Power supply is an important part of modern electronic design, and there are many available components on the market to help engineers design energy-saving and compact power products. These components range from discrete components such as simple diodes to complex power management ICs with advanced semiconductor architectures.
Designing energy-efficient power supplies is a challenging task, and power engineers need these power products to provide as much power as possible (usually higher power than the previous generation) and to make them as small as possible. However, at a higher power level, there will be a lot of heat that needs to be dissipated, and the accumulated heat will have a negative impact on the long-term reliability of power products.
Considering that heat is inversely proportional to efficiency, efficiency is an important topic in almost all discussions on power supply. Increased efficiency means less heat is generated, so less heat management can be required. This is one of the few truly "win-win" product designs in engineering because it reduces power size, increases power density, reduces BOM costs, reduces operating costs, and improves reliability.
The components used in the power supply have a significant impact on overall efficiency. This article will briefly analyze the major component categories.
If pipes are used as an analogy, a diode can be considered a check valve that allows current to flow in one direction (from anode to cathode) but prevents any reverse current flow. Diodes are usually used to convert alternating current (AC) to direct current (DC). Four diodes are arranged back-to-back (as shown in Figure 2). This circuit is called full wave "bridge" rectifier. In Rectifier Applications, the main parameters to be considered are the forward current rating (Amperes) and the withstand reverse voltage. Diodes are also effective in switching applications.
There are several types of diodes available on the market (see Figure 1), and the difference becomes particularly apparent when the current is on, due to the different forward voltages associated with each particular type. Conventional diodes have the highest voltage drop, which leads to more losses and heat dissipation in the diodes. The forward voltage drop of Schottky diode is lower, which makes its loss smaller, but the factor to be weighed is its low reverse breakdown voltage.
The speed at which diodes convert between current conduction and blocking AC current is also important. Diodes made of conventional materials are fast and slow, while Schottky diodes are almost always fast.
New wide band gap semiconductor materials such as silicon carbide (SIC) and gallium nitride (GAN) have been used in diode modules. These new materials can improve all the main performance parameters (such as temperature rating, forward voltage, reverse breakdown voltage and speed). Not surprisingly, these new components are also more expensive at present, but their unit price will decrease with the increase of production.
Zener diodes are a special type of diode used to clamp transient voltages or create fairly accurate voltage references. This unique diode blocks the reverse current up to a certain voltage and then allows the current to flow. Usually, the reverse breakdown voltage should be considered when choosing zener diode.
A transistor is a solid-state switch that can be controlled by voltage. The current can pass between the collector and emitter, depending on the voltage at the base. By driving the base at high or low voltage, the transistor can be used as a hard switch, which means that the current is either full scale or zero. In the case of an intermediate voltage on the base, the transistor operates in its linear region and the current is controlled by the base voltage.
Bipolar junction transistor (BJT) is the simplest type of transistor and is usually used only for low power designs. BJT has several different parameters, but the main parameters include the rated current, the ability to withstand the voltage between collector and emitter when the base is turned off, the operating speed and the current gain (ratio of base current to Collector Emitter Current). According to the polarity of control voltage and switching voltage, BJT can be divided into NPN or PNP type with slightly different symbols, as shown in Figure 3.
Another type of transistor is metal oxide semiconductor field effect transistor (MOSFET). Similar to BJT, they are also three pole devices, but each pole is reassigned. Between the "source" and "drain" pins, called "drain in". The main parameters of MOSFET are similar to BJT, including rated current, drain source voltage and power supply.
For MOSFETs in power applications, the most important parameter is the resistance measured between the drain and source when conducting, which is called "on resistance", and its symbol is RDS (on). The on resistance will cause the inherent power loss of MOSFET, and has a great impact on the overall power loss of power supply design. Another important parameter is the amount of charge required to drive the gate, which is called gate charge and is represented by the symbol QG. These charges need to be provided in each switching cycle, so the loss of high-frequency power supply is more affected.
Since the power loss of MOSFETs is usually lower than that of BJTs, MOSFETs can be used in higher power applications, especially in modern high-speed designs, because they can operate at higher frequencies than BJTs. There are four types of MOSFETs, n-channel and p-channel, as shown in Figure 4. In addition, there are enhanced mode and depletion mode devices. These names also determine the polarity of the device and whether the gate operates in normally off or normally on mode. All MOSFETs can conduct bidirectional between drain and source.
BJT and MOSFET technology can be combined to create another type of transistor, called insulated gate bipolar transistor (IGBT). These devices also have a gate, a collector and an emitter, but because they are relatively slow and older products, they are generally used only in on-off mode. Although IGBTs are usually limited to around 50 kHz, they can cope with higher power levels (typically up to 5kV / 400A). Therefore, they are usually deployed in high-power applications such as motor control, power supply and large inverter.
The last type of transistor is a thyristor, also known as an AC triode (triac) or a silicon controlled rectifier (SCR). The difference is that triac can be bi-directional, while SCR can only be unidirectional. The symbols of these devices all represent the corresponding device types, as shown in Figure 5. Both types are latch switches controlled by gate pins, both of which are well suited for high-power applications.
Broadband gap technology
Since the invention of semiconductor devices, silicon has been the preferred base material. However, in order to improve the performance of power devices to a new level, the above mentioned wide band gap materials (i.e. SiC and GaN) are becoming more and more common. Compared with the similar silicon devices, the broadband gap materials can achieve faster switching, lower loss and higher temperature. SiC and Gan were originally targeted at MOSFETs, aiming at applications that require maximum improvement in efficiency and switching speed.
Power management IC
There are many types of power conversion: AC-DC, DC-DC and DC-AC. just as there are many kinds of power conversion methods, the IC used in power supply application is also diverse. For these, there are many topologies that can be applied to specific application standards. Some of the most popular topologies include buck, boost, bridge, half bridge, etc. The IC provided by some manufacturers can be used as the controller of power system design. Generally, a complete design only needs external MOSFET and some discrete components, thus shortening the development time.
Some designs may require linear conversion, but these are often suitable for professional applications such as medical and scientific instruments that require ultra-low noise. Some linear controllers require external MOSFETs, while others have built-in MOSFETs, so they are often referred to as "three-terminal regulators.". The efficiency of linear conversion is often low, especially when the difference between input and output voltage is large.
In general, switching mode power conversion is more common and there are more such ICs on the market. Devices designed for low-power applications may have MOSFETs integrated into the controller, while for high-power applications, they are usually independent. Some devices can be complex and require the design of multiphase power solutions. There are also many ICs that can be integrated into larger, more complex systems for other power related functions.
Although many control ICs appear to be interchangeable, there is also a great deal of know-how in this area. Sometimes, manufacturers may cooperate through license agreements to share certain technologies, but in many cases, there may be subtle differences between products, which makes it easy to ignore the differences and cause problems in implementation.
Power pack options
Power conversion is one of the few areas where through-hole modules are still in use due to the need to install the module on the radiator. However, most components have surface mount options, and the common types are shown in Figure 6. With the rapid development of packaging technology, many manufacturers have found innovative ways to release heat from chips. This improves performance benchmarks, achieves higher power density, and ensures long-term reliability.
Power design can be a complex field, and designers are faced with great challenges in how to achieve better specifications and meet efficiency and safety standards. There are many devices on the market for power systems, which can be roughly divided into three categories: diodes, power switches (including transistors and MOSFETs), and more complex integrated circuits that provide the functions required for power supply.
In order to make power products provide more functions and higher performance, new devices are also emerging. Efficiency and reliability are the key indicators. Although the price of new broadband gap materials is higher, the development trend of technology is turning to this kind of devices which are more durable and have higher performance.