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Giving diodes a further boost

Posted: 02 Dec 2015     Print Version  Bookmark and Share

Keywords:diode  capacitor  advanced driver assistance systems  ADAS  Schottky diode 

Engineers are aware that one basic guideline for a good design is to "keep it simple" – but do so only to the extent possible. That's why the semiconductor diode is such as powerful component: it's a two-terminal device which serves in countless roles from blocking reverse DC, to AC-line or signal rectification, to capturing and holding a peak value (with an associated capacitor). Diodes come in thousands of sizes and current/voltage ratings to meet these diverse needs.

Determining when and how to shift from a simple device to one which is less simple, at least at first glance, is part of the designer's challenge and dilemma. While diodes are extremely useful, they have several imperfections including their reverse leakage current and well-defined temperature coefficient. One major problem is forward voltage drop, between 0.3 and 0.8 V depending on diode type. This is a problem in two respects: in low-voltage systems, even that sub-1 V loss can be a large part of the available rail potential; in higher-current designs, the drop corresponds to power loss, inefficiency, and thermal dissipation which may cause the diode to fail.

Circuit designers have struggled with and adapted to the voltage-drop weakness, but IC vendors have devised ways around it. While this may seem like adding needless complexity and cost to a simple, component, in order to overcome a minor problem, the reality is that the problem is not a trivial in many situations.

A new product from Texas Instruments illustrates the problem in a niche of a mass-market application and outlines how a clever IC can be part of a solution. It is standard practice to protect automotive electronic sub-systems – including advanced driver assistance systems (ADAS), powertrain, infotainment – against reverse voltages from load dumps and other events. The obvious way to do is to connect a Schottky diode (0.4 V drop) or PFET device in series with the supply rail depicted in figure 1.

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Figure 1: The "obvious" way to protect a system from reverse voltages, whether transient or steady state, is to place a diode with low Vdrop in series between supply and loads.

However, the load current can easily reach 10 A or more in these sub-system for overall dissipation approaching 4 W, which can stress the diode and affect its reliability, in addition to the Vdrop loss (with a 12-V nominal rail, the 0.4 V is a loss 3.3% of nominal output).

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