Power MOSFETs fill many roles in power management applications, but one of their least publicized uses is as static or load switches. These applications require a low on-resistance, or RDS(ON).
Minimizing RDS(ON) requires the appropriate silicon chip as well as a package with low interconnection resistance. Typically, the silicon contributes about 30% of the RDS(ON) and the package contributes the rest. One device that meets the requirements for static-switch applications is Fairchild Semiconductor's FDMS7650, a 30-V, 60-A trench MOSFET with an RDS(ON) of 0.99 mΩ (Fig. 1).
Housed in a Power56 package with a 5- × 6-mm footprint, the FDMS7650 replaces larger packages — like the D2PAK — that require at least five times the space. An exposed drain pad on the bottom of the Power56 enhances thermal dissipation. Low on-resistance silicon combined with a thermally efficient package enables optimal efficiency from a small footprint.
A typical application for the FDMS7650 is in power systems found in server farms, routers, and base stations. These systems cannot tolerate down time so power supplies connected in parallel handle load current equally (Fig. 2).
In this N+1 redundancy configuration, if one supply fails, the ORing MOSFET controller can disable the bad supply while the remaining supplies continue to provide load current. Maintaining this high efficiency requires power MOSFET switches with very low RDS(ON).
In Fig. 2, the ORing MOSFETs are continuously on, so efficiency is important. Low RDS(ON) results in lower conduction losses and higher efficiency. The ORing MOSFET must be thermally efficient because loads can easily exceed 25 A.
In developing MOSFETs, RDS(ON) and total gate-charge are inversely relateds. For example, RDS(ON) determines power dissipation when the MOSFET turns on. Total gate charge affects the device turn-on power; for instance, a lower total gate charge makes it easier to turn on the MOSFET, and vice versa.
ORing configuration does not require dynamically changing gate voltage. Therefore, the FDMS7650's total gate charge of 149 nC does not inhibit its use in an ORing configuration because the MOSFET turns on and off infrequently.
Typical synchronous rectifiers comprise high-side and low-side MOSFETs, each requiring different characteristics for optimal design. Generally, the best high-side MOSFET has a low figure of merit (FOM), calculated as total gate-charge × RDS(ON). Here, total gate-charge includes the gate-to-source charge (QGS) and the gate-to-drain charge (QGD).
Conversely, optimum low-side MOSFETs must exhibit very low RDS(ON) with no dependence on total gate charge. In some applications, the FDMS7650 can replace multiple parallel low-side switches in a synchronous converter.