1. Home
  2. Electronics Design
  3. A question on a typical filter circuit after rectifier in SMPS design

A question on a typical filter circuit after rectifier in SMPS design

Hi all,

As I am studying a typical power supply circuit used in a product and I see two 10 uF capacitors used in series after the rectifier circuit. I also see 4 x 470k ohms resistors used together with the above mentioned 10uF capacitors (please see below).

|---------------| | | > | > | > | > | | | | --------- | --------- > | > | > | > | | | |---------------| | | > | > | > | > | | | | --------- | --------- > | > | > | > | | | |---------------|

Basically I understand that the filter capacitors after recrifier circuit are actually current reserviors to provide the required pulsated current by the SMPS switches, however, I do not fully understand this circuit idea.

My questions are:

  1. The AC mains is 230V. Maximum possible RMS will be 250V (353V) peak. What do you think could be the reason of using 2 x 400V 10uF capacitors instead of 470uF at 600V?

  1. What is the use of 470k ohm resistors parallel with capacitors?

  2. How come (2 resistors//1capacitor) series with another (2 resistors//1 capacitor)?

Thanks and Best Regards

Reply to
Myauk
Loading thread data ...

Your diagram didn't line up, try a fixed width font.

Resistors might be in series to increase voltage tolerance. The average

1/4W is rated for 300V or so, in principle. Don't know why they'd need to, or to stack capacitors. Every 80-265VAC power supply I've seen is stocked with one 400 or 450VDC capacitor.

Tim

-- Deep Friar: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

Sorry for the inconvenience.

What i am really curious about is why specifically should it be (2 resistors//1capacitor) series with another (2 resistors//1 capacitor). Can't it be 4 resistors // 4 capacitors?

e

to,

ked

Reply to
Myauk

Do you see anything else going to the center point of the two capacitors?

Usually this configuration is used because it can be made into a voltage doubler (for 120VAC nominal input) by adding a single link to the bridge input.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

Using two caps serve a dual purpose in common SMPS designs. First, it allows changing the rectifier to a voltage doubler configuration with a switch (manual or auto, mechanical or triac) when used with a 110-120V mains supply, and operate as a bridge rectifier circuit at 220-250V AC. Secondly, the junction of the two caps provide a convenient tap point at half the total DC voltage for the SMPS push-pull output stage.

However, all such configurations I've seen use two 200V or 250V caps. I don't understand how you equate 2x10uF to one 470uF.

Again these serve a dual purpose. First, they ensure that the total DC voltage is evenly distributed between the capacitors, especially when there's little load on the caps from the rest of the circuit. Secondly, they discharge the capacitors of any potentially dangerous voltage when the PSU is turned off and opened for inspection.

I've already explained the purpose of the resistors. I couldn't see a meaningful diagram from your post. Are the two resistors per capacitor in series? If Q3 is asking why 2 resistors instead of 1, it's to ensure a lower voltage stress for each resistor. Resistors have a maximum voltage rating in addition to the wattage rating.

A quarter-watt resistor may have a rating of 250V and theoretically this should be enough for the

Reply to
pawihte

average

need to,

stocked

and

circuit.

The whole circuit strikes me as some kind of surge suppression on the=20 input of a power factor correction circuit.

Reply to
JosephKK

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.