When building a 24V DC system for LiFePO4 battery charging, telecom gear, or high-power LED arrays, pulling 120W from a standard wall outlet requires more than just a basic buck converter. You need an isolated offline smps power supply circuit that handles universal AC input, survives grid surges, and delivers clean DC without melting your enclosure. This guide walks through the exact math, topology selection, and component-level design for a 120W (24V / 5A) offline flyback converter.
Topology Selection: Linear vs. Switching and Flyback vs. Forward
Before picking a controller, we must answer the fundamental question: why not use a linear regulator? The decision comes down to dropout voltage and thermal physics. If you rectify 120VAC, you get roughly 170V peak, which sags to about 160VDC on the bulk capacitor under a 5A load. To regulate 160VDC down to 24V at 5A using a linear pass element, the dropout voltage is 136V.
The power dissipated as heat would be P = V_drop × I_out = 136V × 5A = 680W. Your efficiency would be a dismal 15%, and you would need a heatsink the size of a car battery. A modern smps power supply circuit operating at 88% efficiency only dissipates about 16.3W of heat (120W / 0.88 - 120W), making it viable for a sealed, fanless enclosure.
For isolated AC-DC conversion in the 50W to 150W range, you have three main switching topologies to choose from. Here is how they stack up in real-world bench conditions:
| Topology | Typical Efficiency | Heat / Thermal Mgmt | Noise / EMI Profile | Cost & Complexity |
|---|---|---|---|---|
| Flyback | 84% - 89% | Moderate (Primary switch & secondary diode run hot) | High (Hard switching, high di/dt and dv/dt spikes) | Lowest. 1 magnetic component. Ideal for <150W. |
| Forward (Single/Two-Switch) | 87% - 91% | Lower (Energy transferred during on-time, less RMS current) | Moderate (Better than flyback, requires output LC filter) | Medium. Requires transformer reset mechanism and output inductor. |
| LLC Resonant | 92% - 95% | Lowest (ZVS/ZCS eliminates switching losses) | Lowest (Sinusoidal waveforms, minimal EMI) | Highest. Complex magnetics design, tight dead-time control. |
| Active Clamp Flyback | 90% - 93% | Low (Recovers leakage inductance energy) | Low (Soft switching on primary) | High. Requires auxiliary clamp switch and precise timing. |
For a 120W bench build or custom battery charger, the standard Flyback remains the undisputed king of cost-to-performance. While LLC resonant converters are highly efficient, the magnetics design is unforgiving for hobbyists and small-batch makers. We will proceed with a Flyback topology using a highly integrated primary-side regulator to minimize component count.
Input Stage: Universal Range, Protection, and Rectification
A robust offline supply must handle a universal input range of 85VAC to 265VAC to accommodate global grids and brownout conditions. The input stage conditions the AC line, protects against transients, and limits inrush current.
Protection and EMI Filtering
- Fuse: Use a 3.15A, 250VAC time-delay (slow-blow) ceramic fuse. A fast-blow fuse will nuisance-trip during the initial bulk capacitor charging phase.
- MOV (Metal Oxide Varistor): A 10D471 (470V clamping voltage) placed line-to-neutral. This absorbs standard IEC 61000-4-5 surges. If the MOV fails short, the fuse blows, saving the downstream bridge rectifier.
- NTC Thermistor: A 5D-11 (5 ohms cold resistance, 11mm diameter) limits the inrush current to roughly 32A peak at 265VAC high-line. Bench Tip: For >75W designs, the continuous 0.5W+ dissipation in the NTC wastes efficiency. In high-end designs, a relay bypasses the NTC after 500ms, but for a basic 120W build, leaving it in-line is acceptable if you ensure adequate airflow.
- Common Mode Choke: A 10mH toroidal choke paired with 0.1µF X2 and 2.2nF Y2 capacitors forms the pi-filter necessary to pass FCC Class B conducted emissions.
Rectification and Bulk Storage
A standard full-bridge rectifier like the KBU808 (8A, 800V) converts the AC to DC. For the bulk capacitor, the rule of thumb for universal input flybacks is 2µF to 3µF per watt of output power. For 120W, we need a 220µF, 400V electrolytic capacitor (e.g., Rubycon MXG or Nippon Chemi-Con KXJ series) rated for 105°C. This maintains the DC bus above 100V even at 85VAC low-line conditions.
The 120W SMPS Power Supply Circuit: 24V/5A Flyback Design
To minimize the optocoupler drift and secondary-side shunt regulator complexity, we will use Power Integrations' InnoSwitch4-CZ (INN4073C). This IC integrates the 750V primary PowiGaN switch, the secondary synchronous rectifier driver, and the FluxLink communication path, eliminating the optocoupler entirely.
Transformer Magnetics and Turns Ratio
The heart of any smps power supply circuit is the transformer. For a 24V output using a 750V switch, we target a primary-to-secondary turns ratio (Np:Ns) of roughly 12:1.
- Core: RM10 or PQ26/20 ferrite core (e.g., TDK PC95 material).
- Primary Inductance: 450µH (gapped to prevent saturation at 1.5A peak primary current).
- Windings: Use Litz wire for the primary to mitigate skin effect at the 65kHz switching frequency. The secondary should be a thick copper foil or multi-strand bundle to handle the 5A RMS current without excessive I²R heating.
Ripple, Noise, and the LC Post-Filter
Flyback converters inherently produce high output ripple because the output capacitor must supply the entire load current while the primary switch is on. With a standard 470µF low-ESR output capacitor (ESR ≈ 40mΩ), the expected ripple voltage is:
V_ripple = I_out × ESR = 5A × 0.040Ω = 200mV peak-to-peak.
For sensitive battery management systems (BMS) or analog telemetry, 200mV of noise is unacceptable. To fix this, add an LC post-filter directly after the main output capacitor. Use a 2.2µH shielded power inductor (rated for >7A saturation) followed by a 100µF, 35V ceramic or polymer capacitor. This creates a second-order low-pass filter that knocks the switching ripple down to <25mV p-p without introducing the instability issues you get when trying to compensate a control loop with a massive LC filter on the secondary side.
Thermal Derating and PCB Layout Rules
A 120W supply generating 16W of heat in a sealed plastic enclosure will quickly cook its electrolytic capacitors if thermal management is ignored. The lifespan of an aluminum electrolytic capacitor follows the Arrhenius equation: operating life halves for every 10°C rise in core temperature.
Creepage, Clearance, and Layout
When routing the PCB for an offline smps power supply circuit, safety isolation is non-negotiable.
- Creepage and Clearance: Maintain a minimum of 6.4mm of physical distance (creepage) between any primary-side copper and secondary-side copper. This is required to meet UL/IEC 62368-1 reinforced insulation standards for a 250VAC working voltage.
- Slits: If you cannot achieve 6.4mm of clearance under the transformer, mill a 1mm slot in the PCB directly under the core to force the surface leakage path to meet the requirement.
- High di/dt Loops: Keep the loop area formed by the bulk capacitor, the primary switch (inside the InnoSwitch IC), and the transformer primary pin as tight as physically possible. A large loop area acts as a magnetic antenna, radiating EMI and causing voltage spikes across the primary switch due to parasitic leakage inductance (V = L_leak × di/dt).
- Secondary Loop: Similarly, the loop from the transformer secondary pin, through the synchronous rectifier, and back to the output ground must be minimized to reduce ringing and output noise.
By respecting the thermal limits of your magnetics and capacitors, and utilizing an LC post-filter to tame the inherent flyback ripple, this 120W topology provides a highly reliable, cost-effective foundation for 24V DC power systems.
