Hi all,
anybody familiar with this strange capacitor-diode circuit at the line side of a small switching power supply? See attached picture.
The function seems obvious - the capacitors charge in series (through the middle diode) and discharge in parallel (through the other two diodes) at half the charging voltage.
But what is the purpose?
Thanks!
Perhaps it allows to improve the crest factor (makes current consumption peaks smoother) :
the standard diode bridge+capacitor would have short peaks of current consumption coincident with peaks of the AC voltage. It is not good for AC network - e.g. for the same average current I at low duty factor fraction of ohmic losses increases since it is proportional to R*I^2.
https://en.wikipedia.org/wiki/Crest_factor
May be for it is for redundant operation. Its high voltage side and even if one capacitor fails other can continue to work.
Ok, the purpose of this capacitor , if the half cycle of the wave is passing via the capacitor if the circuit is unstable because the input voltage has a harmonic(multi frequencies)
Dear Zdenek,
Hope you are well!
Your circuit analysis is right. When the the mains is interrupted then the stored charges in the two capacitors will be discharged.
I think this circuit is build in such manner to build a divider with the two separate discharge paths for every capacitor in order to avoid self discharge in each other.
This source may be required for the operation of the switched mode power supply.
I could not get the data sheet to see the requirements of the sp2605F.
Best wishes
As the "charging" and "discharging" circuits show, the output voltage is held to about half the peak input voltage by the capacitors when the ac input is lower than half the peak. I'm not clear on what the advantage to that is, as the rectified voltage will still rise to the peak level at other times.
Perhaps because it allows lower voltage electrolytic capacitors (with their high energy density and low cost) to be used to "valley fill" the rectified voltage? For example, the 400 V rated capacitors could be used up to a nominal 480 Vac input, with a limit of 565 Vac RMS ac input before their rating is exceeded.
Bruce Carsten: could be, but in this case (a small 4 W wall plug adapter, marked 120 V input) a 400 V capacitor is an overkill...
@ Zdenek Svindrych: You're right; I didn't catch that it was only a 120 Vac input, and I still don't see where it's a 4 W power converter. Well, I still haven't got a better idea.
This circuit is called "Valley-Fill rectifier" and is used to improve power factor of this power supply. When absolute instantaneous mains voltage is lower than half an amplitude, the load is fed from capacitors. Otherwise, the load is fed directly from mains voltage through bridge rectifier. This allows to control the waveshape of input (mains) current. Look for "Valley Fill Power Factor Correction" on the internet.
Dear Zdenek,
I see you did not comment my answer!
What is the difference of my answer from that of Bruce??
I come now to your inquiry about the capacitor voltage 400 V. This voltage is the name voltage of the capacitor and not the operating voltage. It is the voltage rating of the capacitor. The voltage rating of the capacitor must be normally greater than the highest operating voltage even it must at least twice as large as the maximum operating voltage. This is a normal practice.
Best wishes.
Best wishes
Because this circuit has a voltage drop of 2×0.7V from peak voltage. So I think that this capacitor with two diodes are used to compensate this voltages drop and decreases the ripple factor at the same time.
Valley fill circuit. But usually a resistor is added on cathode of second diode to limit peak current at discharge.
A cheap way to improve Power Factor on small power converters.
Fateh Singh: Makes perfect sense (only the resistor would limit the charging current of the capacitors, not discharging).
On the other hand, the two capacitors cannot participate in EMI filtering (for the most part), and there needs to be another small capacitor (not shown in my drawing) to create the low impedance source for the switching operation.
Finally, this particular converter emits annoying audible buzz (60 / 120 Hz) when loaded (4 W), probably because only the "valleys are filled".
Indeed Zdenek Svindrych , my mistake.
Concerning the EMI, I think that these HV electrolytic capacitors will not help with HF EMI. If I remember well, their SRF is quite low...
The buzz is due to the periodic nature of the charge/discharge @ 2x60Hz.
Other solution would be much more complex : PFC+Flyback, or some other control scheme for the flyback. These would push the noise to ultrasound, ie inaudible for humans
In this circuit the capacitor is protection the diode's by work a short circuit via the generation higher harmonic frequencies that causes for passing the highest currents...
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Materials Science
- Materials
- Inorganic Materials
- Ceramics
- Glass