Справочник по диодам Шоттки. Преимущества диодов Шоттки в англоязычных источниках Schottky barrier rectifiers : Меньшее по сравнению с обычными кремниевыми диодами падение напряжения 0. Но это касается только диодов Шоттки на напряжение до В. Далее, судя по данным из справочников по Шоттки, это преимущество сходит на нет Меньшая емкость p-n перехода, что позволяет использовать диоды Шоттки на более высоких частотах. С емкостью связан и меньший уровень помех Недостатки диодов Шоттки: При кратковременном превышении обратного напряжения сразу выходят из строя обычные диоды выживают при условии, что не превышена температура перехода.
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The only way for your bridge rectifier to see this capacitance is if it was connected the same way as you measured it - across the primary and secondary. What is important is the stray capacitance, which is parasitic capacitance formed across the two secondary leads. Physically, it is the capacitance formed between the windings of one coil to the other windings to that same coil.
The capacitance is there, even if it has a fairly low value resistor shorting it the secondary winding resistance. A Moment to Reflect Before one can understand a snubber, one must understand the snubbee. The thing being snubbed. This is, of course, ringing. Ringing is caused by reflections. In transmission lines, and or hitting something with a hammer, or any energy flow. When a sudden discontinuity in the characteristic impedance of a current path, it results in some of that energy getting reflected back towards the source.
And a diode, transistor, relay, or other switching element represents more or less the worst possible case of discontinuous impedance - it goes from being the characteristic impedance of that leg of the circuit to effectively infinite save for a trickle of leakage current and often in a matter of nano seconds.
This is bad. That is going to cause a significant reflection. And what will store it? The parasitic capacitances and inductances of our circuit of course! Together, they form an LC tank, oscillating at the resonant frequency as determined by the amount of parasitic inductance and capacitance making up the tank. This is the source of the ringing, and what determines the frequency it rings at.
Reflections in the context of transmission lines and characteristic impedances can get confusing because this is all very abstract. These are actual reflections! The kind you are quite familiar with already: the reflection from a piece of glass, the echo off a rock wall, or the heat reflected off the parabola of a heat lamp. Vibration in a chime struck against a wall.
This is all we are talking about, and it is common to any movement of energy. Stop, Hammer Time Understand that what I am about to say is more just an analogy, but a mechanical equivalence of the same effect.
There is a small resistance to your hammer swing in the form of air resistance. This is the characteristic impedance. However, once the diode slams shut, this is a sharp discontinuity in impedance, one that results in a huge increase of impedance. This is your hammer hitting a hard surface. Some of the energy of your hammer blow is reflected back into the hammer, causing it to bounce and vibrate ring in your hand.
It dissipates quickly though, usually in the form of heat - the hammer head will begin to heat up after blow after blow. This is because some of the energy of each swing is being reflected back into the hammer, and this occurs because of a change in mechanical impedance - from moving through air to suddenly encountering a hard barrier, or even just splashing into water.
With that in mind, the snubber is simply a way to dissipate some of that reflected energy as heat - just like with the hammer. The hammer is already well-snubbed by the steel it is made out of, but our circuit is not a hammer, it is more like a chime. It rings for a long time and loudly after being struck, so our snubber is like placing your hand on it to end the vibration quickly.
You might have guessed what we are trying to do here already: provide a resistive path that matches the characteristic impedance of the circuit in parallel with the switch. This is simply equal to the impedance due to the parasitic capacitance and inductance the same thing that also causes the LC tank and the ringing. This is the important part. However, if we just put that resistor in parallel with our switching element There is an entire alternative path now and a diode is made irrelevant in this way.
So we add a capacitor in series with the resistor to block DC current from flowing, allowing our switch to actually do something useful.
Now, instead of energy getting reflected back towards the source and into the parasitic tank formed by the capacitance across the diode and the inductance of the transformer secondary and any other parasitics at play , it can continue on smoothly through the same impedance it had been in the form of our snubbing resistor, R, and into our snubber capacitor.
The capacitor at a minimum needs to be equal to the parasitic capacitance so it can actually absorb this energy without causing a reflection. The only component that is actually snubbing - or dissipating - this energy is the resistor, R. The imaginary component of complex impedance - reactance - is impedance caused by storing of energy, vs. We want to dissipate, not store this energy, and our snubber gives the energy a dissipative path it can go through, reflection free mostly , when our diode or whatever slams shut like brick wall.
Increasing the resistor value will not allow all of it to flow into our snubber capacitor and get reflected back, and less will not dissipate as much of it as we could be, so this really is the one value here that has an optimum value that we need to pick carefully.
Ok, it does, but not in the way you probably think. Remember, the capacitor stores energy, it is doing nothing to help dissipate this leftover energy from the diode turning off. It does have a more subtle effect however. Again looking at the resonant frequency equation, we can see that quadrupling the capacitance will reduce the ringing frequency by half. In other words, if we pick a C that is 3 times that of our calculated parasitic capacitances, it will cut the ringing frequency by a factor of 2.
This means that it will take twice as long for the reflected energy to flow through our dissipating snubber resistor, and that much more energy and more power, being energy over time will be dissipated in the resistor.
No Answer, No Cry Why not just make the capacitor huge? An ugly one. The on time for a bridge rectifier, assuming 50Hz mains, this would be half the period of 50Hz, or 10ms. The trade off is up to you, the articles you linked give you everything you need, especially the first one, which goes through the practical considerations of the trade off, such as power dissipated in the resistor and switches.
But minimally, you need the capacitor to be equal or greater than the parasitic capacitance, as this is the minimum needed to store all the energy that will be stored in the inductance. And there are even caveats with the resistance: while there is an optimum value for snubbing out that reflected energy as fast as possible, it is not the optimum value if you care more about, say, the peak voltage that results.
Your ringing will be worse but lower in amplitude. There is a useful range of capacitances, but the exact value is a trade off with other considerations that are up to you to figure out or not, and merely use the rule of thumb - which is what I would suggest.
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