A circuit to avoid thermal runaway in parallel LED strings

[Calina]

Avoiding thermal runaway when driving multiple LED strings A current mirror and monitor can save a parallel string from failure.

I just found the above article and thought some of you could be interested:
http://www.ledsmagazine.com/features/6/2/2

 

[Illum]

hmm, nice!

if you really want simple/cheap you could always set in a resettable breaker
I have a couple Raychem "Polyswitch" # RXE-065S-1 circuit protectors...they look like ceramic disk caps, constant current around 600ma...it'll trip when I run over 1200ma through them then reset when I turn the power off.

At $0.33/each...I can't really complain

BD139 - NPN Epitaxial Silicon Transistor Data sheet here

Fairchild Semi sells tham at TO-126 pkgs
ON Semi sells TO-225 pkgs available on digi-key here

 

[Calina]

That Raychem "Polyswitch" # RXE-065S-1 circuit protectors is not a regulator but it is certainy more convenient than a fuse.

I didn't know such small "breakers" existed.
It makes a small inexpensive protection system. Thanks!

 

[65535]

Could use a thermistor spliced into the driver cicuits POT or the current setting resistor. As it heats up the light would dim.

 

[Calina]

I don't think this would work, as the problem is not the total amount of heat produced but the heat at the individual die level.
The same is true with the resettable breaker but I like the idea that such a small breaker exist. It would be useful to prevent general circuits overload.

 

[baterija]

Very interesting article.

An interesting aside, from information in the article, comes from the mention of Vf varying by +- 20%. It puts comparing runtime graphs based on one sample of light into perspective when a constant current driver could be driving a single LED with +-20% difference in power.

 

[likeguymontag]

If you have enough LEDs per string you don't have to do anything special to ensure that the current balances. LEDdynamics says this is permissible with as few as three LEDs per string. From their datasheet for the BuckPuck:

Note that parallel strings of LEDs can be driven directly with no additional circuitry required to insure current sharing. The nature of the LEDs themselves will provide good current sharing if the parallel strings comprise three or more junctions each.

 

[Mr Happy]

If you have enough LEDs per string you don't have to do anything special to ensure that the current balances. LEDdynamics says this is permissible with as few as three LEDs per string.

Note that the referenced article disputes that claim with evidence to the contrary. Either you need to decide who to believe, or you need to do your own experiments to see what's true in your own situation.

 

[likeguymontag]

Note that the referenced article disputes that claim with evidence to the contrary. Either you need to decide who to believe, or you need to do your own experiments to see what's true in your own situation.

Huh, maybe I should actually read the article.

 

[Matie]

Parallel LED Current Balance Circuit and Resettable Breakers

Obsolete Post.

 

[Matie]

Parallel LED Current Balance Circuit and Resettable Breakers

I have a 60V 11A Adjustable C/V power supply CPS-6011.
Looking at the options for high voltage current balancing.

6x 18 LED's in series.
53.64v 1050ma per string.

Could you suggest a suitable IC package.
I would like an efficient design to carry forward with Fixed CC supply.

Found this LED balancing circuit for multiple strings.

LED balancing circuit

The current through R1 is the voltage V1 divided by the resistor R1.
The total LED current is approximately the sum of the current through the emitter resistors R1, R2 and R3.

The current through each string remains approximately equal, even if the LED voltage varies or if one is shorted. This is due to the relatively constant voltage of two reference diodes (D7 and D8, in series) will determine the current through the emitter resistors.

For example if LED (D1) in one string is open, the current through the string (D1 and D2) is zero. The base to emitter junction of Q1 behaves like a diode and a small amount of current passes through R1.

The reference voltage Vr drops, as to the other base to emitter junction voltages, and less current flows through the other strings.

I am having trouble with the math to choose components, if anyone can help out.

Reference: http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00145611.pdf

 

[hiuintahs]

I'll take a stab at it. I didn't take the time to read the application note but I do understand the schematic above. NPN transistors Q1, Q2 and Q3 are configured as "emitter followers". That means the emitter voltage will follow the base voltage. So the first thing to do is determine what current you want to pass through the LEDs. I'll show an example and then you can adapt it to your own design once you see how its done. And also adapt it to the other LED legs.

Let's say these are 5mm LEDs and you want to max them out at their 20mA rating. That means that the voltage at V1 is (R1 x 0.02). That voltage is also equal to the voltage across D8. The voltage across D7 is pretty much equal to the base to emitter voltage of the transistor and that cancels it out. Bipoloar NPN transistors have close to the same voltage drop across them as regular silicone diodes. Let's assume 0.7v. Therefore the voltage across R1, called V1 is the same as the voltage across D8 which would be 0.7v. R1 = (0.7v / 0.02) = 35 ohms. R2 and R3 would be the same. The voltage at the anode of D7 will be (2 * 0.7v) or 1.4v. Since the transistors have a gain of 50 or better, very little current will flow through the base of the them. Rr should be set high enough so that minimal current is wasted down through that leg of D7 and D8. I'd probably put 1mA down through those diodes. That should bias them to the 0.7v on state. That voltage will vary and you're going to have to adjust some values to get everything working the way you want...........but this should get you close. Lets say we put 1mA down through the diodes and 0.5mA left to bias each of the transistors such that they are well into the saturation point......meaning that they are turned on. So that means 2.5mA of current needs to flow through resistor Rr.

With this circuit Vin can vary and you will still get the same current through the LEDs provided that 2.5mA of current flows through Rr at minimum Vin. 5mm LEDs have around 2.0v drop across them. It varies depending on LED. But lets assume 2.0v for this exercise. When transistor is turned on all the way, it has a 0.2v drop between the collector and the emitter. So now we can calculate what the minimum Vin has to be: V1 + 0.2v + 2.0v +2.0V = 4.9V. Any higher voltage will just cause the collector to emitter voltage to take up the excess. Now maximum Rr can be calculated as (4.9v -1.4v) / 0.0025 = 1.4K ohms. You may need to lower a bit if our estimation on base current was too low.

To keep from wasting any more input power than you have to, try matching Vin to the minimum voltage needed for you to get results. That will keep less unnecessary power off of the transistors. With the current through the LEDs being held constant, all excess voltage will be dissipated as power across the transistors. You want to keep that to a minimum and within a range that satisfies your input voltage range if its battery powered.

 

[Matie]

Parallel LED Current Balance Circuit and Resettable Breakers

To keep from wasting any more input power than you have to, try matching Vin to the minimum voltage needed for you to get results.

Here is my attempt at the Maths.

108 XP-G3 LED's
6x 18 LEDs in Series.

XP-G3 LEDs = (2.74 Vf 150mA @25C) - (3.11 Vf 2000mA @25C)

Voltage Range = 49.32 to 55.98v
49.32v = .2 Transistor drop = 49.52v



BD139 Transistors have Maximum Collector-Emitter Voltage of 80.
Collector Current (DC) = 1.5A

If I Building in Voltage clearance and set Supply Current Adjust <1.5A it should be ok..?
The Minimum Voltage could be about 50v up to the Supplies Maximum of 60v!

Determine what current you want to pass through the LEDs.
NPN transistors configured as "emitter followers".

Work out Voltage at V1
V1 = Resistor1 x 1.5A

Bipoloar NPN transistors 0.7v drop across.
0.7v. R1 = (0.7v / 1.5A) = 0.4666666666666667 ohms.

The voltage at the anode of D7 will be (2 * 0.7v) or 1.4v.

1mA down through diodes should bias them to the 0.7v on state.
0.5mA to bias each transistors well into saturation point, turned on.

6 Transistors = 3mA current
1mA through D7/D8 Diodes + 3mA for Transistors.

4mA of current required through Rr.

Rr can be calculated as (50v -1.4v Diode drop) / 0.0040A = 12.15K ohms.

-----------------------

Looking for a suitable IC with 6 thermally connected Transistors.

Will need some help with diode part numbers and wattage rating for Resistors.

Thanks.

BTW: What do you think of this "Current Balancing Circuit" with reference separated from the LED's immediate supply?
Seems like a lot better design!

 

[hiuintahs]

Good job Matie. Looks like you're on your way to build a successful circuit. Those values will get you close and then you can fine tune as necessary. Use at least a 1 watt resistor for r1, r2, & r3 a possibly a 3w for more robustness. Power across the transistors will be LED current x the voltage from collector to emitter. Max transistor power will be at your max Vin. And they will have to be heat-sinked.

Another thought to keeping as much power off of the transistors as possible is to find the ideal (minimum) operating voltage for Vin that keeps Vce (the voltage from collector to emitter) to a minimum and then regulate the Vin to your LED strings to that value. Then the transistors won't have to be as beefy. That of course transfers the heat issue to the regulator circuit rather than the individual transistors.

 

[Matie]

Parallel LED Current Balance Circuit and Resettable Breakers

Another thought to keeping as much power off of the transistors as possible is to find the ideal (minimum) operating voltage for Vin that keeps Vce (the voltage from collector to emitter) to a minimum and then regulate the Vin to your LED strings to that value.

Not sure if I understand this correctly?

The Minimum usable voltage for string of 18 LED's is just under 50v.
Limiting the LED's to 1050mA would require 2.98v per LED or 53.64v Maximum, not a lot of variation.

Very happy to of come so far with this.

Grateful

 

[Matie]

Parallel LED Current Balance Circuit and Resettable Breakers

hmm, nice!
if you really want simple/cheap you could always set in a resettable breaker
I have a couple Raychem "Polyswitch" # RXE-065S-1 circuit protectors...they look like ceramic disk caps, constant current around 600ma...it'll trip when I run over 1200ma through them then reset when I turn the power off.

Resettable Breakers

I have a 60V 11A adjustable Power Supply and wanted to safeguard against someone adjusting to 11A and blowing up Current Balancing Transistors or LED's.

Anybody using these Polyswitches, do you have a known Source Current reference to give some real world experience.
A device like RXE-065S1 would be perfect If the trip Current stayed within the 1.3 to 1.053 Amp variation (20 to 40C).

But when the Data sheet mentions Min Hold Current is 0.65A and Max Trip Current is 1.3A there is a very large gap between.
So the question is, how tight is the Trip Current in real use.

I will have to buy some and check, but welcome any hands on experience!

Thermal Derating Curve
You will see below the Raychem RXE-065S1 trigger current changes 19% between 20 and 40C Ambient Temperatures.
The Littlefuse RKEF065 has very similar specs with tighter trigger Current variation of 13% over 20 to 40C Ambient Temperatures.

Time-to-trip
The other thing is time to trip in seconds, the RXE-065S1 is more like a Slow Burn Fuse at 5.3 Sec.

Which makes me think this might not be suitable for my purpose, but would protect from a slowly rising current?
The RKEF065 has a faster, 1 second trip time.
But would it trip before damage if someone dialed up the current on Power Supply.
Anybody know?
-

Here's some Specs for those who are interested, 40C Trip Currents are derived from graph - double check with data sheets before use.
-------


RXE-065S1 60V - Raychem

% of rated hold and trip current for Ambient Tempretures.
Thermal Derating Curve (81% of hold and trip @40C)

Hold Current (A) 0.65 @20C
Hold Current (A) 0.47 @40C

Trip Current (A) 1.3 @20C
Trip Current (A) 1.053 @40C

Time-to-trip 5.3 Sec
Time-to-trip with Fault Current of 10A = .1 Sec

Min Resistance 0.31 Ohm
Max Resistance 0.48 Ohm
---------------------------------

RKEF065 60V - TE Connectivity or Littlefuse

% of rated hold and trip current for Ambient Tempretures.
Thermal Derating Curve (87% of hold and trip @40C)

Hold Current (A) 0.65 @20C
Hold Current (A) 0.54 @40C

Trip Current (A) 1.30 @20C
Trip Current (A) 1.131 @40C

Time-to-trip 1.0 Sec
Time-to-trip with Fault Current of 10A = .09 Sec

Min Resistance 0.250 Ohm
Max Resistance 0.450 Ohm
---------------------------------

RKEF050 60V - TE Connectivity or Littlefuse

% of rated hold and trip current for Ambient Tempretures.
Thermal Derating Curve (87% of hold and trip @40C)

Hold Current (A) 0.50 @20C
Hold Current (A) 0.38 @40C

Trip Current (A) 1.00 @20C
Trip Current (A) 0.87 @40C

Time-to-trip 0.8 Sec
Time-to-trip with Fault Current of 10A = .06 Sec

Min Resistance 0.320 Ohm
Max Resistance 0.529 Ohm