Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

[ngth82]

Good morning bad caps -

I have an Insignia NS55D510NA17 TV that I bought for $20. The there are 50 backlights in the TV and 48 or so are BLUE and this appears to be due to the yellow film/coating breaking down on each individual LED. (See picture; camera picks this up as purple... they are really blue). The LED itself looks to be an exposed die. There is no plastic package for them. I couldn't find much information about the LEDs or a direct discrete replacement either. These come from the LED strip - SVH550AC3_5LED_REV05_150410.

Anyway, I would like to replace these LEDs with 1W (3.3V @ 300mA) Samsung LEDs that i have on hand. The power board is a Hisense RSAG7.820.6106/ROH. The output specs is 170V @ 700mA ( 3.4V @ 700 mA each == 2.4W).

The LED backlight controller is a MSC LX27901ID. The datasheet is here from the Microsemi website. What i can't find is application information on how to reduce the output current to the LED string. The datasheet references the BRTA pin as the "terminal for programming the analog LED current setting. An analog signal is accepted from this pin to set the reference level for the LED current control loop" -- but there are no equations available in the datasheet that specifies the calculation. I was also hoping to drive the LEDs at a lower voltage, but the datasheet doesn't have a good equation for that.

I'll do some component traces and share the circuit here today.

Maybe i should file this under the "I could spend my time working on something else" projects. I could have just purchased a new set of LED strips for $65 or so from ShopJimmy... but I figure lets just mod the strips and the SMPS instead.

Thank you,
Thomas

[budm]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

"The output specs is 170V @ 700mA ( 3.4V @ 700 mA each == 2.4W)." That is not how you calculate it.
If you look at the LED terminal it looks like there are two LED strings so the current will be spitted between the two LED strings, and then you will also have Vdrops (15 ~ 20v) on the MOSFET that is for maintaining the constant current through the LED strings.
Did you verify that the original LED's are 3V or 6V LED's?
Dis you also verify the foot print of the original LED to make sure that the large pad for heat dissipation matched your replacement LED?

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Hi Budm -
I had some time to look at this again today.

I looked at the board and the datasheet application circuits again. The power board has the same markings and stickers as this one on shop jimmy: https://www.shopjimmy.com/insignia-1...-led-board.htm

The board is configured to use both AOUT and BOUT - Similar to Figure 18 in the datasheet. COUT is a n/c.

Regarding your questions from the other day:

If you look at the LED terminal it looks like there are two LED strings so the current will be spitted between the two LED strings, and then you will also have Vdrops (15 ~ 20v) on the MOSFET that is for maintaining the constant current through the LED strings.

Ah yes, looking at the application circuit more closely - there are two strings. The board provides 170V to both strings. I soldered wires to the back of the LED string connector and measured them simultaneously. Please see the attached scope shot for string 1 and string 2 connected to CH1 and CH2 of the scope, respectively.

Did you verify that the original LED's are 3V or 6V LED's?

Sadly, no i didn't before removing them from the strips. But looking at the configuration per above, I would assume a 6V to be more correct than a 3V LED.

Dis you also verify the foot print of the original LED to make sure that the large pad for heat dissipation matched your replacement LED?

The original LED is tiny - just a bare die with no plastic. There are no LEDs that i could find that match this - nor would i want to rework these small ones as-is. The pads were MAYBE just 1mm wide by 2mm.

I removed the white soldermask from each LED location to expose the top side copper of the positive and negative. I put some Kapton tape in the middle to ensure the positive and negative sides of the target LED don't short out. I've done this before with other incompatible pads. This one won't be driven past 70% since I'm just going to use it as a second garage TV.

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

I’ll see about finding the LEDs that I popped off. I’m sure a few of them are still in my work bench / solder debris bin then characterizing them to the point of failure.

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Update: TV fixed.

In post #3, I said that the SMPS has two outputs for two strings at 170mV and presumed the backlights were wired as two independent strings.

In reality, the backlight is wired as a SINGLE string while the SMPS has two current balanced outputs.

As previously noted, the original LEDs are bare die and tiny. I found 6V LG LEDs that are substantially larger and modified the physical LED strip by removing white solder mask to expose more copper - i used a Dremal and a wire brush to make quick work of this. Kapton tape was added between the positive and negative legs to prevent a short. I added solder paste to the new "pads" and reflowed the LED with a heatgun from the bottom. After that, i checked both pads under a microscope to ensure there was sufficient solder fill. I added more solder paste and reflowed it again if there wasn't.

I rewired the LED strips as two separate strings. 25 LEDs per string at ~6V per LED. Measured current is 350mA per string after I completed the modifications. The backlights completed an hour long burn in test. They worked fine at full brightness.

The TV was reassembled and works fine. Looks like I have a second garage TV going up soon!

The LEDs that i purchased are as follows: https://www.ebay.com/itm/223494097714

-Thomas

[budm]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

So if it has 50 6V LED's connected as one string, the total Vf will be 300V, but power supply is rated at 170V then it cannot be one LED string.
If you have two LED strings with 25 6V LED's then each string will have 150V for Vf + about 20V headroom Vdrops for the PWM control MOSFET.
You can also see that the Cathode return 1N and 2N are tied together, if the LED strings are connected in series then the 1N would have been connected to 2P.
So something is not adding up.
As far as reducing LED current, read ISNS page 5 and 15 of the spec sheet, if you increase the value of the resistor (R100 in application circuit, it shows 0.40 Ohms but your board will be different) the LED current will decrease but it has to be above 100mV and below 600mV. See fig. 19 page 14.
I would reduce the current down to around 200 ~ 250mA max.

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Hey Budm -

Maybe a picture is worth a thousand words.

The top half shows the physical LED topology from the factory. 50 LEDs as a single string. 170V output max and lets assume ~20V drop across the MOSFET (i didn't measure it though and the TV has been reassembled now). That would be 3V LEDs that were originally installed.

Also in one of the earlier posts i said i didn't measure the LED voltage before popping them off the strip. Also, my original assumption was it was two physical strings but that was based on looking and measuring the output from the power SMPS only without looking at the physical connectivity of the backlights, by this time i had already replaced the LEDs with 6V LEDs.

The bottom half of the image shows what i did to the physical LED string connections after installing the 6V LEDS -- I made it as two separate strings - 2 strings of 25 @ approx 6V each. The measured current through each string -- approximately 350mA.

[budm]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

But if you look at the board connector the 1N (Cathode of #1 string) and 2N (Cathdode of #2 string) are connected together, the 1P and 2P are not connected together but connected to separated drive circuits which you can see there are two grey caps (C922/923) one for each string.
350mA is still way to high.
What is the spec of the 6V LED? That will be power dissipation of around 2.1W @ 350mA! I doubt that your LED is rated that high.

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Hi Budm -

To answer a few questions.. i had 30 minutes between calls to take a quick look.

• The implementation uses AOUT and BOUT out of the controller. I think it's closer to Figure 18, not Figure 19 -- which only uses COUT.
• Up close shots of the board are located on ShopJimmy - https://www.shopjimmy.com/insignia-1...led-board.htm#
  
• The LED that I bought was from eBay -- this item: https://www.ebay.com/itm/223494097714 I haven't characterized the curve on it yet. (Probably should have from the beginning... can't go back in time.). I'll install it on scrap LED strip to test.
  
• The output of the string really is 170V at full brightness. No drop across a FET.
  
• Yikes, agreed with confirmed measured values: 6.8V across each LED at 350mA is going to burn it up. (2.38W!)
  
• I'll take a closer look at the board and lower the current down to 250mA... at first glance looks like R944/R945 (they are in parallel) would be the correct resistors to modify.
  

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Scratch the last bullet point. R944/R945 is in parallel and is connected to the 18V output to towards the main board. I'm ohming the SMPS board out now and will provide a markup image tracing the backlight inverter circuits.

Thanks,
Thomas

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Alright, i think i have this mostly traced out and have a working solution

The first image shows the bottom side of the board markup. I tried to find the ISNS components from Pin 10 of the IC. I used Figure 18 in the datasheet as a reference for this portion.

Starting at pin 10, I found a cap immediately "north.". There is also a trace that goes under the backlight IC and veers off to the right, through a 0-ohm resistor then has four pads for resistors. Only the left most pad is populated - a 2kOhm resistor with SMD marking "2001" on top.

If you follow the orange line to the left, there are five parallel resistors. From left to right it's 2.2 Ohm, 5.1 Ohm, 5.1 Ohm, 2.2 Ohm and 2.2 Ohm... 0.57 Ohm equivalent resistance. With this circuit as-is, i measured 350mA across a string of 25 LEDs. ~170V across all 25.

I removed the left most 2.2 ohm resistor (R942). The new equiv resistance is ~0.768 Ohm. At full brightness, the measured forward current is 260 mA, ~166V for the string ==> 6.64V per LED or 1.73W each.

I think this is much more in the comfort range. I'll bump it down a little more. With the linear trend line, i should be able to hit 225mA or so around 0.85 Ohm.

This may be a tomorrow thing. With COVID-19 and the order to stay at home, we're taking advantage of the time for family game night.

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Finished. I made a second mod to the SMPS.

I changed the effective resistance of the ISNS resistor to GND equal ~0.91 Ohm (nominal). In all, i removed a 2.2 Ohm and a 5.1 Ohm resistor from the back side of the SMPS. (The left most two resistors as shown in the first image in the previous post)

This set the string voltage to 163V, and current to 222 mA per string at full brightness. 6.52V * 222mA = 1.45W per LED.

When I set the brightness effectively down to "0" - The avg PWM voltage on the DMM is 132V @ 29mA.


Moral of this story should go: check the darn LEDs carefully before removing them in a rush and double check the assumptions of the SMPS and the LED string configuration. I could have saved myself a lot of headache and time from the beginning.

I'm glad it works and I can hang this on my garage wall knowing it'll work.

As always, thanks for the feedback @BudM

[ngth82]

Re: Modifying Insignia NS-55D510NA17 SMPS for lower current LEDs

Here's the Vf vs If chart for the eBay 2W LEDs for future reference.

For the measurements, this was mounted on a LG compatible footprint LED strip.

Purchased here March 2020: https://www.ebay.com/itm/223494097714