Why Choose IGBT Over MOSFET for Low-Frequency Applications?

[tim9000]

Apologies upfront, I have a backlog of PF threads I have created not replied to...I will endeavour to do so when I can.

So I've read that IGBTs are more efficient than MOSFETs for lower than 20kHz frequencies (and are capable of use for high voltage rating and better performance than ordinary bipolar transistors).

However, the datasheet of the IGBTs that I bought states that the low saturation collector emitter voltage is 2V at 25A. And I read that on the internet: "The main advantages of an IGBT over a power MOSFET are the much lower on-state voltage drops due to conductivity modulation. This feature translates to a smaller chip size compared to that for a MOSFET for the same current and voltage ratings."

I don't really understand what conductivity modulation is (is this wider ability to control the resistance of the emitter to collector with the gate voltage?), but I also thought that MOSFETs were of a similar conductive voltage drop to diodes, ~0.7V.

If this is true, it would seem to me that MOSFETs would have a lower condition loss.

Can anyone shed some light on this? Also, if I was to control a simple ON-OFF 24V DC load using either, which one would I choose?

Thank you

 

[Asymptotic]

Check out "Conductivity Modulation in Semiconductor Structures Under Breakdown and Injection" (from a Springer publication).
http://www.springer.com/cda/content/document/cda_downloaddocument/9781441965646-c1.pdf?SGWID=0-0-45-968746-p173998968

Also, application note IXAN0063, "Insulated Gate Bipolar Transistor (IGBT) Basics" under the General|IGBTs tab.
http://www.ixys.com/TechnicalSupport/appnotes.aspx

if I was to control a simple ON-OFF 24V DC load using either, which one would I choose?

• How much current?
• Is the load resistive, or reactive (i.e. - a relay coil)?
• What is the required switching speed?

Unless the application required those features that can make IGBTs and MOSFETs more desirable why not use a BJT? .

 

[tim9000]

Check out "Conductivity Modulation in Semiconductor Structures Under Breakdown and Injection" (from a Springer publication).
http://www.springer.com/cda/content/document/cda_downloaddocument/9781441965646-c1.pdf?SGWID=0-0-45-968746-p173998968

Also, application note IXAN0063, "Insulated Gate Bipolar Transistor (IGBT) Basics" under the General|IGBTs tab.
http://www.ixys.com/TechnicalSupport/appnotes.aspx

• How much current?
• Is the load resistive, or reactive (i.e. - a relay coil)?
• What is the required switching speed?

Unless the application required those features that can make IGBTs and MOSFETs more desirable why not use a BJT? .

Thank you for the reply.

I will checkout those document links you provided, cheers.

The current will be not insignificant, I was going to drive a DC motor with it, maybe up to 200W peak. I suppose there is no inductance with a DC motor, so resistive.
If it was an inductive or capacitive load (say of 5A), why would that make a difference?

The switching speed would be like 0.001Hz.

EDIT: I suppose it would depend on if it was a brushless (pennant magnet) DC motor or not; I don't know about brushless motors, but I suppose I would need to make a controller for a brushless one, not sure what that switching speed would be though, sorry.

Thanks

 

[Asymptotic]

if I was to control a simple ON-OFF 24V DC load using either, which one would I choose?

Forgive my incorrect assumption. "Simple load" brings to my mind visions of resistance heaters, relay coils and the like, but not motor and their controllers. BJTs would not be appropriate, but an SCR bridge might be in the running.

The current will be not insignificant, I was going to drive a DC motor with it, maybe up to 200W peak. I suppose there is no inductance with a DC motor, so resistive.

200 watts isn't very much power (not when one is accustomed to motors in the 300 kw to 520 kw range). You'd be supposing wrongly about a motor having no inductance, after all, motors are clever arrangements of coils of wires interacting with magnetic fields, and another name for a coil of wire is an inductor.

If it was an inductive or capacitive load (say of 5A), why would that make a difference?

Because switching reactive loads can generate voltages substantially greater than supply voltage. This affects component selection and circuit design.

The switching speed would be like 0.001Hz.

I take it you mean a switching frequency of 1 kHz? A frequency of 0.001 Hz has a period of 1000 seconds.

EDIT: I suppose it would depend on if it was a brushless (pennant magnet) DC motor or not; I don't know about brushless motors, but I suppose I would need to make a controller for a brushless one, not sure what that switching speed would be though, sorry.

I'm not following your meaning. What specific motor are you planning to use?

 

[tim9000]

Thanks for the Reply

... but an SCR bridge might be in the running.

THAT'S A GOOD POINT, I'VE NEVER USED AN SCR, AND SO DIDN'T THINK OF IT.

200 watts isn't very much power (not when one is accustomed to motors in the 300 kw to 520 kw range). You'd be supposing wrongly about a motor having no inductance, after all, motors are clever arrangements of coils of wires interacting with magnetic fields, and another name for a coil of wire is an inductor.

YES, THAT'S TRUE, THE REASON I SAID THAT WAS BECAUSE I REMEMBERED HEARING THAT IF YOU RAN A DC MOTOR WITH AC, IT WOULD WORK, BUT IT WOULD KILL THE PERFORMANCE, BECAUSE OF THE AC INDUCTANCE.

Because switching reactive loads can generate voltages substantially greater than supply voltage. This affects component selection and circuit design.

SO VOLTAGE SPIKES SUCH AS FROM LEAKAGE INDUCTANCE CAN BLOW THE MOSFET WHEN IT SNAPPS OFF?

I take it you mean a switching frequency of 1 kHz? A frequency of 0.001 Hz has a period of 1000 seconds.

ACTUALLY THIS WAS MY FAULT, I WAS BEING A BIT OF A SMARTARSE, 1000 SECONDS IS CORRECT, THE PROJECT WOULD BE FOR AN ELECTRIC SCOOTER (ALSO PRACTICE IN CONSTRUCTING CIRCUITS WHICH CONTROL POWER ELECTRONICS).

I'm not following your meaning. What specific motor are you planning to use?

I DO HAVE AN OLD DC MOTOR, BUT I WAS CONSIDERING ONE OF THOSE NEW CHEAP CHINESE HUB MOTORS, WHICH I THINK USE PERMANENT MAGNETS, SO I MIGHT NEED TO BUILD A MORE COMPLEX CONTROLLER FOR IT

Thanks

 

[Windadct]

24V DC generally - MOSFET, Ihttps://www.infineon.com/dgdl/choosewisely.pdf?fileId=5546d462533600a40153574048b73edc

A 60V + , 20A MOSFET, with heatsink should foot the bill.

It is not clear if you are just turning the motor on an off, one direction (Single switch - simple, or even a relay) , or bi-directional with speed control ( PWM) may need 4 MOSFET, 2 PWM channels - etc.

SCR will not allow turn off - without additional circuitry.

 

[cabraham]

IGBT uses a bjt to carry load current, which is driven by a MOSFET. The bjt collector-emitter voltage drop increases with collector current by a power less than 1. Maybe logarithmic or close. If Ic doubles, then Vce increases less than double.
FETs exhibit a 1st power relation. If Id doubles, Vds doubles. It is linear. At low current FETs exhibit low drop. At high currents IGBT has lower drop.
Lights have lower drop & conduction loss at high current. But switching losses are higher for IGBT. Although new IGBTs are getting faster, they still incur higher loss at high frequency than FETs.
At low voltage FETs are preferred, under 250 volts. At higher voltages, IGBT incurs lower conduction loss, higher switching loss.
A good program or spread sheet can compute losses for both, & selection is eased. Did I help.

Claude

 

[tim9000]

Hi All,
I am sorry I haven't had enough time to read the replies in depth, I am hoping to tomorrow. I have briefly skimmed the replies and I have read some interesting things, I appreciate your responses. Especially because I really haven't explained things very well, partly because I'm not working from a design and convoluting a real project with thought-experiment.
Case in point:

It is not clear if you are just turning the motor on an off, one direction (Single switch - simple, or even a relay) , or bi-directional with speed control ( PWM) may need 4 MOSFET, 2 PWM channels - etc.

Yes you're right, I was thinking about the MOSFET as just an on-off relay, but in practice it would be used for low frequency speed control

Hopefully I will reply again soon, with any luck tomorrow.
Thanks.

 

[Windadct]

Here are a couple reference figures from an Application Manual

 

[Windadct]

Hello Again - I have been asked to cite the above references. It is from the Semikron Application Handbook, complete PDF is downloadable for free from their website : available HERE. ( ~ 25MB ) Paperback available from the SHOP, for about $35 inc shipping.

- Full disclosure, I work for Semikron.

Still I would compare this to the old GE SCR book, as one of THE must have references in the application of power electronics, our market is roughly 10KW and above.

As for a reference site, try www.powerguru.org

 

[DaveE]

Yes, SCRs are rugged and inexpensive compared to MOSFETs and IGBTs (assuming you use them correctly, of course). But they are much harder to control than the transistor alternatives. At 200W, they probably aren't worth the effort. MOSFETs are definitely the easiest to use, but also the most expensive. Don't neglect the design/cost of cooling the device (i.e. heatsinks). Make sure your design takes into consideration the back EMF produced by the motor when your transistors turn off; you must always provide some path for the load current, since it will not stop instantly.