voltage doubler efficiency / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

voltage doubler efficiency

1 2 3 next

Move Thread

LAN_403

IamSmooth

Sun Jan 09 2011, 09:03PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I have check the power in and out of a bridge rectifier/voltage doubler. It takes AC in through a bridge rectifier and out to to 1500uf caps in series. I have experimented with putting a choke in series. I get efficiencies from 85%. Is there any way to improve this? My goal is to get a DC output.

If I lower the capacitance too much I find my voltage waveform sags when under load.

Pinky's Brain

Mon Jan 10 2011, 04:30PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

How much of the loss is simply across the ESR of the capacitors?

IamSmooth

Tue Jan 11 2011, 03:32AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

That would have to be where it is coming from as there is not much else. Depending on my measurements I can get between 85 and 90.

Mattski

Tue Jan 11 2011, 05:03AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Without seeing a schematic for your design I would say that diode voltage drop is also a loss in your system. You can use Schottky diodes which tend to have very small forward voltage drops. Synchronous rectifications with FETs is also an option for this.

Capacitor ESR might also be a factor, without trying to hand calculate I'd simply suggest simulating your circuit with a reasonable ESR value and see how ESR affects total power loss.

IamSmooth

Tue Jan 11 2011, 06:32PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I agree that there is some loss with the rectifier, but schottky's seem like an expensive alternative. My doubler is putting out close to 700vdc @ 15-20A Are there any standard rectifier bridges that have ultra-low voltage drops other than schottkys?

Dr. Dark Current

Tue Jan 11 2011, 07:14PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

700V at 15 amps? That is over 1O kilowatts... You can not possibly lose 1 kilowatt in caps or diodes... I think there is some error in your measurements.

IamSmooth

Tue Jan 11 2011, 09:06PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Interesting. Because when I look at the AC amps and volt in (240vac x 45A) it is close to the AC amps out at the inverter (~240vac x 43-44A). The voltage from the switches sags, so it is not a perfect square wave. These measurements are approximate with analog meters.

Yet, when I measure the voltage and DC amps coming out of the doubler in the mid-stage, it seems I am losing power.

MAINS -> doubler -> SMPS -> INVERTER

I guess it is hard to accurately measure the amps from the doubler?

Mattski

Tue Jan 11 2011, 10:16PM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

wrote ...
Are there any standard rectifier bridges that have ultra-low voltage drops other than schottkys?

Do a digikey parametric search, but if it has performance comparable to a schottky then the price will be too...

wrote ...
Interesting. Because when I look at the AC amps and volt in (240vac x 45A) it is close to the AC amps out at the inverter (~240vac x 43-44A). The voltage from the switches sags, so it is not a perfect square wave. These measurements are approximate with analog meters.

Are your meters true RMS at least? Are you distinguishing between actual power and reactive power? Measuring efficiency takes some work.

GeordieBoy

Wed Jan 12 2011, 12:04AM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The power factor (PF) for a full-wave voltage doubler is poor, and will typically be something in the range of 0.5 to 0.8 This means that the real power component only represents between 50% and 80% of the total AC current drawn from the mains line. Or to put it another way the RMS AC current drawn from the line can be up to twice what you would expect for the amount of real power you are drawing off the DC output side of the voltage doubler. (It's exactly the same for a normal full-wave rectifier arrangement, so this problem is by no means unique to the voltage doubler.)

The lighter the load and the bigger the capacitors are the smaller the conduction angle of the diodes becomes. This causes current to flow through the diodes in very short but incredibly intense pulses! If you think about it, all of the energy that is being continuously drawn from the DC output must be replenished in the capacitors during the tiny fraction of a mains cycle near the peaks where the diodes actually turn-on and conduct. It is this horrendous peaky current waveform that represents "harmonic distortion" and wrecks the power factor.

You basically have three options:

1. Live with it! Whilst the AC line current draw is excessive, it's not real power being wasted, that's why nothing is getting hot as if 1kW was being burnt up somewhere. It's only lots of harmonic current sloshing about and wrecking the quality of the mains supply for everyone else! (Ok, it does cause excessive dissipation in fuses, house wiring, variacs etc, but you can over-rate these components to allow for poor PF.)

2. Design a power factor corrector. These can be either passive or active. The subject is complicated though. This type of poor power factor is not easily corrected with a PFC capacitor like displacement power factor is - In short be prepared to do lots of reading and learn lots of new and exciting maths!

3. Switch to 3-phase power where you can use a 6-pulse or 12-pulse rectifier and achieve a better power factor that way. 3-phase service also typically lets you draw more power too if you intend to scale up.

I hope this info helps,

-Richie,

Dr. H.

Wed Jan 12 2011, 07:41AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Good day

what GeordieBoy sad but in a graphical way. You can see how all the used power have to be replenished for a very short time.

Cheers

1 2 3 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob