Is IRF9640 the best choice for a high-side switching topology?

[CrizzyD]

Hi guys,
as the title asks, for a PI that uses a high-side configuration for the coil is there a better choice (generally speaking) P-channel FET than the IRF9640? I'm in the process of redesigning an old metal detector for someone and one of the limiting factors is that the system needs to be compatible with already established coils. The coils themselves are separated with the Tx being 80uH and the Rx being 2000mH . My Tx pulse width is 60us with a period of 500us. I'd like to increase pulse width further to "push" more current into the coil but the IRF9640 gets substantially hotter if the pulse width is increased from this point (currently stands at about 50C at ambient with a big TO-220 heatsink). Is there a P-channel part that has lower RDson than 0.5 Ohm with similar or better avalanching specs?

[Carl-NC]

If the IRF9640 is avalanching during flyback then that is where most of the heat is generated, so focus on a PMOS with a higher breakdown. In that respect, PMOS selection is rather limited, with many choices being SMT but they are workable (make an adapter bard). Try for at least 400V.

[CrizzyD]

I did also find:
IXYS SEMICONDUCTOR IXTP10P50P

https://uk.farnell.com/ixys-semicond...0ab/dp/3438416

It seems to be much better for avalanche, higher rated energy etc.but the RdsOn is double at 1 Ohm. I'll give it a try anyway, but at a quick glance could this be a better option? If not, could anyone suggest an alternative? Preferably in a TO-220 package.

Edit: the flyback I'm measuring is at -250V and the IRF9640 is rated for -200V

[Carl-NC]

A 200V MOSFET will likely avalanche at 250V so it sounds like it's avalanching. If the flyback peak is flat-topped then definitely. I suspect 1Ω will not make a tremendous difference, if it does you could parallel 2 MOSFETs at the risk of increasing the capacitance.

[CrizzyD]

If the flyback peak is flat-topped then definitely.

the back EMF does appear to "clip" at the 250V so potentially the flyback would be even greater.

[kt315]

put 1 Ohm resistor in SOURCE pin of the mosfet, with 1000 uF +, like you see this in HH2 schematic.
just invert a circuit N-channel into your P-channel view. it must give you decrease of energy on the mosfet.

[profesor]

I would recommend the IRF9630 better.

[CrizzyD]

I would recommend the IRF9630 better.

Why would you recommend this over the 9640? Seems worse for the application; lower RdsOn, lower avalanche ruggedness, same Vds?

[CrizzyD]

put 1 Ohm resistor in SOURCE pin of the mosfet, with 1000 uF +, like you see this in HH2 schematic.
just invert a circuit N-channel into your P-channel view. it must give you decrease of energy on the mosfet.

I did a bit of sleuthing based on your suggestion and I found this schematic on the site. I'm not familiar with a lot of these designs or initialisms as I'm new to the hobbyist side. Aside from the obvious incorrect switching configuration, I'm not sure how the addition of the cap and 1 Ohm resistor will help?

Also I tried the IXTP10P50P alternative and the thermals do seem to be better and interestingly the back EMF cut-off flat-tops now -500V so it's still avalanching but based on the shape of signal, it flat-tops at a much smaller angle. I don't think that I'm going to be able to get a much better part unfortunately. I'm probably stuck. A design switching at -12V with an N channel would definitely be the way forward in the future when I'm not limited in the design.

HH2.PDF

[kt315]

you also can put a HH diode, D6 in your HH2 circuiit. it kills avalanche but slighly decreases the sensitivity.
i did buy them last time on a link, did not still tested. https://www.aliexpress.com/item/1005005221566700.html

[CrizzyD]

I added a 5 Ohm resistor in the path between drain and the coil and that hugely reduced avalanche (and subsequent current draw) meaning that I had to increase the pulse width but obviously increasing pulse width will only do so much. In an ideal scenario I'd be able to have a 100 milli Ohm -1kV FET but that's not going to happen. Interesting, I hadn't considered a diode

[profesor]

Why would you recommend this over the 9640? Seems worse for the application; lower RdsOn, lower avalanche ruggedness, same Vds?

smaller I/O capacity. Important for broadband and pulsed signals.

[CrizzyD]

smaller I/O capacity. Important for broadband and pulsed signals.

Interesting, I would have thought that for the input capacitance, the push/pull circuit would have taken care of issues from a larger capacitance on the gate side? Output capacitance I agree with but I think the difference is negligible.

[Carl-NC]

Adding a series 5Ω resistor reduces the coil current, which is the opposite of what you want. Then, you have to increase the pulse width just to get back to where you were, at which point the FET will be just as hot.

The only practical way to deal with this is to use a higher breakdown FET to avoid avalanche. However, the flyback energy still has to go somewhere, and what you will find is that R58 (damping) and the input clamping resistor will get hotter.

The IRF9630 will not help you here, it has the same breakdown. If that was not the issue, then it would help reduce the FET capacitance and make recovery a little faster. I don't know where KT is suggesting to add a diode.

[CrizzyD]

Hi Carl, thanks for the reply. I had found that with a 60us pulse and no resistor then the FET was reaching 50C ambient. However if I doubled the pulse width and with a 5Ohm resistor then it massively reduced the heat on the FET and increased it on the resistor but with slightly reduced performance. Ideally, I wouldn't have to reduce the current.