Buck Converters

Overview

Buck converters use a switching element, inductor and capacitor to convert an input voltage into a lower output voltage. It is a type of switch-mode power supply (SMPS).

When using a P-channel MOSFET for synchronous rectification, it’s body diode is forward-biased when the converter is in shutdown mode. This can drain the power source into the output. More advanced buck converters have extra circuitry to disconnect this P-channel MOSFET when the device is not active.

Control Methods

Almost all control methods aim to regulate the output voltage.

Constant Frequency, Current-Mode Control

Constant frequency, current-mode control is a very common control method for buck converters. It has the benefits of:

• Permitting smaller output capacitances
• Simplifying the external frequency compensation

Peak current measurement is a common way of “measuring” the average output current.

A transconductance amplifier (amplifier that converts a input voltage to an output current) is used to compare the voltage at a feedback pin (typically labelled FB) to an internal voltage reference.

PCM: Peak current mode

See the excellent SNVA555: Understanding and Applying Current-Mode Control Theory by TI for more information on current-mode control theory.

Inductor Selection

You can use the following equations to select the main inductor for a buck converter.

First, calculate the maximum average inductor current using:

$$ I_L = I_{OUT} \frac{V_{OUT}}{0.8 V_{IN}} $$

where:
\( V_{IN} \) = the input voltage to the buck regulator
\( V_{OUT} \) = the output voltage of the buck regulator

Then, calculate the value of inductance required with:

$$ L = \frac{V_{IN} (V_{OUT} - V_{IN})}{\Delta I_L \cdot f \cdot V_{OUT}} $$

where:
\( \Delta I_L \) = the desired ripple current in the inductor
\( f \) = the switching frequency
and everything else as mentioned previously

Types Of Inductors

Inductors used for buck converters can generally be classified into one of three different types:

1. Wire-wound ferrite core: Copper wound around a ferrite core.
2. Metal composite: Metal powders moulded around copper windings at high pressure.

1. Multilayer: Laminated sheets.

The various advantages of each of explored in the following table:

Capacitor Selection

The output capacitance is primarily determined by the maximum allowed output voltage ripple. This ripple is determined by the capacitance of the capacitor and it’s ESR (equivalent series resistance). The output capacitance of a boost converter can be found using the following equation.

$$ C_{min} = \dfrac{I_O (V_{OUT} - V_{IN})}{f \Delta V V_{OUT}} $$

where:
\(\Delta V\) = the maximum desired output voltage ripple
and everything else as mentioned previously

The actual ripple will be slightly larger than this due to the ESR of the capacitor.

$$ \Delta V_{ESR} = I_O R_{ESR} $$

where:
\(R_{ESR}\) = the parasitic series resistance of the output capacitor

The total output ripple is the sum of the ripple caused by the capacitance, and the ripple cause by the ESR.

Down Conversion

Some boost converters also have a built in regulator to provide regulation when the input voltage exceeds the desired output voltage. This is normally a linear regulator, so your efficiency will drop and you will have to take into account the thermal dissipation. This is normally called down conversion.

The price you pay for this added down conversion feature is a slightly higher cost, and slightly higher quiescent current (e.g. some of TI’s boost converters have 19uA quiescent current without down conversion, and 25uA with down conversion).

Input Voltage Range

Typically, boost ICs with an internal switch (a converter) can support lower input voltages than those that require an external switch (a controller). A typical minimum input voltage for a converter is in the range 0.3-0.9V, while a controller’s minimum is in the range 0.9-1.8V.

Buck Converter Calculator

You can find a buck converter calculator as part of mbedded.ninja’s NinjaCalc web app.

Operation Modes

Continuous Conduction Mode (CCM)

CCM is the basic and default operating mode for most buck converters. It is a synchronous mode, meaning the switching frequency is constant and continuous.

Advanced Asynchronous Modulation (AAM)

AAM is not supported by all buck converters, and is a mode used at low output currents to reduce the power consumption of the SMPS.

Examples

Tiny (Nano) Buck Converters

Texas Instruments released a series of very small (3.5x3.5x1.8mm) buck converter modules in 2015. One of the most impressive features is that this includes the inductor (external capacitors are still required). One example is the LMZ20502, which can provide up to 2A of current with an input voltage range of 2.7-5.5V and a output voltage range of 0.8-3.6V.

Notice how most of the volume on the module is taken up the chip inductor (the big brown thing that dominates most of the image). The dimensions of the package are shown in the diagram below.