How to Select the Diode for our Amplifier's Power Supply

hey guys what's up eddie oh here with kissanalog.com today what we're going to do is we're going to look at the diodes and we're going to look at some waveforms here and i hope it'll be uh interesting and you'll learn something okay uh before we do that let me just score the the board here real quick uh i've seen this mac if you've seen the last couple of videos uh transformed the bridge rectifier and the capacitors so we're going to talk about the diodes all right and there's a couple ratings you know here's the specs i kind of wrote down previous video to say things to think about so one of them is the voltage rating 600 volts you want this reverse uh voltage rating so when the diodes when you when it's forward biased you're going to get a voltage drop current is going to flow through it right so when the waveform up here is positive it's going to come through here the current is going to flow come up here you're going to get this first bump and then it's going to return through this dial so those two diodes are turned on this diode is off but he has voltage there and this guy's tied to here so he's got the full voltage the peak of this waveform when there's a light load the transformer is not being you know loaded down you know the regulation transformer will kind of loosen up and the voltage will peak up so you want to make sure that even if you think oh i got 34 volts out here but no this could be higher maybe 40 volts maybe higher than that even in this case we have 600 volt diodes way more than what we need well that's great but the bad thing is is often the other thing we care about is when this the one that is conducting what's the voltage drop across it we like it to be really small like it's shocking like 0.4 volts or something a lot of times we use regular silicon diodes in these bridge rectifiers and they're you know we think 0.6.7 we put our multimeter across them right but these higher voltage ones and these power diodes these large packages they can be higher like 1.5 volts we're going to look at the data sheet real quick i'm going to show you where you sit look for this stuff data sheet will actually show over two volts because they'll show that based on the rating of the current that it's rated for look at this it's rated for four current with 30 amps these guys are beefy now 30 amps times let's say it was 2 volts that'd be 60 watts you couldn't do that for very long because it'd get really hot but you'd have to heat sink it and keep it cool so you could get that 30 amp stir but it can actually do pulsing uh it can re do repetitive forward current pulses at 70 amps and we're actually going to be doing a kind of more of a pulsating current so it's capable of a lot of current now it can do a single pulse of 325 amps but then it has to cool down again before you can give another 320 fan shot when you first turn on the power supply we're going to get this inrusher current right we're going to use a thermistor that's going to slow that down but you know you get this inrusher current so you know it's good that it can handle high current so this is still higher than what we need but that's always a good thing and you can get lower voltage diodes that can handle high current so that might be something we might want to look at but these dials came with this kit they i think just in low quantities they cost over two bucks so they're pretty good dials they're large packages i think they might be fine but we probably could find something a little bit better now on the positive side these diodes are also what they call fast recovery so when a diode is conducting and then when it stops conducting well as it built is you know as you apply voltage to this pn junction this positive negative junction of the silicon so you have these carriers that are intermixing and once they get a certain charge build up then you get current flow and you get that voltage drop from that that charge is created when you when the voltage you know here changes and reverse biases that diode it doesn't instantly change kind of like the traffic in an intersection light goes red sometimes there's still cars shooting across it right so you got to wait for that stuff to stop that's called reverse recovery this one is on the order of i think 40 50 nanoseconds pretty darn fast another good thing about that is a lot of times like the waveform comes down and then all sudden they recover real sharp and that can any time you get sharp moving transients you can get emi this is kind of a soft knee on it so that's a positive and the fact it's fast you know when you have current flow you have uh power dissipation right even if it's microamps you still get some power dissipation because now you have a larger voltage developed across it so current times voltage is power well uh since these are fast there's not going to be too much dissipation there nothing to worry about but yeah it might be interesting to maybe look for something you still want to be let's say if we thought we had 40 volts here um a 100 volt diode still want like a 2x plus if it's a regulated voltage it gives you a little bit more comfort but if it's not you have to worry about line fluctuations and all that kind of stuff so you got to think about all that so those are the ratings what i i'm also showing is the the thermal resistance r theta junction capacitance and actually this is a mistake the thermal resistance inside that package from the semiconductor to the case you can't do anything about once it gets to the case you can uh let it just radiate you know or you can uh put on heatsink and get it heat up through conduction or convection airflow well uh it's so that that's good to have a real low number these large packages usually have a little number these are less than uh 1 degrees c per watt i think it's 0.7 degrees c per watt we'll look on the data sheet but what i wrote down was the junction to ambient so j a r theta j a and that's 30 i think it's 30 degrees c might have that wrong too but so uh so based on that and a 61 amplifier i understand it's supposed to the power supply runs 150 watts because it's so inefficient so each side 75 watts and so take the wattage divided by voltage to get current and if we have 34 volts that the board asked for then we would have uh 2.2 amps now since each set of the diodes are conducting um half of that then we can divide it by half and say okay these two dots get 1.1 these guys get 1.1 now it's not that's not absolutely accurate but let me explain uh each diode has to supply two amps then this guy takes over and he supplies the two amps and they go back and forth but since they're only on half the time you can kind of look at it like well the average is 1.1 even though they're they're both supplying 2.2 amps when they're on if they're off half the time then it's really only 1.1 amp right that's another way of kind of looking at that and then if you say okay 1.5 volts when it's conducting so 1.1 amp times that voltage is 1.65 watts and if you say you have 1.65 watts we'll times that by this many degrees c per watt you know then you get 50 degrees c rise and that's rise that's not the temperature that it will be at that's the rise above whatever the ambient temperature is so if it's say 20 degrees in here it'd be 70 degrees okay so on the bench i've seen about 44 degrees when we look at these waveforms you'll see that it's not quite 1.5 volts all the time either so you the the average voltage is is less so maybe what if it was half then it's probably closer to what i see here on the bench but and i haven't let it sit on the bench for a long time to run so kind of hard till there too all right so i hope this kind of helps you understand uh the diode a little bit better and you know how to select it a lot of times you'll see those uh bridge rectifier units it's basically four dot well it is four diodes in a package and they're usually large packages with hole in the middle they mount down to the bottom those are great too these are discrete diodes and there's four per side so you you could replace these with two bridge rectifiers uh with four diodes in each package right but and anyway in this case i think they use this because they're easier to mount on a board and they're big now i mounted mine vertical so i could put heatsinks on them just in case i'm gonna run them for a while once we get the amp firing everything going and we'll see how much current and if it really does rise closer to 50 degrees c i don't want to get that hot so more than likely i'm going to be putting heat sinks on them but for now i'm leaving them off just so we can see what happens okay and the immediate calculations show that it's not we're not in some immediate danger by doing that i already kind of went through that before i powered on and left too long but often when i'm powering up a new power supply i power it on turn it down touch fill everything once i know the voltages are safe and then i'll do that until i start getting a better confidence level because some things take a while to heat up what if you had a capacitor that had too much ripple current in it finally heated up and went boom you know you don't want a surprise like that and uh yeah sometimes even something installed backwards won't blow up immediately so yeah i i'm kind of slow and and getting to the point where i feel safe about just letting it run for a long time and then i use my thermal camera and i look at things because then i can look at everything all one time and not be surprised by something over here when i was looking at the transistor over here but all right uh let's come over here i'm going to show you the data sheet real quick just show you where you'll find that data the important data and then let's look at uh these waveforms that i've been picking up pretty cool stuff all right let's do it okay guys this is the data sheet from on semi as you can see 30 amp 600 volt and they call it stealth diode i think that's because it turns off fast and kind of soft so there's the information there two packages we have the big to247 package where this is the 220. some highlights there let's just come down to here where you can look at some of these uh maximum ratings 600 volts 30 amps see here's a repetitive 170 amps single time 325 that's for 60 hertz you know because they know that's what you're going to be using it for 200 watts if you can keep it cool right so let's go down that's the forward voltage drop okay 30 amps at 25 c so it's gonna be in here if it gets hot it actually drops a little bit because that's what um pn junctions do right all right so you know you hear about thermal runaway and that kind of thing right diodes don't necessarily do that but they could if they get real hot okay reverse recovery time 36 nanoseconds okay the other important thing which i think is really important besides voltage and current is this and this is the 0.75 degrees c per watt that stated jc theta j what i was showing is 30 degrees and then the theta j a for the 220 the smaller package look at that twice more than twice as high 62 degrees c per watt okay now let's look at this curve this is an important curve a four current from forward voltage okay so if we're pulsing currents say in this area this 25 c curve it's probably always going to be a little bit warmer than that right but anyway yeah worst case scenario even if you kept it really cool when you first turn it on it's gonna be less than 1.5 so yeah so that should be the worst case for us the 1.5 uh for 10 amp pulses but yeah you can kind of see and if it's obviously it's going to be warming up a little bit it's not we're going to try to keep below the temperature of any of these things so it's going to be somewhere between this 25c and this 100c line now this is also the internal temperature which is going to be you know hotter than the case and just for your interest this is a ford current and this is a reverse recovery time so depending on how much current you have it takes a little bit more time see how the curves go up and something just to be kind of be aware of is diodes do have junction capacitance so you can see right here reverse for voltage so when you have reverse voltage on it you know say up where it's going to be turned off you're going to be mostly looking in this range it's going to be you know down here in the 100 of picofarad 50 picofarad kind of region but that but that 50 people fair just something to be aware of that there is capacitance in your diodes okay and when you really want to delve deep into this it does take a little bit of time to transfer that heat from the junction to the case or to your you know heat sink and this is a pulsating current it shows some time durations based on uh this rectangle wave shape so what that tells you is if you have a really fast rising pulse uh you may not feel it in the temperature even though the dye itself is getting very hot now i just want to point something out when you look at this 36 r3060 you'll see it fall under different manufacturers uh prefix like in this case isl 9r i think the one that came on it's kind of a chinese company prefix i believe but it's you know several companies make this diode and the specs are all very similar so i think yeah it's you're going to get pretty much the same performance regardless of the company especially when we're not pushing it to the limits where you know it has to we're really relying on a certain spec okay if you are then you might want to buy from a company you know and all that kind of stuff it's always that's always a good idea actually okay pretty cool dial to see from the data sheet right okay let's jump over to the scope and take a look at the waveforms there's some cool stuff that happens and a lot of times when we're drawing things on on boards to try to explain things it's kind of hard to explain all the intricacies all the you know all the small things that are happening so a lot of times we're kind of glazing over some of the things just you know saying this is what's happening when there's actually maybe a few little things happening in between so we're going to see that here on the scope come over here and take a look all right in this shot what we're going to see is something pretty cool we're going to look at the diode now i've moved the differential probe channel for the green one here it's 10 volts per division i've moved it to ch uh to look across a diode so you know typically what you'd see is the you see the waveforms say go up when it's reverse bias sine wave come down as it crosses and and turns it on it would be flat maybe a volt in half if you got a volt and a half drop and then as the ac waveform went back up it turned off the diode and you'd see uh the sine wave part okay and that's the part where it blocks the waveform so let's go ahead and just bring up the voltage so you can see there we go and you can see where it's turned on or this was reverse bias and this is where it's forward biased okay let me bring it up to 120 volts there we are okay our outputs are all up let's see what's going on with this guy okay our outputs are all up and it's kind of got a little bit of a funky wave shape you know this uh bulge across the diode okay i just froze the waveform so we can just talk about for a minute now here let me get a pointer here here we go uh okay the green waveform uh the voltage say it's gonna it should look exciting soil is what you would imagine then it comes down and then the diode conducts we see the pulsating current so uh the diode gets forward biased to this region and it's you know in this case let's say it's a vote in half it'd be down a vote in half and then come back up and then back down again right but it's interesting well first of all we know the top the sign wave is kind of flat because that's where the pulse of current is causing the voltage drop in the windings itself so we're getting that kind of waveform but this is interesting like this over here it's kind of you know it's not sinusoidal shape it's it's more almost trapezoidal shape right that's because even though the voltage might look sinusoidal coming out of the transformer the voltage across the diode is the difference between the output voltage and so that's that accounts for the ripple voltage too which is this yellow one and the voltage on the transformer side so it's really the difference between the two and since the voltage on the output is a ripple like this triangular thing it changes the the way the voltage actually looks across the diode so just wanted to point that out so you can kind of see how this kind of angles down this way and this kind of angles up this way and then when this stops then this kind of proceeds to kind of try to take on that siding soil look and then this pulsating current kind of pulls it back down so it kind of messes with the whole waveform so it's not ideal like you'll see people draw on a board like i might draw on board because to draw something like this it's it's real world it's actually how it works but it's not easy to draw and it's a lot easier to show on a scope i guess okay guys so now what i want to do is turn it back on and i want to show you a little bit more about this waveform okay i turned the scope back on it's in run mode so it's not in hold mode anymore it's just running you can't see it moving wiggling around okay now what i'm going to do is i'm going to increase the instead of 10 volts per we're going to go to say a half a volt per we want to zoom in on this bottom part of the waveform okay so let's just go ahead and make it bigger now it's only one amp load right now look at that that's pretty funky looking that's probably not what you expected let's just try to look at one of them let me get the uh triggering solid here okay one thing i love about this scope is is these arrows here how you can just kind of go through the different sources so see i went from the current source or it's purple to the green arrow now it's triggering on the voltage waveform so i can trigger on this funky looking pulse down here now look at that guys notice that when the current pulse happens you get the voltage drop across the diode otherwise the diodes forward biased right through here right through this region right here it's forward biased right there but there's no current flow so there's really not much of a voltage drop see the blue line that's channel two that one has no load so that's why i left it there so it gives you a nice visual line so the greens you know it's half a volt per division so it's about a quarter of a volt below but then when the current actually pulses up it pulses down and you get that um you know this is half volt per so it's about one volt a little bit more than one volt of a forward voltage drop so one amp so just remember that one amp load you get about just over one volt of uh forward voltage drop pretty cool right okay i'm going to freeze this because i want to talk about this i want to show you something else that might not be super obvious right now all right guys now uh what i want to show you is we're going to zoom in on just one of these waveforms here let me just zoom in and i'm going to bring uh that little see where the current is right here where the current drops right there i'm going to bring that to the center right there okay so that's where the current stops all right so now but look what happens here so this is what i'm i was trying to show you on the last video is we're charging up the capacitor and then right up here is where normally you'll see people on a white board or whatever chalk you know on a board show okay cap charges and then for the peak wave form this charges this way but it doesn't really right we have this little dimple this little thing that happens where there's some sharing going on and you can actually see that because right here between right that that peak of that pulse and this line right here in the center of the scope right there is where the current this purple pulse actually stops so that's actually where their current pulses up stops and now the capacitor supply and current through uh well actually from the zero line right here is where the current is actually leaving the capacitor so that little spot there and this little spot if i spread that out a little bit more between the zero line and this this current is leaving the capacitor and meanwhile the green pulse is still there so that's my point is well sorry the purple pulse the green is the voltage on the diode so the the purple pulse of of current is is sharing going to the output plus still charging the capacitor and at this point it turns off and then you see this abrupt change and now the capacitor is just just charging right through here so that is and then you see from this point here to this it's a straight line it's very linear through there and that equation that we calculate how much capacitance we need is based on that so if we look at that it's one division two three four five divisions say five and a half divisions one millisecond so that's 5.5 milliseconds so in that equation i was showing one amp which is what we have right now and right now we have about half a volt peak to peak ripple and all right guys so now if we look up here we see 200 millivolts per division so our ripple voltage is peak to peak is like 200 400 550 like what it shows right here is about 550. so that equation i was showing it was for one amp for one volt peak peak you would need about you know i was saying about eight millifarads well right now what we have is about 16 millifares about double that so it's about half a volt so that's just about right uh now if we had half capacitance that would double so would be right around what i was saying uh that we need now this is a light load and the duty cycle uh part of what i'm showing you here is the duty cycle the charge time is actually changes like this discharge time actually changes it's kind of longer for the light load now watch what happens when we go from one amp to five amps watch what happens to the diode and also we can see how the discharge cycle changes too the pulse width control kind of thing okay let me go ahead and start the scope back up okay we're in run mode i'll bring the voltage back up back up to 120 so we're back to where we started right okay let me get a couple more pulses in there so you can kind of see everything okay now i'm gonna go from one amp two three four five okay let's shrink the current we're going off the screen and we got a good volt drip hole we got that okay i'm gonna go ahead and freeze that i think that's good image and i'll drop the load back down still being extra sensitive or extra cautious okay turn off the power supply all right so now we can talk about this okay you can see how much further the pulse goes down because we have a lot more current now right we're at five amps so here let me spread this out so we can focus on one of these pulses again now you can see how much you know how much wider this uh charging current got so now the diode is supplying current through almost a whole portion of the voltage waveform where it's below this line but not quite we still have some room here where we could pull more current if we needed to but doesn't really need to right okay now look at this the discharge time look how long this current pulse is supplying current as this voltage waveform drops because we're pulling so much current and then right here this corner you can see that's where everything turns off then the capacitor is discharging so let me just line that up in the center line right right there okay so now we've got one two three four just less than four divisions where before we had more right now we only have four milliseconds not even four milliseconds to discharge but also look at this we only have 1.8 volt peak-to-peak even though we have five amps load we're only getting 1.8 now since we have twice the capacitance that i suggest in my formula it would be 2.5 volts peak peak it'd be half of it'd be you know if it's one volt peak peak for every one amp at five amps would have five volts peak peak but we're getting and then since we doubled the size capacitance it'd be 2.5 but it's less because we don't really need that much capacitance it's probably closer to you know if we count this as four milliseconds so if we use that time to calculate how much capacitance we need we'd be a little closer but you know i think the six millifarads per every amp load for every one volt peak peak that's a good starting point i think but yeah this is interesting right this whole waveform the way the voltage develops now look at this at five amps we're at half volt per division so we're one two uh three divisions down so that is it's almost three divisions so it's almost a volt and a half of uh ford volts drop right through there so right about that 1.5 and the current is 5 amps per division so it's from right here it's 5 it's about 8 amps peak and then below the line when the capacitor is supplying the current it's right through here and and we get our ripple voltage our negative ripple voltage dropping down it's right through here and that's five amps that's one division down all right guys uh let me know what you think of this and if you have any questions comments please do that and patreons thank you for your support and uh everybody for watching videos supporting channel that's awesome thumbs up if you like the video that helps the youtube analytics a lot really does and uh oh and you become a patrons for as little as dollar a month throw that out there so i can buy more stuff so you know i'm gonna put this board together and we're gonna make it up to this we're gonna do another test on this put it together with the amplifier got meters crying at me back here uh and then we're gonna run this thing and we're gonna keep moving on to building this amplifier up all right okay guys hey thanks for watching see you next time

Channel: Kiss Analog

Views: 1,510

Rating: 5 out of 5

Keywords: how to select a diode, How Much Capacitance, how much capacitance is needed, how much capacitance do you need, Resistor Derating, Hiraga PS kit, Hiraga, Jean Hiraga, Le Monster, Hiraga Le Monster, Jean Hiraga Power Supply, Hiraga Class A Amplifier, Aleph 5, Aleph Class A amplifier, Aleph 5 Class A Amplifier, Power Supply Kit, Kiss Analog, KissAnalog, Class A kit, Power Supply Design, Le Monster Power supply, Class A Amplifier review, how a class a amplifier works

Id: x7cQMGoQ8fI

Length: 30min 30sec (1830 seconds)

Published: Mon Oct 12 2020