MB3759 Datasheet, PDF (12/27 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Switchable between push-pull and single-end functions)

MB3759
s SWITCHING ELEMENT
1. Selection of the Switching Transistor
It can be said that the success or otherwise of a switching regulator is determined by the choice of switching
transistor. Typically, the following parameters are considered in selecting a transistor.
• Withstand voltage
• Current
• Power
• Speed
For the withstand voltage, current, and power, it is necessary to determine that the area of safe operation (ASO)
of the intended transistor covers the intended range for these parameters.
The speed (switching speed: rise time tr, storage time tstg, and fall time tf) is related to the efficiency and also
influences the power.
The figures show the transistor load curve and VCE - IC waveforms for chopper and inverter-type regulators.
The chopper regulator is a relatively easy circuit to deal with as the diode clamps the collector. A peak can be
seen immediately after turn-on. However, this is due to the diode and is explained later.
In an inverter regulator, the diodes on the secondary side act as a clamp. Viewed from the primary side, however,
a leakage inductance is present. This results in an inductive spike which must be taken account of as it is added
to double the VIN voltage.
chopper regulator
inverter regulator
IC
IN _VCE
L
VO
Q
D
C
IN
D1 L _VO
C
D2
IC
on
IC
on
off
VIN
VCE
VCE
Ton
t
IC
Ton
off
VCE
VIN 2 VIN
VCE
Ton
2 VIN
VIN
t
IC
Ton
t
t
12