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Electronics 102 - Lesson 7

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Switching Power Supplies Parts Selection

When designing a switch mode power supply, parts selection can be a challenge. There are many, often conflicting requirements on components and the multitude of offerings from various vendors does not make the task easier.

The following components are usually the hardest to specify, so I will try to give you some idea of what to consider as you are making your part selection.

  • Power switch (usually a MOSFET)
  • Power Rectifiers
  • Power Transformer and Inductor(s)
  • Capacitors
  • Power Resistors

Power Switch

Nowadays, power MOSFETs are almost universally used in SMPS design, with IGBTs taking the rest. However, a few years ago the designation Power MOSFET did not leave too much to the imagination. Nowadays, there are many different types of Power MOSFETs: the traditional Silicon MOSFET and new parts based on exotic materials such as Silicon Carbide (SiC) and Gallium Nitride (GaN). Let's look at the features of each type of MOSFETs.

Silicon MOSFET

In power applications, N channel MOSFETs are universally used. For a given die size (which ultimately drives the price of a device), N channel MOSFETs have a lower Rdson than P channel MOSFETs and better switching characteristics. This is the reason why P channel MOSFETs are much harder to find and more expensive.

Some of the advantages of MOSFETs (particularly compared to Bipolar transistors) are:

  • In the ON state, a MOSFET presents a small resistance, i.e. at lower current, the voltage drop can be very low and efficiency very high.
  • The gate appears like a capacitor, it only needs current to change state. If you do not need to switch very fast, the gate drive circuit can be very simple.
  • The MOSFET does not have a secondary breakdown mode like Bipolar transistors
  • MOSFETs have an intrinsic "body" diode. In some applications, it can save a component.
  • A MOSFET's Rdson increases at high temperature, which improves current sharing when devices are used in parallel.
  • MOSFETs can take significant abuses with overcurrent (don't ask me how I know, you did not hear this from me :)

Some of the gotchas with using MOSFETS are:

  • For larger devices, the gate looks like a very large capacitor, requiring appreciable power to switch quickly.
  • MOSFETs optimized for switching power supply applications have to be significantly derated when used in linear mode (as in audio amplifiers). This is not always clearly stated in datasheets.
  • The "body" diode has poor characteristics
  • A MOSFET's Rdson increases at high temperature, which makes them prone to thermal runaway.

Insulated Gate Bipolar Transistor

The IGBT is a hybrid device, composed of a N channel MOSFET and a large Bipolar transistor on the same die. The MOSFET drives the Bipolar in a Darlington configuration, giving the device the easy drive of the MOSFET and the high current capability by die area of the Bipolar. IGBTs are limited to lower switching frequencies and have a finite voltage drop. For low voltage applications, MOSFETs are usually prefered but at high voltages >600V, IGBTs can be a lot less expensive than MOSFETs if you can live with the slower switching speed.

Silicon Carbide (SiC) MOSFETs

Silicon carbide devices have been in development for many years, starting with rectifiers. SiC rectifiers use Schottky technology but are able to handle voltages in excess of 1kV (Silicon Schottky rectifiers are limited to 200V, and their performance degrades significantly above about 80V). More recently, SiC MOSFETs have been introduced. The advantages of SiC compared to Silicon MOSFETs are low switching losses, low gate charge and high operating temperature. However, they require a more complicated drive circuit and are still expensive and not widely available.

Gallium Nitride (GaN) MOSFETs

Gallium Nitride is an even more recent development in power technology.The advantage of GaN over other processes is the high power density that can be achieved for a given die area, making miniature devices possible. GaN devices also require very low gate drive power and therefore can make very efficient power supplies. GaN devices are so fast and power efficient that they are now used in microwave amplifiers for portable electronics like cell phones and tablets. However the manufacturing process is more complicated than regular Si devices, so it may take a little longer before they can really compete on cost.

Power Rectifiers

There are variations also in the technologies used to manufacture power rectifiers, and there also performance will depend on the technology used.

Silicon Rectifiers

Silicon rectifiers are the most common and most widely used types of rectifiers. They are available across the entire range of power ratings, from low level signal diodes like the 1N4148 up to monsters rated hundreds of A and > 1000V.

Aside from voltage and current, rectifiers are classified by speed. specifically reverse recovery time. It is the time it takes for the rectifier to switch from a conducting state where the current flows in the desired direction, to a blocking state when the voltage reverses. Standard speed rectifiers have switching speed measured in microseconds while fast and ultra fast rectifiers are measured in 10's of nanoseconds.

If the rectifier is not fast enough for your application, it will get hot and efficiency will be poor. Ultimately, the rectifier may fail because the recovery time tends to get longer (worse) at higher temperature, leading to thermal runaway (the hotter the part gets, the slower the recovery, so the more heat is dissipated in the part which makes it even hotter.)

Within a certain voltage and current rating, you will have to decide how fast you need the rectifier to be. Standard speed devices are OK for 60Hz line frequency rectification. For any kind of switching supply, you will need, fast to ultra-fast rectifiers. The faster rectifiers tend to be more expensive and have a slightly higher voltage drop. This is the price to pay for faster recovery.

Schottky Rectifiers

Schottky rectifiers do not use a conventional semiconductor junction. They use a semiconductor-metal junction and their main characteristics are that the recovery time is essentially non-existant and the forward voltage drop is much lower than conventional silicon junction rectifiers.

That seems like a good deal however as always, there are gotchas. Schottky rectifiers have higher leakage current, particularly at high temperature, and the Schottky process is best suited for low voltage parts. While Schottky diodes are available with as high as 200V voltage rating, the performance of 200V Schottky rectifiers is not much better than that of regular silicon rectifiers at that voltage. Schottky diodes are best suited to applications using diodes rated 60V or less.

Silicon Carbide rectifiers

Here also, SiC has created a new price-performance point. SiC rectifiers are Schottky rectifiers (low voltage drop, no reverse recovery) but they can be used at high voltage (up to 1200V and higher) and at very high temperature without the leakage current problem of silicon Schottky diodes. They voltage drop is not as low as silicon Schottky diodes, so they are not used at lower voltages.

They tend to be very popular with Power Factor Correction circuits where their absence of recovery makes them very efficient.

Power Transformers and Inductors

Now we are getting into an area that is more akin to art than science, even though to get the best of a magnetic design, you have to be well versed in the science as well as the art.

There are many variations and options available to the magnetics designer and many large books have been written about it. I will keep this section short to give you a feel for what is involved.

Types of cores

The core defines the main properties of a piece of magnetics. Cores are optimized for high frequency operation (usually ferrites) while others are optimized for 60Hz line frequency operation (like conventional E-I laminations used on the "older" style of transformer.) For some applications like audio frequencies, tape wound cores can be used.

For inductors, there are also additional choices, like powered iron and Metglas (a proprietary material)

Winding techniques

The winding techniques and materials used to wind the coil of a piece of magnetics can have as much effect on performance as the core. The type of conductor (conventional round wire, square wire, Litz wire, foil) cabn be used to optimize performance depending of voltage, current and frequency of operation.

Capacitors

Capacitors are just as important in defining the performance of a power supply as any of the other components we just looked at. Capacitors are rated by capacitance and voltage of course, but just as important, ripple current and Equivalent Series Resistance (ESR) must be controlled and their effect understood. For switch mode power supplies, there are only really two different types of capacitors in common use, the Electrolytic capacitor and the Ceramic Multilayer capacitor.

Aluminium Electrolytic capacitors

Aluminium Electrolytic Capacitorss have a high capacitance for their size. They are usually polarized (you cannot reverse the voltage across the capacitor or it will fail spectacularly.) They also tend to have a higher ESR than other technologies. They are usually inexpensive and widely available. One particular nasty feature is that the electrolyte in them may leak and since it is acidic, it may cause extensive damage, or they may dry out and see their performance drop dramatically when they are stored for a long time. They may also short when they get older. Typically, Aluminium Electrolytic capacitors have a relatively wide tolerance, reflecting the variability of the manufacturing process. They are usually specified with tolerance like -20/+100%. Tighter tolerance parts tend to be much more expensive.

Multilayer Ceramic Capacitors (MLC)

MLC are the subject of intense developments, with performance (particularly energy density) increasing continuously. They now challenge Aluminium Electrolytic capacitors in many applications with their low cost, small size and excellent performance (low ESR) for switch mode power supplies. Some of the gotchas with MLC are

  • Brittle (may crack and short when the PWB they are solderd to is bent)
  • Acoustic noise (this is often surprising, they are susceptible to electrostriction)
  • Very low ESR may cause power supply instabilities (this is often counterintuitive)
  • High temperature coefficient (depends on the actual material used, high energy density materials like Z5U are worse)
  • Capacitance changes with voltage applied

Tantalum Electrolytic Capacitors

Tantalum capacitors have higher density, lower ESR and lower leakage than aluminium electrolytic capacitors. They can also be specified over a wider temperature range. However, they are expensive, will fail immediately with very small amount of reverse bias and do not like high current, which defeats the purpose of having a low ESR as you cannot use them for decoupling in high current circuits. Now that MLC capacitors are widely available, the need for tantalum capacitors is greatly reduced.

Resistors

The lowly resistor may also be a critical component in many switch mode power supplies. Two critical areas are when they are used for current sensing and in snubber circuits. Many different technologies are used to manufacture resistors and we are going to quickly review them.

Metal film

Metal Film resistors offer the best performance at competitive price. They are stable and can be purchased with high precision and stability ratings. The same technology can be used to make radial leaded parts and surface mounted parts. The metal film is fragile however and can be damaged by electrical stress, so watch out the ratings. They are best used in the precision voltage regulation circuit.

Thick film

A similar techn ology to metal film, but the film is a composite instead of metal. Performance is not quite as high but sufficient for most applications and the parts are typically the least expensive for a given value/power rating.

Wire wound

Wire wound resistors can have high precision and stability but they are usually used for their high power dissipation potential. However, since they are wound with wire in a helix, they may have a significant inductive component which makes them inapropriate in most current sensing circuits. Their application in snubber circuits should also be analyzed. Wire wound resistors are available with extremely high power ratings since there is no technological limit to how big they can be made.

Carbon Composition resistors

Carbon Composition resistors were used in RF circuits because of their low inductance and high peak power rating. However, very poor stability and wide initial tolerance have caused their demise and they are now almost impossible to procure.

Bulk Ceramic resistors

Bulk Ceramic resistors have replaced composition resistors. They have a lot of the same advantages of low inductance and high peak power rating, but with excellent stability. However, they are expensive so they are only used when absolutely needed. They are also available with very high power ratings.