I see spellman/bertan hv power supplies achieve this but are expensive. Is there a circuit that can regulate cheap flyback transformer's voltage ripple just as well? I've seen some circuits referred to as smoothing circuits and filter circuits but it's hard to find quality search results that actually mention design related to voltage ripple control. I would only need around 30 uA to 400 uA.
With those modest current requirements (30-400uA), why not just use an LDO on the output as post regulator? You don't mention the voltages, so that may not be an option. If the voltage is higher than 36V, then you'll have to start looking at RC or LC low-pass filters. For the filters you'll have to know the highest frequency of the ripple and just filter it.
pretty much you need to put active regulation after the rectifier of the flyback. a capacitance multiplier is probably good start.
Disclaimer: I am not responsible for the actions of others, this post is merely educational. Do not make this circuit.
The best way I could find to have high voltage ripple brought to negligible amounts is to drive the flyback transformer(or other high freq. AC source) at the highest frequency possible within the range of 20khz to 100khz, take its AC (remove diode) and use this as input to a full wave Greinacher multiplier, with a parallel filter network. It may be more practical to include a voltage divider after the flyback, as I intend to do.
This spreadsheet calculator helps to calculate capacitor and diode values for low ripple, according to your own desired current and voltage.
I used google translate on this pdf with OCR to learn of this design as Fig. 7. The final ripple is shown in Fig. 9.
Fig. 10 implies that inductors stray capacitors are added; the roughly translated text claims the following:
"Stray capacitors equivalently between two condenser columns Must compensate for the lead current flowing through the As shown in Figure 10, cw circuit capacitors Insert an inductor in parallel between the column and the boost transformer true Sky The gain of the step-down transformer at this time was increased."
I believe google translate mistakenly translated "step-up" as "step-down".
I recommend making a negative voltage version of this circuit to accomplish the ranges of voltages closer to earth ground and to reduce the number of stages and their consequential voltage drop.
Example component values are provided for Fig. 7, where any capacitor labeled "Cf" is equal to Co, and N is the number of stages in the Greinacher/Cockroft-Walton multiplier:
f=100KHz, I=75μA, I2=10μA
I3=10μA, Cp=36pF, Co=3900pF
R1=250KΩ, R2=125KΩ, R3=62.5KΩ
Keep in mind any load resistances affect the low pass filter cutoff frequency. I could not find much of example values for the "RL1" and "RL2" load resistances, seemingly they are just there to show how to connect the circuit to its application.
Due to low capacitance values and high voltages, it is cheaper to make your own capacitors according to this formula so long as large size is not a problem. Caps and diodes minimum ratings should be twice the input voltage and current.
It is best to avoid parallel diodes to achieve current rating because of their tendency to heat up and provide resistance, taking current disproportionately. Also, apply soldering heat conservatively to avoid breaking them.