 | Author | Message | | | eric | | Posted: Mon Oct 09, 2006 11:59 pm Post subject: Switch-on current limiting | | | | Site Admin Joined: 01 Jan 1970 Posts: 311 | G. Kleine 009
In certain direct-current operated circuits, such as DC-to-DC converters,
the switch-on current may be so high that the output voltage
of the power supply cannot reach its nominal level. This difficulty
may be prevented with a limiting circuit as shown in the diagram.
When the input voltage, Uin, is switched on, transistor T1 is off
since capacitor C1 is not charged. The level of current Iat the
moment of switch-on is
I(t=0)=Uin/R2.
Capacitor C1 is charged slowly via resistor R1 until the
gate–source threshold voltage, UGS(th), is exceeded, whereupon
the transistor begins to conduct. The time interval between switchon
and the transistor coming on is determined by time constant
R1-C1 and the ratio Uin:UGS(th). The gate-source voltage, UGS, of
the BUZ20 is in the range ±20 V. If Vin is larger than these values,
or a different transistor is used, UGS must be limited to the
range mentioned by zener diode D2, for which a ZPD18 was used
in the prototype.
Diode D1 enables C1 to be discharged via the load when Uin
is switched off. The
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