SMPS transformer for a computer power supply
I normally work on my laptop, but I still have a desktop computer that I use for software projects. The bulky power supplies used in desktop computers and precision power supplies all use an SMPS transformer for galvanic isolation at the input of the regulator. If you’re designing a switching power supply, you’ll need to evaluate the behavior of all transformers (both for input rectification and in the power transformer section) produces the desired output voltage and that ripple is within your tolerances.
What is an SMPS Transformer?
An SMPS transformer is used in a switching power supply to provide galvanic isolation. The SMPS transformer in an isolated power supply provides a clear barrier that prevents dangerous high voltages from passing to the output, providing safety from electrical shock at the output. The disadvantage of an isolated power supply is its low efficiency and large size. A non-isolated SMPS can have efficiency above ~95%, while an isolated power supply typically has 70% to 90% efficiency.
The low efficiency of an isolated SMPS arises due to the SMPS transformer used in the package. As these power supplies are run at high voltage/current, they pass strong magnetic fields into the core of the SMPS transformer. Hysteresis will occur in the magnetic cores used in SMPS transformers as the input AC signal oscillates. There is a danger that the transformer core will saturate during operation when the input current is very high, which leads to more severe hysteresis and heat dissipation in the core.
SMPS transformer and core material manufacturers generally supply hysteresis curves with their components, which allows a designer to determine the limits on the input current. SMPS units with lower efficiency will heat up to higher temperatures when run at high voltage/current, thus they require some thermal management strategies to remove heat. For high output power supply units, heat sinks are normally used with chassis-mounted fans to provide sufficient cooling.
Placing an SMPS Transformer
An SMPS transformer can be placed in one or more locations in a regulator circuit. Real power supplies normally contain multiple power conversion and regulation stages, and an SMPS transformer could appear in any of these. As the transformer is intended to provide galvanic isolation, a common location for the transformer is between the output regulation stage and the switching stage. The exact location for the SMPS will depend on the power supply topology. Two common topologies that make use of an SMPS transformer are a flyback converter and forward converter.
Simplified block diagram showing the typical placement of an SMPS transformer in a power supply topology.
Choosing an SMPS Transformer for High Efficiency
There are many types of SMPS transformers that use different core materials, winding directions, and coil numbers. Most SMPS topologies will use a 2-coil transformer, while others (e.g., bridge or LLC) will use a 3-coil transformer. A 3-coil transformer is often used to provide multiple output power levels from a single SMPS circuit.
Different core materials will saturate at different magnetic field strengths, which will determine the highest voltage that could be used with the transformer. You should always choose a transformer that operates within the linear hysteresis range for your supply to minimize heating and power loss. Some saturation and loss data are shown in the table below (data source).
Your power supply circuitry can always benefit from simulations before creating a layout with your components. SPICE-based simulations are ideal for examining signal behavior in the device and help you qualify whether alternative components should be used. After you’ve selected candidate components for your SMPS, there are some simple simulations you can perform to evaluate its behavior:
Transient analysis: this will help you visualize ripple on the regulator output.
Parameter sweep: this is helpful for iterating through various values for passives in the circuit. In particular, this can be used to iterate through different inductor/capacitor values on the output as part of transient analysis.
Monte Carlo sensitivity: all components have some tolerances in their ratings. A Monte Carlo sensitivity simulation can show you how the output level, noise, and ripple is affected by these component tolerances.
Temperature sensitivity: most components will give you a temperature rating for a given voltage/current level, but it doesn’t hurt to simulate temperature changes for different input/output configurations and components.
Power factor modeling: this comprises some transient analysis and frequency sweeps as you want to minimize total harmonic distortion and power loss during switching.
EMI/RFI filtering: switching power supplies that run at high current are notorious for producing radiated EMI, which can be received in nearby circuits. Therefore, an EMI filter circuit is normally placed at the rectifier input stage to suppress EMI feedback and suppress any noise on the input AC signal.
As you’re dealing with a switching regulator, you’ll want to use an SMPS component model for your PWM oscillator. In the event such a model is not available for your desired oscillator circuitry, you can simulate the PWM signal with an arbitrary piecewise linear voltage source in your circuit. You can then iterate through different duty cycles to check out the output is affected.
When you need to build precise power regulators with an SMPS transformer, you’ll need to use the best PCB design and analysis software. The simulation and analysis tools in PSpice Simulator for Allegro and the full suite of analysis tools from Cadence are ideal for evaluating signal behavior from any SMPS. You’ll also have access to manufacturer part search tools as you prepare to source an SMPS transformer and other components for your system, and a standout magnetic parts editor that is not present in many other SPICE simulators out there.
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