Larger output choke for cheap class D amplifier - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: August 06, 2020, 02:43:34 am
content: I've got several of these class D amplifier modules.https://www.ebay.com.au/itm/IRS2092S-500W-Mono-Channel-Digital-Amplifier-Class-D-HIFI-Power-Amp-Board-FAN/311844643896?hash=item489b63a438:g:bfcAAOSwU8hY63c-I haven't actually driven a speaker with them yet, only a resistive dummy load. I've got them powered with +/- 67 volts and they switch at IIRC 377kHz or thereabouts. At the output there is about 2V p/p ripple at the switching frequency. If you have a 20kHz sinewave coming out the amplifier at only 10V p/p it looks pretty ordinary, with lots of shaky wiggles. Not only that, the output inductor runs quite warm to the touch, even with no signal. I know for a fact that I would never be able to hear this switching frequency ripple, but I like the idea of reducing it a whole lot so that the signal looks cleaner. With the existing inductor the frequency response starts to roll off at about 45kHz, but I can only hear to 7.9kHz so we can get a bit aggressive with the inductor here. What would be a good starting point for toroid material, size and inductance? Size is not an issue - it doesn't have to fit on the board. The output filter cap looks to be 470nF in the pic.

author: BrianHG
author_rank: Super Contributor
timestamp: August 06, 2020, 05:02:17 am
content: That output inductor supposed to be a tuned circuit to remove the 377Khz. I'm assuming that the 2vpp leakage is at 377Khz. That tuned circuit should have a really negative db notch at that frequency. Changing the inductor/capacitor value for a larger one may simultaneously cut some of your audio band, but, you chance the new circuit will no longer have quite as good 377Khz notch. (This is assuming the design on that board is properly tuned in the first place.)Let's take a look at this from a slightly different angle. +/-67v supply is a 134v p/p 377Khz signal feeding that inductor/cap circuit. The output is only 2v p/p. That's 67:1. Instead of modifying that circuit, what if you were to externally add a second identical filter. That remaining 2v p/p will now be down to 0.03v p/p. If not, the oscillations you measure may be ground bounce through you scope probe and the only way to get a good reliable measurement here is to use a differential probe, or double check the 2v p/p reading with an external true-rms voltmeter.Changing the current inductor with one with a higher Q and or choosing a lower series resistance cap on the output will also lower that 2v p/p. If the heat your module is generating is not because of the poor power mosfet switching timing, but because that the current inductor doesn't have a high enough impedance at 377Khz, then changing that inductor will make your module run cooler. A test to find out may be removing the module's filter cap. If this doesn't affect the amp's drive of the mosfets and the module runs a LOT cooler, then you know that a better inductor may be the way to go.Your inductor choice probably needs to be the same inductance and at minimum capable of handling the same amount of raw current, however, you want an inductor with the really high Q as close as possible to the 377Khz, especially paired with the module's filter cap.As for the inductor selection, or you winding your own, there are others here better suited to answer that question for you.

author: Circlotron
author_rank: Super Contributor
timestamp: August 06, 2020, 06:01:00 am
content: Quote from: BrianHG on August 06, 2020, 05:02:17 amThanks for your reply, BrianHG. I have to disagree with you somewhat. If the cap and inductor resonated at the switching frequency, seeing they are in series they would be practically a short circuit at that frequency. Minimum impedance anyway. BIG current would be flowing. What's more, the voltage across the cap (speaker terminals) would be off the planet. I'm guessing the inductor would be about100uH33uH and in conjunction with 470nF they would resonate at 23.2kHz. Having said that, I wonder why things don't go pear shaped when the amp outputs a 23kHz signal?Also, it's definitely the inductor that gets hot. The rest of the amp is quite okay heatwise. I'm expecting it is skin effect as well as core losses. Litz wire might smarten things up.Edit -> simulation says inductor is 33uH.

author: BrianHG
author_rank: Super Contributor
timestamp: August 06, 2020, 07:19:03 am
content: Quote from: Circlotron on August 06, 2020, 06:01:00 amOpps, did I say resonate. The inductor should have the highest possible series resistance at 377Khz, IE, not conduct the signal through. As for the cap, it should be in parallel across the speaker's +/- output to ensure no residue 377KHz makes it through. I must of made a mistake, but in the past, when we wanted to erase such a tone, we would create a notch filter with a tuned inductor whose impedance would have highest decibel cut/highest resistance at the frequency we wanted to eliminate, the 377KHz.BTW, for the IRS2092, the cap is parallel to GND across the speaker output. The inductor is is acting like a series choke/notch filter as seen in the datasheet example schematic:

author: Circlotron
author_rank: Super Contributor
timestamp: August 06, 2020, 10:30:17 am
content: 20 kHz output, 4V p/p into 8 ohms.

author: Circlotron
author_rank: Super Contributor
timestamp: August 06, 2020, 01:31:48 pm
content: After a bit of reading up on the subject and a bit of simulating, it seems that assuming an 8 ohm resistive load, for any given value of C there is an optimum value of L that works best otherwise it ends up overdamped or underdamped. You can't just chuck on a bigger inductor and expect optimum results.

author: BrianHG
author_rank: Super Contributor
timestamp: August 06, 2020, 06:10:18 pm
content: Yes, but the inductor needs to be tuned and to minimize that 377Khz, the higher the Q the better.Take a look at this data sheet:https://katalog.we-online.de/pbs/datasheet/7447070.pdfNotice a sweet spot between 4&5MHz where the impedance reaches 3KOhm, yet, above 5Mhz the resistance drops.The same thing counts for your current inductor with it's core. It's current highest impedance may already be tuned to 377KHz. An like this chart, if you increase the inductance, you may be improving/increasing resistance for say a 250Khz signal, but, at 377KHz, the resistance may begin to lower because 377KHz is now on the right hand side of that bump.Like I said, did the manufacturers of that module choose the optimum inductor, or just pick something off the shelf to a close value?Did you categorize that inductor knowing where that high impedance notch it located?Is 377Khz centered in the middle of that high impedance tuned point?Maybe the filter cap is the wrong value, you might just need a beefier lower ESR cap to fix the problem.Here is another one:https://media.digikey.com/pdf/Data%20Sheets/Wurth%20Electronics%20PDFs/S14100035.pdfNotice the attenuation in decibels has a unique sweet spot at 1.5MHz, and only 1.5MHz.As you can guess, this inductor was designed to work well with a 1.5MHz calss D amp output filter.A heavy capacitor on that inductor may drag down that 1.5Mhz, but it will begin to heat up.Now, will changing the windings on your current inductors improve their notch if it already isn't at 377KHz, or, will it just muddy up it's Q? What's the best frequency for the core's of those inductors. The kind of look like almost identical to the one in the second datasheet here:https://www.digikey.com/products/en?keywords=S14100035Could this mean the manufacturer use the wrong inductor, they used one designed for a 1.5MHz switching amp?You said your source was 377KHz & 67v power source. In the datasheet for that inductor, there is an almost 20db drop at 400KHz. That leaves you with a 4v signal and an inductor which would get toasty with a parallel cap trying to short out that signal to an even lower voltage.After all this, what I'm basically saying is if you cannot measure you current inductor's series impedance vs frequency, adding coil turns will be a shot in the dark.

author: BrianHG
author_rank: Super Contributor
timestamp: August 06, 2020, 09:56:55 pm
content: I would say add a second external filter, perhaps identical to the first like what I added to the IRS2092 schematic in green.

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2020, 10:31:57 pm
content: These switch-mode amplifiers need to see an inductive load (at the carrier frequency) to the mosfet output junction, since the mosfets are inherently switches with very low resistance and the inductor keeps the peak current within safe limits. In my previous employment, when we ran very long cables from switch-mode servo amplifiers to an appropriate servo motor (very similar to Class-D audio), the manufacturer required an extra choke coil, since the shunt capacitance of the cable could ruin the inductive load of the motor winding.