36C3 ChaosWest: DC/DC Converters: Everything You Wanted To Know About Them

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

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