Chapter 4
Linear Power Supplies
Abstract
This chapter describes linear power supplies and switched linear regulators. Constant current linear regulators can be inefficient in some applications, especially if the voltage drop across the regulator circuit is a high proportion of the overall supply voltage, compared to the load voltage. On the other hand, a switched linear regulator used with rectified AC mains supplies can be efficient if designed correctly and have a good power factor. Both types produce very little or no electromagnetic interference.
Keywords
constant current
efficiency
switched linear regulators
electromagnetic interference
power factor
EMI
Linear power supplies for driving light-emitting diodes (LEDs) are preferred for a number of reasons. The complete absence of any electromagnetic interference (EMI) radiation is one important technical reason. Low-overall cost is an important commercial reason. However, they also have disadvantages: in some applications they have low efficiency and hence they introduce thermal problems. It may be necessary to add a heat sink, thus increasing cost and size.
Switched linear regulator ICs can be used to good effect when the circuit is powered directly from the AC mains supply. In this case, a number of linear regulators are used to drive a long string of low-current LEDs. The total number of LEDs in series has to be high enough for the forward voltage drop to be close to the peak AC line voltage. In a typical 230-V AC application, about 100 LEDs need to be connected in series. For 120-V AC applications, about 50 LEDs must be connected in series. Switched linear regulators are described in Section 4.3.
4.1. Voltage Regulators
Many voltage regulators are based on the LM317 originally from National Semiconductor, but are now made by a number of manufacturers. Inside the LM317 are: (1) a power switch, which is an NPN transistor; (2) a voltage reference set to produce 1.25 V, and (3) an operational amplifier (op-amp) to control the power switch, as shown in Fig. 4.1. The op-amp tries to keep the voltage at the output equal to the voltage at the adjust pin (ADJ) minus the reference voltage.
Figure 4.1 LM317 Regulator.
ADJ, Adjust pin; IN, input; OUT, output; R, resistor.
ADJ, Adjust pin; IN, input; OUT, output; R, resistor.
To produce a certain output voltage, a feedback resistor is connected from the output (OUT) to the ADJ pin and a sink resistor is connected from the ADJ pin to ground, thus creating a potential divider. Usually the feedback resistor is set to 240 Ω, to draw a minimum of 5 mA from the regulator and help maintain stability. A capacitor on the output terminal also helps with stability. The output voltage is given by the equation:
Note, IADJ = 100 μA, worst case.
Variations of the LM317 regulator include fixed positive voltage versions (LM78xx) and negative voltage versions (LM79xx), where “xx” indicates the voltage; that is, LM7805 is a +5-V, 1-A regulator.
The LM317 and its variants need a minimum input to output voltage difference to operate correctly. This is typically in the range 1–3 V, depending on the current through the regulator (higher current requires a higher-voltage differential). This input to output voltage difference is equal to the voltage across the internal constant current generator, as the OUT pin is at the same potential as the voltage reference.
Low drop-out voltage regulators sometimes use a PNP transistor as the power switch, with the emitter connected to the input (IN) terminal and the collector connected to the OUT terminal (Fig. 4.2). They also have a ground pin that enables an internal reference voltage to be generated independent of the input to output voltage differential. A drop-out voltage of less than 1 V is possible.
Figure 4.2 Low Drop-Out Voltage Regulator.
4.1.1. Voltage Regulators as Current Source or Sink
In Fig. 4.3 are shown two circuits using a voltage regulator as a current limiter, one is configured as a current source and the other as a current sink.
Figure 4.3 Constant Current Circuits Using LM317.
As previously described, the LM317 regulates when there is +1.25 V between the OUT and ADJ pins. In Fig. 4.3, a current sense resistor (R1) is connected between the OUT and ADJ pins. Current flowing through R1 will produce a voltage drop, with the OUT pin becoming more positive than the ADJ pin. When the voltage drop across R1 reaches 1.25 V, the LM317 will limit the current. Thus the current limit is given by: 
4.2. Constant Current Circuits
There are many constant current circuits; some using integrated circuits, some using discrete components, and others using a combination of both ICs and discrete devices. In this subsection, we will examine some examples of each type.
4.2.1. Discrete Current Regulators
A simple constant current sink uses an op-amp with an input voltage range that extends to the negative rail, as shown in Fig. 4.4. To set the current level, a voltage reference is required. The voltage drop across a current sensing resistor is compared to the reference voltage and the op-amp output voltage rises or falls to control the current. The voltage reference can be a temperature-compensated precision reference, or a Zener diode. A Zener diode generally has a smallest temperature coefficient and lowest dynamic impedance at a breakdown voltage of 6.2 V. In Fig. 4.4, a 3.9-V Zener diode is used because the supply voltage is only 5 V.
Figure 4.4 Constant Current Sink Using Operational Amplifier (Op-Amp).
4.2.2. Integrated Regulators
There are a number of integrated current regulators suitable for driving LEDs. They can use bipolar or CMOS technology and have a wide range of working voltages. Lower-voltage (typically bipolar) types generally have lower costs, but there are times when high-voltage ratings are needed, particularly for transient suppression. An example where higher voltage is needed is shown in Fig. 4.5.
Figure 4.5 High-Voltage Regulator Example.
The Microchip CL220, in the aforementioned example, is a 20-mA current regulator that has a 220-V rating. When used with a high number of LEDs in series, it can be powered from a rectified AC mains supply. The number of LEDs required depends on the AC mains voltage, but for 230-V AC operation about 66 LEDs are needed. The surprising feature is the good power factor (0.916), which is mainly due to the equivalent series resistance of the many low-current LEDs used. The load is thus mainly resistive. Power factor concerns AC mains powered lamps and will be described in Chapter 8, but suffice to say here is that a power factor of 1 is a perfect resistive load and that regulations require a good power factor in most LED lighting applications.
4.3. Switched Linear Current Regulators for AC Mains Operation
Switched linear regulators intended for AC mains powered applications employ multiple current regulators and are for LED lamps where there is a long string of LEDs wired in series. There are tapping points along this LED string for connection to the linear regulators. For efficiency, the total number of LEDs in series has to be high enough, so that the LED string forward voltage drop is just below the peak AC voltage. There are two types of switched linear current regulators. The first type is integrated, like the Microchip CL8800, so that the switching off of each regulator is controlled by subsequent stages. The second type is modular, using a set of cascaded regulators, like the Texas Instruments TPS92410/TPS92411 chipset.
4.3.1. Integrated Switched Linear Regulators
The principle of switched linear regulators is illustrated in Fig. 4.6. In this case I am just showing four regulators, for clarity. The current limit of each regulator rises as we go from regulator 1 to regulator 4. Note the Zener diode used instead of LEDs for the last regulator; this provides no light but limits the voltage drop across the current regulator. In practice, the last string of LEDs provides little light, and using a Zener diode prevents low-frequency flicker as the AC voltage peak level varies over time.
Figure 4.6 Switched Linear Regulator Principle.
A good example of a switched linear regulator IC is the Microchip CL8800, comprising six current regulators (Fig. 4.7). Connected to the positive side of the supply voltage is the LED string, which is tapped at various points along its length. Each tapping point is connected to a CL8800 current regulator input. A current regulator can only conduct when the supply voltage exceeds the forward voltage drop of all the LEDs connected to it. As higher order regulators have more LEDs between them and the supply, the turn-on voltage for the current regulators are progressively increasing as we go from regulator 1 to regulator 6.
Figure 4.7 Typical CL8800 Application Circuit.
The current limits of the six regulators are set individually by external resistors connected, in series, to the 0-V rail. This series connection of the resistors ensures that the current limits are progressively increased. Regulator 1 has the lowest current limit and regulator 6 has the highest current limit. The regulators are internally coupled, so that when a higher-current regulator starts to conduct, the regulator immediately before it starts to turn off. For example, when current regulator 3 starts to conduct, current regulator 2 starts to turn off. Current regulator 1 is already off and, because of the progressive turn-on voltage, current regulators 4, 5, and 6 are not conducting.
In practice, the number of LEDs in each section of the LED string is progressively reduced. For example, string 1 between the supply voltage and regulator 1 may have 45 LEDs, but string 2 between regulator 1 and regulator 2 may only have 22 LEDs. The last string may have as few as 8 LEDs (or may be replaced by a Zener diode to prevent low-frequency flicker). This arrangement means that the voltage steps, between one regulator turning off and the next taking over, become smaller as the supply voltage increases.
The current steps are not linear either. The current steps are graduated, so they are made large for the first regulator and small for the last regulator. Example current settings are 45 mA for regulator 1, then 60 mA for regulator 2, followed by 80 (reg. 3), 88 (reg. 4), 92 (reg. 5), and finally 95 mA for regulator 6. Note that the schematic shows the current setting resistor connected to ground from terminal set_5, rather than set_6. This is because the reference voltage for the 6th regulator was made higher than the other regulators, for some unknown reason.
The combination of progressively smaller voltage and current steps produced a very high power factor, typically 0.98. The coupling of regulators, so that they are turn on and off gradually, gives a very low-harmonic distortion of 5–15% typically.
4.3.2. Modular Cascade Linear Regulators
A good example of a modular cascade linear regulator is the Texas Instruments TPS92410 and TPS92411 chipset (Fig. 4.8). The TPS92410 is a linear controller for regulating the current. The IC uses changes in the rectified AC mains voltage to vary the current limit. The current is thus made proportional to the instantaneous AC mains voltage, so the power factor is high. The TPS92411 is a LED bypass switch, so that some LED strings are shorted out, depending on the AC mains voltage. This allows the efficiency to remain high.
Figure 4.8 Typical TPS92410/TPS92411 Application Circuit.
The typical application for 120-V AC operation uses three LED strings in series. The LED strings have to be different lengths, with the first string (80-V forward voltage) being double the length of the second (40-V forward voltage). The second string is double the length of the third string (20-V forward voltage).
During operation, as the AC mains voltage rises to about 20 V, the first TPS92411 switch opens and the associated LED string is powered. As the voltage rises further, to about 40 V, the second LED is powered and the first TPS92411 switch closes. Now only the second string is lit. As the voltage rises to about 60 V, the first TPS92411 switch opens again and both first and second LED strings are lit. The next step would be for the third LED string to be lit and both the first and second TPS92411 switches closing, thus extinguishing the associated LED strings. The LED strings are lit in a binary sequence, as shown in Fig. 4.9.
Figure 4.9 Binary Illumination Sequence.
4.4. Advantages and Disadvantages
The advantage of linear power supplies is that they produce no EMI radiation. This advantage cannot be overstated, as the cost of materials and development time spent on eliminating EMI can be high.
A switching power supply may appear to have few components, but this does not take into account the EMI filtering and screening. These additional circuits can double the overall cost of the LED driver. If the LEDs are distributed, such as in channel lighting where there is no opportunity to shield any EMI, both common mode and differential filtering are required. And common mode chokes are expensive!
One disadvantage of a linear LED driver can be low efficiency, which is the ratio of the LED voltage to the supply voltage. The efficiency is low only if the supply voltage is somewhat higher than the LED voltage. In these cases, poor inefficiency causes the introduction of thermal problems. A heat sink may be required, which is bulky and moderately expensive. It should be noted that where the supply voltage is only slightly higher than the LED voltage, the efficiency of a circuit using linear regulator could be higher than one using a switching regulator.
Linear mains–powered LED drivers have the disadvantage of large size because a step-down transformer is almost always required (unless the LED string voltage is very near to the peak AC supply voltage). A 50- or 60-Hz mains transformer is bulky and heavy. Smoothing capacitors after the bridge rectifier are also very bulky. The efficiency will vary as the AC supply voltage rises and falls over a long period because the difference between the rectified voltage and the LED string voltage will change.
Switched linear or modular cascaded linear regulators can be used from the AC mains supply. They have the advantage of not needing a transformer, although the disadvantage of not being isolated from dangerously high voltages. They are quite small and surprisingly efficient because most of the voltage drop is across the LED load. However, they do need a considerable number of LEDs in series.
4.5. Limitations
The main limitation of a linear supply is that the LED voltage will always be lower than the supply voltage. Linear voltage and current sources cannot boost the output voltage so that the output is higher than the input. In cases where the output voltage needs to be higher than the input voltage, a switching regulator is necessary. These will be discussed in the next few chapters.
4.6. Common Errors in Designing Linear LED Drivers
The most common error is to ignore the power dissipation. Power dissipation is simply the voltage drop across the regulator multiplied by the current through it. If the voltage drop is high, the current must be limited to stay within the device package power dissipation limits. A surface-mount D-PAK package may be limited to about 1 W, even when there is some copper area soldered to the tab terminal. Heat sinks are now available for surface mount packages, which eases the problem.
Another error is to ignore the start-up conditions. The voltage rating of the regulator must be high enough to allow for the output being connected to 0 V (ground). This is because at start-up, the output capacitor will be uncharged and thus at 0 V. Only after operating for a short period does the output capacitor charge, which reduces the voltage drop across the regulator. The voltage rating of the regulator should always be greater than the maximum input voltage expected.
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