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DC-DC Converter Inductor Selection Guidelines for High Power Systems

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All electronics need power, and real power provided by energy sources or the grid is never stable or noise-free. When you need to convert grid power to DC power, or you’re working with a fluctuating DC source, you’ll need a DC-DC converter for regulation and to maintain the output voltage to a desired value.

One critical component in a DC-DC converter is the inductor on the output stage, and DC-DC converter inductor selection is one of many important design choices needed for successful design. Switching DC-DC converters use an inductor for voltage regulation, along with some other components. The inductor in the circuit will have slightly different roles in different topologies, depending on whether the system steps up or steps down the voltage. Let’s look at two common groups of topologies where the inductor provides different functions in the circuit and how to select the appropriate inductor size.

DC-DC Converter Inductor Selection

The process for selecting an inductor for a DC-DC converter depends on a number of parameters. Depending on the power levels involved, these inductors can be rather large and may need to be custom-designed for your system. In addition to the required footprint, the following specifications are important and need to be considered in DC-DC converter inductor selection:

  • Current rating and coil resistance. The coil itself will have some DC resistance, which will generate heat and drop some voltage across the inductor. The coil resistance should be as low as possible to prevent power loss as heat. In addition, the current rating will depend on the physical size of the inductor package; wire-wound inductors are used for high power converters, while foil inductors can be used in lower power converters.

  • Saturation current. Ferrite inductors will have some saturation current above which the inductor core will saturate. This leads to hysteresis in the next switching cycle. The inductor should be chosen so that the saturation current is larger than the desired maximum current in the system.

  • Inductance vs. frequency. All components have parasitics. Parasitic capacitance between windings and resistance in the coil create the possibility for self-resonance in the system at high frequencies. Practically, the self-resonance frequency for an inductor is normally very high, so this is not so important unless you plan to use a PWM signal with very fast edge rates and ~MHz frequencies.

DC DC converter inductor selection and types of inductors

Different types of inductors for DC-DC converters.

Last but not least is the inductor value. The required inductor value depends on the topology you’re working with as this will determine what function the inductor needs to perform in a DC-DC converter. There are two groups of DC-DC converter topologies that should be considered here: buck/boost/Cuk/split-Pi converters and resonant bridge converters.

Buck, Boost, and Cuk Converter Inductor Selection

Buck, boost, and buck-boost converter topologies, as well as the related Cuk and split-Pi converters, all make use of an inductor to store energy when the power MOSFET in the circuit is activated during switching. Every time the inductor switches, the current through the inverter begins changing at a rate that is similar to the rise/fall time of the PWM signal, and the inductor will generate a back EMF.

These converter topologies take advantage of this back EMF in the inductor during switching that makes these converter topologies operate with low as the role of the inductor is two-fold:

  • Store and release during switching events. Each time the PWM signal switches the MOSFET in the circuit on and off, the PWM signal changes direction, ultimately directing current to the load and ensuring DC operation. Here, it’s the location of the inductor in different topologies that will help determine whether the system is operating in buck or boost mode.

  • Limit ripple on the converter’s output. The inductor has some reactive impedance that is proportional to frequency, so the ripple waveform that would be generated in each PWM switching cycle is largely dropped across the inductor. This means a larger inductor value is more useful in damping high frequency ripple.

There are many application notes on this topic that will help you determine the best inductor value to use for your buck or boost converter. Some approximate formulas you can use to calculate the value of the inductor in Cuk, boost, and buck converters are shown below. The designer needs to select a desired ripple current value (peak-to-peak) in these equations. As we can see, a larger inductor would provide more damping, which reduces ripple in the system.

 

DC DC converter inductor selection switching converters

Equations for inductor values in Cuk, boost, and buck converters.

 

LLC Resonant Converter

In this type of converter, the output voltage/current is output across a transformer coil connected to a rectifier bridge, which modifies the steps down/up the voltage/current according to the standard transformer turns ratio equation. The primary side of the converter contains a capacitor in series with the inductor and the transformer, creating an LLC circuit. A variation on this circuit is the LCC circuit, where two a series and shunt capacitance are used with the converter’s transformer. Note that the transformer is also basically an inductor, so its coils will create some inductance.

In this switching converter topology, the point of the inductor and the transformer is to exploit some gain on the primary side of the converter. This is done by driving the LLC circuit in the converter near its resonance frequency. By carefully adjusting the frequency of the switching signal, more or less gain can be seen on the primary side, which maintains the output voltage/current at some desired value. The image below shows a half-bridge resonant LLC converter design from Texas Instruments; the MOSFETs on the primary side are driven out of phase to provide the desired switching behavior on the primary side. The high-side and low-side MOSFETs on the secondary side provide rectification, and the capacitor bank provides smoothing and ripple reduction.

 

DC DC converter inductor selection resonant LLC converters

Simulation for a half-bridge resonant LLC converter.

 

The total inductance from the inductor and the primary transformer coil need to be sized together alongside the capacitor to allow the signal on the primary side to exploit sufficient gain. Typical gain values range from ~1.1 to ~1.5, which appear over a narrow range of frequencies. Large inductors (~10 uH) and transformers (~100-300 uH primary coil inductance) are normally used in this type of converter in order to put the resonance frequency of the circuit close to ~100-200 kHz, which is a practical driving frequency used in switching DC-DC converters. Note that, in order to provide smooth DC driving, the duty cycle of the driving signal is normally kept at 50%.

The inductor in this type of converter will normally be quite rugged as LLC converters are normally used in high power systems. This means you’ll need a wire-wound inductor as they are quite rugged. These converters generally require a custom-designed inductor, transformer, or both to set the desired frequency range and gain on the primary side of the converter.

 

DC-DC converter inductor selection for your high power PCB will determine your product’s safety and reliability. The design and layout team at NWES can help you create your next high power product and help ensure your product will comply with important safety and reliability standards. We help our clients stay at the cutting edge with advanced PCB design and layout services. We've also partnered directly with EDA companies and advanced PCB manufacturers, and we'll make sure your next design is fully manufacturable at scale. Contact NWES for a consultation.

 


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