E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

[crepiduse]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

Considering all I have is the main board and the led strips, no seperate driver, how would one test?

[budm]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

The LEDs (I do not see any current limiting resistor on the strips) are connected in the series from what I can see, the return side of the LED (Cathode) is connected the DRAIN pin of the power MOSFET driven by PWM signal, the Source pin is connected to the Current sensing resistor, this junction point is fed to the LED driver IC to monitor the LED current to maintain constant current through the LEDs, another end of the sensing resistor is connected to return GND of the boost converter power supply.
If the 10 LEDs in series then the total Vf is about 33V.
BTW, if you have only one strip connected only, do you get any lights action?

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

I would think he would have to jumper the output connector of the first strip to make a complete circuit. However if the sensing resistor did not knock down the 33 volts he would have to much current going through the five LED's.

[budm]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

The Voltage is not really regulated, the current is regulated to maintain constant current (for steady lights) as the LEDs active resistance varies due to for example the temperature, the MOSFET and the sensing resistor and the feedback are used to maintain the current.

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

This brings me to another question for BudM. Will the sensing resistor cause the unit to shut down if it gets to much current or will it just throttle down the voltage until the current is in the correct reading? If it just throttles down then if a person could not find a LED for the strip one could just jumper the open LED.

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

I read that with the PWM and the drive circuit the duty cycle and the power mosfet inside the drive chip varies causing the supply voltage to be less for the LEDs. In this way they can dim for the ambient light.

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

First the PWM at a giving voltage changes it duty cycle which lowers the DC current over a given time. Then the other example is the Chip that has the power mosfet inside conducts less which is controlled by the feed back circuit to the CPU and shuts down the available voltage.

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

I believe they are changing the voltage to regulate the current. Instead of using the current to regulate the current. If the latter was the case the voltage would remain the same and the current would change by supply more current for a given voltage so if more LEDs were added the available current would increase and not the voltage.

[budm]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

For current to be maintain at constant, the active element in the circuit has to be constantly adjust due to change in the load resistance so basically the Voltage across the load will varies with the change of the load resistance to maintain constant current, in this case the MOSFET is the active element that is being used to maintain the current and the sensing resistor is being use to measure the current through the circuit.
As you can see that when we look at the Voltage output to the LED, the output Voltage is way high due to no load, if it is Voltage regulated, the Voltage would have been constant, so at no load or at high resistance, the output voltage will go up to try to force 100mA or what ever the constant current should be so it max out the output Voltage.

For example, lets say that you have 1A constant current power supply with 1 Ohm load, now if the load is changed to 2 Ohms, then the output voltage must go up to 2V., but if the maximum power supply output Voltage is only 10V, so when you put 100 Ohms load on it and it tries to force 1A through 100 Ohms, that will never happen since it can only put out 10V so the output will show only 10V (MAX OUT) and only 0.100A will be flowing through the 100 Ohms.

[budm]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

When you have two parallel load connect to the constant current source, if one load is open the power supply will try to force the current the existing 1 load.
I.E, two loads, 100mA constant current source, 50mA per load, if one load goes open, the power supply will try to force 100mA through 1 load that is still connected since its job is to maintain 100mA through what is ever connected to it.
You can connect variable resistor to output of the LED drive to see what range of the current regulation it has, and you will be able to see the relationship between the output voltage and the load, for example, some people will bypass the bad LED in the string, if it is well regulated current source then it should function fine, I will need to do the experiment myself to observe the results.

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

So with the constant current in parallel. One would have to have a sensing resistor to each LED circuit to regulate total current in all branches and the cost would prohibit that from happening. Or one could monitor the total voltage in the parallel circuit. If the voltage increased that would mean that one of the LEDs went bad and the CPU could adjust for that and one could also monitor the total current to see if one of the LEDs shorted out and then shut down the circuit. So say if each LED was 10 ohms of resistance. 2 LEDs would be 5 ohms in parallel. And 3 LEDs would be 3.33... Ohms. Even if the LEDs did not behave like resistors one could put a limiting resistor in line with each LED to achive this effect. Then it would be possible to parallel LEDs with constant current.

[budm]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

In the parallel situation, the LED driver circuit goes into shutdown if one of the parallel string goes open circuit that is what I have seen in the panel with parallel connection (it is not really directly parallel since each string has its own MOSFET and sensing resistor).
You cannot really parallel the LED directly since not all Vf are the same, the one with lower Vf will light up but the one with higher Vf may not light up or barely light up, that is why you use resistor with each string of the LED you are doing simple parallel LEDs connections.
Some of the panel that has dynamic contrast will have like 8 drivers for the LEDs so they can do local dimming.

[keeney123]

Re: E320-A0 - Power LED Fades, Sound, Picture (Without Backlight) LED Issue

Thanks BudM for the info