UCC28950: Power management forum

Hello,

The thread was closed because the question/s was answered to the best of our ability.

Hopefully that thread will help other engineers answer the same question you had on that past thread with the information that was given in the thread.

Regards,

Hello,

Your inquiry has been received and is under review.

Regards,

Hi,

It will take a few hours to a few days from your reply until the forum closes. I would like you to wait at least a week before closing it. I think it will be easier for other customers to see this forum if it is all in one thread. I look forward to your reply!

Thanks,

Conor

Hello,

If we have answered the questions we close the thread.  If you have new questions you can repost in the e2e.

I am glad to help where I can.

Regards,

Hello,

Let me start by addressing question Q1.

In regards to Q1.  These equations both calculate average Coss and are not the same.  You are correct that in equation for calculating average Coss that the maximum Vds should be in the denominator and not in the numerator.  The good news is I checked the tool for calculating the Coss_avg for FET QE and QF, and the equation matches (35).  So in the tool (35) and (64) are similar.

Q2.
Is my understanding of the following correct?

>I think this in regards to Q1.  Please see the comment above.

The loss calculated in equation (70) is calculated for one FET (e.g. QE or QF). In a full-bridge converter, two FETs, QE and QF, work as a pair, so the loss of both FETs must be considered. Therefore, in equation (72), this loss is doubled to obtain the total loss of QE and QF. In other words, the value of 14.4 W is the total loss of both QE and QF FETs.

>I checked this equation an PQE in the application note.

>I checked this in the excel tool and it is also doubles the losses in the power budget (PBUDGET)

I have to go now and work on other things.  I will take a look at the rest of your questions later.

Regards,

Hello,

Please see my comments below.

Q3.
Equation (77) may need to be modified in the following two ways. Could you please confirm the following?

A. About POUT
In equation (77), POUT (output power) is used to calculate the input capacitor capacitance, but in reality, the efficiency must be considered. Therefore, it is correct to use POUT / η. This allows the calculation to be based on the actual input power, including the system loss.

>You are correct that this is a better estimate to input power by dividing POUT  by estimated efficiency η

B. About VIN
In equation (77), VIN (input voltage) is used, but in reality, it is appropriate to use the minimum input voltage VINMIN. This is because the capacitance of the capacitor is most important when the input voltage is at its lowest.

> The design used a slightly different approach in calculating the minimum input voltage (VDROP) in equation 76 that was used in 77.

If we calculate based on this correction, CIN will probably be 355uF or more.

>If use a POUT / η you are correct that the bulk capacitor will increase.

>I will notify the applications team so they can select the bulk capacitor based on estimated input power.  I do believe that this is a better estimate.

I have to go now and work on other things.  I will take a look at the rest of your questions later.

Regards,

Hello,

Please see my comments below.

Q4.
Equation (78) may need to be corrected in the following two points. Could you confirm the following?

>O.K.

C. Regarding a1
Equation (78) uses a1, but efficiency (η) should actually be used here. If the calculation is made without considering efficiency, the actual power loss cannot be correctly reflected, so a1 is incorrect and η is correct.

>When it comes to estimating RMS current it is better to over estimate.

>In this case not including efficiency and the reflected DC current is O.K.

>You are correct that adding efficiency is a better estimating.  However, this is still estimated.  The efficiency could be higher and lower based on the components that you choose.

D. ICINRMS (Input Capacitor RMS Current)
When calculated using the corrected formula, it is considered reasonable that ICINRMS = 1.8A.

> This is what the application note calculated for RMS current.

Furthermore, regarding the loss PCIN of the input capacitor, the loss is generally calculated as follows:
PCIN =ESR×I^2 CINRMS

> This is actually the equation that is used in the application note.

Here, ESR is the equivalent series resistance of the input capacitor. If ICINRMS is 1.8A and ESR is an appropriate value, PCIN = 0.5W. Therefore, after correcting equation (78), ICINRMS becomes 1.8A, and based on that, the calculation result is thought to be 0.5W for the capacitor loss.

> This is true based on the application note excerpt that is presented above.

I have to go now and work on other things.  I will take a look at the rest of your questions later.

Regards,

Hello,

Q5.
I have a question about equation (84).

In fact, it is appropriate to use VINMIN for VINMAX used in equation (16) and equation (17). This is because the peak current is maximum at the minimum input voltage, which is the most severe operating condition.

Also, in equation (84), 2*fs (twice the switching frequency) must be corrected to the actual switching frequency fs. The 2 times in this part is incorrect, and the correct value is the simple switching frequency fs.
Based on the corrected equation from above, IP1 (peak current) is thought to be 3.3A. After correcting VINMIN and fs, the correct result is IP1 = 3.3A.

Question 84 is a typo.  It should be Vinmax and not Vinmin.  Even the title state peak current at Vinmin.

I will notify the applications team to update when they have a chance.  

I have to go now and work on other things.  I will take a look at the rest of your questions later.

Regards,

Hello,

Pease see my comments below.

Q6.
Is the result of the calculation of equation (169) 68.1 * 10^3 V/s = 68.1 mV/us? Also, do I need to change VINMAX to 410V?

There is no equation 169 in the application note in the application note.  However, what you have typed does not look correct mathematically.  10^3 would have to be 10^(-3) to equate to 68.1 mV. 

I have to go now and work on other things.  I will take a look at the rest of your question later.

Regards,

Hi Mike,

Thank you for answering so many questions.

Q6.
Equation (169) shows that:　

Also, is VINMAX 410, not 400, the correct value?

I'm also looking forward to an answer to Q7.

Thanks,

Conor

Hello,

These questions were in regards to the application note.  The equation you have shown is not in the application note.

Did that come from the data sheet?

Regards,

Hello,

I checked the data sheet for the UCC28950 and that only goes up to equation 149.

https://www.ti.com/lit/ds/symlink/ucc28950.pdf

I did a search on mMAG in the data sheet and the application note and cannot find this equation anywhere.  If it was mostly from an older data sheet and/or app note. 

https://www.ti.com/lit/an/slua560d/slua560d.pdf

I have to go now and work on other things.  I will take a look at the rest of your inquiry later.

Regards,

Hi Mike,

These seem to be from old data sheets.

https://e2e.ti.com/support/power-management-group/power-management/f/power-management-forum/549441/ucc28950-datasheet-page-59-equation-169

Thanks,

Conor

Hello,

Is Q7 relevant to the latest application note and data sheet?

Regards,

Hi Mike,

Yes, this question relevant to the latest application note.

https://www.ti.com/lit/an/slua560d/slua560d.pdf

Thanks,

Conor