Hi,
Usually you don’t need such a snubber at that place. It was merely in reply of Jan’s remarks. If there is ringing it is the ringing from the leakage inductance of the trannie and the parasitic capacitance of the trannie exited by the fast switch off of the diodes. It is difficult to give any values because it relies on the values of those elements, which can vary much. I wouldn’t bother too much of this. But if there is ringing at that point a general rule is to make the snubber cap 5 to 10 times the parasitic capacitance of the trannie and the resistor small enough to dampen the ringing, but not too small. Normal practice is to use just 10nF – 100 nF caps straight across each diode as pointed out earlier.
Cheers
Usually you don’t need such a snubber at that place. It was merely in reply of Jan’s remarks. If there is ringing it is the ringing from the leakage inductance of the trannie and the parasitic capacitance of the trannie exited by the fast switch off of the diodes. It is difficult to give any values because it relies on the values of those elements, which can vary much. I wouldn’t bother too much of this. But if there is ringing at that point a general rule is to make the snubber cap 5 to 10 times the parasitic capacitance of the trannie and the resistor small enough to dampen the ringing, but not too small. Normal practice is to use just 10nF – 100 nF caps straight across each diode as pointed out earlier.
Cheers
Snubber calculations
The transformer manufactures don't provide enough information to calculate snubbers for the bridge. For the best snubber, it takes experimenting with a scope. What I have been advising to do lately is to just use a cap that draws 1%-5% of the rated winding current. If using a ceramic cap (for the small size), use one rated for >4 times the peak voltage across the windings to reduce the dielectric heating issues inside ceramic caps.
.
The noise from the transformer radiates from both the wiring between the coil and the supply caps and the coil itself. The snubber reduces the high frequency components of this noise. A shorted copper foil shading ring on the outside of the transformer helps with this a lot. Don't place it through the inner holes of the iron where the winding go through for it will make a shorted turn and blow the 115V power fuse. I add insulation between the shading ring and coil, but let the foil touch the core.
The transformer manufactures don't provide enough information to calculate snubbers for the bridge. For the best snubber, it takes experimenting with a scope. What I have been advising to do lately is to just use a cap that draws 1%-5% of the rated winding current. If using a ceramic cap (for the small size), use one rated for >4 times the peak voltage across the windings to reduce the dielectric heating issues inside ceramic caps.
.
The noise from the transformer radiates from both the wiring between the coil and the supply caps and the coil itself. The snubber reduces the high frequency components of this noise. A shorted copper foil shading ring on the outside of the transformer helps with this a lot. Don't place it through the inner holes of the iron where the winding go through for it will make a shorted turn and blow the 115V power fuse. I add insulation between the shading ring and coil, but let the foil touch the core.
I have a new approach- I use an audio signal generator, square-wave and tune RC for critical damping, on power transformers.
Had to do a snubber for a 1.25kV CT transformer and much easier/safer to experiment values at lower voltages. Checking the results operating at 1.25kV the snubber circuit was optimal. Seems to scale well enough, from 10V to 1,250V.
Had to do a snubber for a 1.25kV CT transformer and much easier/safer to experiment values at lower voltages. Checking the results operating at 1.25kV the snubber circuit was optimal. Seems to scale well enough, from 10V to 1,250V.
You are doing this out of circuit, right ? Square wave via a resistor ( 600E ?) to the Transformer coil with the RC snubber and check the wave at the trafo coil ?
You can do it in-circuit if you disconnect the secondary (rectifier, filter caps).
I treat the 50/60Hz mains power transformer like an audio transformer.
Using a signal generator, inject 1kHz square-wave to the primary at say 10Vpp. The power transformer's primary DC resistance is already there, no need for resistor unless the signal generator can be damaged by back EMF. No impedance matching there.
On the secondary, I have a scope and play around with RC values without danger of electrocution. You get several Vpp to work with.
If a center-tapped transformer, then I work with one (half) winding to narrow down the values and then see the effects when this becomes two snubbers, one for each winding, which I use in CT power supplies.
I "tune" for flat response, no overshoot and well-damped ringing. It helped a lot because I was always guessing snubber values.
I treat the 50/60Hz mains power transformer like an audio transformer.
Using a signal generator, inject 1kHz square-wave to the primary at say 10Vpp. The power transformer's primary DC resistance is already there, no need for resistor unless the signal generator can be damaged by back EMF. No impedance matching there.
On the secondary, I have a scope and play around with RC values without danger of electrocution. You get several Vpp to work with.
If a center-tapped transformer, then I work with one (half) winding to narrow down the values and then see the effects when this becomes two snubbers, one for each winding, which I use in CT power supplies.
I "tune" for flat response, no overshoot and well-damped ringing. It helped a lot because I was always guessing snubber values.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.