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Purpose of diodes in this circuit

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vbdev100

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I have came across a circuit for electronic ballast for mercury lamp. I am not able to get the purpose of using the 3 diodes in the following schematic. The middle wire goes to some startup circuitry. Please explain... Thanks

Ballast.png
 
Looks like they are to protect the circuit from back-emf from an inductive load.
 
Maybe D9 do not let to short voltages between C6 and C5.
 
It would probably make more sense if we could see the rest of the circuit :eek:
 
True, but if D9 was not present there would be no need for the other two.
(edit) I think if the purpose of D9 can be understood, it will all make sense.
 
Thanks, I am not having the complete diagram right now. I had saved the image long back while searching for ballast designs.

While studying the circuit some thoughts came to my mind..

1. If we assume the capacitors are in series, each will be charged at 150V DC (as we get 300V DC from 230V)
2. When lower capacitor C5 is supplying current to rest of the circuit, current will flow through upper diode D12. And as the diode will short other capacitor & the diode drop is negligible (0.6V), that means circuit will get 150V DC
3. Similarly when upper capacitor C6 is supplying current, return path will be through D10.

So is this a voltage divider circuit???
 
Hi,

It could be that the start up circuit requires less voltage so the two caps have to be in series (temporary less voltage). After that the start up circuit makes the top cap charge up to some degree. After that we want the lower cap to act as filter for the top line. So the lower cap acts as dual purpose: one to divide the voltage down, and two to act as filter. Without the diode(s) we would need a third cap which is more expensive than another diode.

It's hard to say for sure though because we cant see the load circuitry. That would be necessary for a complete evaluation.
 
It does occur to me the caps could conceivably be switched between parallel and series configurations - it almost looks like a voltage doubler - except for the DC supply.
 
The circuit is of a Passive Power Factor Corrector.
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Valley-Fill PFC Circuit

For low power applications, there's a rectifier circuit known as a 'valley-fill' rectifier. It's simple to implement, but is only suitable where a very high effective ripple voltage on the DC output can be tolerated. This limits its usefulness, but it is found in some low-end LED lighting circuits, and is also suitable for some CFLs and similar lighting products where the high ripple voltage is not likely to cause a problem.

The power factor improvement is much greater than one might expect, and a PF of a little over 0.7 is typical. The current waveform is still quite distorted though, and it's unrealistic to expect too much from such a simple circuit. THD measured 82% in the simulation of the circuit shown. While hardly anything to crow about, it's still better than having over 150% distortion or more.
**broken link removed**
Essentially, the two capacitors are charged in series, but discharged in parallel. This means that when the peak of the applied AC falls, so too does the output voltage, until it reaches a voltage that's roughly half the AC peak (162V less a few diode voltage drops) and is actually the voltage across the capacitors in parallel. The output 'DC' therefore has half the applied voltage of ripple - 158V peak-peak in the circuit shown. As you can imagine, the applications for any rectifier/filter with this much ripple are limited.

It's interesting to see the current waveform, and it is shown below. The 2.2 ohm resistor helps to reduce the sharp peaks that sit on the top of the waveform. Higher values reduce the peaks more and reduce distortion, but result in higher power dissipation in the resistor, wasted power and less power to the load. While it might seem that adding a small inductor (say 10mH) instead of the resistor would be able to eliminate the spike on top of the waveform, it's not as effective as one would hope. The added cost and bulk isn't worth the small gain obtained.


**broken link removed**
If examined closely in a simulation (it's not shown here for clarity), the 'DC' voltage varies from 158 to about 318V - that's a lot of ripple. The mains current waveform looks pretty bad, but it's much, much better than that shown for the standard rectifier and capacitor supply. The power factor is far better than expected, and although there are still some significant harmonics (which result from the distorted waveform), THD is far better than the previous version as well.

As noted though, this type of supply is only suitable where the high ripple is tolerable, and you won't find it used much any more. It's basically an idea that came too late, because cheap PFC ICs that are a great deal better in all respects came along only a short while after this circuit was first used. Until I started working with LED tube lights, I'd never seen it before, and now, only a few years later, I don't see it used in any of the new designs. The latest LED lamps are now using active PFC which is far better than any form of passive PFC can hope to be.
 
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It looks like a Cockroft-Walton/Greinacher multiplier. Cap between the anode and cathode of two diodes in series. That or a variation, it's almost certain it's a voltage multiplier. Factor in that you said it was in a ballast which means you need high voltage output from AC and you can correlate the two. You also have a Graetz bridge upstream to rectify the negative halfwave, and incidentally double the frequency, which would make it a full-wave multiplier kung fu stuff.
 
Yes, except it's functionally the opposite - a voltage multiplier charges the caps in parallel and discharges them in series - this circuit charges them in series and discharges them in parallel. Three's a good explanation here: **broken link removed** (good old ESP, I gain so much from this site!)
 
Thank you ronsimpson, throbscottle & everybody... Sorry for my late reply. I was really puzzled about the circuit, you have been very helpful to me :)
 
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