Avoiding inrush current when testing high power LEDs

12 December 2014

Visible light emitting diodes (LEDs) have gained a reputation for high efficiency and long lifetimes.

They offer a variety of advantages over the traditional incandescent and fluorescent light bulbs. Extensive research has led to the creation of LEDs with higher luminous flux, substantially longer lifetimes, and greater chromaticity. LED lights are capable of producing more lumens per watt of power consumed, which has driven demand. Unlike fluorescent bulbs, they do not contain mercury, so they are environmentally friendly from a recycling and disposal perspective. 

These characteristics are driving the use of LEDs in an ever-growing list of applications, including architectural lighting, automotive displays and exterior lights, backlighting for televisions and video monitors, street lights, outdoor signs and interior lighting, medical equipment, and military systems. Reliable and accurate electrical measurements are vital to maintaining quality when mass-producing these devices, so a solid understanding of them is critical. 

Unlike an incandescent bulb where the lighting source is a filament made of a resistor that lights up when heated, an LED is a two-terminal, semiconductor device that emits visible light when current flows through the device. A diode turns ‘on’ at a characteristic voltage, Vd, in the forward bias operating region when an avalanche of electrons and electron holes start to recombine. One of the properties of an LED during this recombination process is the release of energy in the form of photons, causing the LED to illuminate. The I-V characteristic of a diode in the forward bias region is depicted below, where Vd is the on-voltage of the diode.

Voltage or current?
Although LEDs can be driven either with voltage or current, current is the preferred method. One reason is brightness. LED brightness is proportional to its drive current. As the I-V curve indicates, a small change in voltage results in large variations in current that will lead to drastic and undesirable variation in LED brightness. In addition, temperature and aging can cause Vd to drift over time. Again, this small voltage drift will likely cause unwanted current variations. Furthermore, driving LEDs with excessive amounts of current can result in irreversible damage and lead to much shorter lifetimes. Thus, regulating the drive current at appropriate levels in LEDs is critical.

A common phenomenon that overstresses LEDs is the inrush current. An LED can be modeled as a parallel R-C network; thus, the device is instantaneously a short circuit when a voltage is applied across the device’s terminals. The instantaneous short circuit results in an inrush current, a short duration, start-up current that is of much greater magnitude than the LED’s steady state operating current. For example, introducing an LED to an energised circuit or “hot switching” the LED will lead to dangerous inrush current magnitudes. The circuit shows that when the switch is open, the voltage at the power supply is maintained at the rated voltage of the LED. As soon as the switch closes, the charge stored at the output of the power supply and the wires flows rapidly into the LED until the power supply starts to regulate.

Series 2260B power supplies
To prevent potentially dangerous inrush currents from flowing into loads that have low resistance when power is initially supplied, the Keithley Series 2260B power supplies provide programmable rise time (or slew rate) control. Either the voltage rise time or the current rise time can be controlled. With voltage slew rate control, the voltage rise time can range from a slow 0.1V/s to a high speed of 160V/s (for the 80V versions of these supplies). When operating under constant current control, the output current rise time can be programmed from a low slew rate of 0.01A/s to a maximum value of 144A/s for the 30V/72A version. The programming of a current rise time puts the Series 2260B supplies in a constant current control priority mode in which the current slew rate limits the rate at which the voltage rises across the device under test (DUT). This programmable slew rate makes it possible to operate the power supply in a constant current (CC) mode beyond the traditional programmable constant voltage (CV) mode. When a supply operates in the CV mode, the voltage is regulated while the current may vary. 

Unlike traditional power supplies, the Series 2260B power supplies can be put in a constant current mode independent of the load value. When the power supply is operating in the CC mode, the current is regulated and supplied to the load while the voltage output may vary. This mode eliminates the need for external controlling circuitry and simplifies the approach to “soft start” a LED. The power supply itself is capable of keeping the current input to the LED under control until the LED reaches the on-voltage. Removing the possibility of transient inrush current protects the LED from a significant source of potential damage. 

In addition to the rise time, the fall time can also be programmed and the fall time values can be different from the rise time values. That means the delivery of power to a load can be precisely controlled to prevent overshoot spikes and excessive amounts of inrush current. This capability prevents damage to components, modules, or devices and will generate more precise characteristic I-V curves for LEDs.

The following example illustrates the steps, both front panel and remote operation, to configure a Series 2260B power supply for current limiting, CC high speed priority mode. The example sets the supply’s output voltage to 10V and the current limit to 5A.

Front panel operation

Step 1: Set the power supply to CC high speed priority mode. Press in the function key. The function key will light up, and the display will show F-01 on the top line. Rotate the voltage knob to change the F setting to F-03 (V-I mode slew rate select). Rotate the current knob to set the F-03 setting to 1 for CC high speed priority. Press in the voltage knob to save the configuration setting. ConF will be displayed on the bottom display line when successful. Press the function key again to exit the configuration settings. The function key light will turn off.
Step 2: Set the output voltage and current limit. Press in the voltage knob to highlight specific digits, and then turn the voltage knob to adjust the digits until 10.00V is displayed. Press in the current knob to highlight specific digits, and then turn the current knob to adjust the digits until 5.00A is displayed.
Step 3: Turning on the output. Press the output key. The output key becomes illuminated when the output is on.

Remote Operation 
The following SCPI commands perform the same actions as the steps listed in the front panel operation section:
Keithley Instruments’ Series 2260B power supplies make testing LEDs safe and easy.

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