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.

Brian Blackburn is a senior field applications engineer with ON Semiconductor.