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The inductors in automotive DC-DC converter applications need to be carefully selected to achieve the right combination of cost, quality, and electrical performance.In this article, Field Application Engineer Smail Haddadi provides guidance on how to calculate the required specifications and what trade-offs can be made.
There are about 80 different electronic applications in automotive electronics, and each application requires its own stable power rail, which is derived from the battery voltage.This can be achieved by a large, lossy “linear” regulator, but an effective method is to use a “buck” or “buck-boost” switching regulator, because this can achieve efficiency and efficiency of more than 90%. Compactness.This type of switching regulator requires an inductor. Choosing the correct component can sometimes seem a bit mysterious, because the required calculations originated in the 19th century magnetic theory.Designers want to see an equation where they can “plug in” their performance parameters and get the “correct” inductance and current ratings so that they can simply choose from the parts catalog.However, things are not that simple: some assumptions must be made, pros and cons must be weighed, and it usually requires multiple design iterations.Even so, perfect parts may not be available as standards and need to be redesigned to see how off-the-shelf inductors fit.
Let us consider a buck regulator (Figure 1), where Vin is the battery voltage, Vout is the lower voltage processor power rail, and SW1 and SW2 are switched on and off alternately.The simple transfer function equation is Vout = Vin.Ton/(Ton + Toff) where Ton is the value when SW1 is closed and Toff is the value when it is open.There is no inductance in this equation, so what does it do?In simple terms, the inductor needs to store enough energy when SW1 is turned on to allow it to maintain output when it is turned off.It is possible to calculate the stored energy and equate it to the required energy, but there are actually other things that need to be considered first.The alternating switching of SW1 and SW2 causes the current in the inductor to rise and fall, thereby forming a triangular “ripple current” on the average DC value.Then, the ripple current flows into C1, and when SW1 is closed, C1 releases it.The current through the capacitor ESR will produce output voltage ripple.If this is a critical parameter, and the capacitor and its ESR are fixed by size or cost, this may set the ripple current and inductance value.
Usually the choice of capacitors provides flexibility.This means that if the ESR is low, the ripple current may be high.However, this causes its own problems.For example, if the “valley” of the ripple is zero under certain light loads, and SW2 is a diode, under normal circumstances, it will stop conducting during part of the cycle, and the converter will enter the “discontinuous conduction” mode.In this mode, the transfer function will change and it becomes more difficult to achieve the best steady state.Modern buck converters usually use synchronous rectification, where SW2 is MOSEFT and can conduct drain current in both directions when it is turned on.This means that the inductor can swing negative and maintain continuous conduction (Figure 2).
In this case, the peak-to-peak ripple current ΔI can be allowed to be higher, which is set by the inductance value according to ΔI = ET/L.E is the inductor voltage applied during the time T. When E is the output voltage, it is easiest to consider what happens at the turn-off time Toff of SW1.ΔI is the largest at this point because Toff is the largest at the highest input voltage of the transfer function.For example: For a maximum battery voltage of 18 V, an output of 3.3 V, a peak-to-peak ripple of 1 A, and a switching frequency of 500 kHz, L = 5.4 µH.This assumes that there is no voltage drop between SW1 and SW2.The load current is not calculated in this calculation.
A brief search of the catalog may reveal multiple parts whose current ratings match the required load.However, it is important to remember that the ripple current is superimposed on the DC value, which means that in the above example, the inductor current will actually peak at 0.5 A above the load current.There are different ways to evaluate the current of an inductor: as a thermal saturation limit or a magnetic saturation limit.Thermally limited inductors are usually rated for a given temperature rise, usually 40 oC, and can be operated at higher currents if they can be cooled.Saturation must be avoided at peak currents, and the limit will decrease with temperature.It is necessary to carefully check the inductance data sheet curve to check whether it is limited by heat or saturation.
Inductance loss is also an important consideration.The loss is mainly ohmic loss, which can be calculated when the ripple current is low.At high ripple levels, core losses begin to dominate, and these losses depend on the shape of the waveform as well as frequency and temperature, so it is difficult to predict.Actual tests performed on the prototype, as this may indicate that lower ripple current is necessary for the best overall efficiency.This will require more inductance, and perhaps higher DC resistance-this is an iterative process.
TT Electronics’ high-performance HA66 series is a good starting point (Figure 3).Its range includes a 5.3 µH part, a rated saturation current of 2.5 A, a 2 A load allowed, and a ripple of +/- 0.5 A.These parts are ideal for automotive applications and have obtained AECQ-200 certification from a company with a TS-16949 approved quality system.
This information is derived from materials provided by TT Electronics plc and has been reviewed and adapted.
TT Electronics Co., Ltd. (2019, October 29).Power inductors for automotive DC-DC applications.AZoM.Retrieved from https://www.azom.com/article.aspx?ArticleID=17140 on December 27, 2021.
TT Electronics Co., Ltd. “Power inductors for automotive DC-DC applications”.AZoM.December 27, 2021..
TT Electronics Co., Ltd. “Power inductors for automotive DC-DC applications”.AZoM.https://www.azom.com/article.aspx?ArticleID=17140.(Accessed on December 27, 2021).
TT Electronics Co., Ltd. 2019. Power inductors for automotive DC-DC applications.AZoM, viewed on December 27, 2021, https://www.azom.com/article.aspx?ArticleID=17140.
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