Next-Gen MOSFETs Up Efficiency of Synchronous Buck Converters

Apr 1, 2011 12:00 PM
Davis, Sam, -

Compared with previous MOSFET generations, performance of synchronous buck converters is more efficient by using new devices with improved on-resistance, gate resistance and gate charge, as well as integrating a Schottky diode.

For more than a decade, MOSFET manufacturers have made changes to improve device efficiency by using silicon processing modifications and package innovations. In particular, the synchronous buck converter using two MOSFETs has been the circuit with the most attention. Fig. 1 shows a typical synchronous converter consisting of a high-side and low-side MOSFET, or described another way, the control and synchronous MOSFET. Fig. 2 illustrates the equivalent circuit of a turned on MOSFET, detailing the parasitic elements of the device that affect MOSFET performance.

Using the synchronous converter as a guide, International Rectifier announced its next update. It's a family of power MOSFETs the company claims will be the efficiency standard for 12V input synchronous buck applications that include next-generation servers, desktops, and notebooks. IR says the new improvements maximize efficiency by achieving:

• Low R_DS(ON) (on-resistance)
• Low Q_g (gate charge)
• Low R_G (gate resistance)
• Lowest p.c. board losses
• Integrated Schottky diode

One aspect of the upgrade process was to integrate a monolithic Schottky diode in the synchronous MOSFET. Fig. 3a shows the efficiency curve vs. output current at 300 kHz for a sync FET with and without a Schottky diode. Now, compare this efficiency with Fig. 3b using the same MOSFETs operating at 800 kHz. At the higher frequency, the MOSFET with the integrated Schottky diode provides a greater efficiency difference than the device with a standard PN body diode.

Another part of the improvement was to upgrade the MOSFET package. This was accomplished by a simple redesign of the already successful DirectFET package that has no lead frame, no wire bonds, no molding and <0.8 ppm defectivity rate. Here, the package profile was reduced from 0.7mm to 0.6mm. DirectFET plus refers to the whole product and not just the package; it incorporates all the benefits of the DirectFET package, plus the upgraded silicon features.

The DirectFET package still provides improved thermal performance with better heat transfer from the MOSFET die to the p.c. board (Fig. 4). Plus, the package is lead- and bromide-free. Fig. 5 shows placement of the two MOSFETs on a p.c. board.

An important part of the upgrade was to modify the silicon processing, which can adjust some parameters, while others are interdependent. For example, changing R_DS(ON) affects the Q_G, the total gate charge, and vice versa. In the past R_G, the gate resistance, was ignored because it had a very small effect relative to R_DS(ON). However, with R_DS(ON) in the low milliohm range, R_G does impact performance at higher switching frequencies. Low Rg (<0.5Ω) typically comes at the expense of R_DS(ON), however, IR's process enables a low R_G without trading off R_DS(ON).

Fig. 6a compares the efficiency vs. output current for a MOSFET operating at 300 kHz with R _G of either 0.3Ω or 2.0Ω for a control MOSFET and 1.2Ω for a sync MOSFET. Notice the results in Fig. 6b with the same values of R _G and operation at 800 kHz. Even though the performance improvement is higher at 800KHz, there is still a significant efficiency gain at 300KHz.

A key processing technique is the ability to tailor R_G to better match the impedance of the driver. In the IRF6894 the R_G is slightly higher to better match the impedance of the high efficiency drivers available today.

Noting this, IR proposes that there be a new Figure of merit (FOM) for MOSFETs. Previously, MOSFET FOM was based on the product of R_DS(ON) and Q_G,. The proposed new FOM would be R_DS(ON) × Q_G × R_G. The table lists the proposed new FOMs for some new and older generation MOSFETs.

THE PLUS FACTOR

The first two DirectFET®plus devices in the new family, the IRF6811SPBF and IRF6894MPBF, reduce R_DS(ON) and Q_G compared with previous generation devices, which significantly improves efficiency up to 2 percent. In addition, their low R_G provides an efficiency improvement by minimizing switching losses in DC-DC converters.

The IRF6811SPBF is available in a small package, while the IRF6894MPBF comes in a medium size package. The R_DS(ON), R_G, and Q_G values minimize conduction and switching losses of the 25V DirectFET®plus devices. The IRF6894MPBF features an integrated Schottky diode that reduces losses associated with body diode conduction and reverse recovery.

Footprints of the new DirectFET®plus MOSFETs are compatible with previous generation devices. The packages have the footprint of a MICRO-8. The DirectFET®plus package is compatible with existing layout geometries used in power applications, p.c. board assembly equipment and vapor phase, infra-red or convection soldering techniques. This package allows dual-sided cooling to maximize thermal transfer in power systems, which improves its thermal resistance by about 80%.

These MOSFET packages also exhibit ultra-low inductance, providing a significant reduction in switching losses. This reduces parasitic ringing by minimizing package inductance, which in turn also raises switching speed and reduces EMI. The reduced losses make this product ideal for high efficiency DC-DC converters that power processors operating at higher frequencies. The IRF6811STRPBF MOSFET is optimized for the control FET socket of synchronous buck topologies operating from12 V. The IRF6894MPBF is optimized for parameters that are critical in synchronous buck converter's Sync FET sockets.

TABLE: DIRECTFET AND DIRECTFETPLUS MOSFET CHARACTERISTICS

PART NUMBER	BVDSS (V)	RDS(ON) TYP @10V (MΩ)	RDS(ON) TYP @4.5V (MΩ)	VGS (V)	QG TYP. (NC)	RG TYP. (Ω)	RG×QG× RDS(ON) FOM	PN OR SCHOTTKY DIODE	SYNC FET OR CONTROL FET
IRF6811SPBF	25	2.8	4.1	±16	11	0.4	12.32	PN	Control FET
IRF6894MPBF	25	0.9	1.3	±16	29	0.4	10.44	Schottky	Sync FET
PREVIOUS GENERATION DIRECTFET
IRF6711SPBF	25	3.0	5.2	±20	13	0.3	11.7	PN	Control FET
IRF6794MPBF	25	1.3	2.3	±20	31	0.3	12.09	Schottky	Sync FET

The IRF6894 has a lower FOM compared with the IRF6794, due to the improvement in the silicon R_DS(ON) and Q_G.

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