Appunti di Automotive Electronics Systems

ASIC.

ASICs for Big Production: Companies usually prefer FPGAs for the early stages. But if they're going to make a

• lot of a certain product, like millions of them, then designing a custom ASIC can become cheaper in the long run

because the high design costs are spread out over all those chips.c

7. Output Stages

Giving Power to Action:

The main job of the output stage in an electronic system is to provide the right interface to make things move or

• happen – these "things that make things happen" are called actuators.

From Signals to Power: The information that has been processed by the earlier parts of the system (which might

• be in the form of small voltage or current signals) now needs to be turned into enough power to drive these

actuators.

Actuators Need Power: Most actuators (like motors that move a car or a robotic arm) need a good amount of

• power to do their job. So, the output stage needs to boost the power of the signals.

Power Control: There are many ways to control how much power goes to an actuator. We're going to focus on

• using electrical switches to turn the power on and off in a controlled way.

7.1 Solid-State Switches

Flipping the Switch Electronically:

Solid-state switches are like regular on/off switches, but they don't have any moving mechanical parts. Instead,

• they use special materials (like silicon) to control the flow of electricity.

Different Types for Different Jobs: There are different kinds of solid-state switches, and the best one to use

• depends on how much voltage and current the circuit needs. Some important things to consider are:

Voltage Withstand (Open): How much voltage the switch can handle when it's turned off.

◦ Current Withstand (Closed): How much current the switch can handle when it's turned on.

◦

Common Solid-State Switches:

• BJT and MOSFET: These are transistors that can act as switches, but they're usually used for lower

◦ voltages and currents.

IGBT: These are for medium voltages and high currents and are good for switching on and off quickly.

◦ SCR and Thyristors: These are for very high voltages and currents.

◦

New Materials for Better Performance: Scientists are also working with new materials like Gallium Nitride

• (GaN) and Silicon Carbide (SiC) to make even better transistors for use as switches, especially for things that

need high power, voltage, current, and fast switching (like in electric cars).

GaN and SiC vs. Silicon: These new materials can sometimes do a better job than regular silicon-based

• transistors (like MOSFETs) in certain situations, especially where silicon has its limits.

Why Fast Switching is Good: Switching electricity on and off quickly is important in things like DC-DC

• converters (which change voltage levels in a car)

and in the chargers and motors of electric cars.

Faster switching can make these systems smaller

and lighter.

GaN and SiC Applications: The pictures show

• how GaN and SiC transistors are being used in

many applications, like power supplies, solar

inverters, and especially in electric vehicles.

Comparing Technologies: The graphs also

• compare different types of transistors (including those made of Si, SiC, and GaN) in terms of how little resistance

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they have when turned on (which is good) and how

much voltage they can handle when turned off (also

good). They also show that SiC is good for high

temperatures, and GaN is good for high voltages.

7.2 Bipolar Junction Transistor (BJT)

The First Electronic Switch:

The Bipolar Junction Transistor (BJT) was the very first type of transistor ever made, and it has three terminals

• (like three wires sticking out).

Like Two Diodes Back-to-Back: To understand how it works, you kind of need to know about diodes (which let

• direction).1 way.2

electricity flow in only one A BJT is like two diodes connected in a special (E).3

The Three Terminals: These three terminals are called Base (B), Collector (C), and Emitter Inside the BJT,

• material.4

these terminals are connected to three different layers of a special

BJTs:5

NPN and PNP Flavors: There are two main types of

• NPN: The Collector and Emitter layers are "n-type" (with extra electrons), and the Base layer is "p-type"

◦ (with "holes" where electrons are missing).

PNP: It's the opposite: Collector and Emitter are "p-type," and the Base is "n-type."

◦ flow.6

Current Directions: The arrows in the circuit symbol show the direction of the current For an NPN

• transistor, the current at the Base and Collector goes into the transistor, and the current at the Emitter goes out. For

a PNP, it's the other way around. diode.7

More Complex Than a Diode: BJTs can do more than just let electricity flow in one direction like a They

• can act like a switch (turning current on or off) or like an amplifier (making a small current bigger). We're mostly

interested in how they work as switches here.

Many Different Kinds: Just like solid-state switches in general, there are lots of different BJTs with different

• limits for how much current, voltage, and power they can handle, and how fast they can switch. Bigger, more

powerful BJTs tend to be slower, and faster ones are usually less powerful. BJTs made with Germanium (Ge) are

typically faster than those made with Silicon (Si), but they are also more fragile.

The Table: The table in the picture shows some examples of real BJTs you can buy, along with their ratings for

• power, voltage, current, and how fast they can operate (frequency).

7.2.1 Transfer and Output Curves

How the BJT Behaves:

The BJT is mostly controlled by the current that flows into its Base (B) terminal. Specifically, for the NPN type

• we talked about, the current flowing from the Collector (C) to the Emitter (E) is mainly determined by the current

at the Base.

Like a Small Current Controlling a Big One: It's like a small stream of water (Base current) controlling a much

• larger flow of water (Collector current).

The Relationship: There's a relationship between these two currents, which can be written as:

• I = β * I

C B

Where: I_C is the current flowing from the Collector to the Emitter.

◦ I_B is the current flowing into the Base.

◦ β (beta) is a number that depends on the specific BJT. It's called the current gain, and it tells you how

◦ much the Base current is multiplied to get the Collector current. For typical NPN silicon BJTs, β can be

anywhere from around 10 to 3000. For a common small-signal BJT, it might be around 100.

The Input Curve: The graph on the left shows how the Base current (I_B) changes as the voltage

• between the Base and the Emitter (V_BE) changes. Notice that it looks a bit like the curve for a diode.

To get a Base current flowing, you need to apply a certain voltage (around 0.7V for silicon BJTs at room

temperature). After that voltage, the Base current increases quickly as you increase the voltage.

The Output Curves: The graph on the right shows how the

• Collector current (I_C) changes with the voltage

between the Collector and the Emitter (V_CE) for

different values of the Base current (I_B).

Active Region: When V_CE is above a certain

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small value (around 0.2V for silicon BJTs), the Collector current

(I_C) is roughly equal to β times the Base current

(I_B) and doesn't change much as you increase

V_CE. This is the region where the BJT acts as an

amplifier.

Saturation Region: When V_CE is very small (below that

◦ 0.2V), the Collector current (I_C) doesn't follow the β *

I_B relationship anymore and doesn't increase much even

if you increase the Base current. The BJT is said to be "saturated," like

a sponge that can't hold any more water.

Cut-off Region: If the Base current (I_B) is zero, then the

◦ Collector current (I_C) is also very close to zero (the BJT is "cut-off," like a closed switch).

Acting as a Switch: When we use a BJT as a switch:

• On (Closed Switch): We apply enough Base current to put the BJT into the saturation region, where it

◦ allows a large current to flow from the Collector to the Emitter with a very small voltage drop across it.

Off (Open Switch): We make the Base current zero, which puts the BJT into the cut-off region, where

◦ almost no current flows between the Collector and the Emitter.

PNP Transistors: PNP transistors work in a similar way, but the directions of the currents and the polarities of the

• voltages are reversed.

7.2.2 Regions of Operation

Three Ways the BJT Can Act:

Based on how we set up the voltages and currents at the three terminals (Base, Collector, Emitter), the BJT can work in

three main ways, or "regions":

Cut-off Region (Switch OFF):

• To make the BJT act like an open switch (electricity can't flow between Collector and Emitter), we apply

◦ a very small voltage between the Base and the Emitter (V_BE), usually less than 0.6 - 0.7

Volts for silicon NPN BJTs.

This small V_BE results in almost no current flowing into the Base (I_B is very

◦ low).

Because the Base current is so low, almost no current can flow between the Collector and the Emitter

◦ (I_C is almost zero), no matter what the voltage between the Collector and Emitter

(V_CE) is.

Think of it like a water valve that's completely closed – no water can get through.

◦

Saturation Region (Switch ON):

• To make the BJT act like a closed switch (electricity can flow easily between Collector and Emitter), we

◦ apply a V_BE that's large enough to get a good amount of current flowing into the Base

(I_B is significant).

If the voltage between the Collector and Emitter (V_CE) is very small (smaller than the

◦ "saturation voltage"), then the BJT is in the saturation region.

In this region, even with a larger Base current, the Collector current (I_C) doesn't increase

◦ much more, and the voltage V_CE stays very low (almost like a short circuit).

Think of it like a water valve that's wide open – water can flow through with very little resistance.

◦

Active Region (Amplifier):

• If V_BE is large enough to have a good I_B, and if V_CE is larger

◦ than the saturation voltage, then the BJT is in the active