We can use transistorized low dropout voltage regulator circuits to achieve stable output voltages. These voltages can start at 3 V and go up to 5 V, 8 V, 9 V, 12 V, and more.
They maintain a very low dropout voltage of just 0.1 V.
For example if we build a 5 V low dropout regulator circuit it will provide a steady 5 V output even if the input supply drops to 5.1 V.
A Better Option Than 78XX Regulators
Unlike the standard 7805 regulator which needs at least 7 V input for a stable 5 V output, our circuit has a much lower dropout voltage of only 0.1 V. This makes it more suitable for many applications.
The simple low dropout circuits we discuss are better alternatives to the common 78XX series regulators like the 7805 and 7812. They do not require the input voltage to be 2 V higher than the output. Instead they work well with input voltages that are just within 2% of the output level.
For all linear regulators including the 78XX series and others like the LM317 and LM338 we need to ensure that the input voltage is at least 2 to 3 V above the desired output.
Making a 5 V Low Dropout Regulator
A straightforward low-dropout 5 V stabilized voltage regulator design is depicted in the following image, which will provide you with a correct 5 V regulated voltage regulator regardless of the input supply has fallen below 5.2 V.
The regulator's operation is actually rather straightforward: Q1 and Q2 combine to generate a straightforward high gain common-emitter power switch that permits minimal dropout voltage transfer through the input to the output.
Together with the zener diode, Q3 and R2 function as a simple feedback network that controls the output to a level that is about equal to the zener diode value.
This suggests that adjusting the zener voltage value might likewise alter the output voltage as needed. This is an additional benefit of the concept as it allows the user to alter even the unconventional output values that the fixed 78XX ICs cannot provide.
A 12 V Low Dropout Regulator Configuration
Simply altering the zener settings brings the output to the necessary stable level, as was described in the preceding section. To obtain a 12 V regulated output using inputs ranging from 12.3 V to 20 V, we swapped out the zener diode in the aforementioned 12 v LDO circuit with a 12 V zener diode.
The Current requirements.
The value of R1 as well as the current carrying capability of Q1 and Q2 will determine the current output from these LDO designs.
A maximum current of 200 mA will be permitted by the suggested value of R1, and larger amps could be achieved by suitably reducing the amount of R1.
A high hFE of at least 50 should be provided for Q1 and Q2 in order to guarantee outstanding efficiency. Additionally, as Q2 may also become somewhat heated during operation, it has to have a power transistor in addition to Q1.
Protection against Short Circuits
A noticeable disadvantage of the described low drop circuits is that they don't include short circuit protection, which is often an inherent characteristic of the majority of regular constant regulators.
However, this characteristic may be introduced by utilizing Q4 and Rx to add a current limiting step, as seen as follows:
The voltage drop across Rx is strong enough to switch ON Q4, that starts grounding the Q2 base, whenever the current rises over the predefined limit.
As a result, Q1 and Q2 conduction turn severely limited, and the output voltage stops up until the current consumption naturally returns to baseline.
Soft Start Low-Drop Transistor Regulator
Compared to the popular numerous emitter-follower variations, this high gain voltage regulator with only a few transistors has improved features.
As soon as the circuit was first started up, it had been evaluated in a 30 watt stereo amplifier, and these had rigorous requirements for an extremely controlled supply and an output voltage which might increase steadily from zero volts to the highest.
The power amplifiers' soft-start strategy (about two seconds) allowed the 2000 uF output capacitors to charge without causing the output transistors' collector current to increase excessively.
The typical output impedance of a regulator is 0.1 ohm. The equation is solved to determine the output voltage by:
VO = VZ - VBE1.
The following formula is used to calculate the output voltage's rising time:
T = RB.C1(1 -Vz/V ).
Many digital gadgets require their power supply to have a programmed switch-on sequence. The rising time of the circuit's output might be fixed to give this order or delay duration by setting appropriate RB/C1 values.
Modifiable LDO Circuit
The load is connected to the series transistor T4's collector pin, as shown in the diagram.
This suggests that the voltage across the emitter and collector of this specific transistor might be pushed ON strongly into saturation, resulting in a very small saturation voltage.
obviously the transistor type and current specification affect this particular voltage range.
Parts List
- R1 = 1.2 Ohms
- R2 = 10k
- R3 = 470 Ohms
- R4 = 1.2 k
- R5 = 560 Ohms
- R6 = 1.6 Ohms
- P1 = preset 500 Ohm
- C1 = 10uF/25V
- T1, T3 = BC557
- T2 = BC547
- T4 = BD438
- LED = RED 20mA 5mm
The voltage drop will likely be only 0.2 V in the context of the layout under discussion, which takes into account an ideal current of 0.5 A.
Add to this the voltage drop that is required for current regulation around R6. T3 starts to conduct and limits the output current at around 0.5 V across R6.
In addition to being an indicator, LED D1 also serves as a voltage reference diode, clamping a 1.5 V to 1.6 V reference level at T1's emitter.
The voltage divider, comprising R4, P1, and R5, provides the base driving current for T1. T1 gradually begins to conduct in relation to the voltage differential between the reference and output levels.
The same thing subsequently occurs with T2, resulting in T4 about basic drive. Capacitor C1 is used to filter the output stage.
You could simply swap out BD 438 with a different model, such as BD136, BD138, BD140, and so on.
However, it is possible that such transistors have a slightly higher saturation voltage. It must be noted that D1 would be a red LED as it functions as a reference source; other colored LEDs can have different voltage drop requirements.
How to Use Transistors when Building a Low Dropout Voltage Regulator
I required a voltage regulator having a very low dropout voltage that could provide a steady 5 V at about 3 A. Using discrete components, I constructed the LDO circuit listed below since many three-terminal regulators have a dropout of 2.5 V or more.
This low dropout voltage regulator concept was rather simple, yet it performed similarly to integrated circuit (IC) regulators.
The following test results are obtained from the prototype's schematic, which is shown in the drawing here:
- Dropout voltage (@ 3 A): 0.75 V
- Load Regulation (0-3 A): Below 10 mV
- Line regulation (Vin 6 — 15 V): Below 10 mV
- Ripple rejection (@ 3 A): -63 dB
- Output (no load): 4.96 V
Changes in the output voltage brought on by changes in the outside temperature will mostly depend on the characteristics of the transistor Q3 and the zener diode ZD1.
As a result, these components must be kept separate from sources of heat like the power transformer and the Q1 heatsink.
Since there isn't an unambiguous current restriction in the circuit's current configuration, this feature may be added by combining the parts shown in the illustration as follows.
The dropout voltage will rise by 0.5 V as a result, though. When putting the output to the appropriate highest current and gradually increasing the value of resistor R1 until the output voltage begins to decrease, you may also set a current limit without raising the dropout voltage.
The drawback of this approach is that RI needs to be determined experimentally, and if Q1 is ever changed, recalibration could be required.
Therefore, the process shown in the second diagram above is the recommended option if a current restriction is required and a slight increase in dropout voltage is acceptable.
This circuit's ability to immediately deactivate in the event that a significant load causes the output voltage to drop below about 1.2 V is additional notable feature.
You have to turn off the mains or detach the input voltage, wait a few seconds, and finally turn it back on. Restarting won't happen if the load is just released.
As origin components, components C1 and D1 provide reliable launches even when loads are noticeably capacitive.
Use this simplified design process to modify the circuit to meet different voltage and current requirements (up to about 5 A):
- Find Vo (5 V), the output voltage.
- Choose Io (3 A), the maximum current.
- Select a suitable transistor (TIP2955, 70 V, 10 A) for Q1.
- Vo / (Io / (Q1 hFE)) = 5 / (3 / 20) = 33.3 ohms, or simply 33 ohms, is the formula to calculate R1 max.
- Use a 1 W resistor to calculate the dissipation of R1: Vo^2 / R1 = 25 / 33 = 0.75 W.
- For Q2, pick a suitable device (BD139, 80 V, 1 A).
- Use a 1k2 resistor to find R2 max: Vo / (Io / (Q1 hFE) * (Q2 hFE)) = 5 / (3 / 20 * 40) = 1300 ohms.
- Determine R2's dissipation using the formula Vo^2 / R2 = 25 / 1200 = 21 mW.
- Select a suitable device (BC548, 25 V, 100 mA) for Q3.
- Select a ZD1 zener diode with a voltage of Vo - Vbe (Q3) = 5 - 0.65 = 4.35 V (use a 4V3 zener instead). A 400 mW or 1 W zener can be used because the current flowing through Q3's base is usually quite little.
- To bias the zener, calculate R3 as follows: I2 = 0.2 * (P2 / V2) = 0.2 * (0.4 / 4.3) = 18.6 mA (assuming a 400 mW zener). R3 = 0.65 V / 18.6 mA = 35 ohms, thus use a resistor that is 33 ohms and 1/4 W.
- R4 prevents Q1 from triggering because of leakage; a 100-ohm, 1/4 W resistor is usually suitable, and its exact value is not important.
- R5 may have values as low as 100 ohms and is used as a safeguard to limit excessive current flowing through the base of Q3.
- The starter capacitor is C1, and any capacitor with a voltage rating higher than Vin and a capacity between 1 uF and 4.7 uF should work.
- C2 is used for stability, and 100 uF per amp of load current is a suitable value. The prototype utilized a 330 uF capacitor, which should have a voltage rating higher than the output voltage rating.
- Regarding stability, C3 is essential and ought to be at or near 10 nF.
- Assuming Vin-Vout is low, position Q1 on a suitable heatsink based on the power it dissipates, that will need to be minimal.
Differential Low Dropout Voltage Regulator Circuit
The proposed low dropout regulator circuit is built using ordinary bipolar transistors, and can be effectively used for getting an output virtually zero drop. Meaning suppose if the circuit is intended for getting 5 V regulated from a 5 V to 7 V supply it would give a stabilized fixed output which will be almost equal to a constant 5 V.
In regulator power supply circuits where the input and output voltages aren’t equal, the application of integrated 3-pin voltage regulators like7805, 7812, 7824 etc are not encouraged.
Disadvantage of Standard 3 pin Regulators
The drawback with most of the 3-pin regulators is that they need an input voltage which must be 3 V more than the voltage at the output.
In applications where input and output voltages are nearly identical, then it is better to use a customized low dropout regulator circuit using discrete components as explained below.
In a typical emitter circuit, the series transistor is connected so that there is a lower output voltage than the input voltage but only through the saturation voltage of the transistor. Nevertheless, it will be challenging to deliver short-circuit protection.
Figure 1 represents the series transistor gets its base current from T2 which combines itself with T1 to form a differential amplifier.
This setup makes sure that the D2’s cathode and the junction of voltage divider R4-R5 have equal voltage. The main part for the circuit is T3 which occupies definite current amplification.
However, T2 can only supply the amount of base current that R2 permits. The differential voltage across R2 has a maximum value of the Zener diode sans the base-emitter voltage, VBE, of T2, which is around 4 V. The maximum current that passes R2 is more or less 11 mA.
Output Current Range
When we assume the T3 has a current amplification capacity of 50, the maximum output current would be around 0.55 A. Let’s say a stronger current is drawn, then there will be a drop in the output voltage. Say the drop in voltage is lower than D2’s voltage, the potential difference across R2 will also drop.
As a result, the output current will react as depicted in Figure 2 which describes its fold-back properties. Evidently, we have proof that the series transistor is safeguarded against high, short-circuit currents.
Diode D1 and resistor R1 deliver a soft launch because the voltage across the diode is zero at switch-on mainly due to its connection to the regulator’s output. The circuit may oscillate because of high gain but this can be stabilised when capacitor C1 is added.
How to Select the LDO Output Voltage
The output voltage level, V0, of this low dropout voltage regulator circuit is free to select but you have to make sure it stays within the range of series transistor and D2, R3 and R4. The formula below is used to determine the output voltage level:
V0 = Vz (R5 + R4 / R5)
It is important to equate resistor R2 with the actual current amplification of the transistor. It is noted that a properly cooled BD140 has a maximum dissipation of 5 W.
Moreover, if a noise-free output is needed, an additional 10 µF electrolytic capacitor must be attached in parallel to D2. All this allows a soft start for the circuit with practically no output for about 0.2 s after it is switched on.
Leave a Reply