[Technology Report]
Switch-Mode ICs Promote Efficient Power Management, Part 1: Switch-Mode Fundamentals
Switch-mode ICs play a dominant role in optimizing the efficiency and minimizing the size of power conversion subsystems used in consumer, computer, and industrial equipment.
Sam Davis
|
ED Online ID #20245 |
December 1, 2008
A switch-mode converter circuit uses a controlled power semiconductor switching technique along with an inductor, transformer, or capacitor as an energy-storage element to transfer dc power from its input to its output. In a basic switch-mode converter, a dc-to-pulse-width converter IC accepts a dc input and produces square waves applied to a power semiconductor switch (Fig. 1). The switched output from the power semiconductor is then rectified and filtered to provide a dc output. This circuit becomes a dc-dc voltage regulator by taking a portion of the dc output and feeding it back to the controller IC to make the circuit maintain a constant output voltage.
The major contributor to the efficiency of the switch-mode converter is the power semiconductor switch. Because they have a relatively high switching speed, MOSFETs are the power semiconductor switch of choice in power supplies. In the typical switch-mode power supply, the MOSFET switch applies power to a load when a control signal tells it to do so. The control signal also tells it to turn off.
Ideally, the power MOSFET switch should turn on and off in zero time. It should have an infinite impedance when turned off so zero current flows to the load. Additionally, it should have zero impedance when turned on so the on-state voltage is zero. Also ideally, the switch input should consume zero power when the control signal is applied. However, these idealistic characteristics are unachievable.
In the real world, actual power MOSFETs don’t meet these ideal characteristics. For example, Figure 2a shows a control signal applied to an ideal power MOSFET switch whose output exhibits zero transition time when turning on and off (Fig. 2b). When the MOSFET is off (not conducting current), power dissipation is very low because current is very low. When the MOSFET is on (conducting maximum current), power dissipation is low because its conducting resistance is low.
In contrast, an actual power MOSFET switch exhibits some delay when turning on and off (Fig. 2c). Therefore, most of the power dissipation occurs when the switch goes through the linear region between on and off, which depends on switching speed. A small amount of power also is dissipated when the MOSFET is on and when it is off.
Ideally, you would like the MOSFET to switch at as high a frequency as possible so the output filter can use small inductors and capacitors to provide very low output ripple. Unfortunately, the MOSFET’s power losses begin to increase at some frequency where it affects the power supply’s efficiency. Therefore, there is a practical limit on the MOSFET’s switching frequency, which depends on the characteristics of the specific device.
One disadvantage of the switch-mode converter is switching noise and output voltage ripple caused by the switching process. Specific control techniques and careful component selection can minimize these noises.
Isolated vs. Non-Isolated
In terms of their response to a dc input, there are two types of dc-dc converters: isolated and non-isolated, which depends on whether there is a direct dc path from the input to the output. An isolated converter employs a transformer to provide isolation between the input and output voltage. In the non-isolated converter, there is a dc path from input to output.
For most applications, non-isolated converters are appropriate. However, some applications require isolation between the input and output voltages, which requires a switching transformer. An advantage of the transformer-based converter is that it has the ability to easily produce multiple output voltages, whereas the inductor-based converter provides only one output.
LDOs
The only other possible choice as a power source is the linear low-dropout regulator (LDO). Low dropout refers to the difference between the input and output voltages that allows the IC to regulate the output voltage. That is, the LDO device regulates the output voltage until its input and output approach each other at the dropout voltage. Ideally, the dropout voltage should be as low as possible to minimize power dissipation and maximize efficiency.
LDO voltage regulators are linear devices with the topology shown in Figure 3. The main components are the power semiconductor and a differential error amplifier. One input of the differential amplifier monitors a percentage of the output as determined by the resistor ratio of R1 and R2. The second input to the differential amplifier is from a stable voltage reference (VREF). If the output voltage tends to rise relative to VREF, the drive to the power semiconductor changes to maintain a constant output voltage.
These ICs have simpler circuits than their switch-mode cousins and produce less noise (no switching) but are limited by their current-handling and power dissipation capability. Some LDO ICs are specified as low as 50 mA whereas others can handle up to about 1 A. Linear regulators only step down, and their output-to-input efficiency can be on the order of 50%. In contrast, switch-mode converters can step up, step down, and invert at efficiencies in the 80% to 95% range.
Pulse-Width Modulation
A switch-mode converter varies its dc output current in response to load changes. One widely used approach, pulse-width modulation (PWM), controls the power switch output power by varying its on and off times (Fig. 4). The ratio of on time to the switching period time is the duty cycle. The higher the duty cycle, the higher the power output from the power semiconductor switch.
To generate the PWM signal, the error amp accepts the output voltage feedback and a stable voltage reference to produce an output related to the difference of the two inputs. The PWM comparator compares the error amp’s output voltage with the ramp (sawtooth) from the oscillator, producing a modulated pulse width. The comparator output is applied to the switching logic, whose output goes to the output driver for the power-supply circuit. The switching logic provides the capability to enable or disable the PWM signal applied to the power-supply circuit.
Most PWM controller ICs provide current-limiting protection by sensing the output current. If the current sense input exceeds a specific threshold, it terminates the present cycle (cycle-by-cycle current limit). PWM circuits are available as standalone ICs as well as integrated within dc-dc switch-mode converter ICs with internal power switches.
The TPS40210 and TPS40211current-mode PWM controllers from Texas Instruments are wide-input voltage (4.5 V to 52 V), non-synchronous boost controller ICs (Fig. 5). They suit topologies that require a grounded source N-channel FET including boost, flyback, SEPIC, and various LED driver applications.
Features include programmable soft start, overcurrent protection with automatic retry, and programmable oscillator frequency. Their fixed-frequency current-mode control provides improved transient response and simplified loop compensation. The main difference between the two parts is the reference voltage to which its error amplifier regulates the FB pin. Additional features include:
• Internal slope compensation
• Integrated low-side driver
• Programmable closed-loop soft start
• External synchronization capabilities
• Reference 700-mV (TPS40210), 263-mV (TPS40211)
• Low-current disable function
• Hysteretic converter
The basic hysteretic regulator shown in Figure 6 consists of a comparator with input hysteresis that compares the output feedback voltage with a reference voltage. 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. Then, the comparator output goes high, turning on the switch and allowing the output voltage to rise again.
There is no voltage-error amplifier in the hysteretic converter, so its response to any change in the load current or the input voltage is virtually instantaneous. Therefore, the hysteretic regulator represents the fastest possible dc-dc converter control technique. A disadvantage of the conventional hysteretic regulator is that its frequency varies proportionally with its output capacitor’s equivalent series resistance (ESR). Since the initial value is often poorly controlled, and the ESR of electrolytic capacitors also changes with temperature and age, practical ESR variations can easily lead to a frequency variation on the order of one to three.
Please refresh the page if you have trouble reading this text.
Search Electronic Design
Email Newsletter
Sponsored By:
The Find Power Products monthly newsletter brings you the most important new developments within the world of power design. The newsletter includes exerpts from industry leader Sam Davis's exclusive blog, as well as overviews of the latest new products.
Enter Email to Subscribe
Web Seminar
Sponsored By:
Title: Exploring How Good GUIs Drive Adoption in the Digital Power Management Space
Reprints
