Some applications require loose output regulation and less than 20mA of current. Linear regulators built with discrete components are a cost-effective solution for such applications (Figure 1). For applications that have tight output regulation and require higher currents, high-performance low-dropout linear regulators (LDOs) can be used.
Figure 1: Simple series regulator
There are two design challenges associated with the circuit shown in Figure 1. The first challenge is to regulate the output voltage, and the second challenge is to survive a short circuit event. In this article, the author will discuss how to design a robust linear regulator with discrete components.
Here is an example used to power a microcontroller:
Input Range: 8.4V to 12.6V.
Output range: 1.71V to 3.7V.
Maximum load current: Io_max = 20mA.
Selection of Bipolar NPN Transistors
The NPN bipolar transistor Q1 is the most important component. The author chose this device first. The transistor should meet the following requirements:
The collector-to-emitter and base-to-emitter breakdown voltages should exceed the maximum input voltage Vin_max.
The maximum allowable collector current should exceed the maximum load current Io_max.
In addition to these two basic requirements, it is also a good idea to use components with alternative packages. Having this flexibility will simplify the design process later when it comes to power consumption. The author chose NPN transistors with alternative packages and different power ratings for this application.
Below are the key characteristics of the NPN transistors I used.
When IC = 50mA:
Direct current (DC) current gain hFE = 60;
Collector – Emitter maximum saturation voltage VCEsat = 300mV;
Base-Emitter Maximum Saturation Voltage VBEsat = 950mV.
Selection of Zener Diode Dz
The output voltage is equal to the reverse zener voltage VZ minus the transistor base-to-emitter voltage VBE. Therefore, the minimum reverse Zener voltage should meet the following requirements (Equation 1):
For this application, I chose a test condition with IZT = 1mA and a Zener diode with the following characteristics:
When Vo_min = 1.71V and VBE_max= 0.95V, Vz_min should be greater than 2.65V.
When the reverse current IZT = 1mA, the minimum reverse voltage VZ_min = 2.7V.
When the reverse current IZT = 5mA, the maximum reverse voltage VZ_max = 3.8V.
Base pull-up resistor RB
Resistor RB provides current to the Zener diode and transistor base. Under operating conditions, it should supply sufficient current. The Zener diode reverse current IZ should be greater than 1mA, as discussed by the author in the “Choice of Zener Diode Dz” section. Equation 2 estimates the maximum base current required for operation:
where Hfe_min = 60. Therefore, IB_max ≈ 0.333mA.
Equation 3 calculates the value of RB. I used a resistor with a 1% tolerance.
Therefore, RB should be less than 4.26kΩ. I used a resistor with a standard value of 4.22kΩ.
Add a dummy load resistor for output regulation
When the load current is zero, the output voltage reaches its maximum value. When 1mA ≤ IZT ≤ 5mA, the maximum value of VZ is 3.8. VBE(on) should be greater than 0.1V so that the output of this regulator can meet the requirements. In addition, I added a dummy load resistor to draw collector current under no-load conditions.
Figure 2 shows that VBE(on) can be used as a function of collector current IC. When IC = 0.1mA, VBE(on) is greater than 0.3V.
Figure 2: Base-Emitter Turn-On Voltage vs Collector Current
Equation 4 calculates this virtual resistance:
I added a 36kΩ resistor to the circuit, as shown in Figure 3.
Figure 3: Series regulator with dummy load resistor
Current limiting for short circuit events
A short circuit to ground at the output of the circuit shown in Figure 3 will result in a large collector current. A PSPICE simulation shows that the collector current can be as high as 190mA, see Figure 4.
Figure 4: Short circuit simulation results
The power consumption of transistor Q1 is 2.4W. There is no package that can handle this power consumption.
To limit the short-circuit current, I added a resistor RC (from VIN to the collector of transistor Q1), as shown in Figure 5.
Figure 5: Series regulator with current limiting resistor
Resistor RC will meet the output regulation requirements and dissipate power in the event of a short circuit. The author can calculate the value of RC:
VCE_Test is the collector-emitter voltage used in Figure 1. I chose a 5% tolerance resistor for RC. Using Equation 5, RC should be less than 271Ω. Using this estimate, Equation 6 calculates the worst-case RC power dissipation during a short-circuit event:
This power consumption is about 0.56W. I chose a 1W, 270Ω power resistor. For applications with higher RC short circuit power dissipation, you can place multiple resistors in series to share the power dissipation.
Component Stress Analysis
As far as the resistor RC is concerned, the worst case power dissipation occurs in the short circuit event with the largest input. Using Equation 6, the maximum power dissipation can be calculated to be 0.59W.
As far as transistor Q1 is concerned, no worst case power dissipation occurs during a short circuit event because of the current limiting resistor RC. The power dissipation of Q1 during normal operation is a function of the collector current as shown in Equation 7:
The worst case happens when the following conditions are met:
VIN = VIN_max
VO = VO_min
IC = (VIN_max – VO_min)/(2×RC)
Therefore, the maximum power dissipation of Q1 is (VIN_max – VO_min)2/(4×RC). In this example it is 110mW. I chose a small-outline transistor with a power rating of 350mW in a SOT23 package.
As for the maximum power dissipation of RB, the worst case occurs in the short circuit event with the largest input. The voltage across RB is equal to the input voltage minus VBE(sat). The maximum power consumption is estimated at 38mW.
In this article, the author describes design guidelines for a robust, low-cost linear regulator with discrete components.