Selecting the Right Power Supply Regulators for Automotive Secondary Rail Applications: Page 3 of 6

June 02, 2016 //By Jerome Johnston, Intersil Corporation
Selecting the Right Power Supply Regulators for Automotive Secondary Rail Applications
Today’s car manufacturers are focusing their innovations on the sophisticated cockpit electronics that improve driver safety and the overall driving experience. More and more consumers are factoring a car’s advanced driver assistance systems (ADAS) and infotainment features into their buying decisions. These systems combine several features that require heavy-duty signal processing, such as forward-looking smart cameras for detecting and classifying objects, back-up camera electronic control units (ECUs), and head-unit center information displays, to name a few. As a result, they require higher current power supply regulation at low voltages.
circuit to prevent discontinuous current operation. The synchronous configuration can be designed to enable switch S2 to be turned on under light load conditions. This will allow negative inductor current to flow. While this decreases efficiency at light loads, it allows continuous current flow and prevents EMI.

Therefore, the buck regulator, implemented as a synchronous buck, can provide higher efficiency and lower EMI while occupying less board space than the asynchronous version using a diode. The synchronous buck provides even more benefits if its implementation is optimized for the specific voltage regulation applications. 

A circuit implementation of the synchronous buck would use FET transistors for the upper and lower switches. The lower FET is always an N-channel FET. N-channel devices offer higher electron mobility, and therefore lower resistance for a given size. Nevertheless, the upper FET In a synchronous buck converter can be implemented as either an N-channel or a P-channel. Each has its own advantages and disadvantages.

N-Channel vs. P-Channel High-Side FET Switch

Now we’ll examine why it can be better to employ buck regulators that use a P-channel device as the upper FET in some applications. But first, let’s take a look at the switching section of a synchronous converter with an upper N-channel FET as shown in Figure 4.

Figure 4. Synchronous buck with N-channel high-side FET switch

When an N-channel FET is used for the upper switch, there must be a voltage supply source greater than the voltage supplying the drain of the upper switch. For the N-channel FET to turn on with its source voltage at Vs, its gate voltage must be several volts higher than Vs. This higher voltage is typically generated by using a boot capacitor. When the lower FET is on and the upper FET is off, the boot capacitor is charged by the Vc supply. Note that Vs and Vc may be equal or different in the buck converter that uses

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