TMS320F280039C: Understanding on SFRA behavior using TIDM buck converter FW with 200kHz HW and 20kHz HW setup

Hi Prathamesh,

Can you explain about the hardware setup you are running on? For your custom hardware, how does it differ from the BOOSTXL-BUCKCONV hardware? Is it similar but you have changed some hardware component values? Be sure to update the values accordingly in the powerSUITE file or the settings.h file. You have migrated the code from F280049C to F280039C device? Note the increase CPU frequency on the F280039C device? Have you done any tuning on the control coefficients?

Regards,

Peter

Hi Peter,

Thanks for your response. Following are response to your questions:

a. We have a simple buck converter configuration with input voltage at 600 V and output voltage at 400 V. We have modified the HW components according to the mentioned ratings. 

b. We have updated the switching frequency, voltage and current ratings in setting.h files accordingly.

c. Yes, we migrated code from 49C to 39C and have incorporated increased CPU frequency in the user setting files.

d. We want to perform tuning of control parameters. Hence, we first want to understand the open-loop response of the HW through SFRA.

Thanks & Regards,

Prathamesh

Hi Prathamesh,

Can you provide a high level block diagram of your design? 

For your converter, when testing the buck performance, is everything operating as expected? Can you provide some scope shots of the Vin and Vout for reference to ensure the design is sound.

It would be good to double-check all changes made in the buck converter code to ensure that all changes are accounted for and correctly made conforming to your new design.

Regards,

Peter

Hi Peter,

Thank you for your reply.

a. I am adding high level block diagram of design below:

b. The buck converter performed as expected during testing.

c. Sharing with you the converter scope shots for Vin, Vnode, IL and Vout.

Thanks & Regards,

Prathamesh.

Hi Prathamesh,

Your provided scope shot seems to show correct buck performance for your system. Let me consult with some other experts to understand why you may be seeing differences in the SFRA behavior for your system

Regards,

Peter

Hi Peter,

Do you have any update over my query. I am waiting for your positive response.

Regards,

Prathamesh.

Prathamesh,

Couple of more questions to understand what's going on.

1. I just want to confirm that you are running Lab 1 here? Plant response can be obtained in Lab 1: Open Loop if SFRA enabled and you have to select plant mode in SFRA GUI Page 29 of user guide.

2. In expression window, are you getting correct values for Vout (BUCK_vOutSensed_Volts)? This is because your plant response is based on your ADC data and its scaling correct.

3. Is ISR frequency is same as switching frequency? If yes then does it look like following ?

4. One more thing we can do here is to increase injection amplitude (highlighted) by little (0.01pu) to see if there is any difference in your plot response.

Regards,

Sumit

Hi Sumit,

1.Yes, I confirm that I am running lab-1. I am not getting a plant response (an error has occurred as shown in the figure below, but I have a set of readings recorded in the 'SFRAData' Excel sheet).

2. Yes, I am getting the correct values for Vout (BUCK_vOutSensed_Volts).

3. Yes, the ISR frequency is the same as the switching frequency, as per your image-2.

4. Below is the open-loop response with a '0.01pu' injection amplitude:

Test conditions: 

Vin: 100V, Iin=0.87A
Vout=42.6V, Iout=2A (CC mode of DC electronic load)
Duty=0.45
Injection Amplitude= 0.01
Sweep Freq. = 10Hz to 180kHz.

If anything further require, please let me know.

Thanks & Regards,

Prathamesh.

Prathamesh, 

Considering 10kHz Fsw that is being used, open loop response will also scaled down considering TIDM-DC-DC-BUCK example is running at 200kHz. Notch in gain plot is resonance occurring due to Lout and Coss of fet. Switch node layout can improve that portion. Same thing can be explained regarding  the disturbance in gain and phase plot at frequencies beyond cross-over frequency which is causing non-monotonous roll-off. Is it possible to shorten the switch node routing (switch on path as well as Switch off path) using some thick wire. This will see if we can tone down these disturbances. If you see SFRA user guide, software simply adds disturbance to duty and collects plants response to draw these bode plots. 

Regards,

Sumit

Prathamesh,

One more suggestion, could you please help verify the following:

1. If your this buck converter is operating in continuous conduction mode or discontinuous conduction mode? While obtaining bode plot, TIDM-DC-DC-BUCK example operates in CCM and shows all the graph for the same i.e. inductor current value never touches zero in steady state.

2. If highlighted per unit value is withing 0.0f to 1.0f range

3. Since you are getting issue running in plant mode, Could you please re-run SFRA again with "open loop" to get the the plot that you have shared above and once you get plot then you can change the mode "plant" as shown in image below? Does that bring your plant gain to 0db?

Regards,

Sumit