Electronics > Projects, Designs, and Technical Stuff
Power supply topology - will it work? (Control theory, stability)
salbayeng:
For the pre-regulator, I would suggest a slightly different approach
(a) make a fast, low gain inner loop that controls the voltage output of the pre-regulator so it tracks a reference voltage, the pre-reg output would then have smallish capacitors, and probably a fairly high ripple. The loop gain would be fairly low , e.g. the pre-reg output might droop 10v under full load, (so GH~40) , and there would be no integrators to wind up.
(b) form a reference voltage that is equal to output voltage + desired_MOSFET_drop , so you are making an outer loop that controls the MOSFET_drop
While it might be argued that (a) and (b) together are algebraically the same as what you already have, but in practice you can employ different filtering to the signal path in (b)
(This approach also avoids the voltage divider leaking current across the MOSFET)
Because your buck stage is back to front, in practice the feedback for (a) is a difference of Bus_plus - Vout_minus, and the reference voltage in (b) is generated from another difference amp Vout_plus - Vout_minus. You won't need an opto-coupler either, and you can feed in your desired_MOSFET_drop using the 5vref on the UC3842. The downside is you may now be leaking current across the pre-reg MOSFET.
An alternative is to use a current mirror as a level shifter to replace the Diff amp
---------
An alternative control for the pre-reg might be hysteretic control , this is always stable (or always unstable, depending on your viewpoint!)
-----
Something else to possibly throw in series with one of the supply lines is a largish bobbin type inductor with a diode across it, but depending on where you hook your power supply caps, you probably wont need this (and your supply is current source essentially). This helps avoid extreme levels of short circuit current when discharging capacitors. Basically the inductor sucks up the capacitor voltage for the first millisecond or so, and discharges the capacitor at hundreds of amps instead of thousands, the internal resistance of the inductor also limits the short circuit current, and the large slug of copper sucks up a lot of joules. The diode is there to recirculate the stored current when the short circuit is removed. If there is a semiconductor involved , having the inductor there pushes the locus down in the SOA curves to a safe region, and gives control loops time to respond.
Yansi:
Hello guys!
After some long time, I have finaly found some time for more hobby electronics toying. This is the result:
A fully working CV/CC programmable power supplies 0-300V 0-100mA.
A dual channel 12bit DAC with a 12bit ADC (for output current measurement) have been added together with an isolated DC/DC and digital SPI isolation. The module requires just the unregulated input of 300+ V and 5V for the controls. The floating section is powered from the 5V through the isolated DC/DC.
These are based exactly on what you have helped me a lot with in the above comments. These two will be used in a later project. The larger power supply with the switchmode pre-regulator we have discussed (400V 300mA) is still under development (waiting till I have more time to design some custom PCBs for it).
Many thanks! Will post more when I get more time to finish this supply.
Y.
marcopolo:
Any news?
Yansi:
Hello, unfortunately not many news - what exactly do you have on mind?
The programmable modules are still sitting on the shelf, waiting for the control board to be assembled. (I have some troubles on the control board to be solved there too - I need a DAC output within the range of +5 to -55V).
The original idea of 400V 300mA power supply with switchmode pre-regulator is currently at rest, due to other high priority work and access to a proper 3kW 500V PSU from ElektroAutomatik. I could make some pretty interesting series of episodes of the design failures in those two EA power supplies I have repaired.
Also, I have helped my friend to design a linear power supply using this topology, I will post an image when it'll be finished.
marcopolo:
Hello,
Something like the final schematic :)
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version