High Power DC/DC LED Applications

The solution to this problem of variability in the forward voltage, V1, is to use a constant current rather than a constant voltage to drive the LEDs.

A constant current (CC) LED driver automatically adjusts the output voltage to keep the output current stable and thus the light output constant. This process works with single LEDs or with a chain or string of LEDs connected in series. As long as the current through all the LEDs is the same, they will have the same brightness even if the V1 across each LED is different (see Figure 4).

As the LEDs warm up to their final working temperature, the constant current driver automatically adjusts the driving voltage to keep the current through the LEDs constant, so that the LEDs provide consistent brightness.

One important difference between the options shown above is the input and output voltage ranges. A DC/DC switching regulator has a wide input voltage and output voltage range over which the constant current regulation works well e.g, the RCD-24-0.35 which works from 5V to 36VDC has an output voltage range of 2-34VDC). A wide output voltage range not only allows many different combinations of LED string lengths, but also permits a wide dimming range.

The other two options shown above (Figures 5 and 6) will have power dissipation problems if a potentiometer is used for dimming, as the resistor or linear regulator will have a large volt drop across them which will increase still further the power dissipation losses. The input voltage range has to also be restricted for the same reason.

Two LEDs is not very much! One way around this problem is to either use a boost converter where the output voltage is higher than the input voltage or to use two or more strings of LEDs in parallel. For each paralleled string of LEDs used, the driver current has to be increased to deliver the correct overall current. So if a single string needs a 350mA driver, two strings in parallel will require a 700mA driver, three parallel strings would need a 1.05A source, and so on.

Therefore, the choice of LED driver is dependent on the input voltage available and the total number of LEDs that need to be driven. Figures 8, 9, and 10 show some possible combinations for a fixed 12VDC supply using typical 1W white LEDs.

The majority of high power white LEDs are designed to be run at 350mA constant current. This is because the chemistry of a white light LED sets the forward voltage at about 3V and 3.0V x 0.35A ~ 1 Watt, which is a convenient LED power.

Most DC/DC constant current LED drivers are buck or step-down converters. This means that the maximum output voltage is lower than the input voltage. Thus, the number of LEDs that can be driven depends heavily upon the input voltage.



lf the input voltage is not regulated (e.g. a battery) then the maximum number of LEDs must be reduced depending on the minimum input voltage available.

*Note: lt is a common misconception that the number of LEDs that can be driven is dependent on the maximum V1 given in the LED datasheets. This is not true in practice because when the LEDs reach their operating temperature, the V1 falls significantly. Thus, the Vf given in the datasheet can be reliably used. A datasheet might state that Vf is 3.3V minimum, 3.6V typical and 3.9V maximum at 25°C ambient. However, at 50°C, the figures would be closer to 3.0V minimum, 3.3V typical and 3.6V maximum. Therefore, a fixed 24V supply can reliably power 7 LEDs and a 36V supply can power 10 LEDs, even if the LED driver still needs some voltage headroom to regulate reliably.

lf one string fails in open circuit, the other two strings will be overloaded with 500mA per string. The LEDs will probably cope with this for some time, depending on how well they are heat sinked, but eventually the over-current will cause another LED to fail, whereupon the third string will take all of the 1A current and fail almost immediately.

lf any LED fails short circuit, then the current flowing in the strings will be very unbalanced, with most of the current flowing through the string with the shorted LED. This will eventually cause the string to fail, with the same catastrophic domino effect on the remaining strings as described above.

High power LEDs are reliable in service, so the failures described above may not happen very often. Thus, many LED lighting designers choose the convenience and cost saving of running multiple strings from a single driver and accept the risk that multiple LEDs will fail if any single LED fails.

Another important concern is the balance of currents that flow in multiple strings. We know that two or three strings of LEDs will have different combined forward voltages. The LED driver will deliver a constant current at a voltage that is the average of the combined forward voltages of each string. This voltage will be too high for some strings and too low for others, so the currents will not be equally shared.

Imbalance in LED current flowing through multiple strings:

In the example given above, the current imbalance is not sufficient to cause the overloaded string to fail, so both LED strings will work reliably. However, there will be a 6% difference in light output between the two strings.

One solution to the problem of unbalanced strings is either to use one driver per string or to add an external circuit to balance out the currents. Such a circuit is a current mirror.

Some LED driver manufacturers claim that LEDs automatically share the current equally and such external current mirror circuits are unnecessary. This is not necessarily true. There is always an imbalance, unless the combined forward voltages of the LED strings are identical.

lf, say, two parallel strings are mounted on a common heat sink, then if one string draws more current than the other, it will run brighter and hotter. The heat sink temperature will slowly rise, thus causing the Vf of the second string to fall and causing it to also try and draw more current. In theory, the two strings should then balance out their currents because of the thermal feedback. In practice, this effect can be measured, although that is not enough to guarantee accurate current balancing.

Furthermore, if the two strings are in fact two separate LED lamps, there will be no thermal compensation feedback. The lamp with the lowest combined Vf will draw the most current, will run the hottest and the Vf will still fall further. This will make the imbalance worse and can lead to thermal runaway and LED failure.

Today, the current mirror solution is rarely used. The cost of accurate LED drivers has fallen so low that for high grade lighting it is better to run each string from its own current controller. For low grade lighting, any current imbalance and the resulting reduction in lifetime are acceptable for cost reasons.

In a previous chapter, the consequences of a single LED open-circuit or short-circuit failure were discussed. The larger the number of parallel strings, the lower the danger that a single fault in one string would cause the remaining strings to fail. Thus, if five strings were connected in parallel, then if one LED string were to fail open circuit, the remaining four strings would all be overdriven by only 125%. The LEDs would glow brighter, but they would be unlikely to fail as long as the heat sinking is adequate.

The disadvantage of connecting many strings in parallel is that a driver capable of delivering several Amps will be required, and this could be expensive or hard to find. Also, some care is required with LED drivers capable of delivering many Amps of current; if the LED load is too low, because for example a connector to some of the strings has a faulty connection, the high current will blow the remaining LEDs instantly. Great care needs to be taken that all connections are sound before the LED driver is turned on. Many expensive LED lamp fittings have been permanently damaged by faulty wiring!

In practice, ...