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Application of chip inductors on low power supplies

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2019-12-12 13:39

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【Outline Description】Ultra-low power or ultra-high power switching power supplies are not as easy to select as general switching power supplies. At present, conventional inductors are manufactured for some mainstream designs and are not well suited for some special designs. Inductor selection problem for ultra-efficient Buck circuits.

Ultra-low power or ultra-high power switching power supplies are not as easy to select as general switching power supplies. At present, conventional inductors are manufactured for some mainstream designs and are not well suited for some special designs. Inductor selection problem for ultra-efficient Buck circuits. A typical application example is a small-capacity battery that is powered for a long time. In this kind of circuit, the problem that makes engineers feel difficult is mainly the contradiction between battery capacity (cost and volume) and Buck circuit volume and efficiency. In order to reduce the size of the switching power supply, it is best to choose the highest possible switching frequency. However, the switching loss and the loss of the output inductor increase with the increase of the switching frequency, and it is likely to become the main factor affecting the efficiency. It is these contradictions that greatly increase the difficulty of circuit design.

Inductor requirements for Buck circuits: For engineers, ferromagnetic components (inductors) may be the earliest contact nonlinear devices. However, according to the data provided by the manufacturer, it is difficult to predict the loss of the inductor at high frequencies. Because manufacturers usually only provide parameters such as open circuit inductance, operating current, saturation current, DC resistance, and self-excited frequency. For most switching power supply designs, these parameters are sufficient, and choosing the right inductor based on these parameters is also very easy. However, for ultra-low current, ultra-high frequency switching power supplies, the nonlinear parameters of the inductor core are very sensitive to frequency, and secondly, the frequency also determines the coil loss.

For ordinary switching power supplies, core losses are almost negligible relative to DC I2R losses. Therefore, in general, in addition to the frequency-dependent parameters of "self-excited frequency", the inductance has almost no other frequency-dependent parameters. However, for ultra-low power, ultra-high frequency systems (battery-powered devices), these high frequency losses (core loss and coil losses) are typically much greater than DC losses.

Adjacent magnetic needles with similar magnetic directions interact with each other to form an "alliance". Although these magnetic needles are wrapped by an adhesive material and are physically independent of each other, the magnetic fields between them are interrelated. We call these "alliance" "units." The boundary of the unit is the split surface of the internal "alliance" and the external magnetic needle. Magnetic needles outside the boundaries of the unit are more difficult to unite with the "alliance" within the boundary. We call these boundaries "cell walls", which are often used to explain many of the basic parameters of a magnetic core.

When a magnetic field is applied to the magnetic core (current is applied to the coil), cells having different directions are associated with each other. When a sufficiently strong current forms an applied magnetic field, the cells that are close to the coil are at a stronger magnetic field and will first form a joint (a larger unit). At this time, the unit on the deeper level has not been affected by the magnetic field. The cell walls between the unit and the unaffected unit continue to move toward the center of the core under the action of a magnetic field. If the current in the coil is not undone or flipped, the entire core will be united. The magnetic needles of the entire core are united together and we call it "saturation". The B-H hysteresis loop given by the inductor manufacturer is indicating the process of the core from the initial stage of magnetization to the saturation stage. If the current is weakened, the unit will transition to a free initial state, but some units will continue to remain in a combined state. This incomplete conversion is remanence (can be seen in the hysteresis loop). This remanence phenomenon will manifest as stress in the next unit combination, resulting in core loss.

The hysteresis loss in each cycle is: WH=mH×dI where the integral is the enveloping area in the hysteresis loop, the core from the initial inductance to the peak inductance, and then back to the initial inductance. The energy loss at the switching frequency F is: PH = F × mH × dI It seems easy to calculate these AC losses. However, under high frequency and medium current density, the situation will be extremely complicated. Each circuit has some parameters that affect the core loss, and these parameters are generally difficult to quantify. For example: discrete capacitors, pcb layout, drive voltage, pulse width, load status, input and output voltage, etc. Unfortunately, core losses are severely affected by these parameters.

Each core material has a non-linear conductivity that can cause losses. It is this conductivity that induces a loss of "eddy current" inside the core due to the applied magnetic field. At a constant magnetic flux, the core loss is roughly proportional to the frequency nth power. The index n will vary with the core material and the manufacturing process. The usual inductor manufacturer fits the empirical approximation formula through the core loss curve.

The inductance parameter magnetic induction B can be expressed by the following equation in the forward switching circuit: Bpk=Eavg/(4×A×N×f) where Bpk is the peak AC flux density (Teslas); Eavg is the average AC per half cycle. Voltage; A is the core cross-sectional area (m2); N is the number of turns of the coil; f is the frequency (Hz).

In general, magnetic material manufacturers evaluate the core's rated inductance - AL. The inductance can be easily calculated by AL. L = N2AL where AL is proportional to the doping of the magnetic material and also proportional to the cross-sectional area of the core divided by the length of the magnetic circuit. The total loss of the core is equal to the volume of the core multiplied by Bpk times the frequency in watts/m3. It is closely related to manufacturing materials and manufacturing processes.

Core loss test equipment, the most effective way to test the performance of the inductor is to place the tested inductor on the final switching power supply circuit, and then measure the efficiency of this circuit. However, this test method requires a final circuit and is not easy to use. Now, there is a relatively simple test method that tests the core loss of the inductor (at its set switching frequency) before designing the switching power supply. First, place the cores in series on low-loss capacitor media (such as silver-plated mica). Then, it is driven by a series of resonant modes. The capacitance value of the medium needs to be consistent with the switching frequency of the measured inductor. Finally, the network analyzer is used to complete the entire test process (the signal generator plus a radio frequency voltmeter or power meter can also complete the test).

At the resonance point, the low-loss core can be seen as an L-C resonant circuit. At this point the loss can be equivalent to a pure resistance element (including coil loss and core loss). In the above test equipment, terminals A and R are connected to a 50Ω resistor. The open circuit of this device (excluding the inductor) is equivalent to a 150Ω load oscillator. On the network analyzer, it can be expressed as: 20 × Log (A / R) = 20 × Log (50 / 150) = -9.54dB In this test circuit, the resonant capacitor is 2000pF, the measured inductance is about 2.5mH ~ 2.8 mH, the test frequency is 1 kHz. Among them, the permeability of the magnetic material is a nonlinear function related to frequency, and the test results may be different at higher frequencies.

Core loss test data: a single-layer iron-nickel-molybdenum sheet core with a relative magnetic permeability of 125 mr, a 16-turn multi-core wire wrapped around 10/44, and a double-layer 250-doped nickel-iron-molybdenum magnetic powder core. A multi-core wire of 10/44 is wound around the periphery. The inductance test values were 2.75 mHy and 2.78 mHy, respectively. Although the first inductor is 16 turns, the cross-sectional area is half of the second inductor. Both inductors are highly lossy driven by the same amplitude signal. The equivalent resistance is 360Ω and 300Ω, respectively. In contrast, the other inductor (2.5mHy) uses Micrometals' very low doping material (carbonyl T25-6, relative permeability 8.5). 10/44 multi-core wire 34匝. With the same drive signal, his equivalent loss resistance is 22000 Ω.

There are a number of special considerations for the selection of inductors for low power switching power supplies. For low-power, high-efficiency switching power supply designs, the general device data or the parameters provided by the selection table are not enough. The usual inductors are ferrite cores (non-low-loss materials) that will gradually be phased out in low-power, high-efficiency applications. A relatively simple inductor loss test device can test the loss of the inductor at the designed frequency point and compare the performance of different inductors.

When the design needs to select a low loss inductor, a low doping material should be chosen to obtain a low magnetic field strength parameter -B. Choose a low-loss core or consider a multi-core cable. Also, it is best to use the magnetic components recommended by the chip company, or consult a professional magnetic material expert to meet specific needs.

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