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Old 22nd July 2013, 08:38 AM   #11
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Here is a good application note from IR explaining the linear operation SOA problem
http://www.irf.com/technical-info/appnotes/an-1155.pdf
I don't think any "conventional package" mosfets would allow a single pair to survive 50V rails into real loads
IXYS made some special devices for linear operation, http://www.ixys.com/Documents/Articl...er_MOSFETs.pdf but only in N channel
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Old 22nd July 2013, 11:12 AM   #12
AKSA is offline AKSA  Australia
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Yes, David, well spotted.

This devices are around -12mV/C tempco on the Vgs according to measurements I made a couple of years on the FQA series, which are rated to 40A and 300W.

You need a fairly hefty bias generator to accommodate this soaring rise of current with temperature.

Cheers,

Hugh
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Old 22nd July 2013, 01:14 PM   #13
Bigun is online now Bigun  Canada
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Quote:
Originally Posted by fab View Post
Regarding the output mosfet I am interested in suggestions as long as the VGSoff is high enough for the SKA amp to work properly at full power. As for mods I would put separate source resistor for each mosfet allowing less need for VGS close match...
As far as I can see, most of the vertical MOSFETs we might consider using here will have enough Vgs room to work in SKA and that's the case for the Fairchild devices I'm considering. There are Laterals that have quite low Vgs and for these Greg designed the GB100 - although I don't think it ever came to market. Good idea on the separate source resistors, I wonder why Greg didn't do this, must be a reason. However, I think it's still sensible to match the devices well as the source resistor value used is not exactly generous.

Edit: I'm suspect Greg added this resistor to reduce turn-on current when the devices are cold.

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Originally Posted by fab View Post
If you are able to take out the temperature sensing diode-wired-transistors I would try it also in my CFB version.
I didn't explain well - I will still incorporate transistor temperature sensing elements as they are needed. The thing is, if you look at the schematic I posted above there are three transistors in the current source / sink. They are all temperature sensitive. If you remove the diode-wired transistor you are left with the classic 2-transistor source/sink. In the 2 transistor version you can still place one of the devices (the master device, the one at most left of the diagram, Q6 and Q8) onto the heatsink and as it warms up it will reduce the current level of the source/sink. So I wonder why not do this ?


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Originally Posted by davidsrsb View Post
I had a look at the FQP12P20 QFET and it has -ve Vgs temperature coefficient all the way up to 10A.
Thanks for posting that paper, most relevant. I did look at the datasheet for the FQA40N25 on Digikey's website. Sure enough if you look at the Vgs verses Id curves at around the 100mA level you need to swing the gate 1V to keep current stable between 25degC and 175degC - around 12.5mV per degC. That's a lot but only twice that of the devices Greg used. Well this is interesting, part of the fun of learning about using FETs instead of BJTs.

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Originally Posted by davidsrsb View Post
I don't think any "conventional package" mosfets would allow a single pair to survive 50V rails into real loads
These QFET parts have about twice the SOA of the parts Greg used, why wouldn't one of them do the job of two ? - it's not for welding duties, just music ??
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Last edited by Bigun; 22nd July 2013 at 01:43 PM.
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Old 22nd July 2013, 03:16 PM   #14
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Properly biasing vertical mosfets in common source is hard.
You need 6-10mV/C for ambient compensation and anything up to double that for die temperature rise ABOVE ambient, depending on how well the sensor is tracking the die. GB has done a good job at compensating for ambient, set by the ratio of the 3k and 1k1 resistors times the sense diode tempco, so about 6mV/C. He may well be undercompensating for die temperature rise, depending on heatsink size. This is probably why the SKA has a bit of a reputation for blowing with square waves etc.
I am thinking about using a simple PIC processor to sense ambient and die delta separately and do the maths properly
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Old 22nd July 2013, 04:39 PM   #15
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I will take a look at what I can do to ramp up the compensating tempco so that there is the flexibility to use different MOSFETs.

I didn't hear about square wave issues with the amp - but I'm a bit skeptical about square wave testing myself - I've read more than one well-reasoned report why this type of testing can be misleading. And why a 20KHz high power square wave will fry an output stage faster than you can set the gain on the scope... !
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Old 22nd July 2013, 08:14 PM   #16
MiiB is offline MiiB  Denmark
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It's a bit like a SSA adapted for vertical mosfets and voltage feedback, I am sure it sounds good.

I like the in between resistors compensation.
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Old 22nd July 2013, 08:25 PM   #17
CBS240 is offline CBS240  United States
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Quote:
Originally Posted by AKSA View Post
Yes, David, well spotted.

This devices are around -12mV/C tempco on the Vgs according to measurements I made a couple of years on the FQA series, which are rated to 40A and 300W.

You need a fairly hefty bias generator to accommodate this soaring rise of current with temperature.

Cheers,

Hugh
I found the tempco of these mosfets to be slightly worse than the plural well devices as well. However, at least with a HEC scheme, adequate compensation can be had by using both error amp devices monitoring the thermal difference for larger Vgs muliplication, I seem to recall Bob used one of the devices as the Vgs multiplier in his EC hexfet amp. HEC is source follower (common drain) and I have not used these devices for class AB common source. I suspect source ballast resistors would help to reduce the tempco variation but it may be a challenge to accurately track the tempco. Fortunately mosfets have a larger range of acceptable bias than do BJTs related to the larger variation in Gm. I suggest starting with a slightly higher bias and overcompensating just a bit. You will have to build an actual real circuit and do some trial and error. Don’t be too misled by the Pd max figure, Pd de-rating is significant for these devices. For example, FQP47P06 Pd max is 160W @ 25C, but de-rated at 1.06 W/C. So about 27W at 150C, still quite significant for a TO-220 device. Another small advantage of the planer stripe fets is a lower gate charge vs drain current di/dt. As far as hot-spotting, the type of mosfet is important. Certain types are able to more evenly spread the heat across the die. I have found the planer stripe, at least the Q-fet brand, to be surprisingly quite robust when used as an analog power device. (notice I did not say linear, there is nothing linear about these devices)



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And why a 20KHz high power square wave will fry an output stage faster than you can set the gain on the scope... !
Not always true. I have made amps such as this one that can output much faster square wave than 20KHz and it did not fry.
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Last edited by CBS240; 22nd July 2013 at 08:31 PM.
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Old 23rd July 2013, 01:55 AM   #18
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Attached shows any alternative current source topology. Don't be distracted by the part numbering which is different compared with the master schematic.

PSRR: for improved PSRR you see the small rail capacitor is connected not to the collector of the current source 'master device' Q21, but to the mid-point between the two tail resistors. I've played around with the values a bit and it's possible to tweak up the PSRR in simulations but it can get sensitive to local bias conditions and I would not rely on it to achieve the highest possible PSRR - nevertheless it is an improvement over the baseline published by Greg recently. And if you read around on the internet you'll find that others have tinkered with the baseline current source design, including the use of a zener diode to define a 'regulated' voltage at this junction between the two resistors.

Tempco: In this version of the current source I'm exploring the option to remove the diode-wired transistor and instead I am using the current source 'master transistor' itself as the temperature sensing element which should be placed in physical contact with the output MOSFETs or related heatsink. When this device warms up the output of the current source is reduced and hence so is the bias to the output MOSFETs - which is how we want it to operate. The tempco sensitivity of this arrangement can be adjusted, it depends on the emitter degeneration of Q21, i.e. R93 in this diagram.

In my simulation I varied the temperature of Q21 between 30 degC and 50 degC for a total delta of 20 degC. I then 'probed' the simulated voltage at the gate of the upper output MOSFET to see how much it changed.

With the values shown, the simulated voltage change is 135mV, which is 6.75mV per deg C. This is not far from the tempco of the MOSFETs recommended by Greg.

The largest tempco is achieved when the emitter degeneration resistor on Q21 is set to zero. The value of the 'sense resistor' R49 is adjusted to achieve the desired idle current through the output MOSFETs. Simulating the circuit using these values predicts a tempco of 16mV per deg C. Could this be enough for safe use of the QFETs ?
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Last edited by Bigun; 23rd July 2013 at 02:02 AM.
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Old 23rd July 2013, 02:17 AM   #19
fab is offline fab  Canada
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Quote:
Originally Posted by Bigun View Post
Attached shows any alternative current source topology. Don't be distracted by the part numbering which is different compared with the master schematic.
.....

Tempco: In this version of the current source I'm exploring the option to remove the diode-wired transistor and instead I am using the current source 'master transistor' itself as the temperature sensing element which should be placed in physical contact with the output MOSFETs or related heatsink. When this device warms up the output of the current source is reduced and hence so is the bias to the output MOSFETs - which is how we want it to operate. The tempco sensitivity of this arrangement can be adjusted, it depends on the emitter degeneration of Q21, i.e. R93 in this diagram.

In my simulation I varied the temperature of Q21 between 30 degC and 50 degC for a total delta of 20 degC. I then 'probed' the simulated voltage at the gate of the upper output MOSFET to see how much it changed.

With the values shown, the simulated voltage change is 135mV, which is 6.75mV per deg C. This is not far from the tempco of the MOSFETs recommended by Greg.

The largest tempco is achieved when the emitter degeneration resistor on Q21 is set to zero. The value of the 'sense resistor' R49 is adjusted to achieve the desired idle current through the output MOSFETs. Simulating the circuit using these values predicts a tempco of 16mV per deg C. Could this be enough for safe use of the QFETs ?
Hi Bigun

This is interesting but quite unusual current source with R93 instead of emitter of Q21 connected to power rail.... How do you predict by calculation the voltage across R49?

Thanks
Fab
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Old 23rd July 2013, 04:38 AM   #20
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Not always true. I have made amps such as this one that can output much faster square wave than 20KHz and it did not fry.
It's not the frequency that's the problem with mosfets. A square wave at half rail amplitude is worst case power dissipation in the output devices
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