In a regulated power supply, the load current Ifz flows through the regulating tube. To handle large currents, a high-power adjusting tube must be used. This requires sufficient base current to drive the transistor, which can be challenging for the comparator amplifying circuit. Additionally, in order to achieve better voltage regulation, the adjustment tube needs a higher current gain. However, high-power transistors typically have lower current gain, creating a conflict between performance and practicality.
To resolve this issue, an auxiliary transistor or multiple "assistants" are often added alongside the main adjustment tube, forming what is known as a composite tube. The stabilized power supply circuit using a composite tube as the adjusting element is illustrated in Figure 5-24. When using a composite tube, the reverse leakage current Iceo2 of the second transistor (BG2) may be amplified, especially when a high-power transistor is involved. The reverse cutoff current Icbo increases exponentially with temperature, which can lead to thermal runaway and an uncontrolled rise in output voltage Usc.
To counteract this, the error signal ΔUsc is amplified and applied to the base of BG2, reducing the collector current Ic and potentially turning off BG2. To ensure that the adjustment tube remains in the active region under varying temperatures, a base resistor R7 is often connected to either the positive or negative terminal of the power supply, as shown in Figure 5-24. This resistor may be omitted if the temperature or load variations are minimal, or if a fully silicon-based system is used.
The value of R7 can be approximated using the following formula:
[Image: Formula for calculating R7]
5. Regulated Power Supply with Protection Circuit
In any voltage regulator circuit, short-circuit protection is essential for safe and reliable operation. Ordinary fuses are not suitable for fast protection, so a dedicated protection circuit must be implemented.
The purpose of the protection circuit is to prevent the adjustment tube from overheating or burning out during a short circuit. The basic principle involves turning off the adjustment tube by applying reverse bias when the current exceeds a certain threshold, thereby cutting off the circuit current automatically.
There are various types of protection circuits. Figure 5-25 shows a diode-based protection circuit consisting of a diode D and a sensing resistor R0. Under normal conditions, the diode remains in reverse bias due to low voltage across it. However, when the load current increases, the voltage drop across R0 causes the diode to turn on. As UD = Ube1 + R0 * Ie and the diode's forward voltage is constant, Ube1 decreases, limiting the collector current Ie and protecting the adjustment tube. It is recommended to use a diode with a higher forward voltage for optimal performance.
[Image: Diode protection circuit]
Figure 5-26 illustrates a transistor-based protection circuit, composed of transistor BG2 and resistors R4 and R5. Normally, the base potential of BG2 is higher than its emitter potential, keeping it in an off state. However, during a short circuit, the output voltage drops to zero, causing BG2 to conduct heavily, effectively shorting the base and emitter of the adjustment tube BG1 and turning it off. This prevents excessive current flow and ensures the stability and safety of the regulated power supply.
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