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.