A Simple Method to Provide Open Circuit Protection for Boost Converters

Introduction
 
One approach to driving high-brightness LEDs is to modify the standard boost converter topology to drive a constant current through the load. However, this implementation presents a serious problem because an open-circuit fault in the LED string removes the path for the load current. This can potentially damage the circuit due to the high output voltage from the converter (which is now operating without feedback). This article presents a simple, robust open-circuit fault protection method that uses a Zener diode and a resistor with negligible impact on overall efficiency. The functionality of this topology can be verified by configuring a high-voltage boost converter as a constant-current driver for three high-brightness white LEDs and generating a simulated fault condition at the output. The circuit controls the output voltage to a safe level and reduces the output current in a protected state.

Typical High Brightness LED Boost Converter

Converters are often modified to drive high-brightness LEDs in single-cell Li-Ion, alkaline, and other applications. In these applications, the voltage of the LED string exceeds the battery or power rail voltage. In a standard boost configuration, a voltage divider is used to generate the circuit's feedback voltage, VFB, which monitors the output voltage, VOUT. The converter regulates the output voltage so that VFB is always equal to the on-chip reference voltage, VREF. This topology is adaptive and allows the load to replace the upper resistor in the feedback divider, thereby maintaining a constant current instead of a constant voltage, as shown in the LED string in Figure 1. The load current depends on the on-chip reference voltage of the boost converter and is calculated as follows:

A serious problem with this simple implementation is that an open-circuit fault in the LED string removes the path for the load current. When no current flows through the feedback resistor, RSET, VFB is pulled down to ground. In response, the boost converter increases its operating duty cycle to the maximum possible in order to maintain the correct voltage at the feedback (FB) pin. Using an idealized boost converter transfer function shows that a high output voltage (VOUT) can be produced when the converter is close to its maximum duty cycle. Consider a boost converter with a typical maximum duty cycle of 90% (a common value) and a 5V input:

High voltage at the converter output presents the potential for multiple faults. This voltage may exceed the ratings of internal or external switching devices or passive components. It also presents a potential hazard to the user if the circuit is operated without protection measures, and may damage the load when connected.

Figure 1: LED driver high voltage boost converter structure without open circuit protection


circuit

In the event of an open circuit condition, the load current must have an alternate path. While placing a resistor in parallel with the LED string can provide a path, it is not ideal because it causes significant efficiency losses. An alternative configuration (Figure 2) consisting of a Zener diode and a resistor provides adequate system protection with negligible efficiency losses.

Figure 2: LED driver circuit with open circuit protection

When the load current path is removed, the output voltage rises until Zener diode ZD1 turns on and current flows through RPRO and RSET to ground. The output current is determined by the series combination of RPRO and RSET because VFB is driven equal to the internal bandgap reference voltage VREF. Therefore, the output protection current defaults to:

We choose a voltage for the Zener diode so that no current flows through it during normal circuit operation. To ensure that the diode is completely off during normal operation, the selected voltage should be at least 2V above the maximum load voltage, but less than the specified maximum output voltage of the boost converter. This way, the circuit designer is not often forced to increase the voltage ratings of the output capacitors C2 and C3 and the clamping diode SD1. The output voltage is controlled to be the sum of the Zener diode voltage and the reference voltage:

The RPRO value is chosen by balancing the LED current sensing error and power dissipation during circuit protection. In practice, the value of RPRO should be as large as possible to minimize the power dissipation in the Zener diode:

The errors entering the circuit are caused by the Zener diode leakage current, IZL, and the boost converter internal error amplifier bias current, IFB. Equation 6 is a modified transfer function that includes these errors:

Since both currents are typically less than 1μA, the errors they introduce are very small and can be ignored in most implementations.

As an application example, the TI TPS61170 boost converter IC is configured as a constant current LED driver. It is an ideal boost converter for driving a string of high-brightness LEDs in applications such as backlighting or flashlights. The 3V-18V input range allows the use of a wide range of power supplies, such as 2S to 4S Li-ion or 3S to 12S alkaline battery packs, USB or 12V power rails.

Figure 3: Oscilloscope screenshot of protection circuit activation

The boost converter is configured to drive three high-brightness white LEDs at 260 mA. Using the simplified load current in Equation 7, RSET is calculated for a typical reference voltage of 1.229 V:

We use 1mA as the protection current (IPRO) to calculate the RPRO value:

We choose a 15V Zener diode for ZD1 to exhibit minimal leakage current at an expected load voltage of about 10V, while also controlling the output to a value well below the maximum allowed output voltage of the boost converter (40V). The output voltage is controlled to the Zener diode voltage (VZD1), which is summed with the converter reference voltage by:

With the chosen load current and protection resistor, calculate the deviation from the expected load current (see Equation 10 below). The datasheet value of 200 nA is used for the feedback bias current (IFB), and the 1 μA value is used for the expected Zener diode leakage current, with VOUT of approximately 10 V.

The target load current for the circuit is 260 mA. As we can see, once the theoretical component values ​​are replaced by their effective values ​​in Equation 10, the errors they introduce are far greater than the error introduced by the protection circuit itself.

To test the operation of the protection circuit, a 38Ω resistor decade box was used in place of the LED string to simulate the voltage across the LED string at the designed load current. An open circuit fault was simulated by quickly changing the load resistance from 38Ω to 1038Ω. As shown in Figure 3, this output current change (green trace) indicates the sudden change in load impedance. To compensate, the TPS61170 output voltage (yellow trace) rises to re-reach the designed load current. However, this change does not persist until it reaches its maximum duty cycle, where the output voltage settles to a clamp voltage of approximately 16V.

in conclusion

We have shown you a simple method to provide open circuit protection for a boost converter configured as a constant current LED driver. This circuit consists of a Zener diode and an additional resistor, which limits the output voltage to a safe level while reducing the output current in the event of an open circuit load fault. In addition, this method introduces some error in the load current calculation process and slightly reduces the efficiency during normal circuit operation, but these effects are negligible. A boost converter is configured as an LED driver and a 15V Zener diode and a 1.2kΩ resistor are added for output protection. This demonstrates the functionality of this protection circuit. The demonstration shows that the output of the circuit behaves as expected under simulated load fault conditions