = 25160 - 1;
/* CCR0 reset/set */
TA2CCTL1 = OUTMOD_7;
/* CCR1 PWM duty cycle
* From J1772, the duty cycle is CurrentRating/0.6,
* so CCR1 = ((CurrentRating/0.6)/100)*25160 */
if(CURRENT_RATING <= 51)
TA2CCR1 = CURRENT_RATING * 420;
/* For higher current systems, the duty cycle is defined as
* (CurrentRating/2.5)+64, which results in the following */
else if((CURRENT_RATING > 51) && (CURRENT_RATING <= 80))
TA2CCR1 = 101*CURRENT_RATING + 16160;
else
TA2CCR1 = 0;
There are some magic numbers there and again, maybe I'm missing it but I don't see where "420" comes from (TA2CCR1 = CURRENT_RATING * 420) and similarly for the higher current ratings in lines below. Also, what duty cycle does this result in for the fixed operation.
Lastly, regarding the reasoning for the "fixed" duty cycle; is this simply because the demo application provided does not include a method to receive commands for maximum current to respond to by adjusting the adjusting the duty cycle and therefore, the duty cycle is simply left fixed? Or is there some other reason that I am missing (perhaps a recommended standard duty cycle)?
Many thanks for any clarification on this!
Kevin