10

Rev. A, June 2001

©2001 Fairchild Semiconductor Corporation

Table 1 shows the comparison of the topologies that have been discussed. Note that the rela-

tive cost increases significantly based on whether there is input/output isolation. Efficiency can

range from 70 to 85%. A summary of the first-pass selection of key MOSFET parameters is

shown in Table 2. Parameters of the MOSFETS used in Application Note are shown in Table 3.

Table 1: Comparison of various switching regulator topologies.

Table 2: Power MOSFET voltage and current ratings versus power supply topology.

Table 3: PowerTrench® MOSFETs in SO-8 package for 48V Applications.

Synchronous rectification.

Synchronous rectification may be employed with all the previously mentioned topologies. This

technique consists of replacing the output rectifiers with MOSFETs, striving to lower conduc-

tion losses. The concept is, however, not without its share of problems. The complexity of the

drive and control circuitry increases dramatically. Timing of the gate-drive to the synchronous

FETs is critical to avoid cross conduction. The parasitic capacitances and body diode reverse

recovery characteristics of the output FETs contribute to substantial switching losses in these

devices. The technique is most applicable for 12V and lower output voltages. The lower the

output voltage, the greater the advantage of synchronous rectification. Much above the 12V

level, the large parasitic losses make the approach much less advantageous.

Thermal Considerations

One of the main limiting factors in any power supply design is the ability to keep the junction

temperatures of the power devices within specified limits. When using surface-mount power

devices the primary heat sink is the PC board. The main constituents of PCBs are fiberglass

Topology

Max. Power

(Watts)

Typical

Efficiency (%)

Flyback

30

75

1 Transistor Forward

150

80

2 Transistor Forward

200

83

Push Pull

350

80

Full Bridge

500

85

Topology

Maximum Drain Voltage

Average Drain Current

Flyback

1.5Vin(max)

751.5Pout/ Vin(max)

1 Transistor Forward

2.5 to 3.0 Vin(max)

1.3Pout/ Vin(max)0

2 Transistor Forward

Vin(max)

1.2Pout/ Vin(max)

Push Pull

352.5 to 3.0 Vin(max)0

0.65Pout/ Vin(max)

Full Bridge

Vin(max)

0.60Pout/ Vin(max)

Voltage(DS)

Current (ID max)

ON-Resistance

(@VGS=10V)

FDS3570

80V

9 A

20 m

Ω

FDS3670

100V

6.3A

32 m

Ω

FDS2570

150V

4 A

80 m

Ω

FDS2670

200V

3 A

130 m

Ω