LED Technology
Keeping thermal runaway
under control A
Paul Cheeseman looks at how best to avoid thermal runaway when driving multiple LED strings
n increasingly common method of increasing the light output from a high power LED cluster is to run parallel strings of LEDs from a single constant current source. But this option is not without its hazards.
A typical high power 350mA white LED has a forward voltage (Vf) of about 3.3V, so if a cluster of ten LEDs were required in an application, connecting all the LEDs in series would require a driver capable of delivering at least 33V. If the supply voltage is 24VDC, then an expensive boost converter would be required with all the attendant EMC problems that can create. Connecting the LEDs as two strings of
five wired in parallel requires a higher current 700mA constant current source but only 16.5V output voltage. Thus a low cost buck converter running from 24VDC can be used. The circuit similar to that shown in
Figure 1: Driving multiple LED Strings - a theoretical solution
batch and sequentially manufactured, the Vf of individual LEDs still has a ±20% tolerance. The tolerances mean that the total forward voltage for each string can be very different and therefore the current mismatch significant (see Figure 2). In a test using identical SMD LEDs from a single production batch and using 1 Ohm resistors to help balance out the forward voltages, the currents flowing in each string were measured to be 306mA and 394mA. The LED driver was still doing its job of correctly limiting the current to 700mA, but the over-current flowing through the second string was seriously overdriving the LEDs. Worse, as the LEDs started to get warm, the combined forward voltage of the higher current string started to decrease. This increased the imbalance and more current started to flow through the already
over-driven string. The current through the other string of LEDs reduced as the constant current driver compensated, so they started to cool down and their forward voltage increased. The net result was thermal runaway with
Figure 2: Driving multiple LED Strings - Real life situation (not recommended)
And this circuit is often given as a
recommended application example! What is required is a way of balancing
the currents flowing through the two strings to ensure that they remain approximately equal, even if the combined
Figure 1 can be found in many manufacturer’s datasheets.
The basic assumption made with this
circuit suggestion is that the 700mA regulated current from the LED driver will be shared approximately evenly across both strings of LEDs, i.e. each string of LEDs will see 350mA of current. However, this is rarely the case. Even if the LEDs are all from the same production
12 February 2012 Components in Electronics
the majority of current flowing through one string only, even though the LEDs were mounted on a large metal heat sink. The test was stopped when the current imbalance was 600mA to 100mA. Obviously, if this situation was allowed to continue, the over-driven string would eventually fail and then the entire 700mA would flow through the remaining intact string and destroy that as well.
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