*36C3 preroll music*
Herald: Okay, that was fast two minutes. The next talk is going to be on DC to DC
converters by Zoé Bőle. She's an enthusiast for open hardware and fan of
DIY and has been working on the topic of DC to DC converters for a long time and I
have to keep on talking now because it seems that her computer is not really
communicating with the presentation device. We do have a picture but we don't
get it moving. *troubleshooting whispers in background*
Herald: While they're still having some
issues up here I might remind you that it is very helpful if you take your trash
with you and now please welcome Zoë and we are ready to *inaudible*.
Give her a warm hand!
Zoé Bőle: Hi, my name is Zoë and this is
the first time I'm standing here in a chaos stage so I'm a little bit, like,
anxious but I'm here to talk to you… *applause*
Zoé: I'm here to talk about these DC converters and the talk's called "DC/DC
converters and everything you wanted to know about them" but it's unlikely I can
fit everything into a 50 minute talk, so it's like not everything, but my goal is
to provide you some starting points and give you an overview and hopefully if you
already worked with DC/DC's then you're also not gonna be annoyed and not gonna be
bored. Before I start with the DC/DC topic, I would like ask you to be
excellent to each other and this is not related to my talk but I hear people
starting clapping when someone broke the bottle accidentally and I think it's super
not cool. Yesterday I saw someone breaking down in tears because they just broke a
bottle and everybody was clapping and paying attention to them
and that was like harassment. So, please don't be the one who starts clapping. But
also I'm not here to forbid you to clap and… just know what's happening. So,
brief introduction to DC/DC's and why: quite often you need different voltages
than what you have available. For example you have a microcontroller or you have an
FPGA and you work with the battery then you need to provide a different
voltage for that circuit and the trivial solution is to just use two resistors and
make a voltage divider, but this is totally unsuited for power delivery
because as you start loading the output, the output voltage starts dropping. Also,
this circuit dissipates power even if there is no useful load on the output. So
this is only useful for signals and to have some kind of feedback and regulate
the output to a desired level you can use an LDO, which is the same thing but you
control, one resistor – very simplified – to always keep a desired output voltage.
Of course this can only go lower than your input and your efficiency is limited to
the ratio of the output voltage and the input voltage, and this is even in ideal
situation. So, instead of burning up power in your converter you can just use
switches and this is the idea behind switching supplies that you use a switch
element which is either fully on or fully off.
And if it's fully on then there is no loss on a switch and if it's
fully off there is no current flowing through it so there is no loss either.
There are some practical problems with this approach but
but this works for LEDs and heaters if your switching frequency is high enough.
To think of a DC/DC converter is a box with four terminals. It has an input
side and an output side. Right now I'm talking about buck step-down DC/DC
converters which are non isolated. This means the ground in the input side and the
ground on the output side are connected together inside and this limits certain uses.
Also you should not connect these DC/DC converters in series, so if you have
a block like this and you think "oh, I could use two or three of them and just
connect them in series to give it higher input voltages," that's gonna blow up very
quickly. A block looks like this on a screen and might look like this in reality.
Let's take a look inside. So all of these DC/DC converters consist of a power stage,
a control system, and the feedback. The feedback is there to provide a
regulated output regardless of the operating conditions. So what's inside a
power stage? To have a deeper look inside we can consider this asynchronous buck
converter where a switching element – a MOSFET – is controlled by an analog and
digital circuitry. Feedback is provided from the output voltage and we see a diode
in the middle which I'm going to talk about soon. You also see two capacitors on
the input side and on the output side, which are also very important.
More about them later.
Let's consider the first situation: the switch is on – this is
so-called "the on state" – and this forms a loop from the input to the output. The
input capacitor we can neglect and in an ideal situation the output capacitor is,
um, I will talk more about more about the output later.
*pause*
All right, I don't wanna make this into a lecture and everybody is sleeping in
and the fun part will start very soon.
So, this DC/DC has two states: either the switch is on or switch is off. Right now
the switch is on and you see that the current can flow from the input through
this inductor to the output. The inductor resists the change of current. It's like
pushing a heavy mass and once it starts moving it wants to keep moving. That's why
in this "on" state the input current flows through the inductor and starts to
increase while it's also flowing to the output. Then the converter turns the
switch off, which comes to the off state, and now the diode comes into play, which
will keep the current recirculating. In this "off" state there is no current from
the power from the source to the output, but the output is still powered from this
decaying magnetic field through the inductor. Sometimes you hear about
synchronous DC/DC converters where this diode is replaced by another switch.
In that case efficiency's increased since the voltage drop across MOSFET is lower
than the forward voltage of the diode. In this case, as you can see, current is still
being delivered to the output. And this is the big advantage of the buck converter
that in both an "on" and an "off" state the output is sourced with current.
What the output capacitor does there is it provides the difference
between the inductor current. On the lower end you can see the inductor current.
As the switch is on it ramps up and as switch is off it ramps down, and in the middle
you see this line which is the output current. So you see these triangles and
this is what's provided by the output capacitor. Alright, so this is an actual
part without the simplifications and I would like to talk a bit about the
reference voltage and how that works. So this device creates an internal 0.7V
reference and you can program the output voltage by choosing R1 and R2 on the
left side so at your desired output exactly 0.7V will be at this voltage
divider and this converter will keep regulating to reach the state.
If you're looking for DC/DC converter to your next project then you might see
a bunch of parameters and I'm gonna talk about those.
So first you see a 3.3 volt 2 amp converter. What does it mean?
This depends on how and who specifies that output
because someone says it's two amps if it can provide two amps for a second and
someone says it's two amps if it can continuously provide the two amps even in
a warm environment, so it's important to talk about if it's a peak or continuous
current rating. Then there is this so called "output ripple." You saw that
switching action going on and off and that will create a ripple on the output voltage
so it won't be 3.3V it will be oscillating around that. This can be as low as a few
microvolts and as high as a few volts, depending on the parameters. Also there is
a voltage accuracy: maybe it's labeled as 3.3V but actually it's 3.5 or 3.0.
Load regulation: it's maybe 3.3V when it's unloaded and as you increase the output it
starts changing the output voltage. There is the line regulation which means the
input voltage has influence over the output, which is undesired. Then there is
this maximum input voltage rating. Let's say this converter can tolerate seven
volts on its input so you think "oh let's just hook it up to USB, that's 5V, right?"
Yes, but no, because when you use cables and non-ideal conditions, you can create
transients which overshoot the voltage possibly way above this maximum rating and
this can lead to very nasty surprises. Because sometimes they fail short, which
means they connect their input directly to their output.
In this case the device you connected to the converter might also go up in flames.
So mind the transients and always have some margin between
your desired input voltage and the maximum the converter can tolerate.
Then you might see 95% efficiency and that's also question at
which load because at maximum specified load it will be lower, and at lower/less load it
will be also lower, so there is this efficiency peak. That marketing people love
to specify. There's also this so called "quiescent current" which means your
converter draws current from your input even when there is nothing on its output
and if it runs from a battery this can drain your battery in days or weeks, so
you must pay attention to this. And there is this other factor called "switching
frequency" so how fast, how often the internal switch changes state, but this
might not be a constant value, especially with the previously mentioned quiescent
current feature, the converters that excel at having a low quiescent current don't
have fixed switching frequency, so you might have noise at different frequency
bands and disturb your circuits or radio noise. Let's talk about a few features,
you might want to look for. "Enable": enable functionality. This is very useful to
easily disable your DC/DC converter and without having to interrupt either the
input side or the output side. Let's say you have a 20 amp output converter – you
really don't want to switch the 20 amp with a mechanical big switch. Instead
of that you have a logic input to your DC/DC converter with which you can
turn this completely off. Then there is so called "undervoltage lockout": you might
want to prevent it from running below a certain input voltage to prevent draining
your battery too deep and turning it completely off. There's "power good" that
can provide information to your processor that the output voltage is in regulation
and stabilized. So if you hook up the "power good" output to let's say a reset
line or "enable", then you can be sure that the output voltage is always stable
and your processors are not going to go into glitch. Overtemperature shutdown is
very common these days and that makes these tiny converters almost
indestructible because if they get too hot they just turn off completely before they
get permanently damaged. Efficient standby: this is the so called low
quiescent current option. That means if your output is off, your processor is
sleeping, then it willl reduce switching action to reduce switching losses and
might only draw a few micro amps or even nano amps. Very important for battery
powered applications. Then you might see overcurrent protection which makes the
output very robust. You can even make a short circuit and the overcurrent protection
will limit the output current to this value and this prevents damaging of the
converter and also damage to the cables and switches if they are rated to
withstand the overcurrent protection limit. Now let's talk about noise. The
output ripple is not exactly noise. Output ripple is there because the output
capacitor is non-ideal and usually this this is very low on a properly designed
converter but if you measure the output you might see spikes on the output and
that's not ripple. That's conducted EMI because on that inductor the windings are
coupled very closely, there is some capacitive coupling between the wires, so
the digital on-off action from the switches will propagate to some extent to
the output. This is attenuated by the capacitors but they cannot be completely
filtered off and you will see the switching frequency and even upper
harmonics of it but this can also be filtered. There is also radiated EMI,
which comes mostly from the switching node and capacitive coupling to the ground
plane, and also the inductor – if it's not shielded then a magnetic field can also
radiate out and cause interference. On this picture what you see is that gray
block, that's a shielded inductor, and the two blue connectors at the end of this PCB
are screw terminals. I personally advise against using this style of screw
terminals because the wires can easily slip out, make a short, or you don't
notice that they are not connected, so I prefer a different style of connectors.
It's good to know about non-ideal components. The capacitors that are used
have a so called DC bias. These multi- layer ceramic capacitors are very
sensitive to the DC voltage across the terminals and if they are rated, let's say
20 microfarads, at the rated voltage they might lose up to 90 percent of their
capacity. So you always have to pick a capacitor that's rated to a higher voltage
than what your output is to compensate for this effect, and you also need to put more
capacitors at your output than what you would think in an ideal situation. Mind
the transients! As I said, if you plan to hotplug, connect to live wires to your
converter, you have to keep in mind the inrush current. Those capacitors, when
they are fully discharged and you connect that to the input, then they will try to
charge to the input voltage as fast as the cabling lets that happen, and the
cables have inductance which will store energy and overshoot the input voltage.
When fiddling with MOSFETs, don't forget the ESD protection. MOSFETs are very
sensitive at their gate, because the oxide layer is so thin that even 20V voltage is
enough to break it down, and a 20V ESD strike is something you probably don't
even notice, but it can damage the MOSFETs. And ever avoid the 7800 series
LDO, because it's a very old part and I still see it in new designs, while there
are much better ones with better regulation, less quiescent current, and
it's also an LDO, so it's like just marginally related to DC/DCs. If
you make your own DC/DC converters instead of buying one, you should read the
datasheet and follow the instructions because the manufacturers give you a
proven tested layout, which is typically good advice to follow and you should only
deviate from that if you know what you're doing. *I'm sorry*
*pause* Alright, that mostly concludes what I was trying
to talk about and now it's time for your questions.
*applause*
Herald: Now there are two microphones one there and one over there, usually they
are… ah, here comes the light. Are there any questions? How about the signal angel,
does the internet have any questions? The internet doesn't have a question but
here's one up front. Q: What would you recommend instead of
screw terminals? Zoé: That's a very good question and that
really depends on the application. You can have different kind of screw terminals,
which use either crimped therminals on the cable so you have a cable shoe.
Q: Like a ring or something? Zoé: Yes. Because then there is no way that it can
slip out. For less current you can use dupont connectors, they can take like
2 to 3A per contact. You know the standard pin header and that kind of thing. And
there are also latching connectors from molex and other manufacturers. The problem
is with that you need crimping tools and those can be very expensive. So it first
makes sense to get those when you have a hackerspace or you can share it with other
people. Herald: The next question, please?
Q: Thank you for your talk. On your last slide last point you mentioned stability
analysis. What is your experience with running such converters in parallel for
redundancy and how would you do the analysis there?
Zoé: Running current mode converters parallel is typically okay, but they won't
do current sharing automatically. So this one converter has a certain output voltage
set and the other one has little bit different voltage, and that will create a
difference in their output currents, and there are topologies and there are
converters which are prepared for parallel operation and they can provide current
information to all of the parallel converters, and they can ultimately
synchronize. For stability, that should not influence the stability of it. What I
should have mentioned is stability analysis because we have a control loop.
The control loop takes the output and creates a control signal that influences
the output, but this loop has a delay, and because of this delay, basically you can
make an oscillator of this, and to avoid that, you can use a network analyzer and
inject a signal into the converter. Q: Thank you.
Herald: Yeah, you go ahead, over there. Q: Hi, what would you say the choice is
between a dis-synchronous mode or a forced-synchronous mode?
Zoé: That's a very good question. All right so when I talked about this briefly
and mentioned the synchronous converters, with forced synchronous converters you
have a control switch and those have typically fixed switch frequency. If the
output current is zero, then during half of the period current will flow backward
from the output capacitor to the input side and then the next half period that
current will flow back from the input to the output, so basically energy swings
between input and output and this causes efficiency loss, but this also avoids
operation in discontinuous mode, which reduces ripple and reduces EMI. So it
depends on your application. Q: Thanks.
Zoé: You're welcome. Herald: The next question?
Q: Hi, Zoé, thank you for the talk! I have a question about: you mentioned linear
regulators at the end, what are they used for in this context?
Zoé: you mean 7800 series? Q: Yes.
Q: Not, the one before, I think. Zoé: Those were very good regulators in
the 70s and those are linear regulators, and the problem with the 7800 series is
everybody knows about them because books are full of them but they have quite a few
milli amps of quiescent current. They also have bad regulation against load and line
transients and they are not cheaper than much of their alternatives, so there's
really no reason to use those. You can use for example a DC/DC pre-regulator and then
an LDO afterward to smooth out the voltage.
Q: Okay, thank you. Herald: Go ahead
Q: Thank you very much! My question is, you mentioned the noise coupled via the
inductor to the output. Which sort of filter do you recommend: differential
noise or common mode noise, and input or output? Which is most important from your
perspective? Zoé: So lots of the noise goes actually
back to the input supply and I said that in an ideal circuit the input capacitor is
not necessary, but in a real circuit the input capacitor is critical because the
input inductance is seen by by the switch. If you let me show you. On this chart you
see the inductor current and the input current, it follows the inductor current
only during the on phase which means after the end of the on phase and beginning of
the off phase it falls from maximum value to zero and later on at the end of the off
phase and the beginning of the on phase the current jumps from zero to the output
current, and these jumps in the supply current create an awful lot of EMI if the
input capacitor is not large enough so this is a very critical thing. I saw quite
a few converters where the input capacitor is under dimensioned and when you run it
over longer wires with more parasitic inductance, that can create a lot of EMI.
For ways of reducing the the noise on the output, the best way is to have proper
filtering capacitors. If you use ceramic capacitors and enough high enough value
you can get rid of almost all of the noise. I made a design which had microvolt
noise because I found a capacitor with its resonance frequency exactly at the
switching frequency, so basically all that noise that was coming from switching
action was reflected away and higher frequency ranges where it got filtered
dissipated much faster. You can use PI filters at the output but mind
that you worsen the transient behavior of your converters. So if your load suddenly
needs a lot more power and starts drawing more current, then your converter will
react slower because of the filter you just added. PI filters or RC
filters if you don't need that much current.
Q: Okay, thanks. Herald: Okay great I don't see any more
questions, so everything seems to be fully explained. Thank you and give her a
applause and good night.
*applause*
Zoé: Thank you
*postroll music*
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