High Power DC/DC LED Applications

High Power DC/DC LED Applications Image
What we want to show in this whitepaper is how constant current sources are used to keep the brightness of all single LEDs in a LED system on the same level.

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1. LED Characteristics

The first rule of war is „know your enemy“. lt is the same principle with Solid State Lighting (SSL); if you don‘t understand how an LED works, don‘t be surprised when your application doesn‘t succeed.

LEDs are non-linear devices. lf a low voltage is applied to an LED it does not conduct. As the voltage increases, it passes a threshold value, when suddenly the LED starts to emit light and the current sharply increases. Thereafter, if the voltage continues to rise the LED rapidly overheats and burns out. The trick is to operate the LED in the narrow band between full off and full on.

There is an additional complication, however. The useful operating area voltage is different for different high power LEDs (even among LEDs from the same batch and supplier), and the voltage range changes with ambient temperature and age of the LED.


Fig. 1: Useful operating area for high power LEDs [Ta = 25°C]


Fig. 2: LED Characteristics in detail
Figure 2 shows the useful operating area in greater detail. In this example, we are looking at 4 identical LEDs that according to the datasheet have the same specifications. All LED manufacturers sort LEDs according to the colour of light they emit (this is called „binning“—the LEDs are tested during manufacture and sorted into different bins according to their colour temperature).

The consequence of binning is that the LEDs are all mixed up and one delivery can include several different production batches, and therefore a wide variation in the threshold values, or forward voltage (V1), is to be expected. Most high power LED datasheets specify a V1 tolerance of around 20%, so the wide variations shown in Figure 2 are not exaggerated.

In this example, lf we choose a supply voltage of, say, 3V, then LED 1 is being over-driven, LED 2 draws 300mA, LED 3 draws 270mA and LED 4 draws only 120mA.
Furthermore, these curves are dynamic. As the LEDs warm up to their operating temperatures, the curves all drift to the left (the forward voltage, Vf, reduces with increasing temperature). This shift usually takes place very quickly (within 0.1s of turn on), followed by a more gradual drift as the heat sink slowly warms up.

To avoid these temperature drift effects, manufacturers pulse their LEDs at high rates to give results that are all standardized to 25°C. However, this means that the real life Vf and that given in the datasheets are not the same (see footnote on page 8).

The light output of an LED is, however, directly proportional to the current flowing through it (Figure 3), so in the example given above with a 3V supply voltage, LED 1 will glow like a supernova, LED 2 will be slightly brighter than LED 3 and LED 4 will appear very dim.


Fig. 3: Relationship of light output to LED forward current

2. Driving LEDs with a constant current DC source

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.
Another major advantage is that a constant current driver does not allow any single LED in a chain to be overdriven, which ensures that they all have a long operating life. lf any LED fails short circuit, the remaining LEDs will still operate with the correct drive current.


Fig. 4: LED string example

3. Some DC constant current sources

The simplest constant current source is a constant voltage supply operating the LEDs via a resistor (Figure 5). lf the voltage drop across the resistor is about the same as the forward voltage of an LED, then a 10% change in V1 causes a similar change in the LED current (compare this with the curves shown in Figure 2, where a 10% change in V1 causes an about 50% change in LED current).

This solution is very cheap, but has no current regulation and is very wasteful of power. Many of the low cost cluster-type LED bulbs offered as replacement lamps for low voltage halogens use this method. Needless to say, if any LED fails short circuit, the resistor is overloaded and usually burns out after a relatively short while, and therefore the lifetime of these cluster LED lamps is relatively short.


Fig. 5: Simple Resistor: Cost effective, but inaccurate and wasteful
The best constant current source is a switching regulator (Figure 7). The price of the driver is higher than the other solutions, but the output current accuracy can be as much as <±3% over a wide range of LED loads, and conversion efficiencies can be as high as 96%, which means that only 4% of the energy is wasted as heat and the drivers can be used at high ambient temperatures.


Fig. 7: Switching Regulator: Higher cost, but accurate and efficient
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.

Example: How many 1W LEDs can be driven from a 12V Lead acid battery?

Battery voltage range 9-14VDC
DC/DC driver headroom 1V
Therefore, LED driver output voltage range 8-13VDC
If LED forward voltage, Vf 3.3V typical
Then the maximum number of LEDs that can be driven 2

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.

4. Connecting LEDs in strings

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.

Input Voltage 5VDC 12VDC 24VDC 36VDC 54VDC
Typical # of LEDs in String 1 3 7 10* 15
Table 1: Number of LEDs that can be driven per string vs 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.

5. Connecting LED strings in parallel

Connecting a single string of LEDs to an LED driver is the safest and surest method of driving LEDS (Figure 8). lf any LED fails open circuit, the current to the remaining LEDs in the string is broken. lf any LED fails short circuit, the current in the remaining LEDs remains the same.

Advantages: accurate LED current, fail safe (open or short circuit)
Disadvantages: low number of LEDs per driver (3 LEDs)


Fig. 8: Single string
Driving multiple strings from a single LED driver has the advantage that more LEDs can be driven, but there are dangers if any LED fails (Figure 9). With two strings in parallel, if any LED fails in open circuit, the 700mA constant current will flow through the remaining LEDs and cause the entire string to fail after a very short time.

Advantages: double the number of LEDs per driver (6 LEDs)
Disadvantages: not fail safe, open circuit doubles current in the other branch, short circuit unbalances the current in the strings


Fig. 9: Double string
With three strings in parallel, if any single LED fails, the remaining two strings will share the 1A driving current (Figure 10).

Advantages: triple the number of LEDs per driver (9 LEDs) Disadvantages: not fail safe, open circuit or short circuit unbalances the current in the strings


Fig. 10: Triple string
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.

6. Balancing LED Currents in parallel strings

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:


Fig. 11: Ideal balance


Fig. 12: Real life balance
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.
The first NPN transistor acts as the reference. The second NPN transistor „mirrors“ this current. In this way the currents in the strings are automatically equally shared. The 1Ω emitter resistors are theoretically not required for the current mirror, but in practice they help balance out differences in Vbe between the transistors and give a more accurate current balance.

A current mirror also helps protect against LED failures. lf any LED in the first string fails open circuit, then the second string is protected (the reference current is zero, so the current in the other strings also falls to zero). Moreover, if any LED fails short circuit, then the currents are still equally balanced.

However, if any LED fails open circuit in the second string, then the current mirror will not protect the LEDs in the first string from being overdriven. A modification of this circuit can also protect against this situation, where the first transistor uses a dummy load to set the current in the remaining strings. lt is also possible to extend the current mirror to three or more strings by connecting more transistors with their base connections all paralleled.


Fig. 13: Balancing LED currents using 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.

7. Parallel strings or grid array – which is better?

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, ...

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