Most engineers have difficulty designing robust switching converters. The first problem is stability. Stabilizing their complex control loops can be a daunting task because many converters require a ripple in the output voltage to work properly. Others exhibit subharmonic oscillations, and you must inject a ramp signal into the reference.
When large-value ceramic capacitors became affordable, many engineers substituted them for electrolytic output capacitors. Ceramic capacitors have such low ESR (equivalent-series resistance) that they have essentially no ripple voltage, causing oscillation. The ripple voltage itself may violate design requirements, such as when powering analog circuits. This problem requires postregulation or the use of extra inductive damping.
Another common problem, noise, can radiate back into the input or output power lines or radiate into space as electromagnetic radiation. The worst issue with this problem is that a designer may not notice it until sending the product for FCC (Federal Communications Commission) and CE (Conformité Européenne) testing just before production. Designers can use various techniques to shield this noise from the world and the rest of the system. It is better to not generate the noise in the first place than to later attempt to shield it in tens or hundreds of end-user applications.
As with linear regulators, thermal issues can also be problems in switching converters. Most buck regulators generate more heat in the freewheeling diode than in the FET. A thermal plot from National Semiconductor's Webench online-design tool shows that diode D1 is the hottest component on the board and is heating the IC it abuts (Figure 5). To reduce the heat that freewheeling diodes generate, synchronous buck regulators replace the diode with a second out-of-phase FET.
Most of the above problems are traceable to an inferior pc-board layout. Several articles are available that discuss the pitfalls of laying out a good switching regulator (reference 3 and reference 4). Engineers should always take advantage of the applications-engineering staff of the companies that make the regulator IC. They can avoid an enormous amount of frustration and chaos if the applications engineers can review your design and layout before you commit the board to fabrication.
Offline regulators
This article has so far discussed only dc/dc converters. Another class of converters creates dc power from ac power. The ac power most commonly comes from residential ac-power lines; the converters are thus offline supplies (Reference 5). Other designs use an isolated topology to create one or more dc supplies from raw dc power from the classic rectifier circuit.
Allegro, On, STMicro, Power Integrations, and the Unitrode division of Texas Instruments make these types of devices. Offline-supply problems include inrush currents and harmonic currents. Inrush current is the large flow of current necessary for charging up the input capacitors at the moment of closing the input switch.
This current can stress the rectifier diodes and cause early capacitor failure. Approaches to correcting this problem include the use of NTC (negative-temperature-coefficient) devices in series with the inputs (Reference 6). These devices offer a high resistance when they are cold. As the input current runs into the capacitor, the devices heat up, and the resistance decreases. Drawbacks can be the 190°C operating temperature as well as sensitivity to ambient temperature.
The second problem with offline supplies is that the input capacitors draw in large spikes of current. These spikes top off at every line cycle. Using PFC, which is mandatory on supplies sold in Europe, can reduce these spikes.
Remember to fuse the electrolytic capacitors. Failing UL (Underwriters Laboratories) fire testing just before production is as calamitous as failing FCC and CE EMI/RFI (electromagnetic-interference/radio-frequency-interference) testing.
Another common problem with off-line regulators using a switching IC is the quiescent current of the start-up circuit. You must provide 5 to 10V to the chip before any oscillation and regulation can begin. So, you must often use a large power resistor to feed this voltage to the chip.
If you place the resistor across the 170V or higher dc bus to the 5 or 10V IC power rail, significant power dissipation will occur. Designers can in these cases use 500V Supertex depletion-mode FETs, but that option may be infeasible for low-cost supplies.
Some vendors, such as Power Integrations, have developed alternative architectures to deal with this problem. "Solutions that use an integrated power transistor can derive the power for the control section by using the high-voltage MOSFET as a potential divider and tapping off a small amount of current at low voltage," says Doug Bailey, the company's vice president of marketing. "Power Integrations uses this approach in all of its switching ICs, and it works very well."
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