Designing a Solid State Power Supply: Linear vs Switching for 24V DC Loads

When you need to step down a 24V solar battery bank or industrial DC bus to 5V for an ESP32 sensor node, you are designing a solid state power supply. The direct answer for a 24V-to-5V, 2A load is to use a synchronous or asynchronous switching buck regulator like the Texas Instruments TPS5430DDA. A switching topology will dissipate roughly 1.4W of heat at 88% efficiency, whereas a linear regulator would attempt to burn 38W as waste heat, instantly triggering thermal shutdown or destroying the silicon.

This guide walks through the topology math, input protection requirements, and a complete component-level design to get your 24V system running reliably.

The Solid State Power Supply Dilemma: Linear vs. Switching Topologies

In modern electronics, a 'solid state power supply' simply means the conversion is handled entirely by semiconductor junctions (MOSFETs, BJTs, diodes) rather than electromechanical relays or motor-generator sets. The real decision on the bench is between linear regulation and switching regulation.

Linear regulators act as smart, variable resistors. They drop excess voltage by converting it directly to heat. Switching regulators rapidly toggle a MOSFET on and off, storing energy in an inductor and releasing it to the output, which allows them to step down voltage with minimal thermal loss.

Sizing the Input Stage: Protection and Headroom Math

A '24V nominal' battery system is rarely exactly 24V. A 24V LiFePO4 pack ranges from 20V (empty) to 28.4V (full charge). A 24V lead-acid bank can hit 29.2V during equalization charging, and can experience inductive spikes up to 40V when heavy loads disconnect.

Your solid state power supply input stage must handle this range and survive transients.

1. Reverse Polarity Protection: Skip the standard 1N4007 diode. At 2A, a silicon diode drops ~0.8V, wasting 1.6W. Instead, use a P-channel MOSFET like the Si2301 in the high-side path. It drops only ~50mV ($I^2R$ loss) and blocks reverse voltage inherently.
2. Transient Voltage Suppression (TVS): The TPS5430 has an absolute maximum input rating of 40V. To clamp inductive spikes from long battery cables, place an SMAJ24A TVS diode across the input. Under a surge, it clamps at 38.9V, keeping the IC safely below its 40V destruction threshold.
3. Headroom and Dropout: The TPS5430 requires a minimum input-to-output differential (dropout voltage) of roughly 1.5V to maintain regulation at high loads. With a 5V output, your minimum input must stay above 6.5V. Since our lowest battery state is 20V, headroom is more than sufficient.

Worked Design Example: 24V to 5V @ 2A for Mixed-Signal IoT

Let's spec the exact component values for a TI TPS5430DDA buck converter delivering 5V at 2A continuous. The TPS5430 operates up to 36V continuous (40V transient) and switches at a fixed 500 kHz, which is a great middle-ground for efficiency and EMI.

The Feedback Math: The TPS5430 regulates the FB pin to an internal 1.221V reference. The formula is $V_{out} = 1.221 \times (1 + \frac{R1}{R2})$. To get 5.0V, the ratio $\frac{R1}{R2}$ must be 3.095. Using a standard 10kΩ for R2, R1 calculates to 30.95kΩ. The nearest standard 1% E96 resistor value is 30.9kΩ, yielding an actual output of 4.99V.

Inductor Sizing: According to the TPS5430 datasheet, you want the inductor ripple current ($\Delta I_L$) to be roughly 30% of your maximum load. At 500 kHz, a 15µH inductor yields a ripple current of ~0.6A at maximum input voltage, keeping the peak inductor current well under the IC's 4A internal current limit and preventing core saturation. Always choose an inductor with a saturation current rating ($I_{sat}$) at least 20% higher than the IC's current limit (minimum 4.8A).

Thermal Derating and Ripple Expectations

Even switching regulators generate heat. The internal MOSFETs have on-resistance ($R_{DS(on)}$), and the Schottky diode drops roughly 0.5V during the freewheeling phase.

At 24V in, 5V out, and 2A load, the TPS5430 operates at roughly 88% efficiency. Total power loss is about 1.4W. The DDA package features an exposed thermal PowerPAD. If soldered to a 2oz copper pour with thermal vias on a 4-layer board, the junction-to-ambient thermal resistance ($\theta_{JA}$) drops to roughly 40°C/W.

• Temperature Rise: $1.4W \times 40°C/W = 56°C$ rise.
• Junction Temp: 25°C (ambient) + 56°C = 81°C. This is well within the 125°C maximum operating limit.

Ripple and Noise: A 500 kHz buck converter will inherently produce 30mV to 50mV peak-to-peak switching ripple on the output. For digital logic (ESP32, Raspberry Pi Pico), this is perfectly acceptable. However, if you are powering a 16-bit ADC or a load-cell amplifier, this noise will show up in your readings.

The Fix: Add a passive LC post-filter. Place a 2.2µH ferrite bead or chip inductor in series with the 5V rail, followed by a 10µF X7R ceramic capacitor to ground right at the analog IC's VCC pin. This creates a low-pass filter that knocks the 500 kHz switching noise down to <2mV, giving you the best of both worlds: switching efficiency and linear-regulator quietness.

The Decision Tree: Picking Your Exact Regulator

Don't default to a switching regulator for every single rail on your board. Use this decision matrix to pick the right solid state topology for your specific sub-circuit.

The Default Recommendation: For 90% of 24V battery, solar, and industrial DC-DC conversion tasks requiring 1A to 3A, the TI TPS5430DDA is the definitive pick. It forgives minor PCB layout mistakes better than ultra-high-frequency (2MHz+) alternatives, the SOIC-8 PowerPAD is easy to hand-solder with a hot air gun for prototyping, and the 36V continuous rating provides a comfortable safety margin over 24V nominal battery systems. Pair it with an SMAJ24A TVS on the input, and your power supply will survive the harsh transients of real-world battery banks.