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TIDA-00637: Question about PWM Fixed Duty Cycle in EVSE Reference Guide and Demo Firmware

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

  • Hello Kevin,

    As you stated correctly, the duty cycle indicates the maximum charging current, the EVES can provide to the EV.
    This maximum current is dependent on the hardware within the EVSE and the grid capabilities. This means once the EVSE is installed, the maximum current it can deliver stays the same. Therefore, the duty cycle is set once and is not changed afterwards.

    To calculate the duty cycle the formula 

    (1) Duty cycle(in %) = CURRENT_RATING/0.6

    is used for currents below 51 A and the formula

    (2) Duty cycle(in%) = (CURRENT_RATING/2.5) + 64

    is used for currents between 51 and 80 A , according to the SAE J1772 standard.

    In the software example a period of 25160 clock cycles is necessary to generate a 1 kHz signal. Now to calculate the compare value for TA2CRR1 we need to calculate the duty cycle with (1) if CURRENT_RATING is below 51 A. Therefore we divide CURRENT_RATING by 0.6. Then we divide it by 100 to transform the percentage value in a real value between 0 and 1. The last step is to multiply it by the period, which is 25160. If we now combine these factors: 1/0.6 * 1/100 * 25160 we get a rounded factor of 420.

    As an example if we take a current rating of 20 A, we get a TACCR1 value of 8400, which results in a Duty Cycle of  8400/25160 = 0.334 = 33.4%

    For the second case with a CURRENT_RATING between 51 A and 80 A a similar thing can be done.
    We take (2) and divide it by 100 and multiply it by 25160 and we get 101 * CURRENT_RATING + 16102, as a rounded compare value for TA2CCR1.

    I am not sure why in the SW example it is 16160, but the difference is neglectable.

    So to sum this up, the duty cycle is never changed after the EVSE is installed, since the maximum current it can deliver does not change in a typical application, where the EVSE is installed in a fixed location. Therefore the formulas to calculate the compare value for TA2CCR1 can be simplified as in the SW example.

    As a footnote I have to mention, that this design uses the OPA171 for driving the pilot signal. This amplifier should be replaced with TLV1805 as in TIDA-010071, since the requirements on rise- and fall times of the pilot signal changed, and OPA171 is not fast enough here. 

    Best regards,

    Andreas

  • Hello Andreas, 

    Many thanks for your reply, it's much appreciated, this clears up everything for me

    Best regards,

    Kevin