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36C3 preroll music
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Herald: Okay, that was fast two minutes.
The next talk is going to be on DC to DC
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converters by Zoé Bőle. She's an
enthusiast for open hardware and fan of
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DIY and has been working on the topic of
DC to DC converters for a long time and I
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have to keep on talking now because it
seems that her computer is not really
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communicating with the presentation
device. We do have a picture but we don't
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get it moving.
troubleshooting whispers in background
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Herald: While they're still having some
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issues up here I might remind you that it
is very helpful if you take your trash
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with you and now please welcome Zoë and we
are ready to inaudible.
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Give her a warm hand!
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Zoé Bőle: Hi, my name is Zoë and this is
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the first time I'm standing here in a
chaos stage so I'm a little bit, like,
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anxious but I'm here to talk to you…
applause
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Zoé: I'm here to talk about these DC
converters and the talk's called "DC/DC
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converters and everything you wanted to
know about them" but it's unlikely I can
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fit everything into a 50 minute talk, so
it's like not everything, but my goal is
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to provide you some starting points and
give you an overview and hopefully if you
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already worked with DC/DC's then you're
also not gonna be annoyed and not gonna be
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bored. Before I start with the DC/DC
topic, I would like ask you to be
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excellent to each other and this is not
related to my talk but I hear people
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starting clapping when someone broke the
bottle accidentally and I think it's super
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not cool. Yesterday I saw someone breaking
down in tears because they just broke a
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bottle and everybody was clapping and
paying attention to them
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and that was like harassment. So, please
don't be the one who starts clapping. But
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also I'm not here to forbid you to clap
and… just know what's happening. So,
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brief introduction to DC/DC's and why:
quite often you need different voltages
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than what you have available. For example
you have a microcontroller or you have an
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FPGA and you work with the battery
then you need to provide a different
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voltage for that circuit and the trivial
solution is to just use two resistors and
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make a voltage divider, but this is
totally unsuited for power delivery
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because as you start loading the output,
the output voltage starts dropping. Also,
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this circuit dissipates power even if
there is no useful load on the output. So
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this is only useful for signals and to
have some kind of feedback and regulate
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the output to a desired level you can use
an LDO, which is the same thing but you
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control, one resistor – very simplified –
to always keep a desired output voltage.
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Of course this can only go lower than your
input and your efficiency is limited to
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the ratio of the output voltage and the
input voltage, and this is even in ideal
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situation. So, instead of burning up power
in your converter you can just use
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switches and this is the idea behind
switching supplies that you use a switch
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element which is either
fully on or fully off.
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And if it's fully on then there
is no loss on a switch and if it's
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fully off there is no current flowing
through it so there is no loss either.
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There are some practical problems
with this approach but
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but this works for LEDs and heaters if your
switching frequency is high enough.
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To think of a DC/DC converter is a
box with four terminals. It has an input
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side and an output side. Right now I'm
talking about buck step-down DC/DC
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converters which are non isolated. This
means the ground in the input side and the
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ground on the output side are connected
together inside and this limits certain uses.
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Also you should not connect these
DC/DC converters in series, so if you have
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a block like this and you think "oh, I
could use two or three of them and just
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connect them in series to give it higher
input voltages," that's gonna blow up very
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quickly. A block looks like this on a
screen and might look like this in reality.
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Let's take a look inside. So all of these
DC/DC converters consist of a power stage,
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a control system, and the feedback.
The feedback is there to provide a
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regulated output regardless of the
operating conditions. So what's inside a
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power stage? To have a deeper look inside
we can consider this asynchronous buck
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converter where a switching element – a
MOSFET – is controlled by an analog and
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digital circuitry. Feedback is provided
from the output voltage and we see a diode
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in the middle which I'm going to talk
about soon. You also see two capacitors on
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the input side and on the output side,
which are also very important.
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More about them later.
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Let's consider the first situation:
the switch is on – this is
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so-called "the on state" – and this forms
a loop from the input to the output. The
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input capacitor we can neglect and in an
ideal situation the output capacitor is,
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um, I will talk more about more about the
output later.
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pause
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All right, I don't wanna make this into a
lecture and everybody is sleeping in
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and the fun part will start very soon.
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So, this DC/DC has two states: either the
switch is on or switch is off. Right now
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the switch is on and you see that the
current can flow from the input through
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this inductor to the output. The inductor
resists the change of current. It's like
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pushing a heavy mass and once it starts
moving it wants to keep moving. That's why
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in this "on" state the input current flows
through the inductor and starts to
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increase while it's also flowing to the
output. Then the converter turns the
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switch off, which comes to the off state,
and now the diode comes into play, which
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will keep the current recirculating. In
this "off" state there is no current from
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the power from the source to the output,
but the output is still powered from this
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decaying magnetic field through the
inductor. Sometimes you hear about
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synchronous DC/DC converters where this
diode is replaced by another switch.
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In that case efficiency's increased since
the voltage drop across MOSFET is lower
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than the forward voltage of the diode. In
this case, as you can see, current is still
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being delivered to the output. And this is
the big advantage of the buck converter
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that in both an "on" and an "off" state
the output is sourced with current.
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What the output capacitor
does there is it provides the difference
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between the inductor current. On the lower
end you can see the inductor current.
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As the switch is on it ramps up and as switch
is off it ramps down, and in the middle
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you see this line which is the output
current. So you see these triangles and
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this is what's provided by the output
capacitor. Alright, so this is an actual
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part without the simplifications and I
would like to talk a bit about the
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reference voltage and how that works. So
this device creates an internal 0.7V
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reference and you can program the output
voltage by choosing R1 and R2 on the
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left side so at your desired output
exactly 0.7V will be at this voltage
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divider and this converter will keep
regulating to reach the state.
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If you're looking for DC/DC converter to
your next project then you might see
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a bunch of parameters and
I'm gonna talk about those.
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So first you see a 3.3 volt 2 amp
converter. What does it mean?
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This depends on how and
who specifies that output
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because someone says it's two amps if it
can provide two amps for a second and
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someone says it's two amps if it can
continuously provide the two amps even in
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a warm environment, so it's important to
talk about if it's a peak or continuous
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current rating. Then there is this so
called "output ripple." You saw that
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switching action going on and off and that
will create a ripple on the output voltage
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so it won't be 3.3V it will be oscillating
around that. This can be as low as a few
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microvolts and as high as a few volts,
depending on the parameters. Also there is
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a voltage accuracy: maybe it's labeled
as 3.3V but actually it's 3.5 or 3.0.
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Load regulation: it's maybe 3.3V when it's
unloaded and as you increase the output it
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starts changing the output voltage. There
is the line regulation which means the
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input voltage has influence over the
output, which is undesired. Then there is
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this maximum input voltage rating. Let's
say this converter can tolerate seven
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volts on its input so you think "oh let's
just hook it up to USB, that's 5V, right?"
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Yes, but no, because when you use cables
and non-ideal conditions, you can create
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transients which overshoot the voltage
possibly way above this maximum rating and
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this can lead to very nasty surprises.
Because sometimes they fail short, which
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means they connect their input
directly to their output.
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In this case the device you connected to
the converter might also go up in flames.
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So mind the transients and always
have some margin between
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your desired input voltage and the
maximum the converter can tolerate.
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Then you might see 95% efficiency
and that's also question at
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which load because at maximum specified
load it will be lower, and at lower/less load it
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will be also lower, so there is this
efficiency peak. That marketing people love
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to specify. There's also this so called
"quiescent current" which means your
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converter draws current from your input
even when there is nothing on its output
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and if it runs from a battery this can
drain your battery in days or weeks, so
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you must pay attention to this. And there is
this other factor called "switching
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frequency" so how fast, how often the
internal switch changes state, but this
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might not be a constant value, especially
with the previously mentioned quiescent
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current feature, the converters that excel
at having a low quiescent current don't
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have fixed switching frequency, so you
might have noise at different frequency
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bands and disturb your circuits or radio
noise. Let's talk about a few features,
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you might want to look for. "Enable": enable
functionality. This is very useful to
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easily disable your DC/DC converter and
without having to interrupt either the
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input side or the output side. Let's say
you have a 20 amp output converter – you
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really don't want to switch the 20 amp
with a mechanical big switch. Instead
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of that you have a logic input to
your DC/DC converter with which you can
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turn this completely off. Then there is so
called "undervoltage lockout": you might
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want to prevent it from running below a
certain input voltage to prevent draining
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your battery too deep and turning it
completely off. There's "power good" that
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can provide information to your processor
that the output voltage is in regulation
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and stabilized. So if you hook up the
"power good" output to let's say a reset
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line or "enable", then you can be sure
that the output voltage is always stable
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and your processors are not going to go
into glitch. Overtemperature shutdown is
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very common these days and that makes
these tiny converters almost
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indestructible because if they get too hot
they just turn off completely before they
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get permanently damaged. Efficient
standby: this is the so called low
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quiescent current option. That means if
your output is off, your processor is
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sleeping, then it willl reduce switching
action to reduce switching losses and
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might only draw a few micro amps or even
nano amps. Very important for battery
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powered applications. Then you might see
overcurrent protection which makes the
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output very robust. You can even make a
short circuit and the overcurrent protection
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will limit the output current to this
value and this prevents damaging of the
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converter and also damage to the cables
and switches if they are rated to
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withstand the overcurrent protection
limit. Now let's talk about noise. The
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output ripple is not exactly noise. Output
ripple is there because the output
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capacitor is non-ideal and usually this
this is very low on a properly designed
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converter but if you measure the output
you might see spikes on the output and
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that's not ripple. That's conducted EMI
because on that inductor the windings are
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coupled very closely, there is some
capacitive coupling between the wires, so
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the digital on-off action from the
switches will propagate to some extent to
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the output. This is attenuated by the
capacitors but they cannot be completely
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filtered off and you will see the
switching frequency and even upper
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harmonics of it but this can also be
filtered. There is also radiated EMI,
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which comes mostly from the switching node
and capacitive coupling to the ground
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plane, and also the inductor – if it's not
shielded then a magnetic field can also
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radiate out and cause interference. On
this picture what you see is that gray
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block, that's a shielded inductor, and the
two blue connectors at the end of this PCB
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are screw terminals. I personally advise
against using this style of screw
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terminals because the wires can easily
slip out, make a short, or you don't
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notice that they are not connected, so I
prefer a different style of connectors.
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It's good to know about non-ideal
components. The capacitors that are used
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have a so called DC bias. These multi-
layer ceramic capacitors are very
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sensitive to the DC voltage across the
terminals and if they are rated, let's say
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20 microfarads, at the rated voltage they
might lose up to 90 percent of their
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capacity. So you always have to pick a
capacitor that's rated to a higher voltage
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than what your output is to compensate for
this effect, and you also need to put more
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capacitors at your output than what you
would think in an ideal situation. Mind
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the transients! As I said, if you plan to
hotplug, connect to live wires to your
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converter, you have to keep in mind the
inrush current. Those capacitors, when
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they are fully discharged and you connect
that to the input, then they will try to
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charge to the input voltage as fast
as the cabling lets that happen, and the
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cables have inductance which will store
energy and overshoot the input voltage.
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When fiddling with MOSFETs, don't forget
the ESD protection. MOSFETs are very
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sensitive at their gate, because the oxide
layer is so thin that even 20V voltage is
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enough to break it down, and a 20V ESD
strike is something you probably don't
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even notice, but it can damage the
MOSFETs. And ever avoid the 7800 series
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LDO, because it's a very old part and I
still see it in new designs, while there
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are much better ones with better
regulation, less quiescent current, and
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it's also an LDO, so it's like just
marginally related to DC/DCs. If
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you make your own DC/DC converters instead
of buying one, you should read the
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datasheet and follow the instructions
because the manufacturers give you a
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proven tested layout, which is typically
good advice to follow and you should only
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deviate from that if you know what you're
doing. I'm sorry
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pause
Alright, that mostly concludes what I was trying
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to talk about and now it's time for your
questions.
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applause
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Herald: Now there are two microphones one
there and one over there, usually they
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are… ah, here comes the light. Are there
any questions? How about the signal angel,
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does the internet have any questions? The
internet doesn't have a question but
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here's one up front.
Q: What would you recommend instead of
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screw terminals?
Zoé: That's a very good question and that
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really depends on the application. You can
have different kind of screw terminals,
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which use either crimped therminals on the
cable so you have a cable shoe.
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Q: Like a ring or something?
Zoé: Yes. Because then there is no way that it can
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slip out. For less current you can use
dupont connectors, they can take like
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2 to 3A per contact. You know the standard
pin header and that kind of thing. And
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there are also latching connectors from
molex and other manufacturers. The problem
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is with that you need crimping tools and
those can be very expensive. So it first
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makes sense to get those when you have a
hackerspace or you can share it with other
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people.
Herald: The next question, please?
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Q: Thank you for your talk. On your last
slide last point you mentioned stability
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analysis. What is your experience with
running such converters in parallel for
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redundancy and how would you do the
analysis there?
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Zoé: Running current mode converters
parallel is typically okay, but they won't
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do current sharing automatically. So this
one converter has a certain output voltage
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set and the other one has little bit
different voltage, and that will create a
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difference in their output currents, and
there are topologies and there are
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converters which are prepared for parallel
operation and they can provide current
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information to all of the parallel
converters, and they can ultimately
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synchronize. For stability, that should
not influence the stability of it. What I
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should have mentioned is stability
analysis because we have a control loop.
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The control loop takes the output and
creates a control signal that influences
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the output, but this loop has a delay, and
because of this delay, basically you can
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make an oscillator of this, and to avoid
that, you can use a network analyzer and
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inject a signal into the converter.
Q: Thank you.
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Herald: Yeah, you go ahead, over there.
Q: Hi, what would you say the choice is
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between a dis-synchronous mode or
a forced-synchronous mode?
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Zoé: That's a very good question. All
right so when I talked about this briefly
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and mentioned the synchronous converters,
with forced synchronous converters you
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have a control switch and those have
typically fixed switch frequency. If the
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output current is zero, then during half
of the period current will flow backward
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from the output capacitor to the input
side and then the next half period that
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current will flow back from the input to
the output, so basically energy swings
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between input and output and this causes
efficiency loss, but this also avoids
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operation in discontinuous mode, which
reduces ripple and reduces EMI. So it
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depends on your application.
Q: Thanks.
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Zoé: You're welcome.
Herald: The next question?
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Q: Hi, Zoé, thank you for the talk! I have
a question about: you mentioned linear
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regulators at the end, what are they used
for in this context?
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Zoé: you mean 7800 series?
Q: Yes.
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Q: Not, the one before, I think.
Zoé: Those were very good regulators in
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00:33:13,910 --> 00:33:22,130
the 70s and those are linear regulators,
and the problem with the 7800 series is
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everybody knows about them because books
are full of them but they have quite a few
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milli amps of quiescent current. They also
have bad regulation against load and line
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transients and they are not cheaper than
much of their alternatives, so there's
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really no reason to use those. You can use
for example a DC/DC pre-regulator and then
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an LDO afterward to smooth out the
voltage.
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Q: Okay, thank you.
Herald: Go ahead
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Q: Thank you very much! My question is,
you mentioned the noise coupled via the
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inductor to the output. Which sort of
filter do you recommend: differential
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00:34:18,440 --> 00:34:24,060
noise or common mode noise, and input or
output? Which is most important from your
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perspective?
Zoé: So lots of the noise goes actually
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back to the input supply and I said that
in an ideal circuit the input capacitor is
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not necessary, but in a real circuit the
input capacitor is critical because the
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00:34:46,970 --> 00:35:03,520
input inductance is seen by by the switch.
If you let me show you. On this chart you
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00:35:03,520 --> 00:35:11,660
see the inductor current and the input
current, it follows the inductor current
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only during the on phase which means after
the end of the on phase and beginning of
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00:35:17,720 --> 00:35:25,520
the off phase it falls from maximum value
to zero and later on at the end of the off
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phase and the beginning of the on phase
the current jumps from zero to the output
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current, and these jumps in the supply
current create an awful lot of EMI if the
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input capacitor is not large enough so
this is a very critical thing. I saw quite
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a few converters where the input capacitor
is under dimensioned and when you run it
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over longer wires with more parasitic
inductance, that can create a lot of EMI.
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For ways of reducing the the noise on the
output, the best way is to have proper
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filtering capacitors. If you use ceramic
capacitors and enough high enough value
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you can get rid of almost all of the
noise. I made a design which had microvolt
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noise because I found a capacitor with its
resonance frequency exactly at the
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switching frequency, so basically all that
noise that was coming from switching
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action was reflected away and higher
frequency ranges where it got filtered
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dissipated much faster. You can
use PI filters at the output but mind
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that you worsen the transient behavior of
your converters. So if your load suddenly
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needs a lot more power and starts drawing
more current, then your converter will
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react slower because of the
filter you just added. PI filters or RC
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filters if you don't need that much
current.
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Q: Okay, thanks.
Herald: Okay great I don't see any more
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00:37:44,620 --> 00:37:52,540
questions, so everything seems to be fully
explained. Thank you and give her a
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applause and good night.
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applause
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Zoé: Thank you
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postroll music
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