Appunti completi Automotive Electronic Systems

Understanding BJT Operation and Regions

The IB value at which the BJT operates is crucial in determining the relationship between I and V, which is the "output curve". As we can see:

• When V is under the saturation voltage, I increases with increasing V.
• When V is larger than the saturation voltage, I is set equal to beta*IC.

There are 3 regions of operation:

1. Cut off region: This behavior is achieved by putting a low V, which causes a quite zero I. The BJT is like an open circuit (open switch) between C and E, with B being the control terminal of the switch.
2. Saturation region: V is large enough to have a true I, but V is smaller than the saturation voltage. The BJT is in saturation. Here the output curve is very steep and behaves like a short circuit (closed switch) between C and E.
3. Active region: V is large enough to have a true I and V is larger than the saturation voltage. Here the BJT behaves like an amplifier with I = beta*IC.

The BJT can be used as a device to control the power.

delivered to a load by using its two state ON and OFF.When the base voltage is low, the device is OFF (open switch) and no current can flow in the load.When the base voltage is high, the device is ON (close switch) and depending on V it can be in saturation or activeCEregion. If we assume that V is lower than the saturation voltage, it is in saturation and behave like a short circuit. ThisCEmeans that in first apporximation the current across the load (resistor) is V /R.CCCC2And the power dissipated by the resistor is: V /R.In this approximation the power dissipated by the BJT is zero, while is the maximum value at the load. In real life thereis a power dissipation on the BJT that turns into heat and can affect its behaviour.Ideally the power supplied to the load can be zero or full power. However is possible to control the average powerCC2delivered by using a square wave with a controlled duty cycle so: P = D*V /R.AVGPulse Width ModulationThe goal of this circuit is to generate a

A square wave signal with a well controlled duty cycle. The system is composed by a Ramp voltage generator, a Comparator, a Power switch.

The ramp voltage generator produces a voltage which increases linearly from Vmin to Vmax and then drops back to Vmin. This voltage is compared against V (which is always between Vmin and Vmax). V determines the duty cycle of the square wave like this:

D = (V - Vmin)/(Vmax - Vmin) if Vmin = 0: D = V /Vmax.

MOSFET

It's a four terminal device that transfers power in a similar way to the BJT. It can be n-type or p-type. It can work like a switch or an amplifier. The terminals are: Gate, Drain, Source, Body. Most of the times B is directly connected with S creating a three terminal device.

The main difference between BJT is that a MOSFET can be seen as a voltage-controller current source rather than a current-controller one. In fact, the Gate is isolated from the rest of the circuit and here there is always zero current. While in

the BJT there is a current in the base and it’s even the controlling element. In the MOSFET the controlling element is V. There are 3 regions of operation:

• GS- Cut-off region: V is below a threshold voltage V. The current in the drain is zero. The device works like an open switch.
• Triode region: V is larger than V. V is below V – V = V (overdrive). I increases linearly with V.
• Saturation region: V is larger than V. V is larger than V – V = V. I depens directly on V by the quadratic relation: I proportional to (V – V).

Relays (Electromechanical Switch)

Four terminal device with a pair of “low voltage” and “high voltage” terminals. The low voltage terminals are wrapped with a coil while the high voltage terminals are connected to a mechanical switch normally open. When some current is forced at the low voltage terminals, the coil generates a megnetic field that close the switch.

on the high voltage side and allows the current to flow. Power Consideration As we discussed previously, a real transistor dissipates power which generates heat that could damage it or change its behaviour. A transistor is temperature dependent, so it's important to keep its temperature as small as possible. For this reason, Heat Sinks are used: they are a simple piece of metal placed in contact with the transistor to improve heat exchange with the environment. They are shaped to maximize their surface while keeping the volume small. A typical structure is a cylinder covered with radial lamellae. Another important feature is the Safe Operating Area (SOA): it's a graphic region composed of the operating condition points in which there will be no hazard or damage for the transistor. There can be found three different limits: - Top horizontal limit: a transistor can withstand currents until a maximum value typically determined by the bonding wire limit. In fact, each device terminal

1. The device is connected with the surrounding components with very thin gold wire that could blow up if the current is too large and physically disconnects the device.
2. Right vertical limit: another limit is the maximum voltage that the device can withstand. If a voltage exceeds a critical value, the electric field becomes so large that the material gets irreversibly damaged. Notice that these first limits are not related to the power dissipation.
3. Diagonal limit: this limit is given by the maximum power dissipation. The device will break due to structural limit. Typically a MOSFET has a larger SOA than the BJT. This is due to the fact that: a BJT has a positive dependence between the collector current and the temperature. If the current increases, the temperature increases. While the MOSFET has a negative relation between the drain current and the temperature.
4. H Bridge: This circuit is frequently used to control DC motors. Also called Full Bridge. It allows four configurations: clockwise and counterclockwise rotation.

Free run and break. It's composed by a DC voltage source and 4 switches. By changing the switch positions we can control the DC motor. Notice that with four switches, 16 configurations are available but, 7 of them are forbidden because they could cause damage at different circuit components like by closing two switches on the same branch because the input source will be short circuited. Others configurations generate the same output as the main four.

Let's talk about the 4 main configurations:

1. S1 and S4 closed: the input voltage drops across the motor from left to right, setting it in rotation in a given direction.
2. S2 and S3 closed: the input voltage drops across the motor with the opposite polarity so, it starts to rotate in the opposite direction.
3. S1 and S3 closed (or S2 and S4 closed): the motor terminals are short circuited so, no voltage drops across the motor. This is the brake configuration.
4. All switches are opened: the motor is floating and no current can flow across it.

The motor is free running. In real H Bridges, switches are BJTs or MOSFETs, each one coupled with a diode that prevents high voltage peaks when switching the power on and off. When MOSFETs are used, we can discuss if is better use n-type or p-type. The answer is two nMOSFETs for the low side and two pMOSFETs for the high side. This choice is made because in nMOSFET the current flows from the drain to the source. Viceversa for the pMOSFET. We know that the MOSFET behaviour is controlled by V (gate to source voltage). If we use the configuration previously defined, all four source will be at a well-defined voltage values: the low side sources grounded and the high side sources at V. therefore it's easy to estimate VGS, which is IN needed to drive the switch on and off: V = V - VGS G S (fixed) However this configuration has a limitation: nMOSFET can carry three times more current than pMOSFET so, to have a balanced circuit, we need bigger pMOSFETs and so, more expensive. A cheaper.

alternative is by using all four nMOSFETs and by implementing an additional simple circuit that connects all sources at the fixed voltage values.

Chapter 8 – Actuators

Solenoids

It's made with an ferromagnetic core free to move inside an inductor. By applying some current in the inductor, it generates a magnetic field which pushes or pulls the ferromagnetic core by following the right hand rule. Usually the core is contrained with a spring and some stoppers to avoid excessive excursion.

AC Motor

It's an electromechanical device that converts electric energy into mechanical rotational energy. It's supplied by an AC voltage source. It's made by two compoentes: a stator and a rotor. The stator is fixed and provides the interface between the electrical energy and the motion. The rotor is in rotational motion so, it's mechanically connect to the object that we want to move. The electric energy is transferred to the rotor by an electromagnetic coupling and

So, without any physical contact, the stator consists in a hollow structure with a specific number of coils placed to create a rotating magnetic field. This rotating magnetic field is created by coupling opposite coils and by supplying at each one of them an AC sine wave signal with the same amplitude and frequency but with a well-defined phase lag that actually depends on how many coils are present. Let's explain the behavior with an example: two phases, each one with two coils placed orthogonally. Initially, the green sine wave is at its maximum while the yellow one is zero. Here, only the green coils are producing a magnetic field which is horizontal. After a little bit of time, the sine waves have the same value, so the field is at 45 degrees between them. Then, the green sine wave goes to zero while the yellow one is reaching its maximum. Now, the field is completely vertical. By following this behavior, the stator generates a rotating magnetic field. There could be different coil configurations, for example, 3.

coils alimented by sine waves with 120 degrees of phase lag. Notice that the speed of this rotation is dictated by the frequency of the AC input signals. The amplitude controls the magnetic field intensity.

Now we have to understand how the rotor can move. There are two typologies of motors: synchronous and asynchronous.

Synchronous: its rotor generates his own magnetic field. Due to Faraday's law, the rotor magnetic field will try to align itself with the rotating magnetic field. Therefore they will rotate together at the same angular speed. It can't self-start.

Asynchronous: its rotor is made only with metal bars coupled at their extremities with two metal rings. Opposite bars create a closed loop circuit in with there will be a parasite current due to the magnetic flux.