Electronics > Beginners
Transistor VS MOSFET for linear power supply
(1/5) > >>
Prithul0218:
What are the advantage/disadvantage of using MOSFET/transistor as the main pass transistor of a linear power supply?

I see most professionally made power supplies use transistor, why do they avoid MOSFET? Can I just replace the transistor of a linear power supply with a MOSFET with similar ratings?
rstofer:
I think you will find that MOSFETs make excellent switches but poor linear elements.  If it was simple to pull out the transistor and drop in a MOSFET, there wouldn't be any transistors used for pass elements.  But, no, we use a 55 year old 2N3055 or even several in parallel.

Here's a technical paper on the subject.  It doesn't say you can't do it but it does point out the complications.

https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-AN-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200

Google has more material.
Jwillis:
Mosfets are better suited for switch mode operation because when operated in their linear region  MOSFETs are subjected to high thermal stress due to the simultaneous occurrence of high drain voltage and current, resulting in high power dissipation.When the thermo-electrical stress exceeds some critical limit, thermal hot spots occur causing the devices to fail. To prevent such failure, MOSFETs operating in the linear region require high power dissipation capability and an extended forward-bias safe operating area.Thermal hot spots can also trigger a second breakdown which is also destructive to the mosfet.
Some Linear Mosfets are designed to handle these conditions by suppressing positive feedback of elector-thermal instability and extending the forward bias safe operating area.They'er built much tougher than a standard mosfet .But this also increases the price as much as 10 times the price of a standard mosfet or a BJT.
BJT function much better in linear operation because unlike a mosfet they are current controlled.So simple current limiting is all that is needed to control a BJT.This makes for a simpler and cheaper linesar circuit .
spec:

--- Quote from: Prithul0218 on December 06, 2018, 06:32:00 am ---What are the advantage/disadvantage of using MOSFET/transistor as the main pass transistor of a linear power supply?

I see most professionally made power supplies use transistor, why do they avoid MOSFET? Can I just replace the transistor of a linear power supply with a MOSFET with similar ratings?

--- End quote ---
Hi Prithul0218,

That is a good question, because at first glance you would imagine that power MOSFETs are far superior in all respects to power bipolar junction transistors (BJTs).

The disadvantages of power MOSFETs compared to an ordinary power BJTs are:

* Massive parasitic capacitances, sometimes as high as 15nF, which vary with voltage and current. This makes designing with MOSFETs difficult as the parasitic capacitances act as integrators and limit the frequency response of the overall circuit (frequency response is important in PSU design)
* MOSFETS have a very high frequency response, but this is a double edged sword because, coupled with the high parasitic capacitances,  MOSFETs are susceptible to parasitic oscillations, normally around 4MHz
* Higher voltage drive than a BJT. You can turn a power BIT on hard with about 1.8V, but at least 4V would be required for a power MOSFET. The MOSFETs with a lower drive voltage tend to have correspondingly higher parasitic capacitances
* Generally poorer safe operating area (SOA)
* Generally, lower maximum junction temperature
* Higher costThese are also the reasons, especially the parasitic capacitances which cause distortion, why you don't see MOSFETs used much in high end audio power amplifiers either.

But another reason that MOSFETs are not used that much in PSU is tradition- production designers like to stick to well tried and tested circuits whenever possible and the old workhorse NPN 2N3055 and PNP MJ2955 have a lot going for them, and they only cost around £1UK, and 45p if you buy 25 or more.

Having said that though, there is a trend towards using MOSFETs in PSUs. And the Nitride power FETs look promising.

MOSFETs are used extensively in switch mode PSU because of their high frequency response and and low ohmic on resistance, giving a compact and efficient PSU. But you have to go through all sorts of antics to get them to work. Luckily there is a host of standard chips and other components, especially capacitors and inductors, that have been developed over the years to help use power MOSFEts in SMPSUs and other applications.

Of course, power MOSFETs do have many advantages which have not been described here.
 
MosherIV:
Hi

In a linear, what ever the pass device is, it must be capable of dissapting lots of power. Worst case is when the psu output voltage is at near min and max current, the pass device must drop near max voltage and pass the max current - ie this is the max power it MUST dissipate.

FETs need to have the gate voltage removed rapidly (pulled) when swtching off or they will randomly drift half on/off.

Transistor can be switched off by removing the base current, ie you can add an output on/off switch by putting a switch to the base of the pass transistor and the switch does not need to be capable of switching the full rated psu current.
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod