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eem2am
Joined: 22 Jun 2008
Posts: 753
Helped: 13
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 03 Nov 2008 18:44 p mosfet driver |
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Dear all,
I am doing a switch mode battery charger with UC3909 IC.....
datasheet....
http://focus.ti.com/lit/ds/symlink/uc3909.pdf
....on page 7 of the datasheet, you may see the complimentary emitter follower pair (NPN and PNP) used to drive the P mosfet.
..however, you also see there a zener and a diode in parallel with the upper resistor.
Can any reader explain the purpose of this zener and diode?
They look a waste of time to me.....surely just having the two resistors is enough?
......as long as these resistors are set to make sure there is no overvoltage to the Gate_Source of the P mosfet then who needs the zener?
...also...this is a switching converter...and zeners are all slow devices....surely unsuitable for use in switching converters like this?
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stabby21
Joined: 21 Oct 2008
Posts: 22
Helped: 1
Location: Philippines
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03 Nov 2008 19:18 p mosfet driver circuit |
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| Since you are dealing with SMPS specially Bulk converter with high side switch, there is a tendency that a spike may occur at your output side going to the input. It only act as a limiter so that the input will not experience a greater amount of voltage.
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03 Nov 2008 19:18 Ads |
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eem2am
Joined: 22 Jun 2008
Posts: 753
Helped: 13
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06 Nov 2008 19:52 p-mosfet driver |
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thankyou so much for you reply!
do you mean that the Miller effect will couple a low voltage through the drain-gate capacitance?
will this be to the detriment of the BJT's?
Also, shouldnt the Zener be connected to the gate of the P MOsfet instead of the bases of the BJT's?
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