7004 Datasheet, PDF (10/12 Pages) Bourns Electronic Solutions – Network Interface Device

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.
Topology
Flyback
1 Transistor Forward
2 Transistor Forward
Push Pull
Full Bridge
Max. Power
(Watts)
30
150
200
350
500
Typical
Efficiency (%)
75
80
83
80
85
Table 1: Comparison of various switching regulator topologies.
Topology
Flyback
1 Transistor Forward
2 Transistor Forward
Push Pull
Full Bridge
Maximum Drain Voltage Average Drain Current
1.5Vin(max)
751.5Pout/ Vin(max)
2.5 to 3.0 Vin(max)
1.3Pout/ Vin(max)0
Vin(max)
1.2Pout/ Vin(max)
352.5 to 3.0 Vin(max)0 0.65Pout/ Vin(max)
Vin(max)
0.60Pout/ Vin(max)
Table 2: Power MOSFET voltage and current ratings versus power supply topology.
FDS3570
FDS3670
FDS2570
FDS2670
Voltage(DS)
80V
100V
150V
200V
Current (ID max)
9A
6.3A
4A
3A
ON-Resistance
(@VGS=10V)
20 mΩ
32 mΩ
80 mΩ
130 mΩ
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
©2001 Fairchild Semiconductor Corporation
10
Rev. A, June 2001