Designing a Dual Voltage Power Supply for 12V and 24V Solar Testing

To build a reliable dual voltage power supply delivering 24V and 12V for testing solar inverters, battery management systems (BMS), and DC-DC converters, use an enclosed AC-DC front-end paired with a high-current synchronous buck converter on the secondary rail. Avoid linear regulators for the 12V step-down; dropping 24V to 12V at 10A with a linear regulator wastes 120W as heat, whereas a modern switching topology runs at 92% efficiency and keeps your bench cool.

Topology Showdown: Linear vs. Switching for Dual-Rail Loads

When deriving a 12V rail from a 24V bus to create a dual voltage power supply, the choice between linear and switching topologies dictates your thermal design and noise floor. Hobbyists often default to linear regulators like the LM338 because they are cheap and simple, but the physics of headroom and dropout voltage make them a poor choice for high-current power system testing.

For testing power electronics, switching noise is the primary drawback. A raw synchronous buck output will show 40mV of switching ripple at the fundamental frequency (e.g., 500kHz). However, by adding a secondary LC pi-filter (a 2.2µH ferrite bead followed by 100µF of low-ESR ceramic capacitance), you can easily knock the high-frequency noise down to <5mV, giving you the best of both worlds: switching efficiency with linear-level cleanliness. You can review standard filter design principles in resources like the All About Circuits DC textbook chapter on power supply filters.

Design Example: 120VAC to 24V/12V Dual Voltage Power Supply

This reference design provides a robust 24V rail for testing MPPT charge controllers and a 12V rail for BMS logic and DC-DC converters. We use an off-the-shelf enclosed AC-DC module for safety and isolation, followed by a custom DC-DC stage.

System Specifications

Input Range and Protection Strategy

The Mean Well LRS-350 series accepts a wide 90-264VAC input range, making it globally compatible. However, the inrush current when charging the internal bulk capacitors can exceed 40A at 230VAC. The Amphenol CL-80 NTC thermistor limits this inrush to a safe level, protecting your bench breakers and the module's internal bridge rectifier. On the DC side, the SMBJ24CA TVS diode clamps any inductive kickback from the test loads (like disconnecting a relay coil on a solar controller) to 38.5V, well below the 42V absolute maximum rating of the LM5145 and downstream test devices.

Thermal Management, Derating, and Protection

Power supply design is ultimately an exercise in thermal management. While the switching topology eliminates the 120W heat dump of a linear regulator, you still must manage the ~10.4W of heat generated by the LM5145 MOSFETs and the inductor core losses.

Derating the AC-DC Front End: The Mean Well LRS-350-24 includes a temperature-controlled internal fan. However, according to manufacturer datasheets, the unit begins to linearly derate its maximum output power above 50°C ambient temperature, reaching 60% capacity at 70°C. If your bench supply is enclosed in a cabinet, you must provide forced ventilation to keep ambient air below 45°C.

DC-DC Stage Thermal Notes: The $R_{DS(on)}$ of the LM5145's internal MOSFETs has a positive temperature coefficient. At a 100°C junction temperature, conduction losses increase by roughly 40% compared to a 25°C baseline. To mitigate this:

• Use a 2oz copper pour on the top and bottom layers of the PCB directly under the IC's thermal pad.
• Stitch the thermal pad to the bottom layer with an array of 0.3mm vias to spread heat.
• Ensure the 10µH shielded inductor has at least 5mm of clearance above it for convective airflow; enclosed inductors trap heat in their ferrite cores, which can drop permeability and cause saturation at high currents.

Dual Voltage Power Supply FAQ

Can I wire two 12V switching supplies in series to make a dual voltage 24V supply?

Technically yes, but it is highly discouraged for testing power systems. Most standard off-the-shelf 12V switching supplies (like the LM2596 modules) do not have floating outputs; their DC negative terminal is tied to earth ground or the AC protective earth. Wiring them in series creates a dead short through the ground reference. If you must do this, you have to verify with a multimeter that the DC- terminals are completely isolated from the AC earth pin and the metal chassis. For reliable 24V and 12V testing, a single 24V front-end with a buck-derived 12V rail is significantly safer and avoids ground loop noise.

Why does my dual voltage power supply trip when I connect a 24V inverter?

Inverters have massive input capacitance (often thousands of microfarads) to handle high-frequency AC switching loads. When you first connect a 24V inverter to your power supply, the inrush current to charge those capacitors looks like a dead short, instantly tripping the power supply's overcurrent protection (OCP) or blowing your DC bus fuse. To fix this, add a precharge circuit: wire a 50-ohm, 10W power resistor in parallel with a pushbutton switch or a relay. Engage the resistor first to slowly charge the inverter caps over 2-3 seconds, then close the main contactor or switch to carry the continuous load.

What is the acceptable ripple for testing a 12V lithium BMS?

Battery Management Systems use high-impedance resistor dividers to measure individual cell voltages. These ADC lines are highly susceptible to high-frequency switching noise. If your 12V supply rail has more than 20mV of peak-to-peak ripple, the BMS microcontroller may read phantom voltage spikes, triggering false over-voltage (OVP) faults and prematurely shutting down the MOSFETs. For BMS testing, keep the 12V rail ripple below 5mV peak-to-peak by utilizing the LC pi-filter mentioned in the design example, and ensure your oscilloscope probe is measuring tip-to-barrel directly at the BMS power pins, not via a long ground alligator clip which will pick up radiated EMI.