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Constant-voltage topology for driving LEDs

Posted: 30 Jun 2015     Print Version  Bookmark and Share

Keywords:LEDs  LED driver  AC  DC  EMI 

LEDs are quickly turning into the preferred choice for lighting in many situations due to their efficiency, versatility, flexibility, long life, and cost-competitiveness. But unlike the traditional incandescent bulb, and to some extent the CFL, the issues of both driving and dimming LEDs has challenges which require careful design to overcome.

Basic LED driving options
At a basic level, a single string of LEDs is easy to drive: place a current-limiting resistor between the AC or DC source and the LED; for use from an AC line, LEDs can be used directly or with a capacitor for voltage smoothing (figure 1). These approaches work, somewhat, but have serious drawbacks. For example, the resistor technique is inefficient due to its I2R losses. It presumes that every LED in volume-production product has the same forward-voltage drop, and so will see the same current and thus produce consistent output intensity. As a result, the simple dropping-resistor technique is only viable and cost-effective for driving a small front-panel "power on" indicator and similar non-critical applications. Similarly, the direct-connect and capacitive technique is low cost but offers marginal, inconsistent, and usually unacceptable performance.

Figure 1: While it is electrically possible to drive LEDs directly from the AC line, or with a simple capacitive for filtering, this method is both inefficient and poorly regulated; it is not practical except in highly specialised circumstances.

An improvement on resistor- or capacitor-based drive is the use of a constant-current (CC) or constant-voltage (CV) source. As their names imply, these are power sub-systems which take the supply rail (AC or DC) and deliver a fixed current or voltage, respectively, regardless of load. The choice of which type to use depends on two factors: the pros and cons of each as an AC/DC power source (efficiency, complexity, performance, cost), and the implications of each for the LED user (ability to handle multiple LED strings, flexibility with changing LED load requirements).

Putting aside the design issues associated with each, and focusing on the impact on users, the CC driver is the better choice when the number of LEDs in a series string is undetermined or may change. The CC supply can support one or many LEDs in a series string at the specified current (usually between 20 and 50 mA) as long as the sum of their voltage drops (about 1.5 V each) does not require the supply to deliver this current at a higher voltage than it is designed to deliver (figure 2a). It is a good choice when the number of LEDs per string changes.

However, a CC supply may not be the best choice when the number of strings or LED array topology is changing, or there is a desire to support multiple end products using the same supply. The reason is that the fixed output current will have to divide among the various strings, reducing the per-string current: a 50mA drive for one string will become two 25mA drives for two parallel strings, (figure 2b).

Figure 2: (top) A constant-current source is a good choice for driving one or more strings of LEDs, even as the number of LEDs in the string changes, but (bottom) falls short when the number of strings in the array decreases or increases, as the per-string current will increase or decrease, respectively, and not meet the design requirements.

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