How Do Switch Mode Power Supplies Work? SMPS Theory and Relay Control

The Core Physics: How Do Switch Mode Power Supplies Work?

A switch mode power supply (SMPS) converts electrical power efficiently by using a high-frequency switching transistor (usually a MOSFET) to rapidly turn DC on and off, storing and transferring energy through inductors and capacitors. Unlike linear regulators that burn excess voltage as heat, an SMPS acts like a high-speed electronic valve. By adjusting the duty cycle (the ratio of 'on' time to 'off' time) of the switching waveform, the SMPS precisely regulates the output voltage.

To understand the math, consider stepping 120V AC down to 24V DC using a standard isolated flyback SMPS topology. First, the AC mains is rectified and filtered into high-voltage DC (roughly 170V DC). The switching MOSFET then chops this 170V DC at a high frequency—typically between 50 kHz and 150 kHz in modern 2026 designs utilizing Gallium Nitride (GaN) transistors. This high-frequency AC is fed through a step-down transformer. On the secondary side, the voltage is rectified again and smoothed by an LC (inductor-capacitor) filter. If the controller needs 24V out, it dynamically adjusts the PWM duty cycle to maintain that exact output regardless of input voltage sags or load changes. This process yields 85% to 95% efficiency, compared to the 40% to 60% efficiency of a linear transformer-based supply.

For a deeper look at the underlying semiconductor physics and topologies, the Texas Instruments Power Management guides provide excellent schematic breakdowns of buck, boost, and flyback architectures.

Sizing an SMPS for Electromechanical Loads

When you use an SMPS—such as a 24VDC Mean Well MDR-60-24 DIN-rail supply—to drive electromechanical components like relays and contactors, you must evaluate two entirely different sides of the component: the control side (coil) and the power side (contacts). The most common mistake on the bench is sizing the power supply based on the load being switched, rather than the coil pulling the current from the SMPS.

Here is the rating matrix for a standard industrial relay (e.g., Omron G2R-2-E DPDT) to illustrate which rating column governs your SMPS sizing:

Which rating column governs this load? The Coil Side Rating governs the SMPS sizing. If you are driving ten of these 24VDC relays simultaneously, your total steady-state draw is only 210 mA (0.21 A). A 2.5A SMPS is more than sufficient. However, the Contact Side Rating and Breaking Capacity govern the actual high-power load (like a motor or heater) that the relay is switching on the other side of the circuit.

Wiring the Control Circuit: Coil vs. Contact Side

Proper wiring requires strict separation between the low-voltage SMPS control loop and the higher-voltage contact switching loop.

Coil Side Wiring (SMPS Output to Relay)

Connect the SMPS positive output (+V) to the relay's A1 terminal and the negative output (-V or COM) to the A2 terminal. Use 18 AWG or 20 AWG wire; since the current is under 50mA, voltage drop is negligible over short DIN-rail runs.

Contact Side Wiring (Line and Load)

The contact side operates independently of the SMPS. Wire your line voltage to the Common (C) terminal and your load to the Normally Open (NO) or Normally Closed (NC) terminal. Ensure terminal screws are torqued to the manufacturer's specification (typically 0.5 to 0.8 Nm for standard 10A relays) to prevent high-resistance joints that cause thermal runaway.

Load Selection Decision Path, Testing, and Maintenance

Electromechanical contacts degrade differently depending on the load type. Use this decision tree to select the right contact rating and derating factor when pairing your SMPS-driven relays to real-world loads.

How to Test It Dead and Live

Dead Testing (Power Off): Set your multimeter to Ohms (Ω). Measure across the A1 and A2 coil terminals; a 24VDC relay should read between 1,000Ω and 1,500Ω. An open reading (OL) means a broken internal coil wire. Next, set the meter to continuity and probe the Common and NO contacts. Manually press the relay armature; the meter should beep, confirming mechanical closure without excessive contact resistance.

Live Testing (Power On): Energize the SMPS and the coil. Set your multimeter to DC Volts. Measure directly across the closed contacts (Common to NO) while the load is running. A healthy relay will show a voltage drop of less than 50mV. If you read 200mV or higher, the contacts are pitted or carbon-fouled and are generating dangerous heat.

Repair vs. Replace and Protection Curves

When to Repair: You can 'repair' a relay circuit by cleaning external dust, tightening loose terminal screws, or replacing a blown flyback diode. When to Replace: Never attempt to repair a relay with welded contacts, a burnt coil smell, or visible internal arcing marks. Electromechanical relays are sealed, disposable components. Replace the entire unit. Protection Note: On the SMPS AC input side, do not treat fuses and breakers as interchangeable without considering trip curves. An SMPS draws a massive inrush current (often 40A to 60A for a few milliseconds) to charge its internal bulk capacitors. A standard fast-blow fuse will nuisance-trip on startup. You must use a slow-blow (time-delay) fuse or a Type C or Type D Miniature Circuit Breaker (MCB) that tolerates the brief magnetic inrush spike without tripping the thermal protection.

Frequently Asked Questions

Why does my switch mode power supply whine or buzz under light loads?

This is caused by a feature called 'skip-cycle mode' or 'burst mode.' To maintain high efficiency when the load is very light (e.g., just one 21mA relay coil on a 60W SMPS), the controller stops switching continuously and instead fires short bursts of pulses, then goes to sleep. If the burst frequency drops into the audible range (20 Hz to 20 kHz), the internal inductors and ceramic capacitors physically vibrate due to magnetostriction and piezoelectric effects, creating a whining sound. It is normal and does not indicate a fault, though upgrading to an SMPS with a higher minimum load or forced continuous conduction mode (CCM) will eliminate the noise.

How do switch mode power supplies handle inrush current compared to linear supplies?

Linear supplies use heavy copper-and-iron transformers that naturally limit inrush current via their primary winding resistance and inductance. SMPS units, however, present a near-dead short to the AC mains for the first few milliseconds as the bulk DC capacitors charge. To prevent tripping upstream breakers or destroying the rectifier bridge, modern SMPS units include an NTC (Negative Temperature Coefficient) thermistor in series with the AC input. The NTC starts with high resistance (e.g., 5Ω to 10Ω) to choke the inrush, then self-heats and drops to a fraction of an ohm for normal operation. If you cycle the SMPS power off and immediately back on before the NTC cools, you may blow the internal fuse because the inrush limiting is temporarily bypassed.

Can I parallel two switch mode power supplies for double the current?

Generally, no. If you wire two standard SMPS units in parallel to get more amperage, the unit with the slightly higher output voltage (even by 0.05V) will take 100% of the load until its overcurrent protection trips, at which point the second unit takes over and trips. To parallel SMPS units safely, you must buy models specifically designed with 'active current sharing' or 'trim' pins (often labeled CS or TRIM). These allow a master-slave communication loop where both supplies dynamically adjust their PWM duty cycles to balance the load equally. If you just need redundancy, use an OR-ing diode module rather than direct paralleling.