[snip]
Are you sure there won't be trouble Win, at high load currents, where Vgs approaches 0v, therefore reducing the zener current towards 0mA. It has a sniff of maybe an uncertain operating point.
A 1mA bleed from the 520V won't hurt surely. I've added it back in for you. :)
-- Tony Williams.
Thanks; it has been so long since i have worked with those
4-pin,5-pin and octal devices that i forgot. Now i remember; ground the plate, signal goes in at the plate, and power at F/2 is pulled out from the grid.. (hint..tetrode)
That's a favorite of mine, I bought 100 a while back for use in one of my HV amplifier designs. But what I liked most was its small die, and the resulting low capacitance. So while it has a hefty voltage rating, the small die means it doesn't have a hefty power capability. Compare its high 2.3 C/W thermal resistance (allowing only 54W max for 25C case) to 0.24 C/W (417W) for the FCH47n60 I mentioned yesterday. That's a factor of 10x in the thermal resistance, which after factoring in the thermal resistance of the heat-sink insulator pads, etc., can lead to a 4 to 5x poorer maximum current rating for the regulator.
OK, good call.
One comment from experience, high-voltage zeners have rather high temperature coefficients. This leads to a much larger problem than dynamic impedance, so substantially reducing the zener current, say to 0.1mA rather than 1mA, could help. Or perhaps better yet, change the zener voltage to the 6-volt region, where the tempco is close to zero. But, this would reduce the loop gain with the high feedback attenuation, so better yet, change to a TL431 active zener control chip.
470k, 1/2 W 68k ,--/\\/\\--+--+---+-----/\\/\\-----, | | | | | FCH47n60 ___| | '-- C E -----+ | ------ Z B | | | | | | | --+-----' '--+--, ,---+-/\\/\\-+-+-----> 300V 520V | _|__|_ | | DN2540 ---- | | |____ZA_____| \\ | | / 820k, 1/2 W +----/\\/\\---+ \\ 0.5mA | 8.2k | | | 286k | \\| | | npn |---------+ |
OK, good call.
One comment from experience, high-voltage zeners have rather high temperature coefficients. This leads to a much larger problem than dynamic impedance, so substantially reducing the zener current, say to 0.1mA rather than 1mA, could help. Or perhaps better yet, change the zener voltage to the 6-volt region, where the tempco is close to zero. But, this would reduce the loop gain with the high feedback attenuation, so better yet, change to a TL431 active zener control chip.
470k, 1/2 W 68k ,--/\\/\\--+--+---+-----/\\/\\-----, | | | | | FCH47n60 ___| | '-- C E -----+ | ------ Z B | | | | | | | --+-----' '--+--, ,---+-/\\/\\-+-+-----> 300V 520V | _|__|_ | | DN2540 ---- | | |____ZA_____| \\ | | / 820k, 1/2 W +----/\\/\\---+ \\ 0.5mA | 8.2k | | | 286k | \\| | | npn |---------+ |
OK, good call.
One comment from experience, high-voltage zeners have rather high temperature coefficients. This leads to a much larger problem than dynamic impedance, so substantially reducing the zener current, say to 0.1mA rather than 1mA, could help. Or perhaps better yet, change the zener voltage to the 6-volt region, where the tempco is close to zero. But, this would reduce the loop gain with the high feedback attenuation, so better yet, change to a TL431 active zener control chip.
470k, 1/2 W 68k ,--/\\/\\--+--+---+-----/\\/\\-----, | | | | | FCH47n60 ___| | '-- C E -----+ | ------ Z B | | | | | | | --+-----' '--+--, ,---+-/\\/\\-+-+-----> 300V 520V | _|__|_ | | DN2540 ---- | | |____ZA_____| \\ | | / 820k, 1/2 W +----/\\/\\---+ \\ 0.5mA | 8.2k | | | 286k | \\| | | npn |---------+ |
OK, good call.
One comment from experience, high-voltage zeners have rather high temperature coefficients. This leads to a much larger problem than dynamic impedance, so substantially reducing the zener current, say to 0.1mA rather than 1mA, could help. Or perhaps better yet, change the zener voltage to the 6-volt region, where the tempco is close to zero. But, this would reduce the loop gain with the high feedback attenuation, so better yet, change to a TL431 active zener control chip.
470k, 1/2 W 68k ,--/\\/\\--+--+---+-----/\\/\\-----, | | | | | FCH47n60 ___| | '-- C E -----+ | ------ Z B | | | | | | | --+-----' '--+--, ,---+-/\\/\\-+-+-----> 300V 520V | _|__|_ | | DN2540 ---- | | |____ZA_____| \\ | | / 820k, 1/2 W +----/\\/\\---+ \\ 0.5mA | 8.2k | | | 286k | \\| | | npn |---------+ |
One of these 840 answers might be helpful:
Cheers! Rich
helpful:
Or he can read about it in our book. Doesn't have our book? Then he can read it at Google's book pages, see
Under Google books, right. Awesome! Floats my boat!
Actually I found that your search term did not work for me, but it was the first hit (out of 641!) when I just typed "foldback".
Chris
Now how am I going to get out of this situation? Telling the truth and admitting that I actually have two copies of AoE -- my own at home, and another one at my office but haven't really worked through them? Or should I just say, thanks for the hint, I'll go and buy the book (making Win happy about yet another sale of a 2nd edition copy so he can stall the 3rd a bit more)? Or should I just shut up, stick my nose in the book and then decide where the three foldback resistors need to go in the circuit that's being discussed here? I guess the latter is what I'll do. Thanks.
robert
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