We'll conclude our discussion of power diodes with
several additional important topics.
The first one is the trade off between the blocking voltage or, or
breakdown voltage of the diode versus the forward
voltage drop versus the switching speed. And this is a trade off that is inherent in any of our powered
semiconductor devices. but it's interesting to see how it works
in the, the p-n junction type diode. In any semiconductor device the breakdown voltage is related to the doping
concentration of the silicon mat, material.
Basically silicon with heavy doping has a low on resistance.
But it also breaks down at a lower electric field
strength than if there was light doping. This is a fundamental characteristic of avalanche breakdown in semiconductor
materials. And so in a diode, we have to include a low
doping concentration, high resistivity region in order to
achieve a specified voltage break down. Here's a diagram that's then, a more
accurate description of the diode which, where we have the p region on the left, the n region on the right and a lightly doped region in the
center. Here, it's denoted in n-minus, which means
low concentration of doping. Often, this is also called i for
intrinsic. And this is called a p-i-n diode. so we have this low doping concentration
in the middle of the diode. And under reverse bias constant conditions
the depletion region extends far out into this n minus
region. with electric fields that the, that are
high in strength that the light or low doping concentration
material can withstand. And we get our voltage breakdown from
that. Now the worry is that by having such a
large region that is high resistivity, this will increase the on resistance or
the forward voltage drop of the device. In a, a minority carrier device though,
such as the pn diode that doesn't happen. And instead what happens to the phenomena
known as conductivity modulation. Basically, the holes from the p region can
diffuse into the lightly doped region where they become minority carriers
and they actually reduce the effective resistivity of the material. They provide charges that are able to
conduct current, even though the, the doping
concentration is low. And in fact, electrons from the end region
can diffuse into this n minus region as well, as well, and
so we get, [COUGH] a, a large concentration of minority
carriers in the lightly doped region that gives us a low
on resistance. So, the good news is, we can get a high voltage breakdown with a low on-resistance
at the same time. But the bad news is, we have a lot more stored minority charge in the
n-minus region. And therefore the switching time is longer
because it takes longer to remove all of that charge to turn the
device off. So then here are what wave farms we
actually observe in a real para diode. So we start out with a diode in the off state, blocking negative voltage and
conducting no current. With this large depletion region that is
blocking the voltage. We then try to turn on the, the diode so
the, whatever circuit is connected to the diode will apply positive current to
the diode, and raise the voltage across the
diode. Initially this positive current charges
the capacitance of the depletion region and raises the voltage at
some rate. Eventually the voltage on the diode is
positive and the diode turns on and starts to conduct current.
We may or may not observe some voltage overshoot in, as shown here. What's happening here is that if we turn
the, the diode on quickly, it's possible that the we, we initially observe the
voltage of the. lightly doped region without conductivity
modulation. So we have a high on resistance then and
we have a large forward drop that might be 10 or
20 volts. But then after a short period, we get
minority charge and conductivity modulation that brings the
forward drop back down to something like. 7 10ths or 1 volt, and then the diode after that conducts with the low on
resistance. When it's time to turn off the diode the external circuit
attempts to reverse polarity or reverse the current
through the diode. And as we discussed in the previous
several lectures. We get a reverse recovery transient with
current flowing backwards through the diode to extract the
minority charge. And the only difference here now is that
we have a lot more minority charge from the
conductivity modulation of the, the lightly doped
region. So we observe the reverse recovery, which
involves removing the recovered charge. And also while the voltage is changing,
we're also charging the capacitance of the
depletion region. So [UNKNOWN] as we've already discussed then this re,
rev, reverse recovery can induce substantial
paralize in the mosfet and during the last part of, of the
reverse recovery in the diode as well. but at a minimum we see this large reverse recovery current causing a current spike
in the mosfet. While the mosfet is held on with high,
with voltage across it. and so we get large instantaneous power
loss induced in the mosfet by the diode reverse
recovery. Okay, here is the little terminology and
it's traditional in the power semi-conductor field to give names to
diodes according to their application. A standard recovery diode typically is one
that is intended for low volt, or low frequency applications such
as 60 cycle or 50 cycle rectifiers.
Often these, for these diodes the reverse recovery time is not
specified. But they wont work in a, 100 kilohertz
switching converter because they're reverse recovery
times are too long. Fast recovery or, ultra-fast recovery, or
more recently, other adjatives like super or hyper fast.
refer to diodes where the reverse recovery time is
specified and they are intended for switching converter
applications. So these will have reverse recovery times
for fast recovery diodes that might be between 100
nanoseconds and a microsecond. Or ultrafast might be a hundred
nanoseconds or below. And nowadays we can get, 600 volt pn junction rectifiers with
recovery times as low as 25 or 35 nanoseconds. a Schottky diode we have to mention also. Schot, a Schottky diode has a metal semi-conductor junction instead of a pn
junction. It is inherently a majority carrier
device. Does, does not work on minority carriers
as we've been discussing so far. as a result the Schottky diode for silicon
rectifiers doesn't have conductivity modulation, nor does it have
long reverse recovery times. So that's good news and bad news also. Schottky diodes are limited, for silicon
devices, to low voltage applications. Generally a hundred volts and below, and
they're very good at say, 45 volts and below. they switch very fast, although, they do
have significant junction capacitance. that, that can affect the circuit and
cause switching loss. Recently, we have Schottky diodes built
with wide band gap semi-conductors such as silicon carbide or
even more recently gallium nitride. these wide band gap devices can attain
high high breakdown voltages of 600 or 1200 volts.
in a shocky diode and still have a majority carrier device.
They have larger forward drops. Or maybe one and a half or two volts instead of something a little below
one volt. However they're very fast switching, and very often in, in a power converter
you will find that if you add a, or if you replace your pn diode with a, a
schottky barrier silicon carbide diode. Even though the conduction loss is higher
the switching loss is much lower, and overall the
efficiency is reduced. so these have found there, there are commercial devices that
have found some good industry acceptance. The major problem today with them is that
they, is the high cost. A silicon carbide 600 volt diode might
cost five or 10 times as much. As a comparable silicon diode, but there
definitely is a measurable reduction in, in switching
loss. Okay, let's discuss paralleling diodes.
With this minority carrier, or pn junction diode classically we.
We're not supposed to parallel diodes.
So for example, suppose we needed a10 amp diode, but all we were able to buy, or all we had
in the lab were 5 amp diodes. So you might say, well I'll, I'll take 2,
5 amp diodes and put them in parallel. And in the process we're hoping that the
10 amps will divide evenly between the two diodes so we get 5
amps through each diode. Generally this doesn't work. And the reason for this is the temperature dependence of the equilibrium ID
characteristic of the diode. Basically, if when a diode gets hotter,
its voltage goes down for the same current or conversely
for the same voltage, the current would go up, and this in a minority carrier, say, pn junction
diode. So here's what happens, you, you put the
two diodes in parallel, we apply 10 amps. The, there's some small difference between
the diodes which makes one of the diodes take a little more current than the other
and it gets a little hotter. And so say that this diode one is hotter, runs a little hotter than
diode 2, that makes it take more current. So instead of 5 amps it might take 6 amps. And diode two then will take 4 amps.
Okay? That makes the first diode get hotter yet,
the second diode get cooler. And even more, the current goes through the first diode and less through the
second. And so we have thermo instability where
one diode hogs all the current. And what will happen is, that diode will
hog the current. It will overheat and it will in fact
exceed it's rating. And so it will fail and you know once it
fails, if fails as on open circuit and that makes all 10 amps go
through the other diode and it fails also. So, in general with minority carrier
devices we aren't able to parallel. And with majority carrier devices it the
forward drop goes up as the temperature goes up and it may be
possible to parallel. So if you want to get minority carrier
diodes to, to share current. We have to do what I would consider heroic
measures. So we can sort devices and select match
devices. We can package them on a common thermal substrate so they run at the same
temperature. and in fact if you want to build a high current module of diodes that's
exactly what's done. Or we could try to build some ex, external circuitry that forces the
currents to balance. Usually that circuitry takes up a lot of
work to make work and generally what we would
rather do is find a, a large enough diode and
just use one. One last short topic.
We talked about. How diode reverse recovery induces
switching loss in the mosfet in a, say a buck converter. Sometimes we have circuits where the
diodes is not directly connected to a transistor. And here's an example of one, this often happens in transformer isolated
converters. Or in some other converter circuits that
have effectively an inductor that provides current to the
diode. So in this, this example I'm going to
model the rest of the converter circuit with some switched
voltage source that's shown here. It's initially a positive voltage and then
at some time it switches to a negative
voltage. And what, what's shown here is an
inductor. And then a diode with it's junction capacitance, here as seen, it's shown
explicitly, this is the capacitance of the depletion
region. so initially we have positive voltage applied, that positive voltage makes
positive current flow to the inductor, and that
current flows through the dial. Turning the diode on, okay? So, the diode is on, and basically this
positive input voltage V1 appears across the inductor, and the
inductor current increases with the slope given by the voltage divided by the inductants, and rises to some positive
current. And at this time where we switch the
input, this voltage negative. at this point. So we had negative voltage applied across
the inductor. The inductor current is still positive
initially so, this positive current continues to flow through the
diode, which keeps it on. And so the input voltage appears across
the inductor. And the inductor current. Then ramps it down from a negative applied
voltage, like this. Eventually it reaches zero, and what
happens then? Well, just because the inductor current is
zero doesn't mean the diode turns off. We have to first remove its stored charge
and that takes some time. So during the reverse recovery time of the
diode, the diode stays on. It maintains its same forward voltage
drop. And so we have the same negative voltage
across the inductor, which continues to charge
the inductor current negative. Okay? So we get some negative current in the inductor.
And then at some point right here. Finally the diode turns off because it's,
it's stored charge has been removed by the negative current and at
that point the diode switches off. And what happens to the inducter current? We built up some negative current in the inductor, we're storing energy in the
inductor. One half LI squared, and the inductor current has to
go somewhere. Well, the only place it can go is through
the capacitance now. So the junction capacitance of the
reverse-biased diode will, will form a resonance circuit in
connection with the inductor. That resonance circuit has some initial
condition on current, and so we get the inductor and capacitor forming a resonance
circuit that rings like this. And we get diode voltage that rings as well. Well, eventually that ringing decays to
zero. By parasitic loses in the circuit. So we have resistance of the inductor winding, we have coral loss and other
losses around the circuit and something damps out
the ringing and makes it the K to 0. Whatever that is, whatever damps it,
dissipates the power and so it will get hot. And this is in fact a form of switching
loss. The diode induces energy storage in the
inductor which is then lost after the diode turns
off. Okay. However much energy loss that is, I'm
going to call it W off right now, is the energy lost
during the diode turn off train, or stored in the energy during the diode turn off
transition and then lost. That energy multiplied by the switching frequency is another form of switching
loss. And so in our power converters, when we
see ringing like this that decays, that ringing is an
additional component of switching loss. In this particular case we can actually
calculate how much energy there is. The recovered charge which is the area
under the curve is defined as the integral of the current
during this, this time. As usual. So then the recovered charge is something
that is a property of the diode. You can also calculate the energy stored
in the inductor during this interval. And basically if we integrate the voltage on the inductor, it's negative
voltage, times the inductor current. That's the integral of the power which is
the total energy stored in the inductor from
here to here. Okay? Well, the voltage is this voltage minus
V2. And the current is this current wave form. But we can, we can plug V2 into here, and since V2 is a constant,
the, the integral of the power is actually just
the constant V2 times the integral of the current, and
the integral of the current is the recovered charge at
the diode. So what we get is that the, the, during
the diode reverse recovery time, energy is stored in the inductor
equal to the applied voltage V2, times the diode stored charge or recovered
charge. Okay? That energy becomes stored in the
inductor, so it's equal to one half LI squared at this point right here at the
peak of the current. And that energy rings between the inductor
and the capacitor during these oscillations and eventually gets
dissipated by whatever damps the ringing. So this is the amount of energy we loose
every-time this diode switches off. We multiply it by the switching frequency
to get the switching loss. [SOUND]. And so the diode can induce switching
loss, and things the size of mosfet. this recov, reverse recovery will induce
ringing that, that is another form of switching
loss. [BLANK_AUDIO].