01-10-2012, 04:25 PM
TOPOLOGIES FOR SWITCHED MODE POWER SUPPLIES
1TOPOLOGIES FOR SWITCHED.pdf (Size: 233.32 KB / Downloads: 29)
INTRODUCTION
This paper presents an overview of the most
important DC-DC converter topologies. The
main object is to guide the designer in selecting
the topology with its associated power
semiconductor devices.
The DC-DC converter topologies can be
divided in two major parts, depending on
whether or not they have galvanic isolation
between the input supply and the output
circuitry.
Single switch versus double switch
flyback
In the single switch flyback, an overvoltage
spike is applied across the power switch at
each turn off. The peak value of this
overvoltage depends upon the switching time,
the circuit capacitance and the primary to
secondary transformer leakage inductance.
So, a single switch flyback nearly always
requires a snubber circuit limiting this
voltage spike (see figure 5).
In a double switch flyback, the leakage
inductance of the power transformer is
much less critical (see figure 6). The two
demagnetization diodes (D1 and D2) provide
a single non dissipative way to systematically
clamp the voltage across the switches to the
input DC voltage Vin. This energy recovery
system allows us to work at higher
switching frequencies and with a better
efficiency than that of the single switch
structure. However, the double switch
structure requires driving a high side
switch. This double switch flyback is also
known as asymmetrical half bridge flyback.
Single switch vs. double switch forward
In the single switch forward, the magnetizing
energy stored in the primary inductance
is restored to the input source by
a demagnetization winding Nd. Most
commonly, the primary and the
demagnetization windings have the same
number of turns.
So, at turn-off, the power switch has to
withstand twice the input voltage during the
demagnetization time, and then, once the
input voltage (see figure 9).
The demagnetization and primary windings
have to be tightly coupled to reduce the
voltage spike - more than the theoretical 2 Vin
- occuring at turn-off across the power
switch.