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Power supply topology - will it work? (Control theory, stability)

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Yansi:
I have made this for step testing this supply.  NE555 + suitable mosfet and bunch of resistors (value selected through the switches), trigger out to the scope.

salbayeng:
Nicely made dummy load!
I used to make them neat when I was younger, but after you've killed 4 or 5 of them, you get a bit sloppy!.
See attached for a dummy load and 20v 40A supply for testing a 24VDC distribution system
and a nice looking one (on the outside!) I made maybe 20years back (the DPM and fan is dead and some switches too)
I use these cheap 100W rheostats a lot too, only broken one so far!

Jay_Diddy_B:
Blueskull and the group,

The difference is more than taking the Thevenin equivalent circuit and paralleling R1 and Rload. This is only a very small change. Typically R1 would be 100 times smaller the Rload, just for efficiency reasons.

This is the point that I am trying to make. This circuit:



Which represent the MOSFET stage and R1 as a transconductance amplifier, does not have the same transfer function as this circuit:



In the second circuit, the maximum size of the output signal, is the always less than the input signal. At low frequencies this circuit has very small phase shift.

In the first circuit, which represents Yansi's circuit, the amplitude of the output can be greater the input voltage. At low frequencies, and high value of load resistor, the gain is high and the phase shift is -90 degrees. The circuit is an integrator.

Regards,

Jay_Diddy_B

salbayeng:
Yea, you're right, of course.  (an improvement on "right off course" :-DD)
I first looked at the output stage and thought to myself , "the output impedance is just R1 (as the MOSFET that shunts it in current mode is effectively infinite impedance)"
It's obvious if you consider the output swings 100's of volts but only a few volts across R1.
The maximum gain (at DC obviously) is simply 100k/R1.

The simple circuit you have is sufficient to explain what is going on.
I would add the embellishment of
(a) the gate resistor and miller capacitance but guessing 300R and 1nF  puts a pole in at 500kHz, of no concern
(b) and also add the zero due to ESR 100uF + 1R is only 1.6kHz (I should probably find some real capacitor data and not guess!)

salbayeng:
Tried to check on ESR for typical capacitors.
Most datasheets stop listing ESR above 100v rating, If I do some extrapolating based on tan\$\delta\$ and rated current, the quality 105C electros have ESR's from 3 to 10 \$\Omega\$ for 10uF/450v. (say 3kHz for 5ohm + 20uF)

A datasheet for 85C rated capacitors is here: http://www.mouser.com/ds/2/88/SK-32353.pdf  this shows 26 \$\Omega\$  for 10uF/450V , this gives a zero at ~ 600Hz

If Yansi were to use a 1uF 450v polypropylene  http://au.mouser.com/search/ProductDetail.aspx?R=0virtualkey0virtualkeyECW-FE2J105J , reasonable size (28x17x10) and 66c
This has tan\$\delta\$ < 0.1%  , which is <120milli \$\Omega\$ as an ESR at 1kHz,  This would push the zero out to ~ 200kHz.

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