Editor’s note: Our guest bloggers are:
Donald Schelle is an analog field applications engineer for Texas Instruments and has more than a decade of engineering experience. He received a bachelor’s degree in electrical engineering from Lakehead University, Thunder Bay, Ontario, Canada. Donald can be reached at firstname.lastname@example.org.
Jared Becker is an applications engineer for Texas Instruments. He received his bachelor’s degree in electrical engineering from Arizona State University. Jared can be reached at email@example.com.
Regulating from a very high voltage (>200V) to something usable by modern electronics (like 3.3 V) is a difficult proposition. For applications that require more than 1-2 mA of current, designers eschew a simple resistor/Zener diode combination in favor of a more complex solution. Managing power dissipation while maintaining a low-cost circuit can be challenging as well. The circuit in Figure 1 is a simple low-cost regulator that achieves high step-down ratios at output currents of 5 mA while minimizing cost; less than $0.50 in low volume.
In this article, we present detailed design calculations and tradeoffs needed to meet wide-input voltage-range requirements. We chose the following design parameters for our example regulator:
- VIN(MAX) = 250V
- VOUT = 3.3V
- IOUT = 5 mA
- TA(MAX) = 60o C (maximum ambient temperature)
Managing power dissipation
Managing design thermals is central to a functional first-pass prototype. Maximizing the input-voltage range requires careful component selection.
The pass transistor’s power dissipation will limit the output current. Equation 1 calculates the maximum current as a function of pass transistor power dissipation:
When the transistor is connected to a piece of 1-ounce copper (which acts as a heat sink) with dimensions of 15 x 15 mm, the chosen transistor (FCX458) is specified with ΘJA = 125o C/W. Given these parameters, the transistor will reach the maximum junction temperature of 150o C when the output current is 2.92 mA. Increasing the copper attached to the transistor to 50 x 50 mm reduces ΘJA to 60o C/W and yields a maximum output current of 6.08 mA. This is well within the design specification of 5 mA.
Power dissipation in the shunt resistor, RSH , also will be at a maximum when the input voltage is at its peak. Equation 2 calculates a minimum RSH resistance value based on the maximum resistor power dissipation:
Limiting power dissipated in the resistor to 350 mW yields a shunt resistance of 174 k Ω , which is a standard Energy Information Administration (EIA) 1 percent resistor value. The majority of power dissipated in the circuit occurs in either the shunt resistor, RSH , or the pass transistor, QP . Table 1 summarizes the power dissipation at the minimum and maximum design operating points.
In general, power losses are lowest when the input voltage is at the minimum. Power losses in the resistor are highest when the input voltage is at the maximum. Power losses in the transistor are highest when the output current and input voltage are at their maximums.
Optimizing for minimum input voltage
You can optimize the design for the widest input supply range by choosing a transistor with a high beta, β , and a shunt regulator with a low minimum cathode current, IK(MIN) . A few popular choices for shunt regulators include the TL431 and TLV431. These devices feature IK(MIN) values of 1 mA and 80 µA, respectively. An improved ATL431 reduces the minimum cathode current to 40 µA. Equation 3 calculates the minimum input voltage as a function of the minimum cathode current:
Decreasing IK(MIN) by choosing an appropriate TL431 device and increasing the β of the transistor play pivotal roles in maximizing the input voltage range. Figure 2 plots the minimum input voltage as a function of β . Decreasing the minimum regulation current of the TLV431 device by 50 percent relaxes pass transistor β requirements equally so. Alternatively, you can maintain beta and gain additional input-voltage range. If suitable components are not available to meet the target input-voltage range, increasing the power dissipated in the shunt resistor will reduce the minimum input voltage at the expense of more heat dissipated in the design.
Testing the circuit
When testing circuits that implement high voltages, it’s important to pay careful attention to appropriate safety precautions. High voltages can be dangerous and you need to treat them with care, respect and intelligence. If you are unfamiliar with appropriate safety precautions, seek informed help before turning on the power.
Testing the circuit yielded good results. An output capacitor is required in order to keep the regulated voltage from oscillating. The circuit in Figure 1 shows 10 µF, but we were able to achieve stability with a capacitance as low as 4.7 µF. No-load to full-load transient responses were also within tolerance. The scope capture in Figure 3 (trace No. 2) shows approximately 200 mV of ripple during the transient event. Also key is that the circuit is designed to be functional across semiconductor process limits for the ATL431. Since the typical IK(MIN) is much lower than the maximum, the measured minimum input voltage was much lower than calculated.
We used Excel to automate the design calculations presented here. You can download the free calculator tool here. Note that the tool requires additional add-ins to select standard resistor values. 
1 Donald Schelle. Calculate Standard Resistor Values in Excel, EDN, January 2013.