Understanding Buck Regulators

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ABSTRACT

In this article, we will examine the basics of DC-DC power conversion typified in computer applications by a
Buck Regulator. This brief letter addresses the importance of this device from an overclocker’s stand point.

INTRODUCTION

Every electronic system is designed to operate from a supply voltage, which is usually assumed to be constant.
A voltage regulator provides this constant DC output voltage and contains circuitry that continuously holds the
output voltage at the design value regardless of changes in load current or input voltage (referred to as regulation),
assuming that the load current and input voltage are within the specified operating range for that regulator.

In
portable systems, the input voltage is often a battery or in case of a motherboard, DC is obtained via a rectified
AC by means of the computer’s power supply, the essence being, the driving voltage is a Direct Current.

DC to DC converter
type regulators take such DC input voltage and produce the required output voltage, which could be higher or
lower than the input battery voltage. Buck regulators are what provide DC power to the CPU and DRAM and
are the target of every Volt-modder. Volt modding in essence involves making available a voltage range beyond
what is specified in the BIOS. While the BIOS allows a measure of voltage controlled via the programmable
nature of the buck regulator, going beyond the specs of the regulator itself is not possible in this manner.

One
could say we are overvolting the regulator or modifying the power supply loop characteristics to accommodate
our needs. To attempt such a thing, we have to understand the basics of the component we are trying to modify.
Linear voltage supplies/regulators have been around for quite a while. However, they bring with them several
critical drawbacks, such as low efficiency, which in turn necessitates the use of bulky heatsinks, cooling fans, and
isolation transformers. This in turn makes them unsuitable in today’s world of compact electronic systems.

The
disadvantages of a linear power supply are greatly reduced by the use of an alternative scheme (ie regulated
switching power supply). In case of a Linear regulator, the power is transferred continuously from Vin to Vout.
In case of a Switching regulator, the power is transferred from Vin to Vout in bursts. This brief note will discuss
one such power supply, the Buck-Regulator.

SWITCHING REGULATOR BASICS

What is wrong with a linear regulator?

Linear regulators are okay for powering very low powered devices and devices which are not sensitive to thermal
fluctuations. They are easy to use, cheap and therefore are very popular. However, due to the way they
work, they are extremely inefficient. A linear regulator works by taking the difference between the input and
output voltages and just burning it up as waste heat.

The larger the difference between the input and output
voltage, the more heat is produced. In most cases, a linear regulator wastes more power stepping down the
voltage than it actually ends up delivering to the target device! With typical efficiencies of 40% and reaching
as low as 14%, linear voltage regulation generates a lot of waste heat which must be dissipated with bulky and
expensive heatsinks. Even the new LDO (low drop-out) regulators are still inefficient linear regulators; they just
give you more ?flexibility with input voltage drops.

How is a switching regulator better?

A switching regulator works by taking small chunks of energy, bit by bit, from the input voltage source, and
moving them to the output. This is accomplished with the help of an electrical switch and a controller which
regulates the rate at which energy is transferred to the output (hence the term “switching regulator”).

The
energy loss involved in moving chunks of energy around in this way is relatively small, resulting in a switching
regulator being about 85% efficient. Since their efficiency is less dependent on input voltage, they can power
useful loads from higher voltage sources. Switch-mode regulators are used in devices like portable phones, video
game platforms, robots, digital cameras, and your computer. Switching regulators are complex circuits to design,
and as a result they are not very popular with hobbyists.

What can switching regulators do that linear regulators cannot?

With high input voltages, driving loads over 200 mA with a linear regulator becomes extremely impractical. A switching regulator can easily power heavy loads from high voltages. Certain kinds of switching regulators can also step up voltage. Linear regulators cannot do this. Switching regulators are used to provide clean DC output to the CPU and DRAM.

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How do I tell if I need a switching regulator?

As a general rule of thumb, if your linear voltage regulation solution is wasting less than 0.5 watts of power, a
switching regulator would be overkill for your project. If your linear regulator is wasting several watts of power,
you most certainly want to replace it with a switcher!

The Power Equation

The equation for wasted power in a linear regulator is given as:

Power wasted = (Input voltage – output voltage) * load current

For example, let us say we have a 12V lead-acid battery and you want to power a micro-controller that draws
5 mA, and an ultrasonic range finder that draws 50 mA. Both the micro-controller and the ultrasonic range finder
run off 5V. You use an LM7805 (a very common linear regulator) to get the voltage down to 5V from 12V.

Power wasted = (12V – 5V) * (0.050A + 0.005A) = 0.385W

0.385W is not too bad for power losses. The LM7805 can handle this without a big heatsink. One could
get more battery life if you used a switching regulator, but in this case the power consumption is so low that
the battery life will be very long anyway.

Now let us expand on this example and add two servos that draw
an average of 0.375A each, and also run off the 5V supply. How much power is wasted in a linear regulator now?

Power wasted = (12V – 5V) * (0.050A + 0.005A + 0.375A + 0.375A) = 5.635W

5.6 Watts is a lot of waste heat! Without a large heatsink, the LM7805 would get so hot it would de-solder
itself or melt your breadboard. Even with the heatsink, 5.6W is also a lot of life to suck out of your battery for
no reason. A switching regulator would be very useful in this case and would reduce power losses to around
0.5W.

PULSE WIDTH MODULATION BASICS

All of the switching converters described here use a form of output voltage regulation known as Pulse Width
Modulation
(PWM). In almost all devices which use switching mode power, the goals would be to supply constant
voltage at variable loads. To accomplish this, a feedback loop is used to correct for any swing in output voltage
due to changing operating conditions. The feedback loop adjusts (corrects) the output voltage by changing the
ON time of the switching element in the converter. As an example of how PWM works, we will examine the
result of applying a series of square wave pulses to an L-C filter.

Pic 1

The series of square wave pulses is altered and provides a DC output voltage that is equal to the peak pulse
amplitude multiplied times the duty cycle (duty cycle is defined as the switch ON time divided by the total
period). This relationship explains how the output voltage can be directly controlled by changing the ON time
of the switch. Square waves are usually produced by using a Transistor as a switch. With this background, let
us proceed to look at Buck-Regulators in the next section.

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BUCK REGULATORS

To start off, let us look at this device from a functional unit point of view. When the output voltage set point
is less than the input voltage, such a regulator is called a Buck converter. When the output voltage set point is
higher, it is a Boost converter.

A feedback input is necessary for the regulator to know the state of the output
voltage so that it can be kept with in the tolerances required by the power supply design requirements (more
on this in the next section). These converters control the output voltage to the specifications by comparing the
output voltage (or current or both) to an internal reference. As a quick preview to what will be discussed in the
next section, the general operating topology is as shown below:

Pic 2.1

They are used to reduce a DC Voltage to a lower reference DC Voltage. The most commonly used switching converter is the Buck, which is used to down-convert a DC voltage to a lower DC voltage of the same
polarity. This is essential in systems that use distributed power rails (like 24V to 48V), which must be locally
converted to 15V, 12V or 5V with very little power loss.

The Buck converter uses a transistor as a switch that
alternately connects and disconnects the input voltage to an inductor. Given below are pictures of an Inductive switcher (also referred to as Magnetic switching). In the Inductive switcher, the energy is pulsed from Vin
to Vout through the inductor. The Inductor acts as a reservoir of energy during every pulse. As the voltage
reaches the desired level, only the energy needed by the load needs to be drawn from Vin and transferred to Vout.

Pic 6

A practical circuit implementation is as shown below:

Pic 2

When the switch turns on, the input voltage is connected to the inductor. The difference between the input
and output voltages is then forced across the inductor, causing current through the inductor to increase. During
the on time, the inductor current flows into both the load and the output capacitor (the capacitor charges during
this time).

When the switch is turned off, the input voltage applied to the inductor is removed. However, since
the current in an inductor can not change instantly, the voltage across the inductor will adjust to hold the
current constant (this stems from the property of self-inductance and is given by Lenz’s law).

The input end
of the inductor is forced negative in voltage by the decreasing current, eventually reaching the point where the
diode is turned on. The inductor current then flows through the load and back through the diode. The capacitor
discharges into the load during the off time, contributing to the total current being supplied to the load (the
total load current during the switch off time is the sum of the inductor and capacitor current).

Pic 3

As explained, the current through the inductor ramps up when the switch is on and ramps down when the
switch is off. The DC load current from the regulated output is the average value of the inductor current. The
peak-to-peak difference in the inductor current waveform is referred to as the inductor ripple current, and the
inductor is typically selected large enough to keep this ripple current less than 20% to 30% of the rated DC
current.

In most Buck regulator applications, the inductor current never drops to zero during full-load operation
(this is defined as continuous mode operation). Overall performance is usually better using continuous mode,
and it allows maximum output power to be obtained from a given input voltage and switch current rating. In
applications where the maximum load current is fairly low, it can be advantageous to design for discontinuous
mode operation.

In these cases, operating in discontinuous mode can result in a smaller overall converter size
(because a smaller inductor can be used). Discontinuous mode operation at lower load current values is generally
harmless, and even converters designed for continuous mode operation at full load will become discontinuous as
the load current is decreased (usually causing no problems).

The I/O characteristics of all DC-DC converters can be examined by using the requirement that the initial
and final Inductor current within a period/cycle should be constant (i.ieet energy stored in an inductor is zero), so average voltage per cycle is zero.

Mathematically:

VLOn D + VLOf f (1 – D) = 0

where D is the duty cycle.
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TWO PWM CONTROL TECHNIQUES IN BRIEF

The control circuitry is the most important part of a Buck regulator, more so from an overclocker’s standpoint.

We had already hinted at this when calculating the output voltage and its dependence on the Duty cycle. In
brief, by varying the ON time, the conduction of the transistor is increased and correspondingly the output
voltage increases. The control circuitry senses any change in output voltage and adjusts the duty cycle to correct
such changes.

An oscillator sets the chopping frequency of the converter and a stable temperature compensated
reference is used, to which the output voltage is compared by a high gain error amplifier. An error voltage to
the PWM is used to adjust the duty cycle. Feedback compensation techniques are nothing new and this is also
known as a servo system. Usually a “servo” system has an error amplifier, integrator, and a low pass filter in a
feedback configuration.

The two most common forms of control in dc/dc switching power converters are CM (current-mode) and VM
(voltage-mode) control. Each method has its own advantages and disadvantages.

CM control provides the ease
of loop compensation and inherent line feed-forward, which makes this method a favorite among designers. VM
control is more immune to noise. This characteristic is important in large-step-down-ratio applications in which
the switch has a short on-time that is susceptible to noise pickup. The ideal approach that has been eluding
designers is a practical CM-controlled regulator without noise-susceptibility challenges.

Voltage Mode

This is the more traditional mode of control in PWM switching converters.
In a simple circuit, the components are an oscillator, an error amplifier and
a comparator. The output voltage is sensed with respect to a reference and
the error voltage is amplified by a high gain amplifier; this is followed by a
comparator which compares the amplified error signal with a sawtooth waveform
generated across a timing capacitor.

Let us take a look at the following schematic:

Pic 4

The circuit shown above is a voltage-mode PWM controller in which the
error amplifier output is compared to a voltage ramp from the oscillator to determine the output pulse width. A current mode PWM replaces the oscillator
ramp with a ramp that is proportional to the current in the magnetic element.
A nice place to perform a mod would be to add a DC Bias voltage to the Volt-
age reference. It would be the easiest to do.

However, most buck regulators
have an internal voltage reference and so this may not be ffeasible Tweaking
the compensation network would be a bit more involved, as random adjustments
can lead to huge output instability, resonance and ringing.

The only possible
way to perform mods would be to change the parameters of the feedback loop.
Usually, it is not recommended that one messes with a feedback loop’s theoretical calculations. However, from a modder’s standpoint, performing calculations
can be a bit involved, especially since running components out of spec is our
primary objective! So, an intelligent mix of theory, trial/error and a bit of luck
would be recommended!

Current Mode

This is a more complicated multi-loop control technique, which has an AC
current feedback loop (AC = oscillating current) in addition to the voltage feedback loop. The second loop controls the peak inductor current with the error
signal rather than controlling the duty cycle of the switching waveform. This is
an attractive option in high frequency switching applications.

Alternative Control Schemes: A Practical Constant ON-Time Buck Regulator

Pic 5

Let us look at an alternative to PWM control in brief.

The basic regulator IC consists of a comparator with the input that compares
the output feedback voltage with a reference volt turning off the buck switch
MOSFET (An OpAmp base comparator is very easy to design). When the
feedback voltage exceeds the reference voltage, the comparator output goes low,
turning off the buck switch MOSFET.

The switch remains off until the feedback
voltage falls below the reference hysteresis voltage (hysteresis is a term used in
electronics and electromagnetics to denote a resistance to change). Then, the
comparator output goes high, turning on the switch and allowing the output
voltage to rise again. It reacts extremely quickly to load and line transients due
to its wide bandwidth control loop.

Unlike a pulse width modulation (PWM)
regulator, this loop does not require an error amplifier or frequency compensation. Practical IC’s used in Motherboards are much more complicated and
use more involved techniques to ensure loop stability and maintain voltage regulation. We will look at a practical example in the next installment of this article.

Hope you have found this a useful read; till next time! Auf Wiedersehen!

References

None of the material presented here is original work. The author has drawn
extensively from the sources cited below – the objective here is purely educational.

  1. http://www.dimensionengineering.com/switchingregulators.htm
  2. http://www.elecdesign.com/Articles/ArticleID/12253/12253.html
  3. PWM, Linear Technology App Notes
  4. Power Electronics Design Handbook- Nihal Kulratna
  5. Maxim-Dallas Semiconductor Application Notes
  6. National semi-conductor Analog University tutorials and knowledgebase (http://www.national.com/AU/design/)

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