Power Management 101 Series: Power Management Semiconductors

April 14, 2009
What are power management subsystems? Power management subsystems control the ac and dc power that keeps electronic systems operating properly.

What are power management subsystems?

Power management subsystems control the ac and dc power that keeps electronic systems operating properly. Power management is analogous to the body’s blood vessels that supply the proper nutrients to keep a person alive. Effective power management is critical for reliable operation of electronic systems, particularly as operating voltages decrease and operating currents increase.

What types of semiconductors are used in power management?

The two main categories of power management semiconductors are power semiconductors and integrated circuits. These two types are distinguished by their internal functions, for example, power semiconductors usually employ one or two functions, but integrated circuits can have multiple internal functions. Power dissipation can’t be used as a defining difference between the two types of devices because they can both dissipate power as low as 1 W or 2W. Power management integrated circuits are more likely to be at the low end of power dissipation, whereas power semiconductors are more likely to be found at the high end of power dissipation values.

What types of power semiconductors are used in power management subsystems?

Power management subsystems can use either switching or linear techniques. Switching circuits only have two states, on or off, whereas analog circuits can have an infinite number of states that lie anywhere from on to off. A good mechanical analogy is that a thermostat is digital because it is either on or off. However, a thermometer is analog because it can have an infinite range of values from its high to low temperature. Power management subsystems employ linear and switching techniques.

What power semiconductors are used in switching power management subsystems?

Power semiconductors employed in power management systems include power switches, and rectifiers (diodes). Power switches include MOSFETs, IGBTs, and BJTs (bipolar junction transistors). Rectifiers are used primarily to convert ac to dc and also to impede the flow of current in their reverse direction.

How are these power semiconductors used?

Monolithic power semiconductors may be discrete devices, that is, only a single type in a package, or integrated with other circuits in a package. Monolithic MOSFETs and BJTs can be discrete devices or integrated with other circuits in a single package. IGBTs are usually only discrete devices, or may have an integrated diode. In addition, various types of power semiconductors may combined in a hybrid package, that is, interconnected with other monolithic discrete devices in the same package. Of the available power switches, MOSFETs are the power semiconductor of choice in power supplies. IGBTs are used extensively in UPSs (uninterruptible power supplies).

What are Power MOSFETs ?

Power MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors) are three-terminal silicon devices that function by applying a signal to the gate that controls current conduction between source and drain. Their current conduction capabilities are up to several tens of amperes, with breakdown voltage ratings (BVDSS) of 10V to 1000V.

What is an IGBT?

The insulated gate bipolar transistor (IGBT) is a three-terminal power semiconductor , noted for high efficiency and fast switching. It switches electric power in many modern appliances: electric cars, variable speed refrigerators, air-conditioners.

The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low–saturation-voltage capability of bipolar transistors. It uses an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device. The IGBT is used in medium-to-high power applications such as switched-mode power supply, traction motor control and induction heating. Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amps with blocking voltages of 6,000 V.

First-generation devices of the 1980s and early 1990s were relatively slow in switching, and prone to failure through such modes as latchup and secondary breakdown. Second-generation devices were much improved, and the current third-generation ones are even better, with speed rivaling MOSFETs, and excellent ruggedness and tolerance of overloads.[1]

How does an ideal power semiconductor switch work?

The individual power semiconductor switch (Fig. 1-1) applies power to a load when a control signal tells it to do so. The control signal also tells it to turn off. Ideally, the power semiconductor switch should turn on and off in zero time. It should have an infinite impedance when turned off so zero current flows to the load and zero impedance when turned on so that the on-state voltage is zero. Another idealistic characteristic would be that the switch input consumes zero power when the control signal is applied. These idealistic characteristics are unachievable with the present state of the art.

How does a power semiconductor operate under real world conditions?

In the real world, actual power semiconductors do not meet these ideal characteristics. For example, Fig. 1-2(a) shows a control signal applied to an ideal power semiconductor switch whose output exhibits zero transition time when turning on and off (Fig. 1-2(b)). When the transistor is off (not conducting current) power dissipation is very low because current is very low. When the transistor is on (conducting maximum current) power dissipation is low because the conducting resistance is low. In contrast, an actual power switch exhibits some delay when turning on and off, as shown in Fig. 1-2(c). Therefore, some power dissipation occurs when the switch goes through the linear region between on and off. This means that the most power dissipation depends on the time spent going from the off to on and vice versa, that is, going through the linear region. Thus, the faster the device goes through the linear region, the lower the power dissipation and losses.

What affects power semiconductor reliability?

Excessive operating voltage can cause power semiconductor failures because the devices may have small spacing between their internal elements. An even worse condition for a power semiconductor is to have high voltage and high current present simultaneously. A few nanoseconds at an excessive voltage or excessive current can cause a failure. Most power semiconductor data sheets specify the maximum voltage that can be applied under all conditions. The military has shown very clearly that operating semiconductors at 20% below their voltage rating improves their reliability substantially.

Besides excessive voltage or current what can cause power semiconductor failures?

Another common killer of power semiconductors is heat. Not only does high temperature destroy devices, but operation at elevated, non-destructive temperatures can still degrade useful life. Data sheets specify a maximum junction temperature, which is typically between 100°C and 200oC for silicon. Most power transistors have a maximum junction rating of 125oC to 150°C, the safe operating temperature is much lower.

What is the effect of transients on a power semiconductor?

Unlike electromechanical parts, semiconductors can be destroyed by very short pulses of energy. A major source of destructive transients is caused by turning on or off an inductive load. Protection against these problems involves a careful combination of operating voltage and current margins and protective devices.

What is the effect of dv/dt and di/dt on a power semiconductor?

The terms dv/dt and di/dt reflect a time rate of change of voltage (dv/dt) or current (di/dt); they may be considered a transient, but they aren't because turning on or off a reactive load generates these conditions. These problems can occur in power semiconductor switches because all sections of the device do not behave in an identical manner when subjected to very high rates of change. It is not only important to look at the dv/dt and di/dt values generated within a circuit, but turn on and turn off times as well. Switching power on and off at a rapid rate can cause electromagnetic interference (EMI) that can affect nearby electronic systems. Domestic and international standards define the amount of EMI that can be emitted.

What is Unclamped Inductive Switching (UIS)?

Whenever current through an inductance is quickly turned off, the magnetic field induces a counter electromagnetic force (CEMF) that can build up surprisingly high potentials across the switch. With transistor switches, the full buildup of this induced potential may far exceed the rated voltage breakdown of the transistor, resulting in catastrophic failure.

There are two failure modes when subjecting a MOSFET to UIS. These failure mechanisms are considered as either active or passive. The active mode results when the avalanche current forces the parasitic bipolar transistor into conduction. In the passive mode the instantaneous chip temperature reaches a critical value. At this elevated temperature the parasitic NPN bipolar transistor and causes catastrophic thermal runaway. In both cases the MOSFET is destroyed.

What is the Safe Operating Area of a power semiconductor?

Power semiconductor manufacturers include a curve in their power transistor data sheets that defines the allowable combination of voltage and current, which is called the device’s safe operating area (SOA). The product of the voltage and current represents the watts dissipated in the chip. If you exceed the SOA, the chip will get too hot and fail. MOSFET devices are limited by the SOA; bipolar devices have an additional failure mechanism called secondary breakdown that significantly reduces the SOA.

What are the characteristics of integrated power management semiconductors?

Integrated power management devices are becoming more prevalent because IC manufacturers have developed techniques for combining the control circuits and power semiconductor switch in the same monolithic package. Often, the power semiconductor switch operates at higher voltage than the control section, which requires isolation between the high voltage and low voltage circuits. It is difficult to isolate higher voltages within the small spaces of an IC, and it also consumes extra silicon as well. However, these problems have been overcome.

What are the cost considerations of integrated power management conductors?

Cost is another consideration with integrated power management devices. As a semiconductor chip gets larger its cost grows exponentially. And, there is the cost of the package that houses the integrated power device and the cost of interconnections. If both the IC and the discrete power semiconductor have large die so that die cost dominates the overall cost, it will be much cheaper to use two parts. Integrated power technology obviously makes sense when the die sizes are moderate, or there are multiple outputs. This is so because the package and handling costs offset the increased silicon cost. A major impact on cost is the number of good devices that can be obtained from silicon wafer, usually called yield. Not only does a larger die size mean a disproportionately larger cost, but the distribution of values of key parameters has a major impact as well.

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