In a regulated power supply, the load current Ifz flows through the regulating tube. To handle large currents, a high-power adjustment tube is required. This necessitates a sufficient base current to drive the adjusting tube, and the comparative amplifying circuit may not be suitable for such high current demands. Additionally, the adjustment tube must have a higher current gain to improve voltage regulation performance effectively. However, high-power tubes typically have a low current amplification factor, creating a contradiction.
To resolve this issue, one or more "assistants" can be added to the original adjustment tube, forming what is known as a composite tube. The stabilized power supply circuit using the composite tube as the adjusting element is illustrated in Figure 5-24. When a composite tube is used, the reverse leakage current Iceo2 of BG2 can 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 increase 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 operates in the active region across 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 temperature and load variations are minimal or if a fully silicon-based design 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
[Image: Circuit diagram of a regulated power supply with protection]
In any voltage regulator circuit, it is essential to include short-circuit protection to ensure safe and reliable operation. Ordinary fuses are too slow to react and are not suitable for protecting the adjustment tube. Therefore, a dedicated protection circuit must be implemented.
The primary function of the protection circuit is to prevent the adjustment tube from being damaged during a short circuit by limiting the current flow. The basic principle involves turning off the adjustment tube when the output 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. During normal operation, the voltage across the diode remains low, keeping it in reverse bias. When the load current increases beyond a certain point, the voltage drop across R0 becomes sufficient to turn on the diode. Since UD = Ube1 + R0 * Ie and the forward voltage of the diode is constant, Ube1 decreases, limiting the collector current Ie and protecting the adjustment tube. For optimal performance, a diode with a higher forward voltage should be used.
[Image: Diode protection circuit]
Figure 5-26 illustrates a transistor-based protection circuit, which includes transistor BG2 and voltage divider resistors R4 and R5. Under normal operating conditions, the base potential of BG2 is higher than its emitter potential due to the voltage division provided by R4 and R5, keeping BG2 in an off state (equivalent to an open circuit). However, during a short circuit, the output voltage drops to zero, causing the emitter of BG2 to act as a ground. This turns BG2 into a saturated conduction state, effectively shorting the base and emitter of the adjustment tube BG1, thus turning it off and cutting off the current flow. This ensures the regulated power supply meets its technical specifications and safety requirements.
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