UCC28951: Design Review

Hi,

Thank you for your interest in UCC28951. I am reviewing your schematics and would get back to you the earliest.

Regards

Bishal Mondal 

Hi,

To understand your device selection, I require additional information of your converter parameters. Can you please provide the excel calculator wherein you have input your custom parameters for the design?

Thanks, and Regards

Bishal Mondal

Hi Bishal Mondal,

They are attached in my first post, but no worries, I have attached them here for your reference.

4743.[User's Guide] sluub02a.pdf1172.[User's Guide] sluuat3b.pdf0523.sluc506c_10A.xls3247.SLUC222E_10A.xlsx1680.Design Requirements.docx

Hi Jai-Wei Seah,

Let me go through it and get back to you.

Thanks,

Bishal Mondal

Hi Bishal Mondal,

Good day, wanted to check in if you needed any further information or clarification.

Please do not hesitate to ask!

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

Thank you very much for reaching out for offering more information/clarification. I feel the documents that you have provided has enough information to help review the schematics. I am currently working on it and will get back to you ASAP.

Thanks,

Bishal Mondal

Hi Jai-Wei Seah,

I have reviewed your component selection and found that it is done correctly. I have replied in your annotations commenting on the selection. There are no changes that may be required. I have found small concern regarding output capacitor values (seems not to be matching with the calculator you have provided; however, the component annotated in the pdf meets the design requirement). Other concerns are commented in the annotations. Please find the comments on the selected components in the attached PDF.

Please feel free to reach out to me in case you need further support on the component selection.

Thanks,

Bishal Mondal

Custom_Design.pdf

Hi Bishal Mondal,

Thank you for the detailed review, please allow me some time to go through your comments.

Aside, I have a question regarding the SR side. The EVM is using 2 SR FETs to perform secondary rectification. If we would like to use diode rectification instead (for low current output), would it be recommended for UCC28951? Or since the FETs are a more efficient approach, we should just stick to SR FETs even with low current output/loads?

We are thinking about the possibility of having both the SR FETs and diode footprints on the PCB and test it out during prototyping to see which is more efficient.

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

You can certainly use diodes to perform secondary voltage rectification due to lower current. The only thing you need to care is to connect OUTE and OUTF of UCC28951 to ground using a high value pulldown resistor.

Best Regards,

Bishal

Hi Bishal,

Noted on your recommendation regarding connecting OUTE and OUTF to ground via pull down resistor. I am aiming to use 10kohm.

Should the ADELEF, DELEF and SYNC pins also be pull down to ground via a resistor in the same manner since these pins are related to the secondary voltage rectification function?

And, would you have any recommendations or guidelines on calculating and selecting the diodes? The existing excel sheet covers only the calculations for SR FETs.

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

A 10k between OUTE to GND and OUTF to GND seems fine as per the datasheet. As for the SYNC pin, if it is not used a 10k resistor must be connected between the pin and ground to reduce noise and switching frequency jitter. Since you are going to use diodes, the values of resistors at ADELEF and DELEF pins will not matter as the PWMs (OUT E and OUTF) are no more used. 

For the diode selection, you may have to compute the losses and see if it is within your required limits (the usual process of diode selection). We do not have any section on selection of diodes in the PSFB documents. 

Best Regards,

Bishal

Hi Bishal,

Noted on your recommendations. If we switch from SR FETs to diodes, are there any changes that needs to be made to the SR clamp circuit? Or is it advisable to depopulate the entire circuit since this SR clamp circuit is intended to clamp the drain voltage of the SR FETs when it is off. While when using diodes for rectification, since the diodes are not controlled by the UCC28951, they will always be on.

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

It is recommended to use an RCD clamp to protect the output synchronous FETs from over voltage due to switch node ringing, this ringing may exist even with diodes in place of FETs for the output rectification. Therefore, I would recommend you keep the clamp circuit. You can observe its effect by populating/depopulating the components of the clamp circuit.

Best Regards,

Bishal

Hi Bishal,

I have a follow up question. The initial discussion above uses input 230Vac A 50/60Hz and outputting 48Vdc/10A continuous. Now, if the output is reduced to 48Vdc/4A continuous (192W), is using SR FETs still beneficial or should we consider diode rectification due to lower power output. My consideration is that SR FETs are still more beneficial even at lower current and wattage as it offers more flexibility in control such as able to control the ADEL and ADELEF timings to achieve better ZVS and eventually smaller output voltage ripple while using diodes offer limited/no control.

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

You are right about the use of SR FETs for output rectification even though the output current is low. Given that we have an IC that offers features like ADEL and ADELEF, choice of FETs for output rectification seems more meaningful. Diodes on the other hand reduces circuit complexity and can be used for low output current operation. 

Best regards,

Bishal

Hi Bishal,

Understood. Following the excel template, the DCM threshold is set at 15% which is 1.5A assuming 10A output. I have a requirement to operate UCC28951 in CCM from 250mA to 10A as to avoid using mosfet body diode as it incurs alot of reverse recovery losses. Can we do that by changing the DCM comparator RG and RE resistors? and how low can we set this DCM threshold?

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

I will look into this and get back to you ASAP.

Best regards,

Bishal

Hi Jai-Wei Seah,

Shorting the DCM pin to GND disables the DCM feature and the controller runs in CCM mode under all conditions.

Best regards,

Bishal

Hi Bishal,

Noted on your reply regarding DCM.

I have a question about the cycle-by-cycle current limit protection of UCC28951. From datasheet, I see that there is a way to calculate and set the TCL(on) which is the time in current limit (I understand it as the over current detection window) and the TCL(off) which is the hiccup time before retry. These calculations are based on the CSS (soft-start capacitor) value. Would the TCL(on) be able to go as low as 10us as I want to have a very fast detection time? From the excel calculator, I see that by using a 0.27nF capacitor, I can get 10us, but I wanted to confirm if this is also practically possible. A follow up question to this, what is the time between detection and disabling the OUTA-F pin output in an event that over current is detected?

Secondly, does the UCC28951 have cycle-by-cycle over and under voltage protection?

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

I will require some time to look into this issue. 

Regards,

Bishal

Hi Jai-Wei Seah,

As you mentioned, you can select Css = 0.27 nF and achieve a TCL(on) of 10 us. However, this will also reduce your soft-start time as it depends on Css too. You may have to see if your design can handle such fast startup. 

For your follow up question regarding the time between detection and disabling the OUTA-F pin output, this time will be same as TCL(on).

Also, to answer your second question, I regret to inform you that UCC28951 does not have cycle-by-cycle over and under voltage protection.

Regards,

Bishal Mondal

Hi Bishal,

Good to know that the 10us TCL(on) is achievable practically, and noted on your point regarding fast startup, we will have a look.

To clarify regarding the detection and disabling time, do you mean detection 10us + disabling 10us = 20us from detection to disable?

Does the switching frequency have any effect on the detection/ disabling time since the current limit is cycle-by-cycle?

Best regards,

Jia-Wei

Hi Jai-Wei Seah,

I am looking at the query in detail. Will get back to you on Monday.

Regards,

Bishal Mondal