Power conversion schemes can be straightforward, such as with a set of diodes and a smoothing capacitor, or involving complex integrated circuits with multiple power conversion and regulation stages. However, one component can participate in all these areas and other important applications like radio frequency tuning and ESD protection: the Schottky barrier rectifier. These provide many important functions in electronics thanks to their internal structure and, of course, the Schottky effect.
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If you need a diode for rectification that admits current with low forward voltage and fast switching speed, then a Schottky diode is the standard option. These characteristics make Schottky barrier rectifiers useful in power delivery to wave shaping applications. In this article, we'll look at how these components stack up to their p-n diode cousins and when you might best be served using a Schottky barrier rectifier instead.
In the most basic sense, a Schottky barrier rectifier (or simply Schottky diode) operates in the same manner as a typical semiconductor diode made from Si or Ge. However, its primary purpose is to provide rectification, i.e., allow current to flow only easily along one specific direction. In this way, they are used in typical DC circuits to enable or block current or in AC circuits as part of wave shaping.
The major differences between a Schottky diode include their forward operating characteristics and, most importantly, their structure. Schottky diodes are constructed by depositing a metal electrical contact on a semiconductor; although n-type or p-type materials can be used in Schottky diodes, n-type materials are normally preferred. The reason is that p-type semiconductor Schottky diodes will have lower forward voltage and thus larger reverse bias breakdown current; using an n-type material provides the best balance between forward voltage and reverse bias current. On the other ends of the component, Ohmic contacts are placed to provide non-rectifying connections to the semiconductor.
Schottky barrier rectifier structure.
Compared to p-n junction diodes, Schottky barrier rectifiers have some advantages that make them more useful in switching, high-frequency rectification, and wave shaping applications. Some of the main advantages of Schottky diodes include:
As shown below, the electrical advantages can be seen when comparing IV curves for a Si p-n diode and a Schottky diode. From here, we can see that the larger reverse bias current behavior leads to slower roll-off into the breakdown region, rather than fast avalanche behavior seen in a p-n diode.
IV curves (left) and forward voltage behavior (right) for Si p-n diodes and Schottky diodes.
The three major application areas for Schottky barrier rectifiers include switching converters for power regulation, ESD protection, and in microwave circuits. Both applications place requirements on three specifications: the junction capacitance (determines the recovery time and frequency response), voltage limit, and current limit.
In power systems, Schottky diodes are switched between forward bias and reverse bias in order to direct a DC current across an inductor and to the output of a regulator circuit. The diode needs to switch and fully modulate between the two states at the same rate as the driving PWM signal in the upstream switching power MOSFETs, which requires low junction capacitance. The other two important specifications are the voltage and current limits; the device should be able to provide rectification at the applied voltage during operation without entering breakdown.
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Schottky diodes can be used in microwave circuits that operate up to GHz frequencies in low-load conditions. The recovery time will be limited by the load impedance and the junction capacitance (see below for an example in a 1N Schottky diode). One big advantage of a Schottky diode in microwave circuits compared to a Si p-n diode is its linearity near zero bias, allowing oscillating signals to be collected directly, and without DC offset if needed.
Example junction capacitance in reverse bias for the Vishay 1N-E3/54.
Certain low-voltage or slower transient events can be reliably protected against with Schottky diodes. An example is shown in the circuit diagram below. In this example, two Schottky diodes are placed as pull-up elements to a power rail in reverse bias. The low voltage drop of these diodes enables clamping of moderate currents in reverse bias back to ground from moderately strong ESD events. A very similar application is used in motor control or when driving large inductive loads; placing the Schottky diode parallel around the load in reverse bias will create the same effect and will protect the driver circuit from damage should a back EMF surge event occur. Should very high levels of ESD protection be needed, a good option is something like a gas discharge tube.
ESD protection with Schottky diodes.
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Voltage on VDD with respect to VSS ..... -0.6V to +7.0V
Voltage on SCL, and SDA with respect to VSS..... -0.6V to 12.5V
Voltage on all other pins (A, W, and B) with respect to VSS ..... -0.3V to VDD + 0.3V
Input clamp current (VI < 0, VI > VDD, VI > VPP ON HV pins) ........... ±20 mA
Output clamp current (VO < 0 or VO > VDD) ....................................... ±20 mA
Your statement "I still need to prevent greater than 5volts across the pot" is probably far too strong of a constraint. Where's the datasheet for your digital pot? Since you haven't provided one, I'll use this one as an example (first one that I found): MCP datasheet If you look at what the actual requirements are, in the absolute maximum ratings section:That shows you that not only is a voltage of up to VDD+0.3V (i.e., 5.3V) OK, but it also describes the limits of the included clamp diodes. So you don't need to worry at all about the voltage spec; as long as you keep the shunt currents below 20mA, you'll be fine.For instance, the 100 ohm resistor I mentioned earlier will provide protection for voltages up to 7V (20mA * 100 ohm + 5V). If you use a 1k resistor, then suddenly that goes up to protection up to 25V. This is all without using any extra diodes.If you want to use silicon protection diodes (to avoid leaky Schottky diodes), then it's pointless to put them directly in parallel with the dpot pins because the dpot clamp diodes will die before the Silicon diodes even start conducting. If, instead, you place the silicon diodes before the protection resistor, then you can create an extremely robust solution indeed with quite small resistances.There's basically no configuration in which Schottky diodes make sense, because if you put them directly on the dpot pins, you can't guarantee that the diode will actually start conducting before the clamp diodes in the IC, and if you put them before the resistor, there's no point in them being Schottky because even a normal 0.7V drop diode will work perfectly well and will mess with your signal less.The point is, this is all simulatable and calculatable using the datasheet figures; there's nothing magical or different about the job of protecting inputs.
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