Category Archives: Transistors

How to Select Voltage References

how to select voltage referencesSensing applications use Analog to Digital Converters and Digital to Analog Converters and the accuracy of their readings depends on the voltage reference used. Most often the voltage reference used are very simple components with only two or three pins. However, the performance of these references depends on several parameters and careful attention is necessary when selecting the proper one. Typically, applications use either a shunt or a series voltage reference.

A series voltage reference is basically a high precision, low-current linear regulator. The load current comes through a series transistor positioned between the input voltage and an internal reference voltage. For the shunt voltage reference, a transistor placed parallel to the load shunts excess current to ground. As the series reference has to supply only the required load current, the shunt reference dissipates more power. The bias current of a shunt reference has to be greater than or equal to the maximum load current plus the minimum operating current of its internal reference.

In general, shunt references offer the user greater flexibility in handling higher input voltages and in creating floating or negative references. They also provide better power supply rejection but consume higher power. On the other hand, series references dissipate lower power and perform better for high-precision applications. The typical way of depicting the use of a shunt reference is by showing the symbol of a Zener diode.

Drift or variation of the reference voltage over temperature is a very important factor and has the units of parts-per-million per degree Celsius or ppm/°C. Most monolithic references use the bandgap reference as their base. Special circuitry is required to maintain drifts lower than 20 ppm/°C with the additional circuitry providing some form of curvature correction. Other types of references use a buried Zener diode voltage combined with the base-to-emitter voltage of a bipolar transistor to provide a stable reference voltage of about 7V. Both types have similar drift characteristics, but buried Zener types offer better noise performance.

All voltage references generate internal noise producing a dynamic error degrading the signal to noise ratio of a data converter. Device datasheets typically provide specifications separately for low and for high frequency noise in addition to the broadband noise in rms microvolts over the 10 Hz to 10 KHz bandwidth. You can reduce the broadband noise by adding a bypass capacitor.

Thermal cycling or change in temperature causes references to show thermal hysteresis. This appears as a shift in the reference voltage. Manufacturers define a thermal cycle as an excursion from room temperature to a minimum and a maximum temperature with a return to the room temperature. This is important as the reference may have to be soldered and this can induce shifts from the desired reference voltage.

Continuous operation may cause long-term stability issues resulting in a typical shift in the reference voltage. Manufacturers usually state the shift after six weeks or 1000 hours of continuous use. Since long-term stability is typically related logarithmically to time, the shift in reference voltage in the first 1000 hours provides a rough idea of the stability of the voltage reference over its life.

What are IGBTs?

An IGBT or the Insulated Gate Bipolar Transistor is an amalgamation of a MOS and a bipolar transistor. It combines the best performances of both devices – the easily driven MOS gate and the low conduction loss of the bipolar. This effective device is quickly displacing most power bipolar transistors that were an obvious choice for high voltage and high current applications. IGBTs offer a balance in tradeoffs between conduction loss, switching speed and ruggedness. Manufacturers are now tweaking IGBTs to work successfully in the areas of high frequency and high efficiency that so long were the sole domain of power MOSFETs. In fact, barring applications that require very low currents, the industry trend is to replace power MOSFETs and power bipolar transistors with IGBTs.

When choosing an IGBT for a specific application, answering a few questions will usually narrow down the selection. Zeroing in on the most appropriate device will require a better understanding of the terms and graphs published by the manufacturers. These questions will be:
• What will be the operating voltage? Select IGBTs with VCES rating of at least 120% of the voltage that has to be blocked.
• Will the switching be hard or soft? A Punch-Through or PT type IGBT is best suited for soft switching because tail current reduces.
• What current does the device require to handle? In the part number of an IGBT, the first two numbers are a rough indication of the usable current. When looking for a device to work with hard switching applications, the selection usually depends on usable frequency versus current graph of the device. However, a certain amount of derating may be needed for which you could start with the IC2 rating.
• What is the speed you require to switch? For maximum possible speeds, a PT type IGBT is more suitable. Again, for hard switching applications, refer to the frequency versus current graph of the device.
• Will the device have to withstand short-circuit conditions? If you are driving motors, the device will certainly have to withstand shorts with low switching frequencies. Most often, short circuit capability is not required for switch mode power supplies.

A generic N-channel IGBT is fundamentally an N-channel MOSFET on a p-type substrate. PT type IGBTs usually have an additional n+ layer. Therefore, the operation of an IGBT is similar to how a power MOSFET works.

When you apply a positive voltage from the emitter to the gate terminal, electrons are drawn towards the gate in the body region. When the gate-emitter voltage is equal to or above the threshold voltage, electrons drawn towards the gate form a conducting channel across the body region, allowing current flow from the collector to the emitter or electron flow from the emitter to the collector.

The flow of electrons causes positive ions or holes to flow from the p-type substrate into the drift region near the emitter. Therefore, IGBTs can have simplified equivalent circuits such as:

The price for lower on-state voltage is the IGBT may latch up if operated outside the datasheet ratings. This is a failure mode where the IGBT cannot be turned off by the gate.

Transistors: What Is The Difference Between BJT, FET And MOSFET?

BJTs, FETs and MOSFETs are all active semiconductor devices, also known as transistors. BJT is the acronym for Bipolar Junction Transistor, FET stands for Field Effect Transistor and MOSFET is Metal Oxide Semiconductor Field Effect Transistor. All three have several subtypes, and unlike passive semiconductor devices such as diodes, active semiconductor devices allow a greater degree of control over their functioning.

Depending on their subtypes, operating frequency, current, voltage and power ratings, all the three types of transistors come in a large variety of packages, and all of them are susceptible to ESD or Electro Static Discharge. That means when you handle these devices, you must take adequate precaution against static charges destroying them.

he basic construction of a BJT is two PN junctions producing three terminals. Depending on the type of junctions, the BJT can be a PNP type or an NPN type. The three terminals are identified as the Emitter or E, the Base or B and the Collector or C. BJTs usually function as current controlling switches. The three terminals can be connected in three types of connections within an electronic circuit – Common Base configuration, Common Emitter configuration and Common Collector configurations. All the three connections have their own functions, merits and demerits. The BJT is Bipolar because the transistor operates with both types of charge carriers, Holes and Electrons.

The FET construction does not have a PN junction in its main current carrying path, which can be made from an N-type or a P-type semiconductor material with high resistivity. A PN junction is formed on the main current carrying path, also called the channel, and this can be made of either a P-type or an N-type material. The three leads of a FET are the Source (S), Drain (D) and Gate (G), with Source and Drain forming the ends of the channel and the Gate controlling the channel conductivity. Unlike the BJT, the FET is a unipolar device since it functions with the conduction of electrons alone for the N-channel type or on holes alone for a P-channel type.

The input impedance at the gate of an FET is very high, unlike the BJT, which comparatively has much lower impedance. Additionally, the conductivity of the channel depends on the voltage applied to the Gate, essentially making it a voltage-controlled device, unlike the BJT, which is current-controlled. The voltage applied to the Gate controls the width of the channel, allowing the FET to carry current between the Drain and Source pins. The Gate voltage that cuts off the current flow between Drain and Source is called the pinch off voltage and is an important parameter.

The MOSFET is a special type of FET whose Gate is insulated from the main current carrying channel. It is also called the IGFET or the Insulated Gate Field Effect Transistor. A very thin layer of silicon dioxide or similar separates the Gate electrode and this can be thought of as a capacitor. The insulation makes the input impedance of the MOSFET even higher than that of a FET. The working of the MOSFET is very similar to the FET.

You can read more about transistors in depth here.

How RTPs Help To Save Expensive PCBs from Thermal Runaway PowerFETs

powerfetAlthough powerFETs or power Field Effect Transistors are very robust devices used in the automotive industry – they have their limitations. In the automotive environment, powerFETs go through the tortures of extreme temperature variations together with severe thermo-mechanical stresses. They face noisy short circuits, high arcing, intermittent shorts as well as inductive loads. These shocks can fatigue the device over time, and it can fail in a short, an open or resistive mode.

For example, if the maximum operating voltage of a powerFET is exceeded, failure happens very quickly. The powerFET goes into an avalanche breakdown once the voltage rating goes beyond the maximum allowed. If the energy within the transient overvoltage is more than the rated avalanche energy level, the device will start to fail resulting in generation of smoke, flame or it may even be de-soldered.

In some cases, the powerFET while failing may generate precarious temperatures through I2R heating. This may cause a thermal runaway for the device, but the resulting current may not be large enough to cause failure of a standard fuse protecting the powerFET. This mode of failure is of particular concern, for not only the powerFET, but also for the PCB or the Printed Circuit Board. A power of as little as 10 Watts may generate localized hot spots of above 180°C, which can damage the glass PCB’s epoxy structure leading to a thermal event.

Tyco Electronics has developed a Reflowable Thermal Protection or RTP, which is a reliable and robust surface mount thermal protector to prevent thermal damage on PCBs caused by failing power electronics. This is a secondary thermal protection device, which can replace several components such as redundant powerFETs, heavy heat sinks and relays currently used for such protection in the automotive designs.

To work effectively, the RTP device has to be placed in series and on the power line, very close to the FET. This allows the device to track the temperature of the FET and disrupt the current by opening the circuit before the thermal runaway condition generates a thermally destructive condition on the PCB. Under normal conditions, the RTP device has a low resistance, typically about 0.6mOhm.

Whenever the RTP device detects the generation of unsafe temperatures because of the failure of a power component or any other board defect, it interrupts the current and prevents a thermal runaway condition that could lead to critical damage. An RTP200 device typically opens (high resistance condition) at 200°C, which is a temperature above the normal operating temperature, but below the Lead-free solder reflow temperature.

It may seem like a paradox that the RTP device operates at 200°C but can withstand Lead-free soldering temperatures of 220°C. This is because the RTP is not in an active state unless it has been armed by passing a specific current through it for a specified amount of time. Before it is armed, the RTP can withstand three Lead-free solder reflow steps before it operates. The electronic arming procedure is one-time only and can be implemented to occur automatically or during system testing.

What is a MOS-FET?

Mos-FETMOS-FET, which is an abbreviation of Metal-Oxide-Semiconductor Field Effect Transistor, is a very important kind of transistor. Many IC’s are constructed of arrays of MOS-FETS on a tiny sliver of silicon.

They are very small, easy to manufacture and many MOS-FETS consume a small amount of power making them an excellent choice for many applications.

It is the most common type of transistor available for either digital or analog circuits, replacing the bipolar transistor which was much more common in the past.

The word ‘metal’ in the name is actually now a misnomer because what was originally the gate material (often Aluminum) is now more often a layer of polysilicon (aka polycrystalline silicon).

BUZ11 – a Popular Power MOSFET

BUZ11

BUZ11

The BUZ11 is an N-Channel enhancement mode silicon gate power field effect transistor designed for applications such as switching regulators, switching converters, motor drivers, relay drivers, and drivers for high power bipolar switchng transistors requiring high speed and low gate drive power. The BUZ11 is also used for DC-DC and DC-AC converters and in the automotive environment for injection, ABS, airbags, lampdrivers and more.

It features:

  • 33A 50V
  • Nanosecond Switching Speed
  • Linear Transfer Characteristics
  • High Input Impedance

The BUZ11 is in a TO220 package.

If you are looking at the BUZ11 with the drain (flange) at the top, the left pin is the GATE, the middle is the DRAIN, and the right lead is the SOURCE.