Designing a Switched Power Supply Circuit: Buck Topology Specs, Parts, and Thermal Math

When stepping down a 24V battery bus to a 5V logic rail at 3A, a switched power supply circuit is the only practical choice. By using high-frequency pulse-width modulation (PWM) and energy storage elements like inductors and capacitors, a switching regulator transfers energy in discrete packets rather than burning off excess voltage as heat. For a 24V-to-5V, 3A load, a synchronous buck converter like the Texas Instruments TPS5430 or the classic LM2596 will achieve upwards of 88% efficiency. In contrast, a linear regulator would waste nearly 80% of its input power as thermal dissipation, requiring massive heatsinks that simply do not fit on modern PCBs.

Linear vs. Switching: Topology Comparison for DC-DC Loads

The decision between linear and switching topologies comes down to headroom math and thermal limits. If you attempt to use a linear regulator (like an LM317 or a high-voltage LDO) to drop 24V down to 5V at 3A, the dropout voltage across the pass transistor is 19V. The power dissipated as heat is calculated as:

P_diss = (V_in - V_out) × I_load = (24V - 5V) × 3A = 57W

Dissipating 57W on a standard TO-220 package without active liquid cooling is impossible; the silicon junction will hit its 150°C thermal shutdown limit in seconds. The efficiency of this linear approach is a dismal 20.8% (5V / 24V). A switched power supply circuit solves this by rapidly switching a MOSFET on and off, using an inductor to smooth the current. The power loss is restricted to switching transitions, conduction resistance (Rds_on), and inductor core losses.

For low-noise analog sensor fronts (like precision ADCs), you might use a switched power supply circuit to step down to 6V, followed by a low-dropout linear regulator to hit 5V cleanly. But for digital logic, microcontrollers, and motor drivers, the switching topology wins outright. For deeper topology selection, refer to the Texas Instruments Buck Converter Overview.

Design Example: 24V to 5V / 3A Buck Converter

Let us design a robust switched power supply circuit for a 24V nominal solar battery bank (which can swing from 18V to 32V) powering a 5V, 3A embedded controller. We will use the TPS5430, a widely available 5.5V-to-36V input, 3A continuous step-down converter.

Input Range and Protection Requirements

Battery buses are electrically hostile. You must design for load dump transients and reverse polarity. A 24V lead-acid or LiFePO4 bank can experience voltage spikes up to 40V during alternator load dump or inductive kickback from nearby relays.

• Reverse Polarity: Use a P-channel MOSFET (e.g., Si2301) on the high side. Unlike a diode, a P-FET drops only millivolts at 3A, saving nearly a watt of heat.
• Transient Voltage Suppression (TVS): Place an SMAJ28A TVS diode immediately after the P-FET to clamp voltage spikes above 28V, protecting the TPS5430's 36V absolute maximum VIN pin.
• Input Bulk Capacitance: A switched power supply circuit draws pulsed current from the source. You need low-ESR ceramic capacitors (e.g., 2× 10µF, 50V X7R) placed as close to the VIN and PGND pins as possible to minimize the high-frequency 'hot loop' area.

Component Selection and Headroom Math

The TPS5430 operates with an internal 500kHz switching frequency. To calculate the required inductor value, we first determine the duty cycle (D) and target ripple current (ΔI_L). We typically target a ripple current of 30% of the maximum load (0.9A).

D = V_out / V_in = 5V / 24V = 0.208
L = (V_out × (1 - D)) / (f_sw × ΔI_L)
L = (5 × (1 - 0.208)) / (500,000 × 0.9) = 8.8 µH

We select a standard 10µH shielded ferrite inductor rated for at least 4.5A saturation current (Isat). Shielded inductors are mandatory here to prevent magnetic flux from coupling into nearby high-impedance analog traces.

For a comprehensive breakdown of inductor saturation and core losses, All About Circuits provides an excellent primer on buck converter magnetics.

Thermal Derating and Ripple Noise Expectations

A common mistake on the bench is assuming a regulator's '3A continuous' rating applies at all ambient temperatures and input voltages. Silicon junction temperature (Tj) is the true limiting factor.

Thermal Math and PCB Derating

Assuming an 88% efficiency at 24V input and 3A output, the total power loss in the switched power supply circuit is:

P_loss = P_out × ((1 / Efficiency) - 1) = 15W × ((1 / 0.88) - 1) = 2.04W

The TPS5430DDA package has a junction-to-ambient thermal resistance (θ_JA) of roughly 40°C/W when soldered to a standard 2-layer PCB with a 2-square-inch copper pour on the bottom layer connected via thermal vias.

T_rise = P_loss × θ_JA = 2.04W × 40°C/W = 81.6°C

If your ambient enclosure temperature is 45°C (common in solar charge boxes or engine bays), the junction temperature will be 45°C + 81.6°C = 126.6°C. This is dangerously close to the 150°C thermal shutdown threshold. Derating action: You must add an array of 0.3mm thermal vias directly under the exposed thermal pad, stitching to a solid ground plane on the opposite layer, which drops the effective θ_JA to roughly 25°C/W, keeping Tj at a safe 96°C.

Ripple and Noise Expectations

Unlike linear regulators, a switched power supply circuit inherently generates output voltage ripple. This ripple is primarily dictated by the Equivalent Series Resistance (ESR) of the output capacitors and the inductor ripple current.

Using three 22µF X5R ceramic capacitors in parallel, the combined ESR is approximately 3mΩ (0.003Ω). The peak-to-peak voltage ripple caused by ESR is:

ΔV_esr = ΔI_L × ESR = 0.9A × 0.003Ω = 2.7mV

Adding the capacitive ripple component (which depends on the capacitance and switching frequency), the total expected peak-to-peak output ripple is roughly 8mV to 12mV. This is perfectly acceptable for digital logic, ESP32 microcontrollers, and relays. If you are powering a 16-bit ADC, you will need to add a secondary LC pi-filter or a high-PSRR LDO on the 5V rail to attenuate the switching frequency harmonics.

Switched Power Supply Circuit FAQ