UCC25640EVM-020: DC-DC converter using UCC256404

Hi Abu Bakar,

Thanks for reaching out to us. Please see the responses below:

1.

Abu Bakar said:

If we understood correctly, the capacitors C1, C17 shown in document SLUUBX3B together make the total resonance capacitance. This placement of caps makes the converter looks like a standard half-bridge converter. What are the pros and cons of not placing them as in a standard Half-bridge LLC converter? 

Split resonant cap is advantageous compared to single resonant capacitor:

For more details see the below screen shots:

2.

Abu Bakar said:

Do the capacitors C13, C21 and C14 also contribute to resonance?

C13 and C21 and C14 are part of the sensing components that are needed for the controller to generate the gate pulses.

Here C13 and C21 are stepping down the resonant cap voltage whereas C14 and R14 are for the current sensing.

Please see the UCC25640x datasheet for further details.

3. 

Abu Bakar said:

We are also interested to implement short-circuit protection. How does the controller work in this scenario? Response time? hiccup mode?

OCP1 is primarily used to react to a short circuit event and shut down quickly. OCP2 and OCP3 are similar to an overload protection that have a slower reaction time.

The response to OCP1, OCP2 and OCP3 are the same. The controller will stop switching for 1s. After 1s, the controller will attempt to soft start.

4. 

Abu Bakar said:

Do you have any recommendations to safely implement short circuit protection?

Apart from the OCP1 protection, you can also use the external components to implement the short circuit protection.

Please refer the schematics of the TIDA-010081 reference design | TI.com where over current protection is implemented using the external components. Here, in case of over current, the BLK pin is pulled down. That way gate pulses will be off in case of the fault.

5. 

Abu Bakar said:

We would also like to implement synchronous rectification using UCC24624 controller, any comments regarding the implementation of protection?

All the system level protections are taken care by the LLC controller. So, you do not need to add any additional components around the UCC24624 other than required for proper operation of this controller.

Regards

Manikanta P

Hi Abu Baker,

Please see the responses below:

1.

Abu Bakar said:

Regarding your reply to Q1, this split capacitor approach seems very interesting, what would be the level of stress voltage on power devices then? Could you please share more details/material for this approach? what tools did you use to make the comparison? it would be nice if these could be shared. Material can also be sent via email.

The power devices stress wouldn't change with respect to resonant cap configuration (whether it's a single resonate cap or split resonant cap).

In case of split cap configuration, resonant current is splitting between two parallel capacitors. 

I do not have any other material except those three pictures that I have shared previously for split cap advantages.

For better understanding, you can use the simplis file given here: https://www.ti.com/lit/zip/slum684

2. 

Abu Bakar said:

In the excel sheet tool sluc675c, it splits as an option the total resonant capacitance (C127) but unclear about the case of resonant current value given in C125 is?

The current in split resonant cap would be half the total resonant current (which is given in C110).

3.

Abu Bakar said:

Refer to the same excel sheet, How to select Ln (C55)?, we think it comes from ratio Lm/Lr? which is already calculated. please comment.

Ln needs to be selected such that voltage gain and max frequency requirements are met.

Please see the design example given in the following app note:

Designing an LLC Resonant Half-Bridge Power Converter Article

4.

Abu Bakar said:

How to get values of Rup and Rlow on "compensation and transient" sheet?

Here is the good reference for opto coupler-based compensators: The TL431 in loop control (pagesperso-orange.fr)

5.

Abu Bakar said:

what's the acceptable range of input voltage variation for UCC25640x controllers? allowed plus/minus the nominal input voltage.

You can find this information in the electrical characteristics table of the controller's data sheet.

6.

Abu Bakar said:

What are your recommendations to drive power MOSFETs with this controller? In EVM-020, there is no external driver?

For connecting external driver to the controller see the below screen shot:

7.

Abu Bakar said:

there is also external current in EVM-20 via R9,R12 and U7, Can it also be treated as a short-circuit protection?

R9,R12,U7 are for the adjusting both the burst mode threshold setting and output voltage set point depending on the magnitude of the output current.

You can modify this circuit for over current implementation as well.  Please refer the schematics of the TIDA-010081 reference design | TI.com where over current protection is implemented using the external components. 

8.

Abu Bakar said:

What's different in a latch-up state as compared to a fault state etc..? need to reset mains?

To come out of latch up state, the mains need to be reset. 

Regards

Manikanta P