Half-Bridge Configuration for Class D - Page 1
Forum section: electronics / projects

author: Jon86
author_rank: Frequent Contributor
timestamp: February 12, 2014, 09:38:16 pm
content: I'm designing a Class D amplifier and I'm currently working on the output stage.What's the easiest way to configure the output mosfets? As in, should I be using a Pchannel at the top and an Nchannel at the bottom? Two Nchannels?I don't have any other rails available other than the +-10V supply, is there any way to drive both of them from the same voltage as they're supplied from?I've tried using a Pchannel and Nchannel but it seems to be shorting straight from +v to -v.Any help greatly appreciated.

author: dannyf
author_rank: Super Contributor
timestamp: February 12, 2014, 10:05:38 pm
content: Google deadtime management. A big deal in those things.generally the driver stage is a half bridge plus n-channels.

author: Jon86
author_rank: Frequent Contributor
timestamp: February 12, 2014, 10:17:39 pm
content: Thanks, I'll take a look.

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 12, 2014, 10:32:30 pm
content: Quote from: Jon86 on February 12, 2014, 09:38:16 pmconfiguration depends on the design? by the sounds of it you didn't do that. class d is no joke its a multi discipline area of study, at best you will get it right to play music at moderate levels, after a while it will just destroy itself.I would recommend you to.- Build a circuit around IRS2092, you in safe hands as latching, O.C.P (Over Current Protection), programmable dead-time selection are all baked in.- Work on obtaining a PCB with a tight 2-layer layout (this is just as important as the design itself, forget everything you heard, know regarding class AB and star ground designs)- Understand mosfet parameters (ones optimised for class-d), learn to read and actually understand the datasheets.- Simulate PCB inductance and the effects they have at high currents.- Learn to select a LC filter and selecting core material for class d.

author: Jon86
author_rank: Frequent Contributor
timestamp: February 13, 2014, 07:25:21 am
content: Sorry, I don't think I explained enough.Im actually building a Class D from scratch, with individual Opamps and MOSFET drivers. It doesn't have to be a good quality audio amp but more of a proof of concept. Its for a college project.

author: lewis
author_rank: Frequent Contributor
timestamp: February 13, 2014, 08:15:24 am
content: The easiest way is to use two N-channel MOSFETs and a gate driver IC like thehttp://www.irf.com/product-info/datasheets/data/ir2114ss.pdf.It's possible to reduce the dead time to near-zero, thereby allowing you to increase the switching frequency, by adding some inductance into the output leg of each MOSFET. PKN amplifiers (which are very good indeed) use this technique.http://www.pknc.com/pkn_xe-xd.pdf

author: Circlotron
author_rank: Super Contributor
timestamp: February 13, 2014, 12:12:15 pm
content: I made a class D amp some time ago using the Crown BCA topology. ->http://www.diyaudio.com/forums/class-d/14847-my-very-first-class-d-pwm-switching-amplifier.htmlImpossible to make it shoot through. Both upper and lower mosfets turn on at the SAME TIME. To generate a positive going output waveform the upper mosfet waveform widens out it's on-time symmetrically about the centre line (i.e. leading and trailing edge both move out by the same amount) and the lower mosfet waveform narrows the same way AT THE SAME TIME, and vice versa for negative going output. Extremely docile amp to get running, quite resistant to blowing up.https://www.google.com.au/webhp?complete=0#complete=0&q=crown+BCA+%22class+D%22

author: dannyf
author_rank: Super Contributor
timestamp: February 13, 2014, 02:13:27 pm
content: QuoteDepending on how you define "shoot through". BCA can be argued to have an inherent shoot-through in that both devices are on so there is a current path that goes through both switches. It is n ot a shoot-through in the traditional sense in that you have other components in that current path (ie. output inductors).It is better to think of it as a H-bridge. In fact, you can build it via an H-bridge.

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 13, 2014, 03:04:00 pm
content: Quote from: Jon86 on February 13, 2014, 07:25:21 amYour details are still to nebulous.Class D from scratch, a few op-amps and mosfet drivers the devil is in the details, what modulator are you even going to use? self oscillating, clocked, hysteresis,UcD, is it using POST or PRE feedbackIf you want reliability then build one using a reference UCD design such as this.http://www.nxp.com/documents/user_manual/UM10155.pdf

author: dannyf
author_rank: Super Contributor
timestamp: February 13, 2014, 10:37:02 pm
content: The UCD design can be done with a small mosfet driver, like the TC family.You may also check out the Mhz digital amps from China - a high-performance version of the UCD.IRF has some reference designs with their chips, available on their website.

author: nctnico
author_rank: Super Contributor
timestamp: February 13, 2014, 11:09:11 pm
content: From memory HIP4080 / HIP4081 MOSFETs drivers are a good way to make a discrete class D amplifier.

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 13, 2014, 11:13:36 pm
content: Quote from: dannyf on February 13, 2014, 10:37:02 pmTC family? okay but im sure the point is a cheap to build proven robust design, TC chips are for fets with high gate charge Qg (in a larger applications there is no benefit using this as in the ucd reference design, the one specified works pretty well without exotic semiconductors) the differential pair can be swooped out with a differential output high speed comparator. anyways...Yes im very familiar with those designs (but they not UCD) they look like hysteresis loops , I wont trust a class d amps from china, there are a few companies that build reliable class-d amplifiers, Hypex is one of them and the lead engineer is Bruno Putzeys great engineer! and the man behind the UCD concept, I wont trust a class d that comes from China even the ones using the IR designs they don't make reliable products and probing the the amp normally reveals the horrible EMI emission or even a small AM clock radio should reveal the imperfections and pitfalls of a bad design.Here is a excellent presentation on the subject of class-d amplifier designs and the issues around designing it, this will shed light on this topic far better than I can express in writing.

author: dannyf
author_rank: Super Contributor
timestamp: February 13, 2014, 11:25:39 pm
content: QuoteYou are fully entitled to have that view - not that anyone cares.If you have a particularly sound criticism on the said amplifiers, I am all ears. Otherwise, try to add value to the discussion.

author: diyaudio
author_rank: Frequent Contributor
timestamp: February 14, 2014, 01:22:16 am
content: Quote from: dannyf on February 13, 2014, 11:25:39 pmTrolling I see. okay.You speak with authority yet lack basic knowledge.Who cares what you think! coupled with your sour attitude, your advice of nothingness you gave us, you clearly lack basic common sense in this area, and clearly struggle to even add value or enrich the conversation positively.Care to elaborate on your TC driver recommendation?QuoteI'm all eyes, clearly you have no idea what you recommending hey? just adding wind, you haven't built a class-d amplifier in your life hey??QuoteNow you talking about sound, I thought this discussion was about design? why are we jumping into realms of sound criticism?? unfortunately for you I don't have my audiofool dictionary with me at the moment.I take it you wear a "I give negative feedback t-shirt" everywhere you go and have it in all the colours available.You clearly asked for it! next time show some respect towards others, be humble.Have fun with your china sound, its made for you.

author: Jon86
author_rank: Frequent Contributor
timestamp: February 14, 2014, 06:21:10 pm
content: Quote from: diyaudio on February 13, 2014, 03:04:00 pmIt's a triangle wave generator and a comparator.I'm not looking for the best sounding amplifier, I don't even care if it sounds good. All I'm worried about is being able to get 10W out of it, and that's all really.What's the most simple way of driving a MOSFET half bridge at 300KHz?I want to try and avoid the two separate inputs for high and low, it'll mean I have to invert the signal separately and that'll give me some delay.Is it possible to drive a mosfet half bridge without using any charge pumps or bootstrap caps? Just using the +-10v supply?

author: dannyf
author_rank: Super Contributor
timestamp: February 14, 2014, 06:28:02 pm
content: Yes. P- and n-Ch pairs.

author: dannyf
author_rank: Super Contributor
timestamp: February 14, 2014, 06:43:00 pm
content: You will need to understand that in those cases, you will need to reference the signal to the negative rail, or use a level shifter.

author: Jon86
author_rank: Frequent Contributor
timestamp: February 14, 2014, 06:45:49 pm
content: Quote from: dannyf on February 14, 2014, 06:43:00 pmWell my signal's going to 10 and -10, and I've got a MOSFET driver that's going to +-10, all I need now is the right configuration of MOSFETS.

author: dannyf
author_rank: Super Contributor
timestamp: February 14, 2014, 06:48:00 pm
content: The "right" configuration is dictated by your mosfet driver(s).

author: Jon86
author_rank: Frequent Contributor
timestamp: February 15, 2014, 11:23:26 am
content: Quote from: dannyf on February 14, 2014, 06:48:00 pmMCP1407 at the moment, looks like it should do the trick but I'll happily change it.This is the current system I'm testing, looks like it should be alright.

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2014, 12:35:12 pm
content: Not sure what trick you are referring to, but you can just follow the datasheet for your driver for the right mosfet configuration.

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2014, 12:48:50 pm
content: For your particular application, the mosfet driver can only drive p-/n- channels - it is typically called a low side driver.to drive n Ch mosfets, you need a high and low side driver.actually, for the particular driver, you can build a low power class d driver without external mosfets. Google duyaudio.com - about 10 - 15 years ago.once you have it working, drop in your mosfets and invert input polarity, you are done.

author: Jon86
author_rank: Frequent Contributor
timestamp: February 15, 2014, 01:19:49 pm
content: Quote from: dannyf on February 15, 2014, 12:35:12 pmDoesn't look like there's anything in the datasheet about the mosfet configuration, it's a single mosfet driver anyway not a half bridge.And what you said about the driver, that's what I'm doing. I've got it all up and running without using mosfets, but it's getting way too hot to use properly. All I need to do is implement the mosfets and I'm done.The configuration I posted below, will that be good enough for what I need or should I try using just two N-Channels and a proper high and low side driver?I don't need quality here, just something that'll work well enough to prove the concept.

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2014, 01:54:00 pm
content: Give it a shot. What do you have to lose?

author: Jon86
author_rank: Frequent Contributor
timestamp: February 15, 2014, 01:57:10 pm
content: Quote from: dannyf on February 15, 2014, 01:54:00 pmTime, unfortunately...I don't have the circuit on me at the moment, and I only have a couple of hours to play with it until I've run out of time.

author: dannyf
author_rank: Super Contributor
timestamp: February 15, 2014, 02:52:20 pm
content: You can either spend your time pondering if it works or put the circuit together and know for sure if it works.it is that simple.

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 15, 2014, 02:54:44 pm
content: Quote from: Jon86 on February 15, 2014, 01:57:10 pmBuy it, stick it on a board, done.http://www.ti.com/lit/ds/symlink/tpa3116d2.pdfAs you only mentioned time in the above, not money i see no reason for not doing this.

author: Jon86
author_rank: Frequent Contributor
timestamp: February 19, 2014, 10:40:47 pm
content: Quote from: peter.mitchell on February 15, 2014, 02:54:44 pmNo the point is my whole project is about doing the entire amp from scratch.