mosfet/IGBT swap / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

mosfet/IGBT swap

Move Thread

LAN_403

IamSmooth

Fri Jan 22 2010, 02:15AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

If I can use my 240vac mains I might try to increase the power to my inverter for some other experiments.
1. Can I swap an IGBT for a mosfet if my frequency in not too high?

THis IGBT

says on the datasheet that it has a built in fast-recovery diode.

2. Does this mean that I would not need a body isolation diode before the collector, and not need a freewheeling diode. Is it a good practice to just have them anyway?

Dr. Slack

Fri Jan 22 2010, 08:05AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The MOSFET process has a parastic body diode, so they're always in there for free, albeit not very good.

The IGBT process doesn't create a body diode, so if there's one in there, it's got reasonable specs.

Steve Conner

Fri Jan 22 2010, 10:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Correct, when you go to IGBTs, Richie's freewheel diodes and schottkies aren't needed any more. Just the four IGBTs and nothing else.

(Just make sure you buy IGBTs with internal diodes. They are available both with and without.)

IamSmooth

Fri Jan 22 2010, 12:30PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

If one is operating at a low enough frequency, Steve, why would one want to use a MOSFET then?

Adrenaline

Fri Jan 22 2010, 02:01PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
Location:
Posts: 80

I am using those exact IGBTs in my induction heater halfbridge. I have had them running at 180kHz, however I am modifying my gate waveform to have programmable dead time. I was just using 50% duty cycle from the PLL chip.

The main issue is the tDoff is much larger than the tDon, you need to make sure to have enough dead time to negate shoot through.

IamSmooth wrote ...

If one is operating at a low enough frequency, Steve, why would one want to use a MOSFET then?

At low frequencies the trade off with MOSFET versus IGBT is conduction losses. The linked IGBT has a VCEsat of 2.5V at 60A giving a conduction loss of 150W. The MOSFET behaves as a resistor while conducting. With the resistor you get I2R losses, to get the same conduction losses you would need a MOSFET with an Rds of 41milliohms at 60A.

Steve Conner

Fri Jan 22 2010, 02:16PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

IamSmooth wrote ...

If one is operating at a low enough frequency, Steve, why would one want to use a MOSFET then?

MOSFETs are cheaper, and if you arrange for your load to always be inductive, then the body-drain diodes never conduct and they can do without the extra gubbins.

wylie

Sat Jan 23 2010, 01:57AM

Registered Member #882 Joined: Sat Jul 07 2007, 04:32AM
Location:
Posts: 103

I thought the FET vs IGBT choice was based more on the voltage/power requirements, and not the frequency. Yeah, FETs go to pretty high voltages too, but the Rds(on) keeps getting higher, and those I2R losses prevent seriously high powers. So if you just need to slap around 100-200W of 120-340V (tv flyback?) a FET is fine. 1-2kW at those same voltages would make the FETs roast, and so IGBTs come into play.

Of course, if you want to go higher than .5MHz, then IGBTs just wont cut it anymore. You're stuck with FETs unless you need so much power that tubes become more economical.

This whole post should be considered as a question, but here's a second question:
Where do BJTs still fit in this picture. Though i'd think FETs would be the choice for tv flybacks, they use BJTs instead. Is it economy again, or is there a component benefit? Or am i only familiar with TV's made in the 70's-90's? Do they run the flybacks differently nowadays? That seems like more than just a 2nd question :)

Steve Conner

Sat Jan 23 2010, 02:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

wylie wrote ...

Where do BJTs still fit in this picture. Though i'd think FETs would be the choice for tv flybacks, they use BJTs instead. Is it economy again, or is there a component benefit? Or am i only familiar with TV's made in the 70's-90's? Do they run the flybacks differently nowadays? That seems like more than just a 2nd question :)

As far as I know, BJTs are still used, and it's economy. The HOT has to have a very high voltage rating, and it's cheaper to make a 1200V BJT than a 1200V MOSFET or IGBT.

The BJT technology is slower and has a poorer SOA than the alternatives, but they design the circuit to make it work. The little capacitor across the transistor stops it exceeding its SOA, and in designs where the same transistor drives the flyback and the deflection coils, they use a PLL circuit that compensates for the BJT's turn-off time and its variation with load current and temperature. It does this by sensing the HV flyback pulse at the HOT collector and phase-locking this to the sync.

Because high voltage BJTs are so cost-effective, Fairchild or someone recently brought out a bizarre device called an ISBT. It's a HV BJT and a low-voltage MOSFET, cascoded together in a 4-pin TO-3P package. (The 4 pins are Collector, Base, Gate and Source... take your pick!)

The basic idea, driving a BJT through its emitter with a power MOSFET, has been in use since the 70s, and I've come across it in a few old monitors and off-line SMPS.

I believe late-era monitors started to use a HV MOSFET to drive the flyback, as MOSFETs got cheaper and operating frequencies got higher. CRT monitors got up to line scan rates of nearly 100kHz before LCDs killed them.

IamSmooth

Sat Jan 23 2010, 11:16PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I want to order some IGBTs to try and take my induction heater to the next level. I have found some that say they are suitable for resonant and soft-switching applications. My inverter is suppose to work at the ZVS region, but it could fall out of resonance. Will these IGBTs work, or should I look for the type that don't mention "soft switching" like this one

This one mentions soft switching

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob