Composite Capcitors

I was trying to make a composite cap from two e.g. 350V 150uF caps in series and I needed balancing resistors. I found some information online about what values to use, but nothing satisfying. Intuitively, one would want to use perfectly matched high valued resistors to equally distribute the voltage and not increase leakage current. Eventually I found this equation in the CDE capacitor application guide:

R = (2Vm – Vb)/(0.0015CVb)

Where:
R: Balancing resistor value across each cap in MOhm
Vb: Maximum VDC bus voltage across the two series capacitors
Vm: Maximum voltage one would permit on each capacitor
C: Rated capacitance of each capacitor in uF

However, what this equation says is that as we wish Vm to approach Vb/2 (i.e. the voltage across the two capacitors is evenly distributed), R approaches zero! But as R gets smaller the extra leakage current, approximated with I=Vb/2R, gets larger. So we have a neat optimization problem for R What I ended up doing is setting Vm = 0.8Vc where Vc is the rated voltage for the capacitors. This works as long as 0.8Vc > Vb/2. The equation thus also implies a lower bound (besides the trivial Vb/2 one) on the rated voltage of series capacitors given the bus voltage.

To complete the example, assuming the bus voltage in my case was 500V, ideally R is:
(2*0.8*350 - 500) / 0.0015*150*500 = 60/112.5 = 0.53MOhm
Of course, obtaining an R at that exact value is difficult. So suppose we can find a 0.5MOhm R +/- 1%, then we will have to verify Vm via:
Vm < (0.0015C*1.01R + 1)Vb/2, so Vm < 279 ~ 0.8Vc
and the extra leakage current is I < Vb/(2*0.99R), so I < 0.51mA

Note that I used the rated capacitance in the example, but in reality one would want to measure the capacitance of the two capacitors, and have a good measurement for the bus voltage. Hence it is important that the caps in the series composite are well-matched and as the CDE manual says: "assist voltage sharing stability by using capacitors from the same manufacturing lot and mounting them to operate at the same temperature."

I hope this information helps someone in constructing series composite caps and choosing the values of balancing resistors. Finally, here is the CDE application guide:
http://www.cde.com/resources/catalogs/AEappGUIDE.pdf