Class -D amp boost - Page 1
Forum section: electronics / beginners

author: 0x007
author_rank: Contributor
timestamp: August 19, 2015, 11:20:48 am
content: Hello everyone , is it possible to boost the power of a class D amp using power mosfets ?As far as I understand class D amps use high frequency pwm instead of a linear components , so I don't see why I cant use mosfets.on the other hand I've been wrong before and I rather ask before I buy some thing and brake my head over it.by class D amp I mean the PAM8403 chip it's cheap and performs well the only drawback its rated only at 5V 3W per channel , its nice but I need MORE.Any way thanks ahead .http://www.diodes.com/datasheets/PAM8403.pdf

author: janekm
author_rank: Unknown
timestamp: August 19, 2015, 11:43:33 am
content: It wouldn't make sense to use that chip with external mosfets... If you want to use separate power mosfets on the output, use one of the Class-D modulator chips designed for that configuration. Low distortion class-D amplifiers are very sensitive to specific parameters of the mosfets used in the output stage (one issue being the switching times for the pair of P/N-channel mosfets, if these are different from what the modulator expects you will get distortion).Here's a random example of a class-D modulator chip:http://www.ti.com/product/tas5001This one is designed for a specific companion H-Bridge chip but I have seen ones designed for discrete mosfets before. You can see in the functional diagram for the TAS5111 (the companion chip) that it's performing "timing control" to avoid distortion / switch-over current drain:http://www.ti.com/general/docs/datasheetdiagram.tsp?genericPartNumber=TAS5111A&diagramId=SLES111It also looks like the PAM8403 already has a bridged output stage, so reversing that and driving another set of mosfets from there would be pretty awkward...Another option, if you just need a little bit more power, would be to use a different integrated IC, for example:http://www.ti.com/product/tpa3123d2Two of those in bridge-tied configuration should make for a decent enough 25W (into 8Ohm load, typical hifi speaker) amplifier...

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2015, 03:11:59 pm
content: The maximum power output of an amplifier (irrespective of whether it's class D or AB) is always dependant on the supply voltage and load impedance.Any bridged amplifier run off 5v and driving 4 Ohm speakers will have a maximum theoretical RMS power output of V2/(2R) = 52/(2*4) = 25/8 = 3.125W. Once the voltage drop across the power transistors is taken into account, it's always less than that so the PAM8403 is as good as it's going to get.Adding lower RONpower MOSFETs will hardly increase the power output. To get more power you need to increase the power supply voltage or reduce the load resistance.

author: 0x007
author_rank: Contributor
timestamp: August 19, 2015, 04:40:26 pm
content: Understood , the chip is build in away that does not allow modification.By the way when I mentioned power mosfets and that the chip operates at only 5v I meant that the speaker will be hooked up to higher voltage.thank you for the enlightenment , it's good to learn new stuff.

author: jlmoon
author_rank: Unknown
timestamp: August 19, 2015, 04:47:04 pm
content: Quote from: Hero999 on August 19, 2015, 03:11:59 pmHence: The reason you can run into some car amps out there that produce enough drive voltage and current to put you 6' in the ground. If I recall from my past days, I have measured as much as 240 VDC on those SMPS power supply caps located on some of the high end amps.