Here I am refering to the figu. 6.52 in AoE on page 372 in the MOSFET alternative.
This is about the figure 6.20 page 369, and I am talking about the 750ohm in series with 470uF on the output.
-- vh. Peter Andersen www.mespilus.dk
An argument can be made that no resistor at all is an option. The rational is that an unbalance in the capacitor voltages causes the high-voltage capacitor's leakage to increase, returning "balance" to the circuit. A counter argument is any such leakage may be causing damage to the capacitor's layers of thin dielectric insulation.
So, to your question, you make the resistors small enough so their current exceeds the capacitor's leakage-current spec by some amount. Then, if you've not made the resistors small enough, you can fall back on the rational above.
A good argument for high resistor current, even if they have to be power resistors, is a more rapid discharge of the capacitors after power switch-off, which is a safety issue. Your safety.
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Thanks,
- Win
Aha, there's a $64,000 question. In series power MOSFET circuits the 800# gorilla is the FET's gate capacitance, Ciss, which is so high it swamps all other circuit elements. To answer your question, make R small enough to assert some control over Ciss, based on your need for fast response to load transients, etc. If you find that too small a value of R is required (i.e. taking too much power), add C to suit your taste, taking control of the circuit away from the MOSFET's Ciss. As always, be sure to include the zeners we show.
Such amplifiers are usually used into capacitive loads. The 470pF presents a minimum load for the amplifier for testing, etc. Using a series resistor with this capacitor adds one more modest zero in the feedback loop to improve stability (the value isn't critical). However, the amplifier must still be made stable in the presence of full capacitive loads. I find adding some series resistance at the amplifier's output is helpful.
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Thanks,
- Win
1k, ouch!!! Do you have a sufficient load current to sink all of the series-resistor-chain current?
. series-connected MOSFET pass elements . HV in 22 500mA max . ----+----D S--D S--D S--D S--+-/\\/\\-+-----, . | G G G G | | | . | | | | | 1k | | . | 100k | 100k | 100k | 100k | | | LOAD . +-/\\/\\-+-/\\/\\-+-/\\/\\-+-/\\/\\-|----|------+ | . | pullup | B | | . '------------/\\/\\-----------+--C E----+ gnd . | | . zener diodes not shown +---|
Yes I have the zener. Well, as for now I can't get it to work, i have full voltage on output when i try to build with series connection of the mosfets. I have just tryed with 1K ohm.
Argh, so this is a pF not uF?? there is no notation on the schematic.
BTW: all this trouble with mosfets in series and gate cap. and all, why not use some BU508 in series (to avoid the secondary breakdown), this seem to me like it could be a much easier and more stable design.
Peter
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