Design with TPS40054

Many Thanks for your reply. Sorry, but I have some other questions too: 

1)       How can I understand the crossover frequency? And how can I calculate it? SwicherPro tell me 17,8KHz.

2)       I have calculated the f_LC and the f_ESR. (Equation 20 and 21). Now I will calculate the components of type III network: (C3, R3, C2, R2 and C1: with Equation 24 and 25)

But which values can I use for the poles and zeros fp1, fp2, fz1 and fz2?

Thank you.

Bizhan Behrouz

Thank you for your reply. Next question is about input caps:

I have found the following equation for calculate the input capacitor:

C= (I * Vo) / (deltaV * Vin * fs),

Where I= max. output current, Vo= output voltage,  deltaV= ripple voltage, Vin= min. input voltage and fs= switching frequency.

(http://focus.ti.com/lit/ug/sluu190/sluu190.pdf      page: 8)

Can I use this equation?

I have used 2 x 220µF, 50V (Panasonic EEEFK1H221P, http://www.panasonic.com/industrial/components/pdf/pic_fk_series.pdf )

But both capacitors are going warm. Do you have an idea, what can I do? May the problem be at the RMS current? At the same page you can calculate the max. of RMS current with following equation:

I= Iout * Sqrt (Vout / Vin_min)    In my design: I = 6 A * Sqrt ( 12V / 18V)= 4.9A

Best regards

Bizhan Behrouz

I have found the following equation for calculate the input capacitor:

C= (I * Vo) / (deltaV * Vin * fs),

Where I= max. output current, Vo= output voltage,  deltaV= ripple voltage, Vin= min. input voltage and fs= switching frequency.

(http://focus.ti.com/lit/ug/sluu190/sluu190.pdf      page: 8)

Can I use this equation?

Yes, you can use this equation.

This equation makes 2 assumptions:

1) No AC current is available from the source (infinite AC impedance)

2) Does not account for drop across the ESR of the input capacitor  (ESR drop across capacitor is ESR * Iout)

I have used 2 x 220µF, 50V (Panasonic EEEFK1H221P, http://www.panasonic.com/industrial/components/pdf/pic_fk_series.pdf )

But both capacitors are going warm. Do you have an idea, what can I do? May the problem be at the RMS current? At the same page you can calculate the max. of RMS current with following equation:

I= Iout * Sqrt (Vout / Vin_min)    In my design: I = 6 A * Sqrt ( 12V / 18V)= 4.9A

Yes, I believe RMS current is part of your issue.  The EEEFK1H221P is only rated for 670mA of current with a 180mOhm ESR.  Two in parallels can not support 4.9A.  I would recommend you use the first equation with a deltaV of 1/2 * Irate * ESR = 1/2 670mA * 180mOhms or 60mV to size low ESR ceramic capacitors to place at the power stage (Drain of high-side FET to Source of low-side FET)  This will limit the RMS current in the bulk capacitors to less than 1/2 their rated ripple current.

Ok, but I have already used 2 x 4.7µF, 5mOhm, 50V ceramic capacitors at the power stage (murata, GRM32ER71H475KA88L, Datasheet: http://www.murata.com/catalog/c03e2.pdf)  and they are connected from Drain of high-side FET to GND (also to Source of low-side FET) like suggestion in SwicherPro circuit too.

If you agree and it is really no problem for you, I can send you my schematic separately and other information, which you need.

best regards, Behrouz

I welcome your schematic if you wish to send it to me privately, but it's not necessary at this point.

When you use the C calculation above based on the ripple voltage I estimated, what capacitance do you calculate?

I actually realized I dropped the ripple waveform component to the RMS current.  It is roughly 1/3 of the peak to peak, so the actually ceramic capacitance required would be lower.

This capacitance will operate the electrolytic capacitor at 1/2 it's rated RMS current or 1/4 its rated power dissipation, which is safe, but will still feel warm.  (A 125C rated capacitor would be rated for 100C temperature rise.  At 1/4 power dissipation, it's still operating at 25C temperture rise or 50C temperature)

When you use the C calculation above based on the ripple voltage I estimated, what capacitance do you calculate?

C= (I * Vo) / (deltaV * Vin * fs) = (6A * 12V) / (60mV * 18V * 300kHz) = 222µF

2 x 220µF (electrolytic, Panasonic EEEFK1H221P) as bulk capacitors.

2 x 4.7µF (ceramic at the power stage (murata, GRM32ER71H475KA88L) in power stage.

Is it right? Do you have other suggestion?

But regarding temperature problem, I found out why the caps are going warm. Both electrolytic caps are located near to high side FET. The high side FET produce a heat above 80°C and the PCB are going warm. It was the main reason for the problem. In new re-layout I will use a new place for the caps.

Other questions:

1) The TPS40k Loop Stability tool: It seems to be an interesting tool and I tried to use this Excel file but if I enter any data in green cells, I get no results. The Analysis Toolpak is selected. What is the problem?

2) Output voltage: I have approximately 6.7V at output, if no load is connected. If I connect a 560 ohm resistor at the output, then the output voltage is approximately 12V. But do you mean, is this solution the right way or I must change the resistor divider from network compensation?

Best regards, Bizhan Behrouz

1) The TPS40k Loop Stability tool: It seems to be an interesting tool and I tried to use this Excel file but if I enter any data in green cells, I get no results. The Analysis Toolpak is selected. What is the problem?

I'm not certain what the problem is, but here are some possible actions to check:

1) Check to see if the data you are entering is in the form of a non-zero number. Many values are used in the denominator of calculations and zero values result in errors.  If you are trying to use zero current, look at your feedback dividier and calclate the feedback divider current, which appears as a load.

2) Do not enter units into the datafields.  Excel sees these as text and will not be able to perform math functions on them

3) Verify "Analysis Tool Pack is "checked" under Tools > Add-Ins

If none of those work, enter your data into the tool, save it and then attach it a reply to this forum post and I will take a look at it.  You can attach files using the "Options" tab at the top of the compose page (Compose / Options / Preview) or using the Insert Media icon (This is the second to last icon from the Right in the bottom row of icons at the top of the Compose text box.  It looks like a filmstrip with a + symbol)  The Insert Media icon can be used to insert multiple files of any file type into a post.  If it is a recognized graphic format, it will be displayed.  Other files types appear as clickable links.

2) Output voltage: I have approximately 6.7V at output, if no load is connected. If I connect a 560 ohm resistor at the output, then the output voltage is approximately 12V. But do you mean, is this solution the right way or I must change the resistor divider from network compensation?

Try placing an oscilloscope probe on the output voltage, I suspect that you have a loop staibility issue at very light loading conditions in discontinuous conduction mode due to the TPS40054's diode emulation.  You could try backing off the control loop gain a little for some more stability under this condition, you could add a pre-load if the impact on efficiency is acceptable or you could even reduce the feedback divider component values so the feedback divider sinks enough current to maintain stability.

It is also possible that at very light loading conditions there is insufficient low-side MOSFET conduction time to recharge the bootstrap diode and the high-side gate drive is dropping out under very light loads.  This can be improved by placing an external diode from BP10 to BOOST to provide a better current path to recharge the bootstrap capacitor.

Dear Peter, 

1) Regarding Loop Stability tool, please check the attached file. You can see my data. I’m not sure but I think that the Excel file has a bug.

2) Other question: Calculate the snubber components:

The SwitcherPro circuit uses a resistor and capacitor between Drain and Source of synchronous rectifier. I couldn’t find any information about these components in data sheet of TPS40054. But I found some information about snubber in TI forum and I made a summary. Please check the attacked file and give me your feed back that my summery is correct.

Thank you. 

Bizhan

Bizhan,

I've attached a screen shot of the file you sent me.  It is calculating component values and generating graphs as it is intended when I open it.  I am not sure why you are having an issue.  Could you take a screen shot of the file when you have it open so I can see what error you are having?

Regarding your snubber summery, everything is correct.

Equation 3a and 3b will simplify to: C + Cp = 4Cp or Cp = 1/3 * C

This will help you solve for Cp and Lsl more easily.

Dear Peter,

I use Excel 2003 German. I have checked the problem with other versions of Excel. Here is the result:

- It doesn’t work with Excel 2003 German.

It works with Excel 2007 German. But after opening the file, Excel tries to repair it. You get the same result with original file of TI.

- It works with Excel 2003 English. 

I’m not sure but I think that it’s a bug in the original file. I will use Excel 2007 and it’s ok. 

Best regards, Behrouz

Behrouz,

I am afraid I don't read German, but I asked one of my German counter parts to read the error message and suspect that the issue is a conflict between the script functions and macros used in the spreadsheet and the security settings in the German Excel 2003.  Check your security settings in Excel and see if they are set to allow VBA and ActiveX script execution

Dear Peter,

I have calculated the R_A and C_A on the page 10 of data sheet. The value of R_A is dependent on R_KFF and V_IN(min) and the value of C_A is dependent on R_A and f_SW. Is it right?

SwicherPro always calculates 243kohm and 470pF. But I don’t know why. Is it normal?

Thank you!

Equation 4 for C_A on page 10 of the datasheet has an error.

7.9 should be (8.0 - 7.9)

The equation should be C_A = I * t / DV

I is the current (8V - 3.48V) / R_A

t is the time 1/f_SW

DV is the allowable voltage change on C_A

R1 can really be almost any value.  The limitations are:

If R1 is very large, the leakage current into the FB pin will result in output voltage errors.  The leakage current is specified as 200nA maximum.  Rfb = 100k results in upto 20mV of offest.  10k results in upto 2mV of offset.

If R1 is very small, the compensation capacitors at the output of the error amplifier become large, slowing the large signal response and limiting the transient recovery of the error amplifier.  I typically recommend a value from 10k-50k.

In higher noise or lower bandwidth designs, this can be even lower.

Hi Peter

We are using TPS40054 in our design. Our input supply voltage range is 10V to 28V and output voltage is 5.2V with 2A maximum current.

We have designed a circuit and it is working fine at normal room temperature (+25C). As we reduce temperature to -25 degree C, SMPS stops functioning and both low and high gate drives are 0V. We observed that BP5 and BP10 voltage are proper. What could be the possible reason for such behaviour? Is TPS40054 tested for -45C as per specs?

In our design, we have disabled the voltage feedforward and UVLO mechanism by connecting 20K resistor between BP5 and KFF pin as per application note SLUA310. 

To replicate the same low temperature behaviour at  normal room temperature, we experimented with RKFF (20k) value. We observed that by increasing the value to 22K and above, SMPS stops functioning and gate drives are at 0V. Is this the expected behaviour? We are not able to make out the reason behind such behaviour from datasheet data? What is IKff current range at -25C?

Please provide your inputs on this issue at the earliest. If you need any additional inputs/circuit details, please let me know.

Regards,

Satyajit