Forums

Half bridge DC-DC converter topology

Started by P E Schoen July 5, 2012
There is a thread in=20
http://www.diyelectriccar.com/forums/showthread.php/evnetics-developing-d=
c-dc-converter-72862p5.html=20
which suggests a half-bridge DC-DC converter with the following =
topology:
http://www.diodes.com/zetex/?ztx=3D3.0/application@app~49!top~5!curr~13

Originally I thought it was only useful for low power applications like=20
phone chargers, and I think the capacitor in series with the primary is=20
superfluous because of the two series capacitors across the DC bus. I =
found=20
a similar topology here:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/A=
PPLICATION_NOTE/CD00003910.pdf

I modeled the converter using LTSpice and it seems to work quite well =
with=20
reasonable components, and it seems to have less problem with transients =

than my direct drive push-pull topology. It also seems to be fairly =
tolerant=20
of imbalance and it does allow the use of PWM, although it works best at =

50%.

http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.png

The simulation ASC file is there also:
http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.asc

In this case, it is a high power step-down DC-DC converter. Apparently =
many=20
EVs use a separate 12V battery for accessories so they can use the same=20
components as wiring as the original ICE donor (or transplant recipient) =

car. The 144V is typical for a battery pack and the DC-DC converter uses =

power from that to keep the battery charged and run the lights, fans,=20
wipers, and other usual accessories. But apparently some of the =
commercially=20
available converters are not very reliable or efficient, and just using =
an=20
ordinary switching supply and/or charger such as are available from=20
Mean-Well are prone to failure in an automotive environment.

I may try a similar design for my purposes, which is essentially the=20
reverse. I want to use 24-48 VDC from batteries and boost it to 320 VDC =
or=20
640 VDC for a VFD and three-phase motor. I have the previous push-pull=20
design modified with a capacitor precharge circuit and adjustable PWM =
but it=20
has become complicated, and this topology seems simpler and perhaps =
better.=20
There seem to be many more drive ICs and complete controllers for=20
half-bridge than for push-pull, so maybe it's the way to go.

Thanks,

Paul=20

On Thu, 5 Jul 2012 22:38:49 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>There is a thread in >http://www.diyelectriccar.com/forums/showthread.php/evnetics-developing-dc-dc-converter-72862p5.html >which suggests a half-bridge DC-DC converter with the following topology: >http://www.diodes.com/zetex/?ztx=3.0/application@app~49!top~5!curr~13 > >Originally I thought it was only useful for low power applications like >phone chargers, and I think the capacitor in series with the primary is >superfluous because of the two series capacitors across the DC bus. I found >a similar topology here: >http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00003910.pdf > >I modeled the converter using LTSpice and it seems to work quite well with >reasonable components, and it seems to have less problem with transients >than my direct drive push-pull topology. It also seems to be fairly tolerant >of imbalance and it does allow the use of PWM, although it works best at >50%. > >http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.png > >The simulation ASC file is there also: >http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.asc > >In this case, it is a high power step-down DC-DC converter. Apparently many >EVs use a separate 12V battery for accessories so they can use the same >components as wiring as the original ICE donor (or transplant recipient) >car. The 144V is typical for a battery pack and the DC-DC converter uses >power from that to keep the battery charged and run the lights, fans, >wipers, and other usual accessories. But apparently some of the commercially >available converters are not very reliable or efficient, and just using an >ordinary switching supply and/or charger such as are available from >Mean-Well are prone to failure in an automotive environment. > >I may try a similar design for my purposes, which is essentially the >reverse. I want to use 24-48 VDC from batteries and boost it to 320 VDC or >640 VDC for a VFD and three-phase motor. I have the previous push-pull >design modified with a capacitor precharge circuit and adjustable PWM but it >has become complicated, and this topology seems simpler and perhaps better. >There seem to be many more drive ICs and complete controllers for >half-bridge than for push-pull, so maybe it's the way to go. > >Thanks, > >Paul
The topology that you are admiring works well in the application because the primary voltage is high. The half bridge lends an automatic reduction in turns ratio. It will not work as well, if the primary voltage is reduced by an order of magnitude, to produce a high voltage output, because it pushes the turns ratio in the wrong direction - away from the ideal unity. If you examine the circuit and imagine active switches on the low voltage side, you will see that the low voltage end of the admired circuit becomes a current-fed push pull topology. If you wanted to maintain the utility in reverse power transmission, you should try to preserve this form, as it can function naturally as a boost converter if low voltage switch conduction overlaps. The issue of inrush is not avoided, however. If you expect it to operate into low voltage loads it must be manipulated to run without an overlap on the low voltage side, with some allowance for energy transfer in the non-overlapping period. Some kinds of switched snubber have been employed to do this successfully, although the added switches do tend to increase component count and cost. This is not an issue if you're charging batteries with terminal voltages that don't reduce to abnormally low values - so you should decide whether this thing is intended to operate without batteries early on. In an automotive application, it would be highly abnormal to expect anything to run without the fuel source. RL
The reason it works better is because it has a choke input filter.  Although 
you still get nasty inrush current with a voltage-mode controller, at least 
without a lot of precautions.  Current mode control is best.

Tim

-- 
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

"P E Schoen" <paul@peschoen.com> wrote in message 
news:jt5j46$dv6$1@dont-email.me...
There is a thread in
http://www.diyelectriccar.com/forums/showthread.php/evnetics-developing-dc-dc-converter-72862p5.html
which suggests a half-bridge DC-DC converter with the following topology:
http://www.diodes.com/zetex/?ztx=3.0/application@app~49!top~5!curr~13

Originally I thought it was only useful for low power applications like
phone chargers, and I think the capacitor in series with the primary is
superfluous because of the two series capacitors across the DC bus. I found
a similar topology here:
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00003910.pdf

I modeled the converter using LTSpice and it seems to work quite well with
reasonable components, and it seems to have less problem with transients
than my direct drive push-pull topology. It also seems to be fairly tolerant
of imbalance and it does allow the use of PWM, although it works best at
50%.

http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.png

The simulation ASC file is there also:
http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.asc

In this case, it is a high power step-down DC-DC converter. Apparently many
EVs use a separate 12V battery for accessories so they can use the same
components as wiring as the original ICE donor (or transplant recipient)
car. The 144V is typical for a battery pack and the DC-DC converter uses
power from that to keep the battery charged and run the lights, fans,
wipers, and other usual accessories. But apparently some of the commercially
available converters are not very reliable or efficient, and just using an
ordinary switching supply and/or charger such as are available from
Mean-Well are prone to failure in an automotive environment.

I may try a similar design for my purposes, which is essentially the
reverse. I want to use 24-48 VDC from batteries and boost it to 320 VDC or
640 VDC for a VFD and three-phase motor. I have the previous push-pull
design modified with a capacitor precharge circuit and adjustable PWM but it
has become complicated, and this topology seems simpler and perhaps better.
There seem to be many more drive ICs and complete controllers for
half-bridge than for push-pull, so maybe it's the way to go.

Thanks,

Paul

On Friday, July 6, 2012 4:38:49 AM UTC+2, P E Schoen wrote:
> There is a thread in > http://www.diyelectriccar.com/forums/showthread.php/evnetics-developing-dc-dc-converter-72862p5.html > which suggests a half-bridge DC-DC converter with the following topology: > http://www.diodes.com/zetex/?ztx=3.0/application@app~49!top~5!curr~13 > > Originally I thought it was only useful for low power applications like > phone chargers, and I think the capacitor in series with the primary is > superfluous because of the two series capacitors across the DC bus. I found > a similar topology here: > http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00003910.pdf > > I modeled the converter using LTSpice and it seems to work quite well with > reasonable components, and it seems to have less problem with transients > than my direct drive push-pull topology. It also seems to be fairly tolerant > of imbalance and it does allow the use of PWM, although it works best at > 50%. > > http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.png
It looks like the series version of Peter Baxandall's Class D oscillator. Jim Williams popularised the parallel version (though he didn't credit Peter Baxandall and probably never knew that Peter Baxandall had described it first in 1959). http://home.planet.nl/~sloma000/0344_001_Baxandal.pdf -- Bill Sloman, Nijmegen
"legg"  wrote in message =
news:5nqcv7p8tuombnod9n213ghl9voh0j355o@4ax.com...

> The topology that you are admiring works well in the application > because the primary voltage is high. The half bridge lends an > automatic reduction in turns ratio. It will not work as well, if the > primary voltage is reduced by an order of magnitude, to produce a > high voltage output, because it pushes the turns ratio in the wrong > direction - away from the ideal unity.
> If you examine the circuit and imagine active switches on the low > voltage side, you will see that the low voltage end of the admired > circuit becomes a current-fed push pull topology. If you wanted to > maintain the utility in reverse power transmission, you should try to > preserve this form, as it can function naturally as a boost converter > if low voltage switch conduction overlaps.
> The issue of inrush is not avoided, however. If you expect it to > operate into low voltage loads it must be manipulated to run without > an overlap on the low voltage side, with some allowance for energy > transfer in the non-overlapping period. Some kinds of switched > snubber have been employed to do this successfully, although the > added switches do tend to increase component count and cost.
> This is not an issue if you're charging batteries with terminal > voltages that don't reduce to abnormally low values - so you should > decide whether this thing is intended to operate without batteries > early on. In an automotive application, it would be highly abnormal to > expect anything to run without the fuel source.
I redesigned the circuit for 24V to 320V, and it is not a battery = charger,=20 but a source of high voltage from a small battery pack.. I had some = problems=20 with some extremely high short duration (40 nSec) power surges in the=20 MOSFETs at turn-off, and I found that most large capacitors for the = center=20 tap had fairly high ESR so that they were eating up about 40 watts each. = So=20 I found that I could use much smaller value capacitors which are = actually=20 polypropylene film, with ESR of 2-4 mOhms, and things worked much = better. I=20 had put some hefty snubbers in the circuit but they may not really be=20 needed. It seems like this should work OK for about 1 kW and hopefully = up to=20 2 kW or so. Maybe 5 kW, although for that I might use 36 or 48 VDC = input. I=20 want to keep the battery current under 100 amps. So, here is the simulation: http://www.enginuitysystems.com/pix/Half_Bridge_24V_320V.png and the ASC file: http://www.enginuitysystems.com/pix/Half_Bridge_24V-320V.asc I might see if I can fix my existing push-pull DC-DC converter pretty = much=20 as-is but adding the precharge and some series inductance to lower the=20 inrush. I'll see if I can get it to work at 16 kHz with the iron core=20 toroid. That should be interesting. The new design uses 50 kHz and a ferrite transformer. I need to = implement a=20 precharge and/or current limit as well. I'll package it in a smaller box = and=20 add some hooks for measurement and datalogging. I may actually build it = in a=20 couple of weeks. Thanks, paul=20
On Fri, 6 Jul 2012 08:46:33 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>"legg" wrote in message news:5nqcv7p8tuombnod9n213ghl9voh0j355o@4ax.com... > >> The topology that you are admiring works well in the application >> because the primary voltage is high. The half bridge lends an >> automatic reduction in turns ratio. It will not work as well, if the >> primary voltage is reduced by an order of magnitude, to produce a >> high voltage output, because it pushes the turns ratio in the wrong >> direction - away from the ideal unity. > >> If you examine the circuit and imagine active switches on the low >> voltage side, you will see that the low voltage end of the admired >> circuit becomes a current-fed push pull topology. If you wanted to >> maintain the utility in reverse power transmission, you should try to >> preserve this form, as it can function naturally as a boost converter >> if low voltage switch conduction overlaps. > >> The issue of inrush is not avoided, however. If you expect it to >> operate into low voltage loads it must be manipulated to run without >> an overlap on the low voltage side, with some allowance for energy >> transfer in the non-overlapping period. Some kinds of switched >> snubber have been employed to do this successfully, although the >> added switches do tend to increase component count and cost. > >> This is not an issue if you're charging batteries with terminal >> voltages that don't reduce to abnormally low values - so you should >> decide whether this thing is intended to operate without batteries >> early on. In an automotive application, it would be highly abnormal to >> expect anything to run without the fuel source. > >I redesigned the circuit for 24V to 320V, and it is not a battery charger, >but a source of high voltage from a small battery pack.. I had some problems >with some extremely high short duration (40 nSec) power surges in the >MOSFETs at turn-off, and I found that most large capacitors for the center >tap had fairly high ESR so that they were eating up about 40 watts each. So >I found that I could use much smaller value capacitors which are actually >polypropylene film, with ESR of 2-4 mOhms, and things worked much better. I >had put some hefty snubbers in the circuit but they may not really be >needed. It seems like this should work OK for about 1 kW and hopefully up to >2 kW or so. Maybe 5 kW, although for that I might use 36 or 48 VDC input. I >want to keep the battery current under 100 amps. > >So, here is the simulation: >http://www.enginuitysystems.com/pix/Half_Bridge_24V_320V.png > >and the ASC file: >http://www.enginuitysystems.com/pix/Half_Bridge_24V-320V.asc > >I might see if I can fix my existing push-pull DC-DC converter pretty much >as-is but adding the precharge and some series inductance to lower the >inrush. I'll see if I can get it to work at 16 kHz with the iron core >toroid. That should be interesting. > >The new design uses 50 kHz and a ferrite transformer. I need to implement a >precharge and/or current limit as well. I'll package it in a smaller box and >add some hooks for measurement and datalogging. I may actually build it in a >couple of weeks. > >Thanks, > >paul
If you examine your simulation more closely, you'll find some difficulties exist that are being glossed over. The power train ripple current, determined by the output inductor, could not be larger (producing AC losses in the internal source impedance of over 400W). 20uH is a very small value for the forward topology at 300V. Source internal loss can be reduced by two orders of magnitude if the output inductor value increases by one order. This is not normally the major design criteria in choosing an output filter inductor, but it's a good indication that you don't know enough about this design process to proceed. I suggest more reading and comprehension, with less bravado, in future 'work'. I doubt your simulations will ever accurately predict behavior in any low voltage, high current, high frequency circuit, unless some attempt is made to introduce the physical strays of wiring inductance and leakage inductance accurately. This requires at least physical model for first estimation. Admittedly, these precautions were more commonly enforced in the past, when real hardware, real expense and real safety were involved. RL
"legg"  wrote in message =
news:lt0ev759ms4070r0devag3p0mh4tdjva7u@4ax.com...

> If you examine your simulation more closely, you'll find some > difficulties exist that are being glossed over.
> The power train ripple current, determined by the output inductor, > could not be larger (producing AC losses in the internal source > impedance of over 400W). 20uH is a very small value for the forward > topology at 300V.
> Source internal loss can be reduced by two orders of magnitude if the > output inductor value increases by one order. This is not normally the > major design criteria in choosing an output filter inductor, but it's > a good indication that you don't know enough about this design > process to proceed.
I don't really see how the source losses come into play. I was getting = close=20 to 90% efficiency at about 1 kW power level so there can't be any more = than=20 100W total losses. And I can see that most of that is in the MOSFETs and = the=20 capacitors. Do you calculate the source losses by using I(in)^2 * R(in)? I increased the value to 100 uH and then 50 uH, and both seemed to work=20 well, although output voltage and total power dropped. I will need to = look=20 at practical inductors to see what would be a good fit. I also tried=20 increasing the resistance of snubber resistors R3 and R4, from 0.1 ohm = to 1=20 ohm or 10 ohms, and the higher values produced short duration power = surges=20 of 1 to 2 kW. A 0.2 ohm value brings it down to about 500W, which may be = safe, depending on the actual MOSFETs I use.
> I suggest more reading and comprehension, with less bravado, in > future 'work'. I doubt your simulations will ever accurately predict > behavior in any low voltage, high current, high frequency circuit, > unless some attempt is made to introduce the physical strays of > wiring inductance and leakage inductance accurately. This > requires at least physical model for first estimation. Admittedly, > these precautions were more commonly enforced in the past, > when real hardware, real expense and real safety were involved.
I understand some of the basics but I don't have a strong background in=20 mathematics and magnetism, so my design process tends to be more=20 "instinctive" and determined largely by trial and error simulation. I = also=20 don't have much experience in high frequency circuits, which is probably = why=20 I have had problems with some designs when I have committed them to a = PCB.=20 The information about skin effect and high frequency AC resistance was = an=20 eye-opener. There is an incredible amount to learn in order to be truly=20 proficient, and there's a limit to how much I can learn by reading and=20 comprehension. When the explanation becomes peppered with calculus my mind shuts down. = I=20 seem to do better by running a simulation and trying to observe effects = that=20 I did not expect, I look for probable causes and make changes to see if=20 there is an improvement. It may not be the best way to proceed, but I = have a=20 feel for what will happen in real world circuits and usually they have=20 worked about as expected. My problems have usually been due to ignoring = the=20 start-up transients, and when I deal with them, I think it will be = reliable. These circuits are more or less on a hobby level at this time. I'm using = them for my own electric tractor project and I'm trying to apply what I=20 learn to larger systems such as electric cars and trucks, in the DIY = market.=20 There are some things that I am still learning from the experiences of=20 people on that forum, but I can also see that they often do not have a = solid=20 understanding of some basic principles. It's quite a leap from my own=20 projects of 2 kW or so, to some of their projects which usually involve=20 20-50 kW and in some cases into the megaWatt range. I'm used to dealing = with=20 such power at line voltage levels and 60 Hz, and AC currents in the 10k = to=20 100k range, but their use of high capacity batteries and exotic motors = and=20 controllers is something else. Mistakes at that level, especially in=20 automotive use, can be costly and dangerous, especially when so many = DIYers=20 do not really understand what they are dealing with and what they are=20 measuring. Thanks, Paul=20
On Fri, 6 Jul 2012 19:50:09 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

<snip>
>I understand some of the basics but I don't have a strong background in >mathematics and magnetism, so my design process tends to be more >"instinctive" and determined largely by trial and error simulation. I also >don't have much experience in high frequency circuits, which is probably why >I have had problems with some designs when I have committed them to a PCB. >The information about skin effect and high frequency AC resistance was an >eye-opener. There is an incredible amount to learn in order to be truly >proficient, and there's a limit to how much I can learn by reading and >comprehension. > >When the explanation becomes peppered with calculus my mind shuts down. I >seem to do better by running a simulation and trying to observe effects that >I did not expect, I look for probable causes and make changes to see if >there is an improvement. It may not be the best way to proceed, but I have a >feel for what will happen in real world circuits and usually they have >worked about as expected. My problems have usually been due to ignoring the >start-up transients, and when I deal with them, I think it will be reliable. > >These circuits are more or less on a hobby level at this time. I'm using >them for my own electric tractor project and I'm trying to apply what I >learn to larger systems such as electric cars and trucks, in the DIY market. >There are some things that I am still learning from the experiences of >people on that forum, but I can also see that they often do not have a solid >understanding of some basic principles. It's quite a leap from my own >projects of 2 kW or so, to some of their projects which usually involve >20-50 kW and in some cases into the megaWatt range. I'm used to dealing with >such power at line voltage levels and 60 Hz, and AC currents in the 10k to >100k range, but their use of high capacity batteries and exotic motors and >controllers is something else. Mistakes at that level, especially in >automotive use, can be costly and dangerous, especially when so many DIYers >do not really understand what they are dealing with and what they are >measuring. > >Thanks, > >Paul
There is nothing shameful about working with circuits below 1KW. It's the most practical method of developing preliminary physical models and prototypes. The principles involved apply at all power levels. What changes is the physical limitations of materials and methods, when scaled. Your circuit is a good example. Try running the simulation with reduced transformer Lp, while maintaining turns ratio. Even with a coupling coefficient of 0.99, your current values are dominating permissible Di/Dt in the power transfer, due to leakage terms. In real life, Lp will be selected only to be high enough so that magnetizing energy doesn't dominate function unintentionally and copper losses are minimized, so long as core losses remain manageable. Another example - see what happens when 50nH is present in battery lead wiring.... Neither of these factors would be so noticeable at 100w. One of the major purposes of modeling in software is that these effects can be predicted before hardware is constructed. There's usually not much calculus required, it's usually just basic algebra, with a little trig thrown in. http://cp.literature.agilent.com/litweb/pdf/5952-4020.pdf http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF DIYers may try to use readily available subassemblies and materials from similar, if not directly related applications. It's just as important to understand the potential and limitations of these materials as it is when developing from scratch. RL
On Thu, 5 Jul 2012 22:38:49 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>There is a thread in >http://www.diyelectriccar.com/forums/showthread.php/evnetics-developing-dc-dc-converter-72862p5.html >which suggests a half-bridge DC-DC converter with the following topology: >http://www.diodes.com/zetex/?ztx=3.0/application@app~49!top~5!curr~13 > >Originally I thought it was only useful for low power applications like >phone chargers, and I think the capacitor in series with the primary is >superfluous because of the two series capacitors across the DC bus. I found >a similar topology here: >http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/APPLICATION_NOTE/CD00003910.pdf > >I modeled the converter using LTSpice and it seems to work quite well with >reasonable components, and it seems to have less problem with transients >than my direct drive push-pull topology. It also seems to be fairly tolerant >of imbalance and it does allow the use of PWM, although it works best at >50%. > >http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.png > >The simulation ASC file is there also: >http://www.enginuitysystems.com/pix/Half_Bridge_144V-12V.asc > >In this case, it is a high power step-down DC-DC converter. Apparently many >EVs use a separate 12V battery for accessories so they can use the same >components as wiring as the original ICE donor (or transplant recipient) >car. The 144V is typical for a battery pack and the DC-DC converter uses >power from that to keep the battery charged and run the lights, fans, >wipers, and other usual accessories. But apparently some of the commercially >available converters are not very reliable or efficient, and just using an >ordinary switching supply and/or charger such as are available from >Mean-Well are prone to failure in an automotive environment. > >I may try a similar design for my purposes, which is essentially the >reverse. I want to use 24-48 VDC from batteries and boost it to 320 VDC or >640 VDC for a VFD and three-phase motor. I have the previous push-pull >design modified with a capacitor precharge circuit and adjustable PWM but it >has become complicated, and this topology seems simpler and perhaps better. >There seem to be many more drive ICs and complete controllers for >half-bridge than for push-pull, so maybe it's the way to go. > >Thanks, > >Paul
This IR driver chip is pretty nice: https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf at least for fixed duty cycle.
"John Larkin"  wrote in message=20
news:s62hv7purs4b8qi6ekob2snu621fijeea1@4ax.com...

> This IR driver chip is pretty nice:
> https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf
> at least for fixed duty cycle.
I have some similar products, IR2104, IR2136, IRS2001, IRS24531. The=20 IRS2153D http://www.irf.com/product-info/datasheets/data/irs2153d.pdf = seems=20 similar to the IR21531 and in fact the added 1 on the part number = signifies=20 a smaller deadband. I like the idea of the built-in timer for=20 self-oscillation although I also like to drive with a PIC for more = accurate=20 frequency. The topology of your application shows the transformer primary driven=20 through capacitors, and that might be even better than what I have with = a=20 center tap between two capacitors. It seems to me that an output voltage = control of sorts could be done by adjusting the frequency of the square=20 wave. And the start-up surge would be easily controlled by starting with = a=20 small duty cycle and ramping up to 50%. That would require a bridge = driver=20 like the IRS2001 which has separate high and low drivers. For my low voltage high power application the capacitors would need to = be=20 able to handle high ripple current with low ESR. Those are somewhat rare = and=20 expensive. Here is a 22,000 uF 63V cap with 20A ripple for $38. http://www.digikey.com/product-detail/en/PEH200MJ5220MB2/399-5653-ND/2193= 731 For a 24V system with half-bridge I figure that's about 12V RMS so for = 1000=20 watts I need about 80 amps. So four of those would be $150. It seems better to use polypropylene film. Here is a 10uF 300V capacitor = with 2.9mOhms ESR and 15A ripple, for $3.51ea/10: http://www.digikey.com/product-detail/en/B32674D3106K/495-2915-ND/1277679= The high voltage is "wasted" but the cost is much better. 6 in parallel = is=20 still reasonable, $21. And I can easily go to a 48V system or even = higher=20 with no worries. In the EV world it would be very useful to have a = 144V-144V=20 booster to get 288V for a 240 VAC motor drive. Probably about 20 kW. And = probably best to build with multiple 2 kW units in parallel. $50/kW = would be=20 an acceptable selling price. Thanks! Paul