Complementary pair MOSFETs for high speed switching

[Ian Greenhalgh]

These devices consist of a complementary pair of MOSFTs in a single 8-pin package and they are incredibly cheap so I grabbed a few of three types while ordering some other stuff from the same supplier. The main characteristic emphasised in the datasheets is their low RDS(on) and they are intended for highspeed switching rather than analogue operation in an amplifier.

However, the things are so cheap, I figured why not see what they sound like in a simple topology as a headphone amp.

One discouraging comment I did find on the forum about such devices:

The elephant in the room with dual complementary MOSFET's intended for switching applications is that they're not actually anywhere near matched in linear conditions, so having them in the same package is really quite pointless as the thermal matching doesn't do anything useful for you.

Oh, so the idea of them being matched is out the window, but some feedback in the amp design should hide that issue I think.

Has anybody built anything using this type of device before?

What sort of design should I look at that might be suitable for such devices?

Anachip Corp. AF4502C Dual NPN+PNP MOSFET

Features

rDS(on)=13.5mΩ @VGS=10V 10.0 (N-Ch)
rDS(on)=20mΩ @VGS=4.5V 8.4 (N-Ch)
rDS(on)=19mΩ @VGS=-10V -8.5 (P-Ch)
rDS(on)=30mΩ @VGS=-4.5V -6.8 (P-Ch)
Low rDS(on) Provides Higher Efficiency and Extends Battery Life
Miniature SO-8 Surface Mount Package Saves Board Space
High power and current handling capability
Low side high current DC-DC Converter applications

General Description

These miniature surface mount MOSFETs utilize High Cell Density process. Low rDS(on) assures minimal power loss and conserves energy, making this device ideal for use in power management circuitry. Typical applications are PWM DC-DC converters, power management in portable and battery-powered products such as computers, printers, battery charger, telecommunication power system, and telephones power system.


Bitek B3942 Dual NPN+PNP MOSFET

General Description

B3942 is the N- and P-Channel logic enhancement mode power field effect transistor using high cell density, DMOS trench technology in production. This high density process is especially tailored to minimize on-state resistance. The device is particularly suitable for low voltage application such as cellular phone and notebook computer power management and other battery powered circuits where high-side switching and low in-line power loss are needed in a very small outline surface mount package.

Features

RDS(ON)=24mΩ@VGS=10V (N-Ch)
RDS(ON)=38mΩ@VGS=4.5V (N-Ch)
RDS(ON)=25mΩ@VGS=-10V (P-Ch)
RDS(ON)=26mΩ@VGS =-4.5V (P-Ch)
Super High Density Cell Design for Extremely Low RDS(ON)
Exceptional On-Resistance and Maximum DC Current
SOP-8 Package

Applications

Power Management in Note book
Portable Equipment
Battery Powered System
DC/DC Converter
Load Switch
DSC


Alpha & Omega Semiconductor Ltd. AO4614 Dual NPN+PNP MOSFET

General Description

The AO4614 uses advanced trench technology MOSFETs to provide excellent RDS(ON) and low gate charge. The complementary MOSFETs may be used in H-bridge, Inverters and other applications. Standard Product AO4614 is Pb-free (meets ROHS & Sony 259 specifications). AO4614L is a Green Product ordering option. AO4614 and AO4614L are electrically identical.

Features

RDS(ON)=31mΩ@VGS=10V (N-Ch)
RDS(ON)=45mΩ@VGS=4.5V (N-Ch)
RDS(ON)=45mΩ@VGS=-10V (P-Ch)
RDS(ON)=63mΩ@VGS =-4.5V (P-Ch)

 

[abraxalito]

Great parts for building classD amps, not so great for linear applications as then you're generally constrained by dissipation and having two transistors in one package means you'll probably not want to use one of them at all.

For headphone amps Toshiba's range of FETs I've found highly suitable, they're not expensive and they have low threshold voltages (of the order of 1V) which becomes important in battery powered applications.

SSM3J327R,LF Toshiba Semiconductor and Storage | Discrete Semiconductor Products | DigiKey

 

[Ian Greenhalgh]

That's a pity, I thought I could build a simple amplifier with them, something like this:

 

[abraxalito]

Why not try anyway? The more mistakes you make, the more you stand to learn.

That amp is a complete disaster but finding out why could be quite educational.

 

[Ian Greenhalgh]

Well, that's not a working schematic, hence the bias diodes are not marked with values.

I will have to find a better schematic to work from.

 

[Ian Greenhalgh]

Some visual musings over how these things might be use to create a simple amplifier, although I'm sure I've over simplified things with just two resistors.

 

[Monte McGuire]

The dissipation issue is definitely a problem when trying to use modern trench FETs in linear mode. Most users run them fully on or fully off most fo the time, so dissipation is usually not such a big problem. However, you can buy them in packages that have die attach pads, and this means that you can heatsink them to the PC board or to some other sort of thermal structure.

 

[Ian Greenhalgh]

Aah, so these ones in the SOP-8 package are less than ideal due to heat issues.

I have 20 of the AO4614 so maybe I could parallel them 10 per channel and that way each individual one is only running at a fraction of it's rated maximum?