Designing Efficient LED-based Interior Lighting

Light-emitting diodes (LEDs) are being exploited by many automotive lighting applications. Interior lighting is one area where LEDs are well suited because of their small size and high efficiency. Consequently, with recent advancements in efficiency and manufacturing to lower cost, these light sources have become the technology of choice for automotive interior lighting systems. But, driving these devices for optimal performance, is an art.

Since LEDs require a specific operating current, the method and precision of setting the LED forward current, in the midst of a typical wide ranging, automotive battery, and charging system, requires circuitry beyond the usual series-limiting resistor.

An innovative use of a standard N-channel depletion mode JFET will be shown to have better advantages when compared to a resistor to adjust the LED operating current. In simple terms, a JFET can be considered as a voltage-controlled resistor. By simply adjusting the gate-source voltage, a relatively constant current can flow out from the source contact. The source can then be a current source for the series connected LED(s). As the drain voltage is connected to the unregulated switched battery connection, it can then provide a relatively constant current and, therefore, be more effective than a standard resistor.

The list of potential interior lighting LED applications include:

instrument cluster backlighting;
switch cluster backlighting;
dome lighting;
convenience lighting; and
RGB mood lighting.

Each application requires specific attention to light output and optical design, LED circuit topology, driver current requirements, and thermal management. Nearly all automotive LED lighting applications incorporate the circuit as shown in Figure 1. In particular, if the current is below 100 mA (the majority of interior lighting applications are for some type of back-lighting or switch illumination, and currents are typically 30 mA). The resistor value is calculated to take into account the Vfwd across the series connected LED string. If a specific supply voltage such as 13.5 V is used, a specific resistor can be chosen:

Vsupply - Vsw_bat - Vrpp -I_led*R1 - 2 Vfwd = 0 V

Vsw_bat = 0 V

Vsupply = 13.5 V (typical)

Vrpp = 0.8 V

Vfwd = 3.5 V

I_led = 30 mA

R1 = 13.5 - 0.8 - 2*(3.5) = 190 Ω/30

This method for setting the current with a specific resistor is well known and by knowing the LED's worst case Vfwd drop, a specific range of resistor value can be chosen. However, as the supply voltage varies from 9 V to 18 V, the current changes in the LED. With the same 190 Ω resistor and 9 V, rearranging equation for the I_led value yields 6.3 mA. Assuming all the parameters remain constant and the supply is elevated to 18 V yields and I-led value is 53 mA.

Instrument clusters need to be backlit such that the gauges and indicators can be read in low ambient light conditions. Dimming capability is paramount and should provide a 100: 1 dimming ratio. In addition, to backlighting several telltale lights are required for driver diagnostics such as air bag check out, powertrain diagnostics, fluid levels, etc. Typically, as many as 30 LEDs could be used in a cluster application.

A multi-element extension of Figure 1 is shown driving six strings of LEDs with a low side set of transistors to provide pulse-width modulated (PWM) dimming. The resistor calculated previously is used to set the LED's forward current and, therefore, at a given supply voltage, the total current is fixed in the LED strings. Again, as the supply voltage varies from 9 V to 18 V, the current changes in the LED. For instance, at 9 V supply, sufficient light needs to be generated to safely read the instrument cluster. At 18 V, thermal issues on the printed circuit board (PCB) may arise, and is where worst-case conditions need to be assessed.

Depending on the color of the LED used for the backlighting, the forward drop of 2.4 V per LED in the red, orange, green, and amber colors to as high as 3.8 V for blue and white LEDs. It should be appreciated that a third LED could be placed in series if the former colors are used. For this example, we will assume a typical white light for cluster backlight, and will limit our series string to two LEDs. A reverse polarity diode is required for the misapplication of the vehicle's battery terminals during maintenance, and may be as high as -15 V. LEDs have typically max reverse rating of -5 V and, therefore, need a blocking diode to protect the LEDs during this reverse polarity condition.

A method for dimming the LED series string is provided on the low side of the circuit. The use of bias resistor transistors or digital transistors such as the MMUN2211 family, help provide a simple interface from the host controller within the cluster. The transistor has integrated Rb and Rbe resistors such that a logic level signal can adequately drive the base emitter circuit. Using such a transistor, and controlling the PWM duty cycle at a single frequency, provides a wide dimming range for the cluster.

A need exists for a low-cost, solid-state constant current regulator for LED drive as well as other applications. Hence, factors favoring a solid-state current regulator include:

low cost;
maintained current regulation over wide forward voltage;
low dropout behavior for low forward voltage operation;
power-limiting behavior at extreme high forward voltage;
behaves like an ideal two-terminal current source for parallel application;
provides high-frequency PWM control for LED dimming;
resists susceptibility to direct-injected RF energy; and
maintains a high level of ESD immunity.

The circuit in Figure 3 is identical to Figure 1, except the 190 V resistor is replaced with depletion mode N channel JFET. Simply shorting the gate to the source and biasing the drain to source by greater than 1 V causes a current to flow in the series LED circuit. The unique part of using a JFET in place of a resistor to set the LED's forward current is that as the drain source voltage increases (battery voltage varying), the current stays relatively constant. The JFET's constant current behavior is depicted in Figure 4, and can be best understood by examining its current-voltage characteristic over the supply voltage's normal operating range.

The first region is a linear region in which the current through the device rises in a linear fashion as the drop across the drain-source terminals increase. This region occurs over a relative narrow voltage range (up to 1.5 V above the LED forward voltage drop). If we compare the LED currents from Figure 1 and Figure 3 in a 9 V input supply, and assuming 0.8 V drop across the reverse polarity diode leaves 1.2 V across the 190 V resistor. This sets the LED current at 6.3 mA. In contrast, the current in the Figure 3 circuit under the same 9 V battery condition, and 1.2 V drop across the JFET, allows 21 mA to flow in the LEDs. Therefore, a JFET bias approach allows for a 3.5 times greater current at low line voltage. This is akin to a dropout specification in a linear regulator. This low dropout behavior, therefore, provides a higher LED current and greater illumination at low vehicle battery conditions.

The next region in Figure 4 is the constant current region and occurs over a voltage range from 1.5 V to approximately 6 V (Vbattery 9.2 V to 14.5 V) above the LED forward voltage drops. This constant current region is defined by the JFET's Idss capability. With the gate shorted to the source, the Idss value becomes the constant current over this region, and is specifically selected for this parameter.

The LED current in Figure 1 is determined at one voltage value (13.5 V). The current is certainly constant at this voltage. In contrast, the JFET constant current region is just that — a region and not a single supply voltage bias point. Extending from the linear region, as the voltage drop across the JFET increases, the JFET's drain current essentially goes in pinch-off mode and rate of change in the current abruptly decreases. The instantaneous slope or admittance decreases. The result is a well-behaved and fairly constant current over a broad battery voltage range (9.5 V to 14.5 V).

It is this region where the most benefit comes with the use of a JFET versus a resistor. The manufacturing of LEDs produces a normal distribution of Vfwd value at a single current. This spread in Vfwd has to be addressed for the user of Figure 1 in order to maintain 30 mA flow at 13.5 V. The user of Figure 1's circuit must be able to place a range of values of resistors that compensates for the specific forward voltage drop in the LED string. Purchasing LEDs with forward voltages over the entire population (binned for forward voltage) tends to reduce cost but ironically forces the user to store as many different resistor values. Instead, if the Figure 5c circuit is used, the constant current source or JFET provides a selected constant current regardless of the forward voltage drop of the LEDs.

The third region can be viewed in Figure 6, which is an extension of Figure 4 up to 40 V. At bias supplies from 6 V up to 40 V, a JFET exhibits a current fold-back due primarily to an electric field effect within the device's channel. The effect of this channel field is to essentially sweep carriers out of the channel and, therefore, have the net effect of decreasing the current and the power dissipation in the JFET. This inherent self-protection feature makes JFET constant current drive ideal for LED bias where extreme operating voltages are encountered.

Up to now we have considered 18 V as a worst-case continuous supply voltage. Double battery conditions, however, exist. They must be considered for one minute up to 26 V, as well as large inductive transients due to vehicle generator/alternator load dumps that last for several hundred milliseconds, and 40 V peaks or higher depending on the cluster and vehicle load dump protection scheme (centralized load dump protection, versus module level load dump protection).

Figure 7 is the calculated power in the BSR58, and an ideal constant current that would remain constant up to 40 V. Note the power savings in a JFET circuit. A 190 Ω resistor for a 30 V drop is 4.7 W and 156 mA I_led. These types of currents cause excessive power dissipation and reduce LED life.

Now that the JFET is better understood as used for LED bias, it would be good if a simple technique existed to trim the value of Idss up or down. The circuit in Figure 8a is a simple parallel combination of the 30 mA JFET producing a total of 90 mA for the LED string bias. Figure 8b is a simple parallel resistor of 2.7 kΩ that slightly trims up the Idss current and would provide a very flat constant current source past 20 V. Figure 8c shows a series resistor of 200 Ω in the source terminal. This resistor has the effect of decreasing the Vgs voltage and, therefore, reduces the Idss current.

It has been shown that the simple resistor biased circuit used for many automotive LED applications can greatly benefit with use of the BSR58 JFET acting as a constant current source. These constant current sources have benefits at low, medium and high line voltages. In addition, these JFETs can easily be trimmed up and down with the addition of resistors or additional parallel JFETs. However, the best benefit to JFET LED bias is to eliminate the large range of resistor values that must be uniquely chosen to compensate the LED's natural variation in its Vfwd.