Designing an LED strip with 12v supply but can be boosted to 48v. No more than 1.2 amps with 70 LEDs and max 35 watts. Other than power dissipation resistors would half watt LEDs work well on the strip?

 

The vf is 3volts so yes I know I cant put more than 12 in a series string but worried about the power consumption

 

Just to clarify I would have the LEDs in series but parallel strings, actually 72 total LEDs

 

The vf is 3volts so yes I know I cant put more than 12 in a series string but worried about the power consumption

If you put 12 in a string then it won't work off 12V

 

Design it for something like 9V then have a buck converter LED driver as the power supply.

 

Or you could even use an LED driver IC.

 

As long as it is a switch mode version, otherwise the lost power in series current control elements will add up to a lot of heat.

 

I was targeting a buck boost with 48v rails from diodes inc external nmos but I need to string a lot of them with LEDs in series for current control and possibly the strings in parallel from each other if that is a good idea

 

As long as it is a switch mode version, otherwise the lost power in series current control elements will add up to a lot of heat.

A power dissipation resistor or 2 per string I am thinking

 

As long as it is a switch mode version, otherwise the lost power in series current control elements will add up to a lot of heat.

I had in mind something like LM3402, ILD8150 or AL8862

 

I had in mind something like LM3402, ILD8150 or AL8862

It will be more than one strip so a controller board with one strip of LEDs and another board with just LEDs and resistors for thermals with pulsed battery lines for pwm so I want to disable the pwm signal. Also targeting amber and white dual diode at the moment. Non graded school project as volunteer for a drl/pos with indicator. Rigid flex boards other than the controller board which is just fr4

 

How should I string these out as anode and cathode lines from the harness? Also what adjustments need to be made for dual diode led? The vf will remain the same at the moment

 

Welcome to AAC.

Could you provide a bit more of a specification including a description of how the project will look when it is used?

I am finding it hard to understand why you have certain specific requirements without knowing the problem this is a solution to.

 

Welcome to AAC.

Could you provide a bit more of a specification including a description of how the project will look when it is used?

I am finding it hard to understand why you have certain specific requirements without knowing the problem this is a solution to.

The requirements are from the harness of an ev we are working on at a university. So that is my allotment of power I can use. I will attach a rough picture of it shortly. The headlamp design look

 

The actual power in each string of LEDs is equal to the current in the string multiplied by the sum of the voltage drops. The supply voltage for the rated light output must be at least equal to the sum of the forward voltage drops in the string. The LEDs will illuminate with a bit less voltage but not deliver the rated illumination. And for adding up the voltage drops, an approximation (a guess) will only provide an incorrect number. So unless the rated voltage drop is 3.000 volts, 4 LEDs in series with a 12.00 volt supply will not be very bright. Ith that many LEDs on a single circuit board, if you have a 48 volt supply, and if the LEDs are all the same part number, you can have the LEDs in series groups of 12 if the forward voltage is 3.5n volts, and use a series resistor for each group. Then all of the groups can be fed in parallel. The voltage drop on the resistor will be less than 6 volts and the current will be probably less than 100 mA, 0.10 amps, so the power should be less than 100 milliwatts. Not a huge amount of heat to deal with, but you should use one watt resistors. OR you could get away with 13 LEDs in each string and a lower resistance.
The point being that you do need to know the actual forward drop and the actual specified current. APPROXIMATIONS AND GUESSES WILL NOT BE CLOSE ENOUGH!!

 

You need to be far clearer in the requirements. You said you have 72 LEDs of 1/2W each.

But how are they supposed to work?

Must they be individually controllable, or in groups that are all lighted at the same time?

Do they require dimming, or just to be lit at one level of brightness?

You mention boosting the 12V to 48V so that you can put more in series. That is a good idea, assuming all in the series are lit at once. You could have up to 15 in a series string if the Vf is really 3.0V. But most white LEDs are more like 3.3 to 3.6V, so it may be fewer, maybe 12.

With 12 in series you would need 6 such series and 6 current controlled drivers.

It would be wasteful to boost first to 48V and the lower it to drive a string. I would make 6 boosters that are current controlled.

Now, if you need individual control or group control, that makes it more complicated.

You also mentioned using resistors to control current. That is not recommended at this power level. You need constant current drivers.

 

They will all be lit at the same time, was hoping to use 2 AL8871 bucks. one for each of the 2 channels

 

You need to be far clearer in the requirements. You said you have 72 LEDs of 1/2W each.

But how are they supposed to work?

Must they be individually controllable, or in groups that are all lighted at the same time?

Do they require dimming, or just to be lit at one level of brightness?

You mention boosting the 12V to 48V so that you can put more in series. That is a good idea, assuming all in the series are lit at once. You could have up to 15 in a series string if the Vf is really 3.0V. But most white LEDs are more like 3.3 to 3.6V, so it may be fewer, maybe 12.

With 12 in series you would need 6 such series and 6 current controlled drivers.

It would be wasteful to boost first to 48V and the lower it to drive a string. I would make 6 boosters that are current controlled.

Now, if you need individual control or group control, that makes it more complicated.

You also mentioned using resistors to control current. That is not recommended at this power level. You need constant current drivers.

I want to boost from 12V to 48V to account for LED vf so I can put more in each string. LEDs in series, strips parallel maybe

 

You need current control for each series string. Resistors would produce too much heat.

If you used fewer higher power LEDs, you could get away with one driver. That is the way I would go. 12 3W LEDs in series, and one current controlled boost converter.

 

Now I have seen the arrangement it looks like all will be lighted at one time. The first question is what is the basic source voltage? is it 12 or 48, as many EVs tend toward higher battery pack voltages. Does the light as shown need to be intensity controlled? As for DRLs, possibly??