Using IGBT or Mosfet

S

sabu31

Advanced Member level 1
Joined
Jan 28, 2011
Messages
468
Helped
12
Reputation
24
Reaction score
13
Trophy points
1,298
Visit site
Activity points
5,370
Hi all,

I need to make a simple boost converter for boosting from 48 V to 192V at 1.2kW.
I have need a suggestion regarding which would be apt device for the active switching device (IRF300P226 Mosfet (2 in parallel) ) or (FGH50T65UPD IGBT (2 in parallel)). Which would give lower overall loss. Switching Frequency is around 50kHz.

Also presently I am using Diode SL80F040 for the boost diode, should i replace the diode with mosfet and go for synchronous boost.
 

Sort by date Sort by votes
It seems everyone is computing power incorrectly and not including dynamic power loss.
For Miller Capacitance I recall the dynamic peak will escalate rapidly above static loss at if the date drive resistance is not low enough <= Rg input. Then the dynamic FET loss Pavg increases with f but does not change with d.f. since there is high boost voltage gain on Vds/Vgs feedback on boost regulators. There is an optimum balance between static + dynamic loss with gate exponential current, RdsOn and Ciss. Although Mitsubishi must have assumed this in their choice of components.


I wonder why has no one commented on Mitsubishi's equations for power loss in FETs.
Power is always the average of 1 or N cycles but may be the product of Pavg=Vrms*Irms for 1 or Ncycles.
 

Upvote 0 Downvote
dynamic losses are heavily dependent on the gate drive - this usually rests with the designer and his/her expertise ( or otherwise ) hence it appears fruitless to comment on dynamic ( switching ) losses until the layout and gate drive are known.
 

Upvote 0 Downvote
D.A.(Tony)Stewart said:
Power is always the average of 1 or N cycles but may be the product of Pavg=Vrms*Irms for 1 or Ncycles.
Click to expand...


for power dissipation of a transistor this almost never is true.
(indeed I can only think of two extreme cases: 0% duty cycle and 100% duty cycle)

Example: I have an 10mOhms MOSFET switching a 10 Ohms resistive load with 50% duty cycle:
* then we have a square wave of about 0V/10V with 50% duty cycle, making it 7.07V RMS across the MOSFET (this is what a true RMS meter shows across DS)
* and the current is a square wave of about 0A/1A with 50% duty cycle, making 0.707A RMS through the MOSFET (this is what a true RMS meter shows as I_D)

According the formula P_MOSFET = V_RMS x I_RMS = 7.07V x 0.707A = 5W (the true conduction power dissipation is about 0.005W)
But indeed the P_tot = integral [from 0 to t_period] (V(t) x I(t)) /t_period

The formula is true for:
constant ohmic loads .... which a switching transistor never is.

Where am I wrong?

Klaus
 

Upvote 0 Downvote

Seems to me ADI is pointing out its senseless, which does give it standing (sense), its senseless :

You do not want to calculate the rms value of the ac power waveform. This produces a result that is not physically meaningful.
Click to expand...

To be or not to be that is, or is not, the question .....

IGBT based welding at 100 Khz : https://www.millerwelds.com/resourc...nables-lighter-more-powerful-welding-machines

Hypertherm, Lincoln more like 50 - 60 Khz.


Knight
 
Last edited:

Upvote 0 Downvote
Thanks Easy-Peasy for answer and along with calculation. I needed a decision between IGBT and Mosfet, a mosfet is way to go for this application.

However, there must some cut-off limit say input voltage/ current where IGBT starts making more sense as compared to Mosfet or vice-versa.
--- Updated ---

Yes. I am seeing IGBT application in commercial half bridge series resonant inverter based induction cooking . They also operate in range of 40k to 60k.
 

Upvote 0 Downvote
The AD's datasheet plot below indicates for almost same Imax rating in Si MOS vs IGBT, shown for static loads.

IGBT's are the same as FET when using 50% or rated current then IGBT's have lower loss from lower Rce than RdsOn in spite of 0.7V diode offset.
This crossover threshold drops as the temperature rises to max 150'S. Using 50% of I max assumes Tjcn rising is 50% above 25'C with the same system thermal resistance.
T
One can choose I max current rating that is 2x nominal use for reliability using the typ. plot @ 25'C or could use 5x for FETs, but then dynamic power loss must be considered from L/R, RceCce << RdsOnCoss and other time constants. So one must not assume FETs are faster with LC reactive effects and lower Rdson means higher Coss. and longer L/R = T time constant and energy dissipation.

e.g. IGBT @ 25'C Rce (1.35-0.7V)/25A = 26 mohm
vs SJMOS, @ 25'C RdsOn=2.2V/25A = 88 mohm (Silicon Junction Metal Oxide Semiconductor)

Of course, this is subjective, comparing only one pair with ~same I max rating. I marked up below from AD link from @danadakk

 
Upvote 0 Downvote
@sabu31 IGBT's are cheaper - and can be quieter for EMI due to their tail losses - but they generally require more heatsink.

larger die IGBT's can have 10uS short ckt capability - which makes them very rugged compared to mosfets - hence you never see mosfets in motor drive applications.
 
Upvote 0 Downvote
Correction: #26 link was Tosbiba' not AD https://toshiba.semicon-storage.com/eu/semiconductor/knowledge/faq/mosfet_igbt/igbt-002.html
--- Updated ---

Hi Sabu,

You ought to know that you cannot choose an optimal design without details on dynamic and reactive load impedance ( time and frequency) Please define Z(f) and Z(t), along with voltage error tolerance, but preferably do not restrict yourself to devices or switching frequency (implementation-specific design-spec error) unless you state a good reason.

Cheers from Toronto,
Tony
 
Last edited:
Upvote 0 Downvote
You must log in or register to reply here.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…