 Okay, that was fast two minutes the next talk is going to be on DC to DC converters By Zoe Böhle 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 Computer is not really communicating with the presentation device We do have a picture, but we don't get it moving While we'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 Zoe and We are ready to go We're a warm hand hi My name is Zoe and this is the first time I'm standing here on a chaos stage. So I'm a bit Anxious, but I'm here to talk to you. I'm here to talk about DC DC converters and The talks 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 DCs, then you're also not gonna Not gonna be annoyed and not gonna be bored so Before I start with the DC this topic. I would like ask You to be excellent to each other and this is not related to my talk, but I I hear people starting clapping when someone broke some battle accidentally and I think that's super not cool. So yesterday I saw someone breaking down tears because They just broke a battle 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 DCs 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 a battery Then you need to provide a different voltage for that Circuit and the trivial solution is to just use two resistors and 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 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 an 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 the 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 this works for LEDs and heaters if Your switching frequencies high enough To Think of a DC-DC DC DC DC converter is a box with four terminals it has an input side and an output side and Right now. I'm talking about Back 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 They have a higher input voltages That's gonna blow up very quickly A block looks like this and on a screen and might look like this in reality and Let's take a look inside So all of these DC-DC converters consist of a power stage a control system and the feedback and 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 There 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 them on more them about them later Let's consider the first situation. This switch is on this is so called the on state and This forms a loop from the input to the output so the input capacitor we can neglect and In an idea situation the output capacitor is I Will talk more about more about the output later All right, I don't want to make this into a Lecture and everybody's sleeping in and the fun part will start very soon so This DC-DC has two States either the switch is on or switches 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 if 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 just starts to increase While it's also flowing to the output Then the converters to converter turns the switch off which comes to the off state and And now the diode comes into play which we'll 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 magnet magnetic field Through the inductor and And Sometimes you hear about synchronous DC-DC converters in where this diode is replaced by another switch In that case Efficiency is increased since the voltage drop across Mosfet is lower than the forward voltage of the diode So in this case as you can see Current is still being delivered to the output and this is the big advantage of the Back converter that in both in on and both in off states the output is sourced with current and so 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 the switches off it ramps down and in the middle you see this Line which is the output current So you see this triangles and this is what's provided by the output capacitor All right, 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 0.7 volt reference and You can program the output voltage by choosing our van and our two on On the left side so at your desired output Exactly 0.7 volt will be at this voltage divider and This converter via keep regulating to reach the state There is if you 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 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 provided two amps Even in a warm environment So it's important to to talk about if it's a peak or continuous countering 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 3.3 volt it will be oscillating around that this can be as Low as a few micro volts and as high as a few volts depending on The parameters Also, there is a voltage accuracy So maybe it's labeled as 3.3 volts, but actually it's 3.5 or 3.0 Load regulation It's maybe 3.3 volts 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 five volts, right? Yes, but no because when you use cables and non idea connect nine non idea conditions you You can create transients which overshoot the voltage Possibly way above this maximum rating and This this can lead to very nested surprises because sometimes they fail to Fair 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 say I 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's this efficiency peak, but marketing people left to specify There's this also this so-called quiescent current, which means your converter draws Current from your input even when there's 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 frequencies. So how fast how often the internals which 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 the Let's say you have a 20 amp output converter you don't really don't want to switch the 20 amp with a mechanical big switch and instead instead of that You have a logic input to your DC-DC converter with which you can turn this completely off Then there is a so-called undervoltage look out 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 this 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 already stable and It's not gonna go your processor not gonna go into glitch Over-temperature shutdown is very common these days and That makes these tiny converters almost industrious in dust 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 will 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 over current protection Which is which makes the output very robust. You can even make a short circuit and the over current protection via limit the output output current to this value and This prevents damage Damaging of the converter and also damages to the cables and switches If they are rated to withstand the over current protection limit And let's talk about noise The alpha triple is not Exactly noise alpha triple Present as is there because the output capacitor is none 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 in on-off action from the switches We are propagated to some extent to the output and 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 also this can also be filtered and 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 that's a shielded inductor and the two blue Connectors at the end of this PCB are Screw terminals and 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're not connected. So I prefer a different style of connectors Okay, so it's good to know about non-ideal components the The capacitors that are used have a so-called DC bias these ceramic capacitors multi-layer summary capacitors are very sensitive to the DC Voltage across the terminals and if they are rated, let's say 20 micro farads 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 To compensate for this effect and you also need to put more capacitors at your output As what you would think in an idea situation Mind the transients as I said if you plan to hotplug Connect to live wires your converter. You have to keep in mind the inrush current those capacitors When they are fully discharged and you connect that to to the input then They will try to charge to their to the input voltage as fast as the Cableing let's 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 and MOSFETs are very sensitive at their gate Because the oxide layer is so thin that even 20 volt Voltage is enough to break it down and A 20 volt ESD strike is something you probably don't even notice but It can it can damage the MOSFETs And I will avoid the 7800 series LDO because That's a very old part and I still see it in new designs While there are much better ones with better regulation less Less quiescent current and that's also an LDO so it's like just marginally related to This it is If you 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 doing I'm sorry That that mostly concludes what I was about what I was trying to talk about and Now it's time for your questions Now there are two microphones one there and one over there Usually there are here comes the light Are there any questions? How about the Signal angel does the internet have any questions? Internet doesn't have a question, but here's one up front. Hi. What would you recommend instead of screw terminals? That's that's a very good question and that's really depends on the application you can have different kind of screw terminals which use It is cable Like crimped crimped terminals on the cable so you have a Cable shoe like a ring or something. Yes like a ring because then there is no way that it can sleep sleep out for less current you can use DuPont connectors they can to take like two or three amps per contact You know the standard pin header and and that kind of thing and There are also Latching connectors from Molex and other manufacturers also the problem is with that that you need crimping tools and Those can be very expensive So it first Makes sense to get those when you have a heifer space or you can share it with other people The next question, please 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? running Current mode converters Parallels typically okay But they won't do current sharing automatically so as one converter has a certain output voltage set and the other one has different a Little bit different voltage and that we 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 automatically synchronize for stability That should not influence the stability of it what I was 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 if because of this delay if There are Yeah, basically you can make an oscillator of this and To avoid that you you can use a network analyzer and inject Signal into the converter and then you know the drill. Thank you Yeah, you go ahead over there Hi What do you what would you say the choice? Is between this synchronous mode or a forced synchronous mode of That's a very good question All right, so when I talked about this briefly and mentioned the synchronous converters with force synchronous converters you have a controlled switch and Those have typically fixed see switch frequency If the output current is zero Then during half of the period currently flow backward from the output capacitor To the input side and then the next half period that currently 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 this continuous mode which reduces ripple and reduces Emi yes, so it depends on your application. Thanks You're welcome the next question Hi, Zoe. Thank you for the talk I had a have a question about You mentioned linear leg regulators at the end. What are they used for in this context? You mean the 7800 series Not the one before I think yeah so those were very good regulators in the 70s and Those are linear regulators and the problem in the 7800 series everybody knows about them because Books are full with it for with them, but they have quite a few a few milliamps of quiescent current They also have bad Bad regulation against load and line transients and They like not cheaper than much for their alternatives. So there's really no reason to use those And you can use for example a DC DC free regulator and then an LDO afterward to smooth out the voltage Okay, thank you Go ahead Yeah, 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 or And input or output which is most important from your perspective So lots of the noise goes actually back to the input supply and I Said that 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. So if you let me show you the on this chart you see the inductor current and the input current is 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 for a 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 low to the outputs Current and This 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 and I So 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 Vays of reducing the the noise on the output the best way is to have proper filtering capacitors If you use Stereomy capacitors and enough high enough value you can get rid of the like almost all of the noise I I made a design which had micro volt noise because I found a capacitor with its Resonance frequency exactly at a switching frequency. So basically all that noise that was Coming from a switching action was reflected away and higher frequency ranges where it got filtered Disappeared in much faster You can use by filters at the output but be mind that You worsen the transient Behavior of your converters. So if you if you load suddenly needs a lot more power and Starts drawing more current then your converter will will react Slower because of the the filter you just added Yeah, five filters or RC filters if you don't need that much current Okay, thanks Okay, great. I don't see any more questions. So Everything seems to be fully explained Thank you and give her applause and good night Thank you