When you design dc/dc converters, it is often necessary to generate a bias supply to operate the control circuitry from the raw input voltage. Many methods are available for configuring the bias supply, each with its own benefits and shortcomings. Some methods use a "trickle-charge" start-up circuit, with a back-feed winding to provide power under normal operating conditions. With a low parts count and cycling short-circuit protection (if leakage inductance to the back-feed winding doesn't cause cycling to stop), this configuration is widely used in the power-supply industry. However, the configuration suffers from a long turn-on time and a limit on the capacitive load into which the converter starts up. Another popular approach uses a transistor-based pass regulator, often in conjunction with an overwinding to keep power dissipation low in normal run conditions. Although this method can provide a fast start-up, it generally exhibits high power dissipation during an output short circuit. Attempts to gate this type of circuit can be messy and complicated. A buck regulator overcomes the disadvantages but can be complex and costly. The circuit in Figure 1 uses IC1, an LT1431 shunt regulator, low-cost transistors, and an off-the-shelf inductor to form a high-voltage converter for use as a bias supply.
|Figure 1.||A shunt-regulator IC and a handful of low-cost components form an efficient, robust bias supply.|
The circuit needs no bleeder resistors or tertiary winding. The selection of Q1 and Q3 limits the maximum input voltage to 80 V in the circuit as shown. The circuit operates as a hysteretic regulator, also called a ripple regulator, relaxation oscillator, or bang-bang controller. Positive feedback comes from R6 and R7; negative feedback comes from C2, R8, and R9. With a 48 V input, inductor current is discontinuous, and the switching frequency is 50 kHz. Efficiency is typically 74% with a 48 V input and a 50-mA load, an acceptable figure for bias-supply power. Q2 and Q4 provide short-circuit protection; thus, the design is robust. Short-circuit current is typically 120 mA. You can repeatedly hot-plug the circuit with no adverse effects.
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