Noise-sensitive applications in harsh automotive and industrial environments require low noise, high efficiency buck regulators that can fit into tight spaces. Monolithic buck regulators, which include the MOSFET power switches in the package, are often chosen because of their small overall solution size relative to a traditional controller IC and external MOSFETs. Monolithic regulators that can operate at high frequency—in the 2 MHz territory well above the AM band—also help to reduce the size of external components. Furthermore, if a regulator offers a low minimum on time (TON), the regulator can directly operate from higher voltage rails without intermediate regulation, saving space and complexity. Low minimum on times require fast switching edges and minimum deadtime control to effectively reduce the switching loss and allow high switching frequency operation.
Another way to save space is to reduce the number of components required to meet electromagnetic interference (EMI) standards and thermal requirements. Unfortunately, in many cases, simply shrinking the converter makes meeting these requirements more difficult. This article presents state-of-the-art solutions that save space while also achieving low EMI and excellent thermal performance.
Switch-mode power converters are chosen for their efficiency, especially at high step-down ratios, but one trade-off is the switching-action induced EMI. In a buck converter, EMI arises from the fast current changes (high di/dt) in the switches and from switch ringing due to the parasitic inductance in the hot loop.
EMI is just one of the parameters that system design engineers must struggle with when trying to design a compact, high performance power supply. A number of critical design constraints are often at odds, requiring critical compromises within the design limits and time to market.
Improving EMI Performance
To reduce EMI in a buck converter, one must reduce the radiating effect of the hot loop as much as possible and minimize signals from the source. There are a number of ways to reduce radiated EMI, but many also reduce the performance of the regulator.
For instance, in a typical discrete FET buck regulator, the switching edge is slowed down, with an external gate resistor, BOOST resistor, or snubber, as a last rescue method to meet the stringent radiated emissions standards in the automotive industry. Such a quick fix for the EMI comes at the price of performance; namely lower efficiency, higher component count, and larger solution size. Slow switching edges increase switching losses, as well as duty ratio loss. The converter must operate at a lower frequency—for instance, 400 kHz—to achieve satisfactory efficiency and pass mandatory radiated EMI emission tests. Figure 1 shows typical switching node voltage waveforms with a fast switching edge and a slow switching edge, respectively. As shown, the switching edge is significantly slower, resulting in increased switching losses, and a significant increase in the minimum duty cycle, or step-down ratio, not to mention other negative effects on performance.
Slowing the switching frequency also increases the physical size of the converter inductor, output cap, and input cap. Meanwhile, a bulky π filter is necessary to pass the conducted emissions tests. The inductance, L, and capacitance, C, in the filter get bigger as the switching frequency goes down. The inductor current rating should be larger than the maximum input current at low line full load. Therefore, a bulky inductor and multiple capacitors are required on the front end to help pass the stringent EMI standards.