john_ellis said:
In some of the tests I drove the gate with a pretty low source impedance, on the order of 100 ohms. In others, I drove it with source impedance up to 2K. The nature of the anomaly was pretty much unchanged over this wide range of source impedances, so I don't think gate capacitance, as seen at the gate terminal, is the issue. However, I did not try measuring Cgs and Cgd as a function of frequency.
My guess is that there is some stray capacitance field originating at the drain and interacting directly with parts of the channel to modulate it, kind of like an additional virtual gate. This virtual gate might be nearly floating, with a high discharge resistance, accounting for the fairly long time constant that must be involved for a frequency response anomaly to occur in the low kHz range.
Cheers,
Bob
Bob Cordell said:
I think that's the most logical explanation on the table. Too
bad we don't have access to that virtual gate - we could have
a solid state Tetrode
In any case, I think of it like the weather - there it is. If it's
a problem you can move elsewhere, to Harris, KS or Fairchild, WI,
where the weather is different.
Nelson Pass said:
I think that's the most logical explanation on the table. Too
bad we don't have access to that virtual gate - we could have
a solid state Tetrode
In any case, I think of it like the weather - there it is. If it's
a problem you can move elsewhere, to Harris, KS or Fairchild, WI,
where the weather is different.
Good point. I'll probably move south when I retire.
Cheers,
Bob
I’d like to resurrect a previous discussion on the 10mS pulse rating of the IFRP240 and the comparative robustness of BJT’s with similar power dissipation and DC current ratings.
At the time I mentioned (partly from experience) the ON Semi MJL21193/4 as devices of similar robustness, but this was dismissed because the datasheets for these devices unfortunately omit 10mS pulse SOA curves.
I’ve just been perusing a whole stack of BJT datasheets for the selection of the output devices for a Class A amplifier design I’m finalising. Here's what I've found.
From the graphs, the 10ms pulse specs for the IRFP240 are:
10V / 30A
50V / 6A
100V / 3A
Being a MOSFET, there is no secondary breakdown, so it’s basically 300W all the way.
The datasheet for the ON Semi MJL1302 (fT=30MHz, Vce=260V, P=200W, Ic=15A), which has an almost identical DC SOA to the MJL21193, fortunately shows a 10mS pulse SOA. Specs are:
10V / 30A
50V / 11A
100V / 3.5A
200W, 180V, 17A, 50MHz fT Sanken BJT, 2SC2922’s 10mS SOA is as follows:
10V / 35A
50V / 12A
100V / 4.5A
200W, 230V, 17A, 60MHz fT Sanken BJT, 2SC3264’s 10mS SOA is as follows:
10V / 35A
50V / 12A
100V / 5A
These are just a few examples. There are many more devices in the ON Semi catalogue of similar robustness in the 4 to 40MHz fT range and Sanken make a few others too.
Cheers,
Glen
At the time I mentioned (partly from experience) the ON Semi MJL21193/4 as devices of similar robustness, but this was dismissed because the datasheets for these devices unfortunately omit 10mS pulse SOA curves.
I’ve just been perusing a whole stack of BJT datasheets for the selection of the output devices for a Class A amplifier design I’m finalising. Here's what I've found.
From the graphs, the 10ms pulse specs for the IRFP240 are:
10V / 30A
50V / 6A
100V / 3A
Being a MOSFET, there is no secondary breakdown, so it’s basically 300W all the way.
The datasheet for the ON Semi MJL1302 (fT=30MHz, Vce=260V, P=200W, Ic=15A), which has an almost identical DC SOA to the MJL21193, fortunately shows a 10mS pulse SOA. Specs are:
10V / 30A
50V / 11A
100V / 3.5A
200W, 180V, 17A, 50MHz fT Sanken BJT, 2SC2922’s 10mS SOA is as follows:
10V / 35A
50V / 12A
100V / 4.5A
200W, 230V, 17A, 60MHz fT Sanken BJT, 2SC3264’s 10mS SOA is as follows:
10V / 35A
50V / 12A
100V / 5A
These are just a few examples. There are many more devices in the ON Semi catalogue of similar robustness in the 4 to 40MHz fT range and Sanken make a few others too.
Cheers,
Glen
that is not true.
Most Mosfets do not shown secondary breakdown but some do and for those types the reduction in SOA at higher voltages is quite significant.
Being mosfet does not equal constant power at all voltages, it appears to depend on the manufacturing process adopted for each FET type.
The majority of FETs have constant power ability and a ratio of DC to transient SOAs.
The bigger BJTs suffer from secondary breakdown and must be de-rated for elevated Vce. However the de-rating factor is less for transient effects and as a result BJTs have a very good SOA for high voltage AND transient currents. Your extracts confirm this characteristic.
I'm afraid the pro FET lobby chooses to ignore and probably deliberately hide this BJT property.
AndrewT said:
OK, but I wasn't speaking of MOSFET’s in general, I was specifically referring to the IRFP240’s 10mS SOA.
jacco vermeulen said:
The MJL1302A is a PNP. The ON Semi datasheet gives a single SOA graph for both the MJL1302A and it’s NPN complement, the MJL3281A.
With the Sanken devices, it’s true that the PNP complements are not as good as the NPN’s, but the 10mS SOA’s for the PNP complements for the transistors I listed still better the IRFP240.
Yup. 600W 10mS @ 50V. That's pretty good.
Cheers,
Glen
jacco vermeulen said:
Hi Jacco,
How are you so sure about the sound of APT....have you tried it so far or just assumptions based on your thinking....
The SOA curve is of APT20M22LVR only...
There are numerous hi power devices above its rating also...but that doesnot mean they sound good or bad unless one has tested them
K a n w a r