MB3759 Datasheet, PDF (13/27 Pages) Fujitsu Component Limited. – Switching Regulator Controller (Switchable between push-pull and single-end functions)

MB3759
The figure below shows an example of the ASO characteristics for a forward-biased power transistor (2SC3058A)
suitable for switching.
Check that the ASO characteristics for the transistor you intend to use fully covers the load curve. Next, check
whether the following conditions are satisfied. If so, the transistor can be expected to perform the switching
operation safely.
• The intended ON time does not exceed the ON-time specified for the ASO characteristic.
• The OFF-time ASO characteristic satisfies the intended operation conditions.
• Derating for the junction temperature has been taken into account.
For a switching transistor, the junction temperature is closely related to the switching speed. This is because the
switching speed becomes slower as the temperature increases and this affects the switching losses.
Forward-biased area of safe operation single pulse
2SC3058A (450 V, 30 A)
IC (Pulse) max.
50
IC max.
20
10
D.C.
TC = +25˚C
Single pulse
Pw
= _{500 µs}
5
2
1
0.5
0.2
0.1
0.05
5 10 20
50 100 200 500 1000
Collector - emitter voltage VCE (V)
2. Selecting the Diode
Consideration must be given to the switching speed when selecting the diode. For chopper regulators in particular,
the diode affects the efficiency and noise characteristics and has a big influence on the performance of the
switching regulator.
If the reverse recovery time of the diode is slower than the turn-on time of the transistor, an in-rush current of
more than twice the load current occurs resulting in noise (spikes) and reduced efficiency.
As a rule for diode selection, use a diode with a reverse recovery time trr that is sufficiently faster than the transistor
tr.
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