TPS92310: Ripple output at the output..

Hi,

The Led been used is "MLCAWT-A1-0000-000XE5" , we have total of 21 LEDs been connected in 7 series and 3 parellel string combination.

Due to ripple voltage(apx 5Vpp) at output we see flicker at LEDs. Also from the datasheet in page 17 not sure what is 30% of ripple talking about, if it is 30% then how about the flicker.

Rgds

Chandra

Hello Chandra,

The datasheet for that LED shows it as being a two die LED, are you connecting these in series or parallel?  Are you using both die as one LED or each die as the LED in your design?  What is your output current to the LEDs?  To calculate the dynamic impedance I need to know how the LEDs are configured for the output and the output current as well as if you are using both LED dies as one or individually.  Or an easier question, are you using 11 MLCAWT LEDs or 21?

For example I calculated the LEDs operating near the maximum specification:

If one die of the LED is used as a single LED in your 7 series, 3 parallel configuration the output current at maximum for the LED specification would be 525 mA and the dynamic resistance would be about 9.3 ohms.  This would use 11 of the MLCAWT led packages with one LED die not being used (11 LEDs X 2 die = 22 die minus one to get to 21 die)

If both die of the LED is used as a single LED in you 7 series, 3 parallel configuration the output current at maximum for the LED specification would be 1050 mA and the dynamic resistance would be about 4.7 ohms.  This would use 21 of the MLCAWT led packages.

The equation is for setting the LED current ripple to 30% of the average output current (the calculated DC output level).  This number can be changed to fit your design, if you reduce the ripple percentage the output capacitor will get larger.  What are you expecting for LED ripple percent?

It would be helpful if you could measure the LED current with an oscilloscope.  It would also be helpful to know if the TPS92310 is operating correct.

Regards,

Hi,

Sorry for the confusion created, I have 7 LEDs in series, as below. 

Rgds

Chandra

Dear Irwin,

Yes 50R means 50 ohm resistor. They are current limiting resistors. Our goal is to have 200mA current.

Rgds

Chandra

Hello Chandra,

Each 50 ohm resistor at 200 mA will have a 10 volt drop across it.  Two 50 ohm resistors will have a total of 20 volts drop, they will also need to be rated for at least 2.0 watts of power dissipation.

The LED string of seven dual die LEDs will have a total voltage across the LEDs of 24.5 volts.  Add the 20 volts from the resistors the total string voltage will be 44.5 volts.

The dynamic impedance of the string not including the resistors will be approximately 18.2 ohms, with the resistors it will be 118.2 ohms.  A 470 uF capacitor across this should have very little current ripple so something else must be happening if you visually see flicker.  I assume the output capacitor is tied across all of the LEDs and the two resistors? 

Also, if 200 mA is run through the seven LEDs and two resistors the efficiency of the string only (not including the power supply) will be 55%, 45% of the power is being lost in the two 50 ohm resistors.

Perhaps the added voltage drop from the resistors is causing OVP protection to kick in (44.5 volts)?

The TPS92310 should be able to regulated the current in the LED string without the 50 ohm resistors.

Using the Cout equation without the 50 ohm resistors the output capacitor for 30% ripple would be 413 uF, with the two 50 ohm resistors 64 uF.

Regards,

Hi Irvin,

Last week I was not able to spend time on this issue.

We checked with current probes current is switching. I have more hints, When we probe across output diode we see transformer output switches on and switches off may be due to it see over voltage or VZCD,

Below is the waveform,

Most importantly, we do not have any snubber circuit, does it play any role. In TI EVM it is suggested as optional(C2 and R3) we are not sure why 

In additional to above board is tested with 7 LEDs in series

Rgds

Chandra

Hi Irwin

if you have seven three volt LEDs unless the current is much too high. I recently found out that your resistors are not 50 ohm but are 1 ohm, is this true? If the 50 ohm resistors are still there that will be a problem, they should be shorted or reduced to a low value that doesn't cause a large voltage drop. -- yes it is shorted.

Are you using transformer 750341087 with 7.615:1 ratio? --> yes we are using same.

Below is one more waveform together with Output voltage and transformer output,

Rgds

Chandra

Hi Irwin,

Attached the xls with calculation for our design, one more observation is from xls we get,

ZCD pin resistor R10(In our case R65) 643 kΩ(In EVM and our case 66.5K)

ZCD pin resistor R12(In our case R58) 13 kΩ(In EVM and our case 11K)

TP9S92310_Calculation_tools__REV1 3a.zip

Rgds

Chandra

Hi Irwin,

In one of the board we changed the value of R65 to 499 and R58 to 30K and added RCD snubber(R=SMF3W150KJ, C=1812B102K202CT, D=1N0007), now we are able to see constant voltage and no hiccup of OVP , also we see expected current output for 7 series LEDs.

But now big problem started, We see U15(TPS92310DGS), R55(3 ohm resistor), Q1(STD3NK80ZT4) getting damaged in some of the boards even with the some of working boards also. We are sure rework is good.

PS: we observer this type of behavior(U15, R55, Q1 getting damaged) even without snubber circuit when we removed R50(Current limiter) and added 0.5 Amps fuse in that place. We removed R50 and tried with fuse because this resistor was frequently getting damaged.

Rgds

Chandra

Hi Irwin,

Sorry it is 499K Ohm typo error.

what are your resistor values in series with the LEDs, I saw 50 ohms and someone stated they were changed to 1 ohm? --> I already mentioned that 50 Ohm is replaced with Short.

Also, We expect the Vout of 23V to 24V becuase 7*3.2V(LED VF) = 22.4 V

Rgds
Chandra