Power System Performance Requires The Right Actives And Passives

Power System Performance Requires The Right Actives And Passives

Switch-mode power supplies (SMPSs) have largely taken over as the de-facto standard for creating multiple supply rails where efficiency is of paramount importance. However, there are many different ways to design a power chain.

We can use buck (step down) converters, boost (step up) converters, buck-boost (step up and step down) converters, and quite a few other topologies. All of these choices, though, share a need for well-behaved external active and passive components to make the system work optimally.

Some power IC solutions may require as few as three external components, such as an ADP2108 buck regulator. Because it has internal power switches, this switch-mode regulator requires three external components—an input and an output capacitor and one inductor.

For cost, performance, and system reliability reasons, the designer must know which parametrics are critical in choosing the correct components. This article looks at the external passive and active components in a typical SMPS design, including resistors, capacitors, inductors, diodes, and MOSFETs.

Resistors

Resistors are widely understood, and their impact on a SMPS is fairly limited. However, where they are used, it’s important to understand their potential impact. When using an adjustable regulator, an external resistor divider network will be employed to divide down the output voltage to provide feedback for the regulator.

Resistor tolerance will come into play here, as will resistor temperature coefficients (tempcos). Newer FPGAs and processors, with their lower core voltages, are placing tighter tolerances on the supply voltage. For an FPGA with a 1-V core voltage, a 5% tolerance is only 50 mV.

Figure 1 shows how resistor tolerance as well as resistor tempcos can drastically affect your final design. The ADP2301 buck regulator has a 0.8-V reference. The output voltage will be:

If we define the gain of the circuit to be:

Designing for a 1-V output voltage, we’ll choose R2 = 10 kΩ and calculate R1 = 2.5 kΩ. The gain of the circuit will be:

If using 5% tolerance resistors and margining for worst case, our gain is:

or:

This amounts to a ±2% tolerance on the output voltage. In a system that needs 5% tolerance on a supply voltage, we’ve already eaten up a large part of our error budget. The same design using 1% tolerance resistors has ±0.4% error.

The resistor temperature coefficient will also cause an error in the system. If R1 is rated at +100 ppm/°C and R2 is rated at –100 ppm/°C, a 100°C temperature rise will add an additional 0.4% error. That’s why resistors with 1% tolerance or better are recommended. Resistors with tempcos as low as 10 ppm/°C are readily available but will increase system cost.

Capacitors

Capacitors perform several functions in SMPS designs, such as energy storage, filtering, compensation, and soft-start programming. As with all real devices, designers must be aware of capacitor parasitics. In the context of SMPS energy storage and filtering, the two most important parasitics are effective series resistance (ESR) and effective series inductance (ESL). Figure 2 shows a simplified drawing of a real capacitor.

An ideal capacitor’s impedance versus frequency will decrease monotonically with increasing frequency. Figure 3 shows the impedance versus frequency for two different 100-µF capacitors. One is an aluminum electrolytic type, the other a multi-layered ceramic capacitor.

At low frequencies, the impedance drops off monotonically with increasing frequency, as expected. But because of the ESR, this impedance reaches a minimum at some frequency. As frequency continues to increase, the capacitor starts to behave more like an inductor, and the impedance will increase in frequency. The impedance versus frequency curves are called “bathtub” curves, and all real capacitors behave in this manner.

Figure 4 illustrates the capacitor functions in a buck converter design. The input capacitor will see large discontinuous ripple currents. This capacitor needs to be rated for high ripple currents (low ESR) and low inductance (ESL). If the input capacitor ESR is too high, this will cause I*R power dissipation within the capacitor, reduce converter efficiency, and potentially overheat the capacitor.

The discontinuous nature of the input current will also interact with the ESL, causing voltage spikes on the input and introducing unwanted noise into the system. The output capacitor in a buck converter will see continuous ripple currents, which are generally low. The ESR should be kept low for best efficiency and load transient response.

Figure 5 illustrates the decoupling capacitor function in a boost converter. The input capacitor will see a continuous ripple current. The capacitor should be chosen to have low ESR to minimize the voltage ripple on the input. The output capacitor will see large discontinuous ripple currents. Low-ESR and low-ESL capacitors are required here.

In a buck-boost converter, the input and output capacitors will see discontinuous ripple currents. Low-ESR and low-ESL capacitors need to be used with this topology.

It may be wise to use several capacitors in parallel to build a larger capacitance. Capacitance will add in parallel. In addition, ESR and ESL will decrease in parallel. By using two (or more) capacitors in parallel, you’ll get a larger capacitance and lower inductance and resistance.

There are many different capacitor types to choose from. Aluminum electrolytic, tantalum, and multi-layered ceramic are the three most commonly used types.

Aluminum electrolytic capacitors offer large values at low cost. They represent the best cost/µF of all the options. The chief disadvantage of aluminum electrolytic capacitors is the high ESR, which can be on the order of several ohms. Be sure to use switching-type capacitors, which will have lower ESR and ESL than their general-purpose counterparts.

Tantalum capacitors use a tantalum powder as the dielectric. They offer large values in smaller packages than an equivalent aluminum capacitor, though at higher cost. ESR tends to be in the 100-mΩ range, which is lower than aluminums. Since they do not use a liquid electrolyte, their lifespan is longer than the aluminum electrolytic type, making them popular in high-reliability applications.

Also, tantalum capacitors are sensitive to surge currents, and they sometimes will require series resistance to limit the inrush currents. Be careful to stay within the manufacturer’s recommended surge current ratings as well as voltage ratings.

The multi-layered ceramic capacitor (MLCC) offers extremely low ESR (<10 mΩ) and ESL (<1nH) in a small surface-mount package. MLCCs are available in sizes up to 100 µF, though the physical size and cost will increase for values greater than 10 µF.

Be aware of the voltage rating of MLCCs as well as the dielectric used in their construction. The actual capacitance will vary with applied voltage, called the voltage coefficient, and the variation can be very large depending on the dielectric chosen. Figure 6 shows the capacitance versus applied voltage for three different caps.

X7R-type dielectric offers the best performance and is highly recommended. Ceramic capacitors, because of the piezoelectric characteristics of the dielectric, are sensitive to printed-circuit board (PCB) vibration, and the voltage noise generated can upset sensitive analog circuits such as phase-locked loops (PLLs). In these sensitive applications, tantalum capacitors that are immune to vibration effects may be a better choice.

Inductors

An inductor is a magnetic energy storage element, and current flowing through the inductor will induce a magnetic field in the core. This magnetic field is the mechanism for the energy storage. Because current in an inductor cannot change instantaneously, when a voltage is applied across an inductor, the current will ramp. Figure 7 illustrates the current waveform in an inductor.

When the switch closes, the full voltage (V) appears across the inductor. The current in the inductor will ramp at a rate of V/L. When the switch is opened, the current will ramp down at the same rate, and a large voltage will be generated as the magnetic field collapses. This magnetic field is the energy storage mechanism. Figure 8 shows a simplified model of an inductor.

In addition to the inductance, there will be a series resistance (DCR) and a shunt capacitance. The DCR is mainly the effect of the wire coil resistance, and it will be important when calculating power loss in the inductor. The shunt capacitance, along with the inductance, can cause the inductor to self resonate. The self-resonant frequency can be calculated from:

A good rule of thumb is to keep the switching frequency one-tenth the self-resonant frequency of the inductor. In most designs this will not be an issue. Power losses within an inductor will cause a temperature rise within the inductor as well as lost efficiency. There are two main categories of power loss in inductors. Designers need to understand both.

Winding resistance (DCR) losses are simply I^2*R losses within the wire conductor, also known as copper losses. The other contributors to power loss in inductors are known as core losses. Core losses are a combination of magnetic hysteresis and eddy currents within the core. They’re much more difficult to calculate, and may not even be available on a datasheet, but will cause power dissipation and temperature rise within the core.

Figure 9 illustrates the inductors function in both buck- and boost-mode power-supply designs. The primary function of the inductor is energy storage, but it also acts as a filter. Inductor value selection begins with determining the maximum ripple current desired.

A good starting point is to use 30% of the dc load current for buck converters and 30% dc input current for boost converters. With this, inductor value can be calculated using the equations in Figure 9. Inductor tolerances can be as much as ±30% out of the box, so be sure to include this in your calculation. Also be sure to choose an inductor with:

where ISat is the inductor’s saturation current. The saturation current is the current at which the inductance will drop by a certain percentage. This percentage will vary by manufacturer, ranging from 10% to 30%. When choosing an inductor, be sure to note the saturation current change over temperature, as your inductor is likely to be operating at high temperature.

Operating at a 10% reduction in inductance is generally acceptable, providing this is a worst-case scenario. Using inductors that are larger than necessary will take up more PCB real estate and are usually more expensive. A higher switching frequency will allow the use of lower-value inductors.

There are two main core materials used in inductors for SMPS: powdered iron and solid ferrite. A powdered-iron core has air gaps within the material that provide for a “soft” saturation curve. Because of the soft response to saturation, inductors using this core material will be better suited to applications that require large, instantaneous currents. Ferrite cored inductors will saturate more quickly, but cost less and will have lower core losses.

Choosing the right value of inductance for your circuit is not a simple calculation, but most designs will work within a fairly wide range of inductance values.

Multilayer chip inductors are relatively new players in the inductor family. They’re available in very small physical sizes (0805) and allow for a very small overall design. The inductance values are currently available up to 4.7 µH, so they generally lend themselves to designs with higher switching frequencies.

The small size also limits the current-handling capacity, approximately 1.5 A, so they aren’t viable for higher-power designs. They’re smaller and offer lower DCR and cost than standard wire-wound inductors, so they may be right for your application.

Shielded Versus Unshielded Inductors

While shielded inductors are more expensive and have a lower saturation current (for the same physical size and value), they greatly reduce electromagnetic interference (EMI). It is almost always worth using the shielded inductors to help avoid any EMI issues with your design.

Diodes

Asynchronous switching power-supply designs employ a passive switch. The switch usually takes the form of a diode. However, because of the diode’s forward voltage drop, asynchronous designs are generally limited to less than 3-A output. Otherwise, the efficiency drop will be too great.

For all but the highest-voltage designs, Schottky diodes are the recommended choice for asynchronous regulators. They are available in breakdown voltages up to approximately 100 V. The lower forward voltage drop of Schottky diodes, compared to silicon diodes, reduces the power dissipation considerably. The effectively zero reverse-recovery time also prevents switching losses in the diode.

Schottky diodes are available with ultra-low forward voltage drop as well. These are only available in breakdown voltages up to approximately 40 V and will cost a bit more, but they will reduce power dissipation in the diode even further.

When selecting a diode, consider the forward voltage drop, breakdown voltage, average forward current, and maximum power dissipation. Choose a device with a forward drop as low as possible, but be sure to use numbers from the data sheet that reflect the forward voltage drop at the current that will be seen in the design.

Often, forward voltage drop will increase greatly with increasing forward current. A higher forward voltage drop will cause greater power dissipation in the device. This, in turn, will decrease converter efficiency and may overheat the diode.

Diodes have a negative forward voltage temperature coefficient, which will be a double-edged sword. As the temperature of the diode rises, the forward voltage drop will decrease, decreasing the power dissipated within the device. But because of this effect, the paralleling of diodes to share current is not recommended, as one diode will tend to dominate and hog all the current in a paralleled system.

The diode’s breakdown voltage should be rated above the voltages in the system. The forward current rating should be greater than the designed rms current for the circuit’s inductor. And of course, the diode needs to be able to dissipate enough power to avoid overheating. Choose a device with a maximum power dissipation specification larger than the design requires. ADIsimPower, Analog Devices’ online power design tool, has a large database of diodes and will strive to choose the best one for your application.

MOSFETs

The “switch” in switching power supplies is generally a MOSFET. Very high-voltage and high-current designs may use an IGBT-type transistor. MOSFETs come in two main varieties: N-channel and P-channel.

N-channel enhancement-mode devices require a positive gate-to-source voltage for turn on, have lower on resistance than P-channel (for the same size), and are less expensive. P-channel devices require a negative gate-to-source voltage for turn on, have higher on resistance, and are more expensive.

Because of the positive gate-to-source voltage requirement, N-channel devices tend to be more difficult to drive, as the gate may need to be driven above the main supply in the system. A simple bootstrap circuit usually handles this, but it adds cost and complexity to the system. P-channel devices, on the other hand, are much easier to drive, and no additional circuitry is required. The consequences for using P-channel MOSFETs are higher cost and higher on resistance.

In choosing a MOSFET, one has to be aware of some key performance parameters, such as RDS, VDS, VGS, CDSS, CGS, CGD, and PMax. MOSFET devices will be rated for maximum current and maximum power dissipation. These ratings must be adhered to. Internal power dissipation comes from two main sources: I^2*RDS and switching losses.

When the MOSFET (switch) is on, the only power dissipation comes from the I^2*RDS loss. When the switch is off, the device dissipates no power. But during transitions, the device will dissipate power. The dissipation during transitions is called switching loss.

Figure 10 shows how the switching loss manifests itself. It is mainly caused by capacitance on the gate, both gate-to-source and gate-to-drain capacitances. These must be charged and discharged to turn on and off the MOSFET. Figure 10 also illustrates the waveforms of the voltage and current.

During turn-on, there’s a period when there’s both voltage across the device and current flowing through the device. This will cause V*I dissipation within the device. Switching losses are greater at higher frequency. This is one of the many tradeoffs in SMPS design. Lower frequency means larger inductors and capacitors and better efficiency. Higher frequency means smaller inductors and smaller capacitors, but more losses.

Summary

When designing a SMPS, the supporting cast of components often takes a back seat to the choice of controller or regulator IC. But the choice of active and passive components will have a huge effect on overall power-supply performance. Efficiency, heat generated, physical size, output power, and cost will all rely in some way on the external components that are selected.

Careful analysis of the required performance is needed to make the best selections. The use of an integrated design tool, such as ADIsimPOWER from Analog Devices, will simplify this process.

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