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Topic: Ideas for automotive constant current LED driver (Read 3469 times) previous topic - next topic

Ideas for automotive constant current LED driver

Hi all,

I was recently asked by a colleague to build a LED driver for the fog lamps in his bike. The LED is 10W and the supply is 12V from the SLA battery. The LED pulls around .95A @10.5V, But we plan to operate it at ~0.5-0.6A owing to a small heatsink in the lamp casing.

I've tested couple of designs,
1. A transistor current source similar to http://http://www.edn.com/design/led/4363924/Transistors-drive-LEDs-to-light-the-path. This needs a very low headroom and works well upto the maximum rated LED current. But the power dissipation in the transistor is high and this needs a separate heatsink in the lamp casing which is difficult to accommodate.

2. I also tested a constant current source with MC34063, But this suffers from reduced headroom. I can only get about 0.3-0.4A with this.

Can someone suggest other solutions for this application?

Since this is for automotive application, I figured it needs a good input protection. What I have in mind is a 40V MOV at the input followed by an LC filter and protection diodes for polarity inversion. Please suggest other methods commonly used.

Thank you

Re: Ideas for automotive constant current LED driver

Reply #1
You can use something like NCP3170 and use a current feedback instead of a voltage feedback using a current-sense resistor in the load line to ground. The important parameter is to find a buck converter that has a low feedback voltage on the error amplifier to minimize the loss in the current sense resistor for the feedback. With 0.8V of the NCP3170 at 0.6A you'd dissipate ~0.5W there.

A simple alternative is to add a small 1:2 to 1:4 opamp amplifier so you can reduce the dissipation in the sense resistor. That means that a lot of controllers become available (most have a 1.25V or 1.275V feedback line). However, such amplifier needs to be of relatively good bandwidth and good phase linearity and stability for the control range.

 

Re: Ideas for automotive constant current LED driver

Reply #2
Look for "mr16" led drivers on ebay, dealextreame, or other china electronics source.  The rectifier diodes may be bypassed to reduce voltage drop, and dimming may be set with a resistor or pwm source.  I've gotten 10 for less than $2 on ebay.

Re: Ideas for automotive constant current LED driver

Reply #3
[quote author="blarson"]Look for "mr16" led drivers on ebay, dealextreame, or other china electronics source.  The rectifier diodes may be bypassed to reduce voltage drop, and dimming may be set with a resistor or pwm source.  I've gotten 10 for less than $2 on ebay.[/quote]

Do you mean this one ? http://http://www.ebay.com/itm/1pcs-Power-Driver-for-1x10w-LED-Light-Lamp-950mA-12V-MR16-/121080902070?pt=LH_DefaultDomain_0&hash=item1c30fbf1b6
My colleague already had one of these that came as part of the lamp. One of them blew up completely. So we thought there wasn't sufficient protection to withstand the surges that are there in power rails.

In the mean time, I tried a few more designs, One of the designs (http://http://www.eeweb.com/blog/extreme_circuits/switchmode-constant-current-source) using LM2576T-ADJ seems to work well. LM2576 has a fairly low dropout voltage and during my tests I was able to easily get ~0.6A through the LED. I had assembled this on a breadboard.

Can anyone explain what are the advantages/disadvantages of running switching converter where the output voltage is ~90% of input voltage?

Thanks

Re: Ideas for automotive constant current LED driver

Reply #4
[quote author="aramosfet"]Can anyone explain what are the advantages/disadvantages of running switching converter where the output voltage is ~90% of input voltage?[/quote]
It is a disadvantage because the switch is in the "on" state a lot of the time. That means that the dissipation of the switch is a major concern and source for losses. The second disadvantage is that your region of control is smaller. The startup takes longer and you risk controller shutdown if you cannot come up to the target voltage (or current with current feedback) withing a reasonable time, but that depends on the controller you use.