IGBT Pdmax ratings "what the heck"

[jollwah]

Hi all, I am looking into inverter style plasma cutter power supplies mainly out of interest, anyway I started looking into some commercial machines and found that their power output generally exceeded the Pdmax ratings of the IGBT's used.

For instance the thermaldyne 250 tig although it uses SCR's to control the power still only has a IGBT rated to 780W IIRC. I believe that 200A at 18V is more like 3600W

I am a bit of Noob, at power electronics so I maybe missing something

Regards Jollwah

 

[dick_freebird]

Are you factoring in the advertised duty cycle rating,
which always seems to fade, big-time, at higher output
currents?

Also, power delivered >> power dissipated in a switching
type power supply.

 

[jollwah]

The 3600w is the delivered peak at the output so the IGBTs would need to pass at least this much power.

I am now starting to wonder what sort of heatsinking is used to determine the PD max value if any

Looking at a fuji IGBT datasheet at the moment

25*C ambient to 150*C max is 125*C increase

The IGBT is rated at 0.034 *C/W therefor 125/0.034 is 3670W

Whis is what the datasheet says that this IGBT module is rated for. So with effective heatswinking the power could be higher?

 

[jollwah]

I guess the question is, is the limting factor to how much power an IGBT can tolerate the junction temperature reached.

Are IGBT modules rated without external heatsinking?

 

[dick_freebird]

Still, I don't believe you are properly distinguishing between
power passed to the load, and power dissipated in the device.

The latter would be something like on-duty*Ron*(Idelivered**2)
(on-duty being the clock-rate duty cycle, not the use duty
factor). Of course there are also switching losses but assume
these are a tolerably small fraction of the big-end loss.

 

[FvM]

A good switching mode inverter can achieve efficiencies up to 98 or 99 %, in other words, not more than 1 or 2 % of transferred
power are dissipated, mostly in power semiconductors and inductive components.

 

[jollwah]

I understand that the power dissipated within the device is only a small percentage of the total power passed through the device.

I also realized that in a full bridge where 2 devices would be used that they would share the load.

I guess that my question was not worded that well. Basically as there does not seem to be any Ron values on the datasheet. How does the Rth (j-c) and Rth (c-f) come into it. Do I add the two values eg. Rth (j-c) max 0.12 °C/W + Rth (c-f) typ 0.025 °C/W = 0.145°C/W. Target temp rise 100°C therefore 100/0.145 = 689W as long as VCES and Ic are not exceeded.

I don't know if that makes sense it's late.

How do they work out the power handling of IGBT's?

Regards
Jollwah

 

[jollwah]

For instance fuji recommend these IGBT's for 28 - 33 KVA inverters yet each IGBT is rated for 660W.

See I would've expected that 2 of these on a full bridge would do around 1.2 KVA