Target Audience and Cost-Efficient Design Focus
This circuit design primarily caters to development engineers seeking an economical step-down converter with a broad input voltage range. Typically, such circuits utilize integrated switching elements within the step-down converter. However, opting for a more discrete solution can significantly lower the overall cost of the converter, especially when produced in large quantities. The project incorporates the TL5001A, a budget-friendly PWM controller ideally suited for this purpose. The step-down converter outlined here operates within an input voltage range spanning from 8 V to 30 V, generating a 5 V output with a maximum current output of 1.5 A. Upon applying the input voltage, IC1’s PWM output is activated, grounding one end of the voltage divider composed of R1 and R2.
Voltage Divider and Driver Stage Design
In this circuit, the current passing through the voltage divider is limited to a maximum of 25 mA. This value is calculated by dividing the maximum input voltage (30 V) minus the output driver’s saturation voltage (2 V) by the total resistance of the voltage divider (1.1 kΩ). T1 and T3 work in tandem to create an NPN/PNP driver stage. Their role is to swiftly charge the gate capacitance of the P-channel MOSFET, T2, during activation and efficiently discharge it during deactivation.
When the PWM output is active, a conducting state is reached in the base-emitter junction of T3. Consequently, T3 conducts from collector to emitter, rapidly discharging the gate capacitance of T2 to around 800 mV, allowing the P-channel MOSFET to conduct. In the absence of activation, a negligible current flows through R2 due to the open-collector output deactivation, leading to T1’s base being raised to the input voltage level. This activates the base-emitter junction of T1, charging the gate capacitance of T2 to the input voltage level, preventing the P-channel MOSFET from conducting.
Voltage Limiting and Switching Frequency
Diodes D2 and D3 serve as voltage limiters for the P-channel MOSFET, which has a maximum gate-source voltage of 20 V. When the Zener voltage of diode D2 is exceeded, it conducts. If the forward voltage of diode D3 is also surpassed, the two diodes collectively clamp the gate-source voltage to approximately 19 V. The circuit’s switching frequency is set at around 100 kHz, striking a balance between efficiency and component size.
Component Selection and Specifications
In component selection, various factors are considered. All resistors utilized are 1/16 W, 1%. Except for the electrolytic capacitor C1, ceramic capacitors are employed throughout the circuit. The larger capacitors, C2 and C5, are specific types: C2 is a Murata ceramic capacitor (GRM21BR71C105KA01) of 1 μF, 16 V, X7R, 10%, while C5 is a Murata ceramic capacitor (GRM32ER60J476ME20) of 47 μF, 6.3 V, X5R, 10%. The Schottky diode D1 is a Fairchild type (MBRS340T3) with a voltage rating of 40 V and a current capacity of 3 A.
For coil L1, a Würth power choke (744771147) with specifications 47 μH, 2.21 A, and 75 mΩ is used. The bipolar transistors T1 (BC846) and T3 (BC856) are complementary transistors with voltage ratings of 60 V, current capacity of 200 mA, and power dissipation of 310 mW, sourced from Vishay. The PWM controller IC1 (TL5001AID) is a low-cost PWM controller equipped with an open-collector output, designed by Texas Instruments.