Preregulator for power supply

Description: This SCR pre-regulator maintains the filter capacitor voltage (Vc) in a variable output power supply at a few volts above the output voltage (V0). The advantages include reduced heat dissipation by the pass transistor, resulting in a smaller heatsink, cooler operation, and improved efficiency, particularly at low output voltages. Components Q1, R1, R2, D1, and D2 create a constant current source for zener diode Z1, ensuring that the output current contribution remains consistently between 2-3 mA. The Darlington pair consisting of Q2 and Q3 keeps the SCR in an off state. The voltage Vc decreases until it equals V0, at which point the Darlington pair activates the SCR, charging the filter capacitor to a higher voltage (VC1) in less than half of the input voltage period. The specified component values are intended for a power supply with a range of 0 - 250 V and a current rating of 3 A.

This SCR pre-regulator circuit is designed to optimize the performance of a variable output power supply by managing the voltage across the filter capacitor (Vc). The primary function of the circuit is to ensure that Vc remains slightly higher than the output voltage (V0), which enhances the efficiency of the power supply while minimizing heat generation in the pass transistor.

The circuit employs a combination of a constant current source comprised of Q1, R1, R2, D1, and D2, which provides a steady current to zener diode Z1. This setup allows the circuit to maintain a consistent output current contribution of 2-3 mA, which is critical for stabilizing the operation of the zener diode and ensuring reliable voltage regulation.

The Darlington pair formed by transistors Q2 and Q3 plays a vital role in controlling the SCR's state. When the voltage Vc drops to match the output voltage V0, the Darlington pair triggers the SCR, allowing it to conduct. This action rapidly charges the filter capacitor to a higher voltage (VC1) within a short timeframe, specifically in less than half of the input voltage period. This quick response is essential for maintaining the desired output characteristics of the power supply.

The component values specified for the circuit are tailored for a power supply capable of delivering up to 250 V and 3 A. This design ensures that the system operates efficiently across a range of output conditions, making it suitable for various applications requiring stable and adjustable voltage supplies. Overall, the SCR pre-regulator circuit is an effective solution for enhancing the performance and reliability of variable output power supplies.This SCR pre-regulator keeps the filter capacitor Vc, in a variable output power supply, a few volts above the output voltage V0. The benefits include: less heat dissipated by the pass transistor and therefore small heatsink, cooler operation and higher efficiency, especially at low output voltages.

Ql, Rl, R2, Dl and D2 form a constant current source for zener Zl, so that the contribution to the output current is always a few mA (2-3 mA). The Darlington pair Q2, Q3 keeps the SCR off. The voltage Vc decreases until Vc = V0 = V at which point the Darlington pair fires the SCR, charging the filter capacitor to a higher voltage VC1 in less than half the period of the input voltage. The component values, shown are for a 0 - 250-V, 3-A power supply.