Transcript 05. Intro_DC_DC_Conversion

Introduction to DC-DC
Conversion
EE174 – SJSU
Tan Nguyen
OBJECTIVES
• Introduction of DC-DC Converter
• Types of DC-DC Converters
• Linear regulator (LR)
• Switching mode power supply (SMPS)
• Advantages and Disadvantages
Introduction
• Batteries are often shown on a schematic diagram as the source of DC
voltage but usually the actual DC voltage source is a power supply.
• DC to DC converters are important portable electronic devices used
whenever we want to change DC electrical power efficiently from one
voltage level to another.
• A power converter generates output voltage and current for the load from
a given input power source.
• Depending on the specific application, either a linear regulator (LR) or a
switching mode power supply (SMPS) solution to be chosen.
Typical Application of DC-DC converter
• Car battery 12V must be stepped down to 3-5V DC voltage to run DVD/CD player
• Laptop computers or cellular phone battery voltage must be stepped down to run
several sub-circuits, each with its own voltage level requirement different from
that supplied by the battery.
• Single cell 1.5 V DC must be stepped up to 5V operate an electronic circuitry.
• A 6V or 9V DC must be stepped up to 500V DC or more, to provide an insulation
testing voltage.
• A 12V DC must be stepped up to +/-40V or so, to run a car hifi amplifier circuitry.
• A 12V DC must be stepped up to 650V DC or so, as part of a DC-AC sinewave
inverter.
Linear Regulator
How a Linear Regulator Works
In an embedded system, a 15V bus rail is available from the front-end power supply. On the
system board, a 10V voltage is needed to power an Op-Amp circuit. The simplest approach to
generate the 10V is to use a voltage divider from the 15V bus, as shown below:
Does this circuit work well? The answer is usually no.
The output 10V is unchanged under the following
conditions:
• Vin is stable.
• The resistor values are unchanged under any condition.
• The load is not vary under different operating conditions.
The Basic Linear Regulator
A linear regulator uses a voltage-controlled current source to force a fixed voltage to appear at the
regulator output terminal. The control circuitry senses the output voltage, and adjusts the current
source (as required by the load) to hold the output voltage at the desired value. The design limit of the
current source defines the maximum load current the regulator can source and still maintain
regulation.
Linear Regulator Operation
The pass device (Q1) is made up of an NPN Darlington driven by a PNP transistor. The current flowing out
the emitter of the pass transistor (load current IL) is controlled by Q2 and the voltage error amplifier. The
current through the R1, R2 resistive divider is assumed to be negligible compared to the load current.
Ideally, VX = VREF  Error Amp = 0  VOUT is constant.
VX
Q2
When VOUT decreases, VX < VREF  The error amplifier
will be high which turns on Q2 and Q1  VIN apply to the
circuit that adjusts the output to desired level.
When VOUT increases, VX > VREF  The error amplifier
will be low which turns off Q2 and Q1  VIN disconnect
from the output that adjusts the output to desired level.
Linear Regulator Types
• Standard (NPN Darlington) Regulator
• Low Dropout or LDO Regulator
• Quasi LDO Regulator
Note:
The single most important difference between these three types is the dropout voltage, which is
defined as the minimum voltage drop required across the regulator to maintain output voltage
regulation.
THE STANDARD (NPN) REGULATOR
In order to maintain output regulation, the pass transistor requires a minimum voltage across it given by:
VD(MIN) = 2 VBE + VCE
Allowing for the -55°C to +150°C temperature range, this minimum VD(MIN) is set at 2.5V to 3V by the
manufacturer to guarantee specified performance limits. The actually falls out of regulation will be
between 1.5V and 2.2V
Example:
1) If VIN = 12V, what is the VOUT_max?
2) If VIN = 12V, VOUT = 6V, R1 = 1 kΩ,
R2 = 2kΩ, what is VREF?
R1
VX
R2
Solutions:
1) VOUT_max = 12 – 3 = 9V
2) R1 = 1 kΩ and R2 = 2kΩ  VX = 4V
 VREF = 4V
THE LOW-DROPOUT (LDO) REGULATOR
The minimum voltage drop required across the LDO regulator to maintain regulation is just the voltage
across the PNP transistor:
VD(MIN) = VCE
The maximum specified dropout voltage of an LDO regulator is usually about 0.7V to 0.8V at full current,
with typical values around 0.6V.
R1
VX
R2
Example:
1) If VIN = 10V, what is the VOUT_max?
2) If VIN = 10V, VREF = 2.5V and VOUT = 5V,
what are values of R1, R2?
Solutions:
1) VOUT_max = 10.0 – 0.8 = 9.2V
2) Want VX = VREF = 2.5V so for VOUT = 5V
 R1 = R2 = 1 kΩ
THE QUASI LOW-DROPOUT REGULATOR
The minimum voltage drop required across the Quasi-LDO regulator to maintain regulation is given by:
VD(MIN) = VBE + VCE
The dropout voltage for a quasi-LDO is usually specified at about 1.5V(max). The actual dropout
voltage is temperature and load current dependent, but could never be expected to go lower than
about 0.9V (25°C) at even the lightest load.
R1
VX
R2
Example:
1) If VIN = 5V, what is the VOUT_max?
2) If VIN = 9V, VREF = 2V and VOUT = 5V,
R1= 3 kΩ, what is value of R2?
Solutions:
1) VOUT_max = 5.0 – 1.5 = 3.5V
2) Want VX = 2V for VOUT = 5V and
R1 = 3 kΩ  R2 = 2 kΩ
Comparison of the Linear Regulators
≈ 0.8 V (WC)
≈ 1.5 V (WC)
≈ 3 V (WC)
Example:
Given VIN = 5V, require output VOUT = 3.3V and 2 Amax, what is the best choice for the design?
Solution: Quasi-LDO
LINEAR REGULATORS
• The linear regulator is a DC-DC converter to provide a constant
voltage output without using switching components.
• The linear regulator is very popular in many applications for its
low cost, low noise and simple to use.
• It was the basis for the power supply industry until switching
mode power supplies became prevalent after the 1960s.
• Power management suppliers have developed many integrated
linear regulators.
• The linear regulator has limited efficiency and can not boost
voltage to make Vout > Vin.
ADJUSTABLE LINEAR REGULATORS
A typical integrated linear regulator needs only VIN, VOUT, FB and optional GND pins. Figure below shows a
typical 3-pin linear regulator, it only needs an input capacitor, output capacitor and two feedback resistors to
set the output voltage.
LINEAR REGULATORS DRAWBACK
• A major drawback of using linear regulators can be the excessive power dissipation of its series transistor Q1
operating in a linear mode.
• Since all the load current must pass through the series transistor, its power dissipation is PLoss = (VIN – VO) •IO.
• The efficiency of a linear regulator can be estimated by:
LINEAR REGULATORS DRAWBACK
• The linear regulator can be very efficient only if VO is close to VIN.
• The linear regulator (LR) has another limitation, which is the minimum voltage
difference between VIN and VO. The transistor in the LR must be operated in its linear
mode. So it requires a certain minimum voltage drop across the collector to emitter of
a bipolar transistor or drain to source of a FET. When VO is too close to VIN, the LR may
be unable to regulate output voltage anymore.
• The linear regulators that can work with low headroom (VIN – VO) are called low
dropout regulators (LDOs).
• The linear regulator or an LDO can only provide step-down DC/DC conversion.
LINEAR REGULATORS APPLICATIONS
There are many applications in which linear regulators provide superior solutions to switching supplies:
1. Simple/low cost solutions. Linear regulator or LDO solutions are simple and easy to use, especially for
low power applications with low output current where thermal stress is not critical. No external power
inductor is required.
2. Low noise/low ripple applications. For noise-sensitive applications, such as communication and radio
devices, minimizing the supply noise is very critical.
3. Fast transient applications. The linear regulator feedback
loop is usually internal, so no external compensation
is required.
4. Low dropout applications. For applications where output voltage is close to the input voltage, LDOs
may be more efficient than an SMPS.
We see that price sensitive applications prefer linear regulators over their sampled-time counterparts.
The design decision is especially clear cut for makers of:
• communications equipment
• small devices
• battery operated systems
• low current devices
• high performance microprocessors with sleep mode (fast transient recovery required)
LINEAR REGULATORS VS SWITCHING REGULATORS
Regulators Linear regulators are less energy efficient than switching regulators. Why do we
continue using them?
Depending upon the application, linear regulators have several redeeming features:
• lower output noise is important for radios and other communications equipment
• faster response to input and output transients
• easier to use because they require only filter capacitors for operation
• generally smaller in size (no magnetics required)
• less expensive (simpler internal circuitry and no magnetics required)
Furthermore, in applications using low input-to-output voltage differentials, the efficiency is not
all that bad! For example, in a 5V to 3.3V microprocessor application, linear regulator efficiency
approaches 66%. And applications with low current subcircuits may not care that regulator
efficiency is less than optimum as the power lost may be negligible overall.
References:
http://en.wikipedia.org/wiki/DC-to-DC_converter
https://www.jaycar.com/images_uploaded/dcdcconv.pdf
Linear Tecnology - Application Note 140
http://www.ti.com/lit/an/snva558/snva558.pdf
http://www.micrel.com/_PDF/other/LDOBk.pdf