Author Topic: High Wattage white led with LM2576 & variable intensity problem  (Read 1966 times)

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Offline Vindhyachal.takniki

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High Wattage white led with LM2576 & variable intensity problem
« on: January 04, 2017, 04:38:37 pm »

1. I have to drive a white led. Input voltage is 6V-8.4V. I need to vary the intensity using a potentiometer.

2. Using the ckt as in attached image. Not using optional output ripple filter. Ckt shows use of HV version, but I am using normal version of LM2576

3. Directly connected the outout to white led. Max output voltage set to 3.7V. Instead of using 1N5822, I have used SS34 diode.
R1 = 5K fixed
R2 = 10k(pot)
Calculated using formula:
Vout = Vref(1 + R2/R1)
Vref = 1.23V, R1 between 1K & 5K
By fixing r1=5K, R2 value comes out to be 10K


Problem:
1. When I rotate the pot from one end to another, led brightness smoothly decreased.
2. But when it is at max end, i.e output voltage at 3.7V, & slighlty rotate the pot in another direction, suddenly the output voltage drops to 3.05V  & there is noise starts from regulator also. Which causes sudden dim in led
And if further move down the pot, the voltage now rises to 3.5V & after that voltage decreases smoothly.

This sudden drop/noise problem only occur when I slightly drop the value from 3.7Vout.

 
 

Offline DmitryL

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #1 on: January 04, 2017, 04:51:06 pm »
In brief:
* LED needs to be driven with a current, not voltage. 
* build a proper LED driver that supplies a given current, not a given voltage.
 

Online Zero999

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #2 on: January 04, 2017, 10:22:52 pm »
In brief:
* LED needs to be driven with a current, not voltage. 
* build a proper LED driver that supplies a given current, not a given voltage.
Yes. The LM2576 isn't suitable for this. I started designing a circuit using the LM2576 with a transistor to implement the current limiting, then decided it was a bad idea because: the only way to dim it is to PWM the shut down pin which might not be fast enough to eliminate flicker, the current regulation would be poor and it might be unstable.

EDIT:
I missed a pull-down resistor from the BJT collector (pin 4 on the LM2576) to 0V.
« Last Edit: January 05, 2017, 08:33:57 am by Hero999 »
 

Offline strawberry

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #3 on: January 05, 2017, 12:23:34 am »
Add bipolar tranzistor paraled it to R2 and feed load current to it.
 

Offline mariush

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #4 on: January 05, 2017, 03:32:36 am »
LED Drivers dude...

Example: 
LM3405AXMY/NOPB  Led Driver, 1 Output, Buck (Step Down), 3V-22V input voltage , 1.6MHz switch, up to 20V/1A out, HVSSOP-8 : http://uk.farnell.com/texas-instruments/lm3405axmy-nopb/led-driver-buck-1-6mhz-hvssop/dp/2492180
Can control current through feedback resistor ( recommended for at least 200mA and up), and through pwm / enable pin when below 200mA

ON SEMICONDUCTOR  NCL30100SNT1G  Led Driver, 1 Output, Buck (Step Down), 6.35V-18V in, 700kHz switch, 1.2A out, TSOP-6  : http://uk.farnell.com/on-semiconductor/ncl30100snt1g/ic-led-driver-buck-cntrl-6tsop/dp/1830588

ON SEMICONDUCTOR  NCP3065DR2G  Led Driver, 1 Output, Buck, Boost, 3V-40V in, 250kHz switch,1.5A out, SOIC-8 : http://uk.farnell.com/on-semiconductor/ncp3065dr2g/ic-dc-dc-converter/dp/1652395

TEXAS INSTRUMENTS  TPS92512DGQT  LED Driver, Buck (Step Down), 4.5V to 42V input, 557 kHz switch-freq., 40V/2.5 A out, HVSSOP-10 : http://uk.farnell.com/texas-instruments/tps92512dgqt/led-driver-buck-557khz-hvssop/dp/2471141

TEXAS INSTRUMENTS  TPS92691PWP  LED Driver, Buck-Boost, Flyback, SEPIC, 4.5V to 65V in, 700 kHz Switch. Freq., 65V/5A out, HTSSOP-16 : http://uk.farnell.com/texas-instruments/tps92691pwp/led-driver-mutli-topology-htssop/dp/2522587

etc etc ....

 

Online Zero999

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #5 on: January 05, 2017, 08:41:31 am »
If you must use the LM2576, then your best bet is to set the output voltage to 300mV or so above the LEDs' maximum rating and power the LEDs from a low drop-out linear current regulator.
 

Offline Vindhyachal.takniki

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #6 on: January 07, 2017, 08:04:03 am »
At 3.7V, where led works ok, is around 300mA as checked by digital multimeter.
I will place a 1ohm series resistor & check if it resolves the problem.

Otherwise I will use PWM ckt.
 

Offline mikeselectricstuff

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #7 on: January 07, 2017, 08:19:14 am »
As others have said, you're doing it wrong. You need to be regulating current, not voltage.
Most voltage regulators can be used as current regs by putting a current-sense resistor in the -ve leg of the load and connecting the reg's feedback pin to the top of this resistor, however the typical 1.2 or 0.8v reference voltage of most voltage regs makes this inefficient due to the loss in the sense resistor. purpose-designed LED drivers typically have a reference of about 0.1v to reduce losses in the sense resistpr
 
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Mike's Electric Stuff: High voltage, vintage electronics etc.
Day Job: Mostly LEDs
 

Online Zero999

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #8 on: January 07, 2017, 06:50:16 pm »
As others have said, you're doing it wrong. You need to be regulating current, not voltage.
Most voltage regulators can be used as current regs by putting a current-sense resistor in the -ve leg of the load and connecting the reg's feedback pin to the top of this resistor, however the typical 1.2 or 0.8v reference voltage of most voltage regs makes this inefficient due to the loss in the sense resistor. purpose-designed LED drivers typically have a reference of about 0.1v to reduce losses in the sense resistor
One way round that is to add an amplifier across the sense resistor to reduce the voltage across it but that may cause problems with oscillation. I agree, it's a bad idea, just use a proper constant current LED driver IC.
 

Offline Vindhyachal.takniki

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #9 on: January 12, 2017, 12:09:24 pm »
THis is one circuit, I found in old local market which works fine.
It works exactly fine, on varying pot value, led intensity varies accordingly.
Which IC it could be:

1. Pin1: Input
2. Pin4: Ground
3. Pin7: led output conected

Its not LM2593, since LM's 7 pin is N_SS
I am making its reverse ckt right now.
 

Offline Vindhyachal.takniki

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Online Zero999

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #11 on: January 12, 2017, 11:19:47 pm »
If it works fine, then why not use it?

There doesn't appear to be a part number on the IC.

If you want to build your own LED driver, then why not use the correct IC? All the main semiconductor manufacturers make them. Look at the links below:

http://www.linear.com/products/led_driver_ics
http://www.st.com/en/power-management/led-drivers.html
http://www.ti.com/lsds/ti/power-management/led-driver-overview.page
 

Offline jazer

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Re: High Wattage white led with LM2576 & variable intensity problem
« Reply #12 on: January 14, 2017, 02:25:24 am »
Slightly off topic, but what's the general experience with LM2576 robustness? I've killed one inadvertently shorting the output briefly. They're supposed to handle that according to the specs. Did I just get a bad counterfeit or something?
 


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