Flogging the FREDs
Voltage fed converters with transformer isolation will exhibit ringing in the secondary. Ringing is caused by parasitic inductances and capacitances in the circuit, with the dominant elements will being the transformer leakage inductance (\$ L_ {\text {Lk}}\$) and junction capacitance ( \$ C_j\$)of the bridge diodes. The diode data sheet shows \$ C_j\$ of 32pF. I'm going to make a naive guess at \$ L_ {\text {Lk}}\$ of 500nH, but it will have to be measured to really know. So, an LC of 500nH and 32pF is what must be snubbed.
Spike amplitude without snubbing will be \$ 2 n V_ {\text {in}}\$, where \$ n \$ is transformer turns ratio and the factor of 2 is what you get for a high Q resonance.
There are different types of voltage snubbers; Clamping, Energy transfer resonant, and Dissipative. The clamping and resonant types require more parts and some involvement of active switches which I think make them impractical for this case. So, I am only going to cover dissipative snubbers because they are the most simple and work well with passive switches (like diodes or synchronous rectifiers).
The form of dissipative snubber that I will cover is a series RC placed in parallel with each bridge diode.
Some facts about RC dampening snubbers:
- They are all about impedance matching. You don't get to choose the snubber resistor value \$ R_d\$. The parasitic LC determines that for you by characteristic impedance Zo.
- You do get to choose the value of the snubber cap \$ C_d\$. That's important since the cap value sets the snubber loss (\$ P_ {\text {Rd}}\$)as \$ C_d F V^2\$ . Where V is the pedestal voltage and F is switching frequency. The snubber cap must provide a low impedance at the LC resonance of the parasitics, so it needs to be several times \$ C_j\$.
Some guidelines, and what to expect with RC dampening snubbers:
For \$ L_ {\text {Lk}}\$ of 500nH and \$ C_j\$ of 32pF, Zo will be 125Ohms. So, \$ R_d\$ would be 125 to match Zo. You may have to fine tune this a little since \$ C_j\$ is non-linear and falls off with reverse voltage.
Choosing the snubber cap \$ C_d\$ : Choose \$ 3 C_j\leq C_d\leq 10 C_j \$ . Higher values in the range do provide better dampening. For example, \$
C_d\$ of \$ 3 C_j\$ will result in a peak diode voltage of \$ 1.5 n V_ {\text
{in}}\$, while \$ C_d\$ of \$ 10 C_j\$ will result in a peak diode voltage of
\$ 1.2 n V_ {\text {in}}\$.
Dissipative snubber performance will not improve for \$ C_d\$ values
greater than \$ 10 C_j\$.
Power loss \$ P_ {\text {Rd}}\$, with a pedestal voltage of 1250V and F of 50KHz.
- If \$ C_d\$ is \$ 3 C_j\$ or 100pF, \$ P_ {\text {Rd}}\$ = \$ C_d F V^2\$ or 7.8W.
- If \$ C_d\$ is \$ 10 C_j\$ or 330pF, \$ P_ {\text {Rd}}\$ = \$ C_d F V^2\$ or 25.8W.
\$ C_d\$ of \$ 10 C_j\$ gives the best dampening with peak voltage of 1.2 time the pedestal voltage, but you can save some power with smaller snubbing caps if you can stand the higher peak voltage.
You would have to disconnect the ground on the transformer center tap, but I would not recommend doing this because the insulation between primary and secondary (or between secondary and case/mounting) may not withstand 12 KV.
If you need 12 KV, you should get a proper 12KV power supply.
Best Answer
A few specs are important: input voltage, output voltage, output current, whether the input is single-phase or three-phase, acceptable output voltage ripple, and output voltage tolerance.
For typical single-phase full-wave rectification, you'll need four diodes and a capacitor. Three-phase would require an additional two diodes.
The DC voltage output will be roughly the peak of the AC line. You'll need a transformer to step it up or down from your source. Even if you just want to rectify the AC line, a transformer is probably a good idea, just to help limit the available energy on the output side in case something goes wrong. The transformer needs to be spec'd to supply at least as much power as you'll be pulling with your load.
Make sure the diodes can handle all the current you want to pull, and the voltage of the DC bus. You can use individual diodes, or a package containing multiple diodes already arranged as a rectifier.
The capacitor should be selected to handle the DC bus voltage. The capacitance and the load current set the ripple voltage. For a 60 Hz line, the cap will be charged once every 8.3 mS. How far the cap drops in between line peaks is determined by the equation. I = C dv/dt, or rearranged, dv = dt I/C. How much ripple is acceptable depends on your application.
Even with relatively low ripple, you'll need a regulator downstream of the capacitor if you want any sort of precise voltage, as the AC line will fluctuate over time.
Also, appropriate use of fuses is important. Put a fuse with a rating somewhat higher than your expected load current in the DC output link. Having a switch in that link may also be nice, just for ease of use, depending on your application.