High Power DC/DC LED Applications: Ensuring Consistent Brightness and Efficiency

2019/02/05
RCD-24シリーズ
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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LED Characteristics

The first rule of war is „know your enemy“. It’s 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. If a low voltage is applied to an LED, it does not conduct. As the voltage increases, it reaches a threshold value, at which point the LED suddenly begins to emit light and the current rises sharply. If the voltage continues to increase beyond this point, the LED quickly overheats and burns out. The key is to operate the LED within the narrow range between fully off and fully on.

There is an additional complication, however. The useful operating voltage range differs between high power LEDs — even among LEDs from the same batch and supplier — and it also changes with ambient temperature and the age of the LED.
Useful operating area for high power LEDs

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

Fig. 2: LED Characteristics in detail
Figure 2 shows the useful operating area in greater detail. In this example, we examine four identical LEDs that, according to the datasheet, share the same specifications. All LED manufacturers sort LEDs based on the color of light they emit (a process called „binning“—the LEDs are tested during production and categorized into different bins according to their colour temperature).

As a result of binning, LEDs from a single delivery can originate from several different production batches, which leads to a wide variation in threshold values, or forward voltage (V1). Most high power LED datasheets specify a V1 tolerance of around 20%, so the significant variations shown in Figure 2 are not exaggerated.

In this example, if we choose a supply voltage of, say, 3V, LED 1 is 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 shift to the left (the forward voltage, Vf, decreases with increasing temperature). This change typically occurs very quickly — within 0.1 seconds of turn-on — followed by a more gradual drift as the heat sink warms up.

To minimize these temperature drift effects, manufacturers pulse the LEDs at high frequencies to produce standardized results at 25°C. However, this also means that the actual operating Vf and the value listed in datasheets may not match (see footnote on page 8).

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

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

Driving LEDs with a Constant Current DC Source

The solution to the problem of variability in forward voltage (V1) is to use constant current rather than constant voltage to drive the LEDs. A constant current (CC) LED driver automatically adjusts its output voltage to maintain a stable output current, and therefore consistent light output. This approach works with single LEDs as well as with chains or strings of LEDs connected in series. As long as the current through all LEDs is the same, they will emit the same brightness—even if the V1 across each LED differs (see Figure 4).
LED string example

Fig. 4: LED string example
As the LEDs warm up to their final operating temperature, the constant current driver continuously adjusts the drive voltage to keep the current constant, ensuring uniform 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.

DC Constant Current Sources

The simplest constant current source is a constant voltage supply operating the LEDs via a resistor (Figure 5). If the voltage drops 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 about a 50% change in LED current).

This solution is very cheap, but offers no current regulation and wastes a significant amount of power. Many low-cost cluster-type LED bulbs, sold as replacements for low voltage halogens, use this method. Needless to say, if any LED fails short circuit, the resistor becomes overloaded and usually burns out after a relatively short time — resulting in a short lifetime for these cluster LED lamps.
Simple Resistor

Fig. 5: Simple Resistor: Cost effective, but inaccurate and wasteful
The next simplest constant current source is a linear current regulator (Figure 6). There are several low-cost LED drivers on the market that use this method. Alternatively, a standard linear voltage regulator, such as the LM317, can be configured for constant current operation.

The internal feedback circuit regulates the current to within about ±5%, but excess power is dissipated as heat, so adequate heat sinking of the regulator is essential. The main drawback of this approach is its poor efficiency, which runs counter to the goal of using high-efficiency SSL devices.
Linear Regulator

Fig. 6: Linear Regulator: Cost effective and accurate, but still wasteful
Switching Regulator

Fig. 7: Switching Regulator: Higher cost, but accurate and efficient
The best constant current source is a switching regulator (Figure 7). While the cost of the driver is higher than that of the other solutions, the output current accuracy can be better than ±3% across a wide range of LED loads. Conversion efficiencies can reach up to 96%, meaning that only 4% of the energy is lost as heat. This makes switching regulators suitable for use in high ambient temperatures.

One important difference between the options shown above is the input and output voltage ranges. A DC/DC switching regulator offers a wide input and output voltage range over which constant current regulation remains effective. For example, the RCD-24-0.35 operates from 5V to 36VDC and provides an output voltage range of 2–34VDC. This wide output range supports many different LED string configurations and also allows for a broad dimming range.

In contrast, the other two options (Figures 5 and 6) experience significant power dissipation issues when a potentiometer is used for dimming. The resistor or linear regulator will have a substantial voltage drop across it, which further increases power losses. For the same reason, the input voltage range for these solutions must also be limited.

Connecting LEDs in Strings

The majority of high power white LEDs are designed to operate at a constant current of 350mA. This is because the chemistry of white light LEDs sets the forward voltage at around 3V, and 3.0V × 0.35A ≈ 1 Watt, which is a convenient power level for LED operation.

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


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

*Note: It is a common misconception that the number of LEDs that can be driven depends on the maximum V1 value listed in LED datasheets. In practice, this is not the case, because as LEDs reach their operating temperature, the forward voltage (V1) drops significantly. Therefore, the Vf value stated in the datasheet can be reliably used for design. For example, a datasheet might list Vf as 3.3V minimum, 3.6V typical, and 3.9V maximum at 25°C ambient. At 50°C, however, the values shift closer to 3.0V minimum, 3.3V typical, and 3.6V maximum. Based on this, a fixed 24V supply can reliably power 7 LEDs, and a 36V supply can drive 10 LEDs — even accounting for the voltage headroom required by the LED driver to regulate properly.

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 are not very much! One way around this limitation is to 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 additional parallel LED string, the driver current must be increased to supply the correct total current. For example, if a single string requires a 350mA driver, then two parallel strings will require 700mA, three will need 1.05A, and so on.

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

Connecting LED Strings in Parallel

Connecting a single string of LEDs to an LED driver is the safest and most reliable method of driving LEDs (Figure 8). If any LED fails open circuit, the current to the remaining LEDs in the string is interrupted. If any LED fails short circuit, the current in the remaining LEDs remains unchanged.

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

Fig. 8: Single string
Driving multiple strings from a single LED driver allows more LEDs to be powered, but it introduces risks if any LED fails (Figure 9). With two strings in parallel, if one LED fails open circuit, the full 700mA constant current will flow through the remaining LEDs in the other string. This overcurrent condition can cause the entire second string to fail in 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
Double string

Fig. 9: Double string
Triple string

Fig. 10: Triple string
With three strings in parallel, if any single LED fails, the remaining two strings will share the 1A drive 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

If one string fails open circuit, the other two strings will be overloaded with 500mA each. The LEDs may tolerate this for a while, depending on how well they are heat-sinked, but eventually the overcurrent will cause another LED to fail. At that point, the remaining third string will receive the full 1A current and will likely fail almost immediately.

If any LED fails short circuit, the current distribution becomes highly unbalanced, with most of the current flowing through the string containing the shorted LED. This will eventually cause that string to fail, triggering the same kind of catastrophic domino effect in the remaining strings as described above.

High power LEDs are generally reliable in service, so such failures may not occur frequently. For this reason, many LED lighting designers accept the convenience and cost savings of running multiple strings from a single driver, despite the risk that one LED failure could lead to several more.

Balancing LED Currents in Parallel Strings

Another important concern is the balance of currents flowing through multiple strings. It’s well known that two or three LED strings will have slightly different combined forward voltages. The LED driver supplies a constant current at a voltage that reflects the average of these combined voltages. However, this average will be too high for some strings and too low for others, resulting in unequal current distribution.

Imbalance in LED current flowing through multiple strings:
Ideal balance

Fig. 11: Ideal balance
Real life balance

Fig. 12: Real life balance
In the example above, the current imbalance is not large enough to cause the overloaded string to fail, so both LED strings will operate reliably. However, there will be a 6% difference in light output between them. One solution to this imbalance is to use a separate driver for each string, or to add an external circuit that equalizes the currents. One such circuit is a current mirror.
Balancing LED currents using a current mirror

Fig. 13: Balancing LED currents using 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 not strictly required for the current mirror to function, but in practice they help compensate for differences in Vbe between the transistors and improve current balance accuracy.

A current mirror also provides protection against LED failures. If any LED in the first string fails open circuit, the second string is protected (since the reference current drops to zero, the current in the other strings also falls to zero). Likewise, if any LED fails short circuit, the currents remain equally balanced.

However, if an LED fails open circuit in the second string, the current mirror does not protect the LEDs in the first string from being overdriven. A modified version of the circuit can prevent this, using a dummy load with the first transistor to set the current in the remaining strings. It is also possible to extend the current mirror to three or more strings by connecting additional transistors with their base connections paralleled.

Some LED driver manufacturers claim that LEDs inherently share current equally, making external current mirror circuits unnecessary. This is not always true. An imbalance will exist unless the combined forward voltages of the LED strings are exactly the same.

If, for example, two parallel strings are mounted on a common heat sink, the string drawing more current will run brighter and hotter. As the heat sink warms up, the Vf of the second string drops, causing it to draw more current in turn. In theory, this thermal feedback should help balance the currents, and in practice the effect can be observed — though it is not reliable enough to ensure accurate current sharing.

Furthermore, if the two strings are part of separate LED lamps, there is no thermal compensation between them. The lamp with the lowest combined Vf will draw the most current, run hotter, and experience further Vf reduction. This positive feedback loop can worsen the imbalance and may lead to thermal runaway and eventual LED failure. Today, current mirror solutions are rarely used. The cost of accurate LED drivers has dropped so far that, for high-grade lighting, it is better to drive each string with its own current controller. In low-grade lighting, some imbalance and the resulting shortened lifespan are generally accepted as cost trade-offs.

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 risk that a fault in one string will cause the remaining strings to fail. For example, if five strings are connected in parallel, and one string fails open circuit, the remaining four strings will be overdriven by only 125%. The LEDs will glow brighter, but they are 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, which could be expensive or difficult to find. Additionally, extra caution is needed with LED drivers that deliver high current. If the LED load is too low—such as due to a faulty connection to some strings—the high current will immediately damage the remaining LEDs. Great care must be taken to ensure all connections are sound before turning on the LED driver. Many expensive LED lamp fittings have been permanently damaged by faulty wiring!

In practice, ...

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