Capacitor Design - Audio Amp Power Supply Design #2 #capacitorsize #capacitorbanksize

ruticker 02.03.2025 12:34:32

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Recognized from a YouTube video by YouScriptor.com, For more details, follow the link Capacitor Design - Audio Amp Power Supply Design #2 #capacitorsize #capacitorbanksize

Hey guys, what's up? Y'all over here with KISS Analog Comm. So today we are going to jump into the second part of our power supply design. We're going to go over a block diagram of the matrix components of the power supply and how to select those parts: the bulk capacitor, bridge rectifier, power transformer, and the fuse. We're going to talk about how the selection of one affects the selection of the next, the trade-offs, and so that we can choose the right parts. So let's jump in! ### Block Diagram Overview This is our simplified block diagram using the KISS method. This is sort of the power supply design for the audio amplifier. It breaks down to input power coming into our circuit protection, then into our transformer, our step-down transformer, because we're stepping down the voltage. In the last video, we saw that we needed about 32 volts, maybe 33 volts, for the amplifier circuit to work correctly. We're going to talk about the voltage we need because of the power supply. So we have some circuit protection, transformer, and the AC/DC block. The AC/DC block is the bridge rectifier with the bulk capacitor. I've drawn it here in pencil. This is our AC voltage source, our fuse, and our transformer primary winding, along with our two secondary windings. The secondary windings are tapped off in the middle to ground. You can think of these transformer windings as just in series, so we have a plus and a minus voltage. The center tap, kind of like a voltage divider, is the zero voltage point, which is ground. ### Bridge Rectifier The plus and minus voltages come off the transformer and into this bridge rectifier. If you want to draw one of these, just draw a square, kind of like a corner, and then draw the diodes. You can see the arrows point in the direction of the current flow. This is our plus voltage out, so the arrows both point to this one. This is our minus, so the arrows point away from it. The AC comes in here; when this side is positive, it can go this way. When this side is negative, it'll go that way. The same with this one: as positive, it goes this way; when it's negative, it goes this way. The steering diodes just tell the voltage where to go. I made a video to talk about diodes and bridge rectifiers in power supplies and how that works, and how the capacitor and the transformer play a part in this. You can watch that video on how bridge rectifiers work in power supplies to see that. ### Capacitor and Waveform Off the output of this plus-minus voltage, we put two capacitors, and the center point of those acts like another voltage divider. The center point is the zero voltage point, which is referenced here. We have a minus capacitor across, which gives us some high voltage, and it passes through, giving us a plus voltage. These capacitors are in series, and anything in that way across the full voltage. I've drawn a waveform here—sorry, AC waveform—that's what we see on this side, and that's what we see on this side. But then, once it goes through this rectifier, we see this pulsating DC waveform. I kind of got a little dashed line; I hope you can see that. So we got this AC waveform, and instead of going negative, it turns into a positive side. These diodes steer it, so we have positive. ### Charge Cycles The time for a cycle is equal to about 16.7 milliseconds, and that's because we have a 60 Hertz frequency. Of course, time is related to frequency: \( T = \frac{1}{F} \). So, \( T = \frac{1}{60 \text{ Hz}} \) is 16.7 milliseconds. Half a waveform is roughly 8.3 milliseconds. That's the time it takes between positive pulses. This capacitor over here is going to give us a cleaner ripple. The voltage can charge up, drop to some point, and then charge up and drop to some point, repeating over and over. So we get two charge cycles, and what these little pointy things are are the current that is going to these diodes as charges to these caps. I'm just showing the positive cap at this point; the same thing is happening on the negative side for this capacitor. ### Capacitor Selection Now, let's see when you select a capacitor, for instance, it affects these other parts. The larger the capacitor, the higher peak currents you get going to these diodes. This capacitor section affects the voltage that current uses in the diodes and also this transformer. The transformer steps down the voltage and also gives us isolation from the primary side of the circuit to the secondary side of the circuit. Current here is traveling in a circle from the voltage to the fuse, transformer, and back, but this current here does not cycle here; it's magnetically coupled. So we have magnetic isolation. The current here comes off this transformer and cycles through these diodes. Like I said, this circle current is a little more complex because one side of the waveform goes one way around, and the other cycle comes when it's positive down here; it goes this way through the capacitors and then back up to the top. ### Capacitor Sizing Equation Now, let's look at the capacitor sizing. This is how we began. This is for this power supply design for the audio amplifier. This is our 60 Hertz coming in. Our power and this voltage is what we have to decide. The time from here to here for one cycle is 16.7 milliseconds. Now, once we go through a good rectifier off our transformer, we'll take this negative-going peak and bring it up here, and it'll be another positive pulse. So we'll get two charge cycles in one 60 Hertz cycle. The time between charge cycles is 8.3 milliseconds. Now we have some math down here, an equation. The equation for capacitors is: \[ I = C \cdot \frac{\Delta V}{\Delta t} \] We know the current is 2.5 amps RMS for the 50-watt condition, and the change in time from cycle to cycle is 8.3 milliseconds. The change in voltage is how much ripple we're going to allow on our power supply. For the music, we said it's going to peak at 20.28 volts, and we're going to give 4 volts of headroom for the audio amplifier, so that's 32.28 volts. For the parasitic, we're going to need some water, so let's say we give ourselves 4 volts like the audio amplifier dead. ### Solving for Capacitance This is likely to change; basically, the transformer selection might force us to have a higher or even lower voltage. But let's see what we do with 4 volts. For this equation, we know three of these things; we just don't know our capacitance. So we solve this equation to put our capacitor on one side. We cross-multiply to get rid of the time by multiplying this side of the equation by time. We end up with: \[ C = \frac{I \cdot \Delta t}{\Delta V} \] Plugging in the numbers, it comes out to 5.25 millifarads, or 5,250 microfarads. That's the minimum size of the capacitor we want if we're going to allow for voltage ripple. ### Worst Case Scenario Now, let's look at another case for capacitor size based on a different impedance. I'm calling this a worst-case scenario: 20 Hertz, the lowest frequency in our audio band. The time for a cycle at 20 Hertz is 50 milliseconds. The good part about that is our AC power comes to our transformer, charges the capacitors, and we get to get three charge cycles because 20 Hertz is three times slower than 60 Hertz. So we've got three charge cycles closed on our 60 Hertz power coming in. We said before we're going to let the voltage drop 4 volts peak, so now we can say 3 times 4 volts is our change in voltage in our equation. ### Final Thoughts So, we went from roughly 5 millifarads to 29 millifarads. We know we need a capacitor somewhere in the range of 5,000 microfarads to 29,000 microfarads, and we have a maximum of 36.28 volts on a rail that we need or want. I took the 36.28 volts, added 5% for the line host input voltage, and 10% for our transformer regulation. This gives us a capacitor rating of about 63 volts. Let's jump into the capacitors, and I think we're ready to go to Mouser.com for that. ### Capacitor Selection on Mouser On Mouser.com, I looked at 63-volt caps. I found a bunch of 10,000 microfarad caps at 63 volts. Some are snap bands, and some are screw heads. I found one for $21, with a 10,000-hour rating and an 11.5 amp rating. But I think I'm going to put two of these in parallel, even though this one is within our range. I want to be able to handle higher peak currents than that. So I think I'm going to do two of these in parallel for the plus and two in parallel for the minus. Alright guys, what do you think? Thumbs up!

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