Operating principle of SBD (Schottky barrier diode)

For the past three years or so, it's become standard practice to attach SBDs (Small Block Diodes) to DC motors in hobby products.

Many people probably use FETs, but from the perspective of ease of use and cost, SBDs are unlikely to become obsolete.

While the role of an SBD is somewhat clear from reading the instruction manual, I wondered what it actually is, and why a standard diode wouldn't work to prevent reverse current. So I did some research.

From what I've researched, there are either overly detailed explanations for general circuits or simple explanations for electric airsoft guns, so I'll try to fill the gap between the two.

My goal is to be able to select the appropriate diodes myself and build my own circuit.

If you're interested, please join us!

Let's start with a review of semiconductors (we explained this with FETs, but we'll go over it again).

If you already know this, please scroll down to SBD.

semiconductor

First, let's consider the basics of semiconductors, which are a hot topic these days.

semiconductor

In short, it is an object that possesses properties intermediate between a conductor (which conducts electricity) and an insulator (which does not conduct electricity).

When a substance is added to a semiconductor, it becomes an object with various properties. For example, when current and voltage are applied, it can lower the temperature (Peltier element) or emit light (light-emitting diode).

Let's review the two most representative types of semiconductors: p-type and n-type semiconductors.

n-type semiconductors are materials that contain free electrons within them. Since the movement of electrons is what causes electric current, it could be said that they possess the fundamental elements of electricity. Basically, they move around freely, so they are called free electrons.

Negative ions are similar, but the effect they have on the human body has been a mystery for decades. I think they have almost no effect.

Next, p-type semiconductors are materials that have pathways (holes) through which electrons can move freely. Imagine them as holes.

In high school chemistry terms, this is a state where the outermost electron shell is not full of electrons, but rather lacking them. In other words, as long as there is electricity in the power source, electrons can be absorbed, and electricity flows.

However, since it does not possess electrons itself, it cannot conduct electricity on its own. For some reason, it is often represented as a pore. This is sometimes called a carrier.

Basically, diodes are made by using these two types of semiconductors to control the movement of electrons.

diode

Let's review some typical pn-type diodes.

Let's connect p-type and n-type semiconductors in a diagram.

Let's connect a wire to this and run some electricity through it.

Connect the positive terminal to the p-type and the negative terminal to the n-type. Since electrons are basically negative, they are attracted to the positive terminal and move from one hole to the other.

This allows electricity to flow. The electrons that are lost in the n-type circuit will not be lost because they are supplied by the power source or battery.

This is called forward bias.

Next, if we connect a negative charge to the p-type and a positive charge to the n-type, electrons will be attracted to the positive charge, so electrons will not go to the p-type.

Furthermore, if all the electrons in the n-type semiconductor are attracted to the negative side, nothing will move (the negative terminal of the power supply balances the electrons inside the n-type semiconductor).

This prevents electricity from flowing. This is called reverse bias.

Furthermore, a region completely devoid of electrons (yellow in the diagram) is created, which is called the depletion layer. While its width can be calculated, we will omit that step for now.

This characteristic allows us to determine the direction in which electricity flows. Therefore, it is called a commutator (or diode).

What is an SBD (Schottky barrier diode)?

Let's set aside p-n type diodes for now and look at SBDs.

SBDs are composed of a metal (apparently called a barrier metal) and an n-type semiconductor.

Apparently, the metal part is called the barrier metal, its electrode is called anode A, and conversely, the electrode on the semiconductor side is called cathode K.

Now let's connect the electrodes to the SBD.

Connect the positive terminal to the metal and the negative terminal to the semiconductor (forward bias).

When a positive electrode is connected to the metal side and a negative electrode to the semiconductor side, electrons behave as follows:

This is called forward bias.

Just like in p-n type systems, electrons have a negative charge, so they are attracted to and move towards the positive electrode. Metals are basically conductors, so electrons can pass through them.

This is how electrons move, and electricity flows (the direction of electricity flow and the direction of electron movement are opposite).

Naturally, electrons tend to move in the positive direction, but since electrons are supplied from the power source, electricity will continue to flow as long as the electrons in the power source are not depleted.

Connecting the negative terminal to the metal and the positive terminal to the semiconductor (reverse bias)

Next, let's try connecting the negative electrode to the metal and the positive electrode to the semiconductor.

This is called reverse bias.

Electrons have a negative charge, so they are attracted to positive charges. In other words, all the electrons are attracted to the positive side of the n-type semiconductor, and their movement stops (the positive charge and electrons balance each other out). As a result, no electricity flows.

The region where there are no electrons at all is called the depletion layer.

Although there are slight differences in the properties of SBDs depending on the thickness and type of metal, they seem to me to be almost identical to p-n type semiconductors up to this point.

Next, let's compare pn-type diodes and SBDs.

Comparison of pn-type diodes and SBDs: Forward bias

First, let's compare the case where the electrodes are connected in the forward direction.

The top one is a pn-type diode, and the bottom one is an SBD.

The presence or absence of a hole makes a significant difference.

SBD Advantage 1: Fast switching

A pn-type diode has a p-type semiconductor attached to it, and electrons from the n-type semiconductor travel through holes to reach the electrodes. This process seems to take a little time.

On the other hand, SBDs only have metal attached, so they don't have holes. The material passes straight through the metal part, so it doesn't take long to reach the electrode.

This is one of the advantages of SBDs: their fast reaction time, meaning that current flows immediately once the electrodes are connected (switched on). Research shows that the difference can be as much as tens of microseconds.

Looking at this alone, you might think that a difference of a few tens of microseconds is insignificant, but for example, if you fire an electric airsoft gun in semi-automatic mode as fast as you can, it can fire about 15 or 16 shots per second. In that case, the firing time for each shot is about 0.075 seconds, so a difference of a few tens of microseconds is a characteristic that affects its usability.

Even if someone says, "No, I don't press the button that many times," human perception, while unreliable, is also sensitive, and a difference of 0.01 seconds is noticeable to many people.

SBD Advantage 2: Low power loss and low heat generation during operation.

In pn-type semiconductors, electrons must work hard to pass through holes to reach the electrodes.

In a pn-type circuit, this effort results in losses, so current will not flow unless a certain voltage is applied. This is called voltage drop and is represented by VF. If the current that flows at this time is IF, the loss is expressed as VF × IF [W].

On the other hand, SBDs have no holes, and the only thing blocking the n-type semiconductor is the metal. Since this metal is a conductor, electrons pass through easily. Because current flows without electrons having to work hard, it has the characteristic of low voltage drop VF. Similarly, the power loss VF × IF [W] is also small.

This lost energy is essentially converted into heat and released into the atmosphere. Therefore, when energized, SBDs generate less heat compared to pn-type diodes.

Comparison of pn-type diodes and SBDs: Reverse bias

Now let's connect the electrodes in reverse and compare the characteristics of the reverse bias. The top one is a pn-type diode and the bottom one is an SBD.

Disadvantage 1 of SBDs: Relatively high leakage current.

Conversely, when electrodes are connected (reverse voltage is applied), a pn diode first has the barrier of holes in the p-type semiconductor, and then electrons must overcome the depletion layer before they can reach the positive electrode. However, there are a few tenacious electrons that manage to move, and electricity flows.

This is called leakage current (IR).

pn-type diodes have very low leakage current (IR), making them excellent rectifiers.

On the other hand, SBDs only consist of metal (barrier metal) and a depletion layer.

Since metals are basically conductors, electrons simply pass through (depending on the type of metal) and break through the depletion layer, so a relatively large number of electrons reach the positive electrode.

In other words, an SBD is a rectifier element with a higher leakage current (IR) compared to a pn-type rectifier.

This is the ultimate high-cycle electric gun. When tuning for ultra-high flow rate, the battery current flows to the SBD when the electric gun fires, so the current going to the motor decreases slightly, which may slightly reduce the motor's torque.

It also doesn't seem to be a good match for super fuel-efficient cars.

Disadvantage 2 of SBDs: High leakage current results in high loss V × IR.

Since SBDs are often connected in parallel with motors, the same voltage as the motor is applied as a reverse voltage.

The motor draws electricity from the battery until it's almost at its maximum capacity, so with a 7.4V LiPo battery, the voltage will be around 8.4V when fully charged.

In other words, the SBD loses 8.4V × IR [W] every time the electric airsoft gun operates, and most of this power is converted into heat.

In the worst-case scenario, the device's own heat causes a decrease in performance, increases leakage current (IR), generates even more heat, and this creates an endless combo that leads to destruction.

Comparison of characteristics between p-n type diodes and SBDs

Now we compare them using graphs with forward current on the y-axis and forward voltage on the x-axis. The characteristics of the pn-type diode are shown by the blue line, and the characteristics of the SBD are shown by the red line.

SBDs have a small voltage drop VF when current flows in the forward direction, resulting in low losses, good response time (said to be a few microseconds), and low heat generation.

Conversely, when a reverse voltage is applied, the leakage current (IR) is large, resulting in significant losses and making the element prone to overheating.

Due to its structure, it only has a barrier metal, so its reverse voltage tolerance is inferior to other diodes. It seems to depend on the material and thickness of the barrier metal, but it appears that the limit is around 100V.

Characteristics of SBD

We will now examine how the properties of the SBD change with temperature.

When electricity flows in the forward direction, the following graph shows the result (red indicates high temperature).

While there are limits, it has the interesting characteristic of performing better at higher temperatures.

Next, let's consider the case where electricity flows in the reverse direction.

Under reverse voltage, the leakage current (IR) appears to increase exponentially as the temperature decreases.

This would increase the device's own loss (V × IR), leading to a negative spiral of performance degradation due to heat, and potentially causing rapid deterioration and failure.

In the case of using an electric airsoft gun, a reverse voltage is always applied when the gun is operating, so electricity is consumed as leakage current, slightly reducing efficiency. Furthermore, the heat from the gun itself and external heat from the motor etc. are added, increasing the leakage current, and in the worst case, it seems that thermal runaway and damage can occur.

In other words, prolonged full-auto firing increases the heat generated by the SBD, reducing its efficiency, and in the worst case, there is a high possibility that the SBD will overheat and break down.

So, it seems best to let the grip cool down as soon as it starts to get even slightly warm.

Conversely, if a back electromotive force is generated in the motor, the forward voltage (VF) is small, so electricity can be quickly passed through it to recover the energy in the motor.

Moreover, it seems to regenerate electricity even more effectively when the motor and other components are extremely hot.

Leakage current and voltage drop characteristics

SBDs can be broadly divided into three types depending on the type of metal and barrier metal used. Roughly speaking, the types can be divided into low voltage drop and high leakage current, standard, and high voltage drop and low leakage current.

If we plot the leakage current (IR) on the vertical axis and the voltage drop (VF) on the horizontal axis, these three factors appear to have the relationship shown in the graph below.

Their respective characteristics are as follows:

If you're considering an electric airsoft gun, I think the yellow one with high VF and low IR is a good choice.

When the electric airsoft gun is operating, the leakage current (IR) is low, so the efficiency reduction is minimized. On the other hand, when back EMF is generated in the motor, the response speed is somewhat slow, but regeneration works properly.

However, under voltage conditions below the voltage drop VF, electricity will flow through the switch, so there might be some sparking, depending on the value of VF.

Conversely, a low VF and high IR setting may slightly reduce the efficiency of the electric airsoft gun's operation (depending on the IR value), but it seems to respond quickly and reliably to the motor's back EMF and regenerate power effectively.

Summary

What I learned from this research is that SBDs are chosen because of their excellent switching response speed.

Although not mentioned in the main text, SBDs are more compact than pn-type diodes because they do not have a p-type semiconductor (the metal part is also thinner).

This is also a good point, as many electric airsoft guns don't have much space.

On the other hand, a drawback is that the leakage current is a concern.

When pursuing efficiency, a potential bottleneck and problem is that the device overheats due to its own leakage current, leading to a negative spiral, runaway operation, and eventual damage. If efficiency is the priority, an FET might be the better option.

In the case of electric airsoft guns, even if the SBD (Slide Block Deposition) is damaged, the gun can still be used normally, so it's possible that users might not notice the damage and continue using it until the switch burns out.

A damaged FET is much better than being constantly powered on and running on full auto like a berserker.

I'm also concerned about the low voltage tolerance. Even a low-performance motor can easily generate 60V or more in back EMF, so I feel that the back EMF of the increasingly common high-performance motors will enter a dangerously high-voltage range (SBD voltage tolerance is around 100V).

Therefore, carefully selecting an SBD element type that matches the specifications of your electric airsoft gun and your playing style will likely reduce problems.

It seems that simply buying any electric airsoft gun part will suffice, and that's not the case.

Next time, we'll consider the implementation circuit for SBDs.

I'll look for other rectifier elements that I can use. I have a feeling a varistor might work.

My next customization series will be about electric airsoft gun motors (DC motor edition). After that, I'd like to explain the popular brushless motors. I'll do my best to cover them, as you can't calculate back EMF without understanding them.