diodesRF
I have been coming across so-called "band switching diodes" recently, but I can't figure out what they actually are. Is this some type of RF switching PIN diode?
I came across these while I was searching for a cheap alternative to step recovery diodes. Someone recommended using the BA682, which is a band switching diode.
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:
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.
\$ 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.
After repairing a returned unit, take a very careful oscillogram across the Schottky with a short 10:1 scope probe (directly connect the tip and use a very short ground wire) and no bandwidth limiting on your good quality (100MHz or better, digital storage preferred) scope.
An example of a short probe, borrowed from elsewhere on EE.SE:
What you're looking for is high voltage (a spike, or ringing) on the leading edge of the voltage waveform. Any time you switch an inductive load, there's the potential for high voltage - unless you carefully look for it (i.e. with a short probe and a good scope) you won't see it. Your layout will have a great influence on whether or not there will be nastyness across the Schottky, so measure to be sure.
If you do end up seeing something ugly, you may need to introduce an RC snubber across the Schottky to keep the diode from avalanching.
Best Answer
Planar silicon diode, specified for this application (dynamic resistance) and made with particularly low capacitance (and relatively high forward drop) compared to a jellybean switching diode.