Giving Power and Efficiency the Old Switcheroo

It seems that the issue surrounding consumer, computer and industrial equipment is becoming less about power and more about efficient power management. A recent two-part article by Sam Davis in Electronic Design illuminates the subject of switch-mode ICs and how they are changing the landscape of modern-day device power management.

ED Switch-mode ICs transfer DC power from its input to its output through the use of a controlled power semiconductor switching technique along with an inductor, transformer or capacitor. The basic switch-mode IC takes the DC input and converts it to square waves that are then applied to the power semiconductor switch. The switched output is then rectified and filtered to provide a dc output.

By taking a part of the DC output and looping it back to the controller IC, the circuit is transformed into a DC-DC voltage regulator that provides a constant voltage output.

In a perfect world, the power MOSFET switch, which is the standard semiconductor for switch-mode circuitry, should turn on and off in zero time so that no current is flowing to the load. This would result in zero power loss during the application of the control signal. In the real world, MOSFETs have shown some delay when turned on and off, which leads to a slight loss of power.

The key is to get the MOSFET to switch at a higher frequency. Using small inductors and capacitors would then lower the output ripple. That has proven elusive since certain devices lose power efficiency with increases in MOSFET frequency.

While switch-mode converters are still the best for device efficiency and power management, they do result in switching noise and a minimal output voltage ripple. But, by carefully selecting the proper components, these relatively minor negatives can be mitigated.

Pulse-Width Modulation


Pulse-Width Modulation
When there is a load change, switch-mode converters can vary their DC output current. For instance, pulse-width modulation alternates the on and off times, thus controlling the switch outlet power. This power switching is based on a duty cycle.

A PWM signal is created with the help of an error AMP. The error AMP accepts the output voltage feedback and produces an output related to the difference of the two inputs. The PWM comparator then produces a modulated pulse width that is applied to the switching logic. This provides switching logic with the ability to activate or deactivate the PWM signal.

Current-limiting protection is a fundamental benefit of most PWM controller ICs. So, if the input is greater than what is acceptable, the cycle is terminated. PWM circuits are available as standalone ICs as well as integrated within DC-DC switch-mode converter ICs with internal power switches.

Synchronous Rectifiers

Synchronous Rectifiers In a switch-mode IC that uses two transistors, a synchronous rectifier provides an output that emulates a diode rectifier. It uses two transistor switches, allowing the current to pass in only one direction. These transistors are typically MOSFETs, with one turning on and the other turning off at the same time.

Although a synchronous rectifier usually costs more than the Schottky rectifier, it makes up for the cost in thermal and packaging considerations. The cost comparison really comes down to a matter of its application.

Multiphase Converter ICs

To meet the growing trend for higher-current, lower-voltage microprocessors, multiphase converters have become a necessity. Multiphase converters use two connected converters to replicate a cells' output.

Single-phase converters are seen as lacking when it comes to higher current and lower voltage. A multiphase converter operates at a common frequency, but phase shifts so that conversion happens at regular intervals and is controlled by a common chip, which staggers the switching time in each converter. Controllers monitor each cells usage of current to ensure that no cell uses too much of the current but that it is shared among the cells. Multiphase converters have proven to be much more effective than their single-phase counterparts, providing better performance and significantly less decoupling capacitance.

The number of phases in a converter will continue to increase with current requirements. Optimization in the design of multiphase ICs is needed based on the number of phases, current per phase, switching frequency, cost, size, and efficiency. Higher output current and lower voltage require tighter output voltage regulation so look for new developments in multiphase ICs to come.
Article written by Sam Davis, at Electronic Design.