Bridgeless PFC using C200 Piccolo MCU

Hello. Thanks for the reply. I am proceeding with my design of the charger and am facing these problems:

1) The macro M2 & M3 in the PFC schematic mentioned above , is a TI custom made power supply. Can you guide me to an off the shelf IC to obtain similar functioning ? In case its not available can you please tell me how to proceed to design such supply on my own.

2) I am starting to modify the power stage for higher power. Hence i was wondering if TI has some software where I can use the given TI schematics and edit it. Kindly also tell how can I access (what software to use for) the example files etc. given in the EXAMPLE PROJECTS section in control suite.

3) Further, i wanted to simulate the whole project ( power stage + control stage + programming), before finally implementing the hardware because I have to optimize many parameters in the design. All of my ICs, components and control schemes are TI suggested, except for mosfets and diodes (for which I already have the SPICE models). Does TI have any software where I can edit the schematic of power and control part and later code the MCUs to obtain the desired outputs ? If not, how can i proceed with this poblem ?

Adding to my previous doubts:

4) I find the component lists of MACRO M3 (DC-PwrEnry) and M5 (ISO-USB-to-JTAG) in BOM, but there is no schematic referring to these macros (http://www.ti.com/tool/tidm-blpfc) . Kindly provide that.

5) What is the exact part no of the control card that goes inside the DIM100 socket. Further, if I wish to upgrade he project for higher power, can i safely assume that the circuit in control will not change ( only the power stage changes).

6) It would be really helpful if you could provide an email id or phone no for faster doubt clarification.

Regards.

Hello.

1) Can you also tell me what is the kind of signal in "I fb-1" in PFC-Bridgeless Macro of the schematic of BL PFC EVM. I mean is it a rectified (only positive) signal or full sinusoidal signal.

2) Also kindly tell what is the range of values (0-3.3V or 0-5V ?) that the I fb-1 has. As mentioned above I am redesigning the power stage and hence I have to upgrade the current transformer and the sense circuit, accordingly. If it is more than 3.3 V, how is this voltage being fed to the Microcontroller ADC input ?

3) While at the secondary side of current transformer, the sense circuit in the schematic and the sense circuit in Design Guide of EVM (Application report, TIDU312), page no 3 is different. So which is the design that I should go ahead with. One gives sinusoidal I fb-1 and other gives rectified I fb-1.

Regards.

Here are the answers to your questions:

1. The signal Ifb-1 is the PFC switch current sense signal. This switch current has some relationship with the PFC rectified current but it is not the same. It is neither a sinusoidal signal.

2. All ADC signal range for this MCU based design is 0~3.3V

3. Page 3 of TIDU312 is just a block diagram, not an accurate schematic. So use the schematic and the BOM together to get the accurate design/part information.

Shamim

Thank you. 

I understand that the current in the sense output will not be pure rectified because the current is not continuous but in PWM form. But when I tried to simulate this part of circuit with exact components as in schematic, I was getting a sinusoidal output at both Ifb-1 and Ifb-2, instead of only +ve half cycle at Ifb-1 and only -ve half cycle at Ifb-2. This is my worry as I am not able to appreciate the function of diode D1 and D2.

Further, what is the need of the voltage supply (12V) and voltage divider circuit ? In most current sense circuits, we just have a diode and a resistor at the secondary side. Also, i am assuming the peak rms current is close to 3.5 A (peak power of 300W divided by min rms voltage of 85V). Am I right ?

Regards.

D1, D2 allow sensed current signals applied to ADC pin only when PFC MOSFETs are on and reset the CT windings when the MOSFETs are off.

The voltage divider circuit applies a small offset to sensed current signal for better measurement. This can be compensated in s/w.

Peak current is much higher allowing for efficiency and some additional margin. The controlled current is not the peak signal. It is the average current that is being controlled. So the peak current in this demo is about 16.5A. This makes the peak sensed voltage = (16.5/50)*10=3.3V.

Thanks.

1) I understand that the maximum value of rms current will be slightly higher than 3.5A (300W/85V) owing to efficiency and margin, but i don't get how is it close to 16.5 A (rms value=16.5/sqrt(2)=11.66 ) which is almost 3.5 times. Further, in above calculation you have multiplied the current at secondary side (16.5/50) by 10, however the sense resistor, R12 is 33 Ohms in schematic. Hence, the peak sense voltage will be (16.5/50)*33= 10.89 V. Where are we going wrong ?

2) We still are curious, why we are sensing pulsed sinusoidal voltage across R12 (as mentioned in earlier doubt), as to our knowledge, Ifb-1 should have only +ve cycle current and Ifb-2 should have -ve cycle current. Where is the ambiguity ?

Thanks for the quick replies, we appreciate it a lot.

1) Peak of Sinewave input current is Im = 1.414*300/80.

Adding 20% margin and 85% efficiency, Imax = 1.414*300/80)(1.2/0.85)= 7.5A.

Now the switch current is measured at the mid point of switch on time. So the actual peak of the sensed current from the current transformer can be twice as high as peak of the actual sine current. Therefore the sensed peak is 2*7.5 = 15A. So the final components  we chose made the peak sensed current as 16.5A.

R12 and R19 are both 10 ohms in the schematic and in the BOM. It is not 33 ohms.

2)Current sense volt across R12/R19 are pulsed current with ramp envelope. We are trying sense the mid point value (sensed volt) of that ramp envelope. Again these mid-point values of the ramp envelope may not track the envelope of the average inductor (input) current (at least under DCM condition). So the mid-point values of the switch current are translated to average input current by a software macro (see BLPFC user-guide software block diagram). This macro block (PFC_BL_ICMD) actually translates the ref input current  command into ref switch current command. Then the switch current command and the switch current feedback are compared and the error is applied to the 2p2z current loop controller.

Thank you for the detailed answer. Its very helpful. Further, I see that there is a difference in the values of the resistors R12 and R19 in your schematic and mine. And hence, i would request you to please share your schematic of this EVM so that we can update ours, if there are any other additional changes.

Regards.

Hello, if the peak current at the primary side of the current transformer is 16.5 A, kindly also share the new BOM for this whole EVM, as the current sensor as per my BOM says the T1 part no as : CTX16-18294 (digikey 283-4353-ND), which is a 5 A Eaton Bussman current sensor. Thanks.