What is the advantage of a snubberized power supply

[srinath]

What does a snubberized power supply do, and can I use something like this to get it -

Sharp S202S12 Zip 4 SIP Type SSR with Snubber Circuit And | eBay

A snubber in the rectifier bridge ?

Thanks.
Srinath.

 

[godfrey]

can I use something like this

No, that's a whole different animal. To find out what it is, just Google the part number (S202S12).

 

[Johno]

What does a snubberized power supply do

This is a seriously misunderstood subject.
Transformers have a phenomenon called leakage inductance which manifests as a fairly high Q inductor in series with the secondary windings (also primary but that is of little consequence in a power transformer).
The apparent secondary leakage inductor forms a tank circuit with the secondary load. In the case of the load being a rectifier with capacitive filtering the secondary current is a short high current pulse as the capacitors charge near the voltage peaks each half cycle. As the secondary voltage passes peak and begins to fall the rectifier diode(s) reverse bias and block the secondary current flow, quickly.
Therefore the energy stored in the leakage inductor tries to dissipate in the secondary tank circuit, ie the rectifier junction capacitance and the secondary winding capacitance as high frequency ringing (can be in the 20MHz region). The radiated interference is pulsed at 100/120Hz for a full wave rectifier and this is what we hear rather than the RF carrier frequency.
Adding an R-C snubber network across the secondary terminals has 2 effects...
- first is that the frequency may be some what reduced to the point where radiation is less effective
- second is to tune the resistor value to critically damp the NEW tank circuit. It is the snubber resistor that kills the ringing - the capacitor's role is to minimise the resistor 50/60Hz power dissipation.

It is the current flowing/ringing in the leakage inductance and the stray capacitance of the secondary windings that causes the problem so putting the snubber anywhere else is detrimental since you are creating a longer aerial without damping the source.

I refer you to Jim Hagerman's 'Calculating Optimum Snubbers' for the math.

John

 

[Mark Johnson]

It is the current flowing/ringing in the leakage inductance and the stray capacitance of the secondary windings that causes the problem so putting the snubber anywhere else is detrimental

Morgan Jones hammers this same point in his Linear Audio v5 article: Snub the transformer secondary, not the rectifier(s)!

 

[srinath]

The part then that has me scratching my head is that the rectifier bridge is the first stop from the transformer. Wouldn't that be the perfect location to counteract it ?
Cool.
Srinath.

 

[tinitus]

maybe he meant to say that it should be on bridge AC side (and not DC side)
opposed to the 'oldfashioned' way mounting accross each diode in the bridge

but isnt that usually just a film cap ?
I thought 'snubber' meant mounting a resistor in series with the cap, to control the Q ringing

 

[PMI]

... opposed to the 'oldfashioned' way mounting accross each diode in the bridge

At least in industrial applications, this would be for a different reason, not because of the transformer.

I thought 'snubber' meant mounting a resistor in series with the cap, to control the Q ringing

In English, the word just means some kind of component or device to suppress a transient (any component).

Same word is also used in hydraulic systems to suppress a pressure transient, like one that forms when a valve opens or closes.

Most common use in engineering is probably a diode in parallel with the coil of a relay. The diode is installed so it block current when the coil is energised, and conducts current when the coil voltage is removed, such as through a pilot relay or a transistor driver, to help dissipate the "inductive kick" from the relay coil...

which sort-of brings me full circle to the transformer, and what happens when current ceases to flow through the diode bridge... ... 😀

 

[Johno]

"maybe he meant to say that it should be on bridge AC side"

More accurately it is the transformer secondary that is snubbed - as close as possible to the transformer terminals - because the energy causing the RF noise is stored in the transformer itself.

"I thought 'snubber' meant mounting a resistor in series with the cap, to control the Q ringing"
Exactly.
The resistor provides a path for current to dissipate the energy stored in the leakage inductor. The value of the resistor is chosen to critically damp the tank circuit formed with the resistor and capacitor across the leakage inductor. The capacitor value is chosen arbitrarily to be low impedance to the natural frequency of the tank circuit and to be high impedance to the 50 or 60 Hz secondary output.

To give you a mental picture of snubbing values, I have calculated a capacitor value of 1n5 in series with a 2k2 resistor for 500VA with 2x55Vac secondary toroidal transformers based on the measured short/open circuit parameters. Secondaries in series so a snubber across each.

 

[AndrewT]

2k2 for a snubbing resistor seems unusually high.
What are the secondary parameters that led to this 2k2?
Could you show the calculations?

 

[srinath]

I am confused, so the rectifier with snubber - Sharp S202S12 Zip 4 SIP Type SSR with Snubber Circuit And | eBay wont work, but I should run a huge resistor 2k2 or somewhere like that and how many watts ? (mine is a single 35v 20 amp transformer) across the outputs of the transformer.

Thanks.
Srinath.

 

[macboy]

I am confused, so the rectifier with snubber - Sharp S202S12 Zip 4 SIP Type SSR with Snubber Circuit And | eBay wont work, but I should run a huge resistor 2k2 or somewhere like that and how many watts ? (mine is a single 35v 20 amp transformer) across the outputs of the transformer.

Thanks.
Srinath.

Sharp S202S12 is not a rectifier! It is a solid state relay.

 

[PMI]

I am confused, so the rectifier with snubber - Sharp S202S12 Zip 4 SIP Type SSR with Snubber Circuit And | eBay wont work...

what Godfrey and macboy said... this is a link to a SSR, a solid state relay. This part is not a rectifier at all, so the issue is not whether it works or not, it probably works just fine for its intended use.

With all due respect, did you perhaps post the wrong link, meaning, looked at one part, then posted a link to another?

I ask because rectifier diodes like that do exist, for example:

www.mouser.com/ds/2/149/ISL9R30120G2-244115.pdf

(also sold on ebay)

 

[srinath]

I dont quite know - I thought it looked like a rectifier bridge and had a snubber in its description. I think I just searched for snubber on ebay and found that which looked like a rectifier bridge.

I guess the answer is - I meant to post that one, but thought since it had a AC pair of terminals and a + and - its a rectifier bridge ...
Sorry about that, I'll look that mouser one now.
Cool.
Srinath.

 

[Johno]

2k2 for a snubbing resistor seems unusually high.
What are the secondary parameters that led to this 2k2?
Could you show the calculations?

The principles are found here: http://www.hagtech.com/pdf/snubber.pdf

And Tom Gootee, on this forum, published an excellent little LTSpice model that does the calculations for you, attached

 

[srinath]

I cant open that file in a format that makes sense. Its all like "wire - 720, 160, etc etc. Looks like a spreadsheet, but I cant make out what it says.
Cool.
Srinath.

 

[PMI]

I cant open that file in a format that makes sense...

Do you have LTSpice installed? If you do, then clicking on the file should open it in LTSpice.

If you don't, it is a free program. Even if you do not simulate your own circuit, you can download a file that someone else created, like the one above, change the values of the components to suit yourself, and see what happens. At a minimum, you can see someone else's simulation full screen, and not as a screen capture.

In other words, you don't have to become an LTSpice expert to use this.

 

[AndrewT]

John,
where do I see the answer?
Are the values in the input section what you measured for your 500VA?

 

[Johno]

John,
where do I see the answer?
Are the values in the input section what you measured for your 500VA?

Tom's model is useful for measuring transformer parameters from which the snubber values are calculated. I do not fully understand it or its implications and it omits the winding capacitance. I followed Hagerman's recipe for transformer measurement and had similar results to the Tom model which basically uses a 50Hz "sig-gen".

My measures:
Leakage inductance: 1.7mH
Resonant freq: 225KHz
Diode capacitance: 250p
Winding capacitance: 295p

Hagerman also suggests using a capacitor in parallel with the snubber, something I think is superfluous as the damping is optimised which ever way is used. However if Hagerman's Cx is chosen large enough then the leakage and diode capacitance would be small enough to ignore and this maybe simplifies the measurement process.

The snubber calculated from my measured values was under damped when viewed on the scope so I increased the snubber resistor value until the ringing current reduced to a single overshoot. The snubber capacitor value is arbitrary.

 

[AndrewT]

...........................The snubber calculated from my measured values was under damped when viewed on the scope so I increased the snubber resistor value until the ringing current reduced to a single overshoot. The snubber capacitor value is arbitrary.

This description sounds odd.
A correct resistor value will give an optimum damped wave shape. This will not have any overshoot.
Underdamped will show a sharper knee in the curve. Severely underdamped will show overshoot.
Very severely underdamped will show ringing.

It appears you have increased the resistor value to severely underdamped stage.

What was the original predicted resistor value that you discarded.
Did this resistor show ringing? Maybe the capacitor was too small in value. The capacitor that your declare "is arbitrary".

 

[godfrey]

The snubber capacitor value is arbitrary.

That doesn't sound right. For decent damping, the snubber capacitor should be larger than the capacitance causing the ringing. I'd guess you need to increase the snubber capacitor and decrease the resistor.