EMRFD Message Archive 5388

Message Date From Subject
5388 2010-10-27 11:50:27 Tim PMOS keyclick-removing PA switch
I took the EMRFD fig 6.114E W7EL-credited PNP integrating transmitter switch, and I think I've updated it to use a small PMOS MOSFET.

It's uploaded under the "Photos"/"N3QE Spice Simulations" section of the Yahoo groups website as "PMOS keying", or go to http://groups.yahoo.com/group/emrfd/photos/album/1536449451/pic/713470941/view

A 2N7000-type switch and a couple of resistors and capacitors go around the PMOS transistor, just me (unencumbered by the thought process) transliterating 6.114E into MOS.

Nice 5ms rise and fall times, I plan to use it to key a 40W class PA and remove keyclicks with the ramps. Actually the transitions at the top and bottom are not purely linear ramps but are

But I'm far from an expert at PMOS switches especially when I'm using a trick to generate a linear ramp with them. I envisage switching 4 or 5 amps with this. As the voltage ramps up and down, I think the MOSFET will have a peak dissipation of circa 2 amps times 7 volts, or 14 watts, but only for milliseconds at a time. Is this an "OK" application of a TO-220 type PMOS switch without a heat sink, or is there some gotcha lurking to get me and burn up even more transistors? :-)

Tim N3QE
5389 2010-10-28 10:12:38 Glen Re: PMOS keyclick-removing PA switch
There IS a potential "gotcha" in that the thermal mass of the 2N7000
chip is not known. Only the static case of thermal dissipation is
given in the data sheet, and not the dynamic case...
For your example numbers, lets assume average 10 wpm. That's roughly
280 dissipation events per minute. You've assumed each of these events
dissipates 14W in 2ms. That averages out to about 40-50 mW. At
312 degrees (C) per watt, chip temp
5390 2010-10-28 13:06:13 Glen Re: PMOS keyclick-removing PA switch
The STATIC situation will burn too, much worse than the dynamic...
You should design the keying transistor for 100% key-down without
overheating. The 2N7000 has about 2 ohms "ON" resistance. This
transistor has to pass the full 4A transmit DC current, so it would
dissipate about 4^2 x 2 = 32 watts. It wouldn't last long.
You'd really want a PMOSfet having less than about 0.1 ohm Rds(on).
5391 2010-10-28 13:14:51 WA0ITP Re: PMOS keyclick-removing PA switch
I must be missing something, doesn't the FDS6375 switch the
load? It looks like the 7002 only switches the FDS6375
gate. The FDS6375 would need to be hooked up "backward"
with the source to the + side in order to reverse bias the
internal protection diode.
I love this radio stuff !
72, 73 Terry, WAƘITP

----- Original Message -----
5394 2010-10-29 06:54:04 Tim Re: PMOS keyclick-removing PA switch
I didn't mean to mislead anyone with my mention of the 2N7000... the subject is in fact using a PMOS to key a PA and what sort of power dissipation might happen there. In the SPICE simulation the 2N7000 (NMOS I thought everyone knew) is just there to invert and key the FDS6375 (the PMOS part I picked randomly out of the LTSPICE library). In LTSPICE the pin terminology for a PMOS part is indeed confusing compared to what we usually draw in a schematic.

I actually built the PA waveshaping secti
5395 2010-10-29 10:07:16 Glen Re: PMOS keyclick-removing PA switch
Sorry Tim, my bad.
Your whole keying circuit does look OK. I was thinking that you were
trying to key with a 2N7000! The FDS6375 PMOSfet is
hefty enough that 100% keydown shouldn't dissipate more than half
a watt. The SO-8 package would get warm, a TO220 package shouldn't.
I'd guess the thermal mass of either chip would absorb the
transient power of a keydown/key-up event without thermally being
damaged. Your PMOS gate drive is a bit low at 2.5v. Increasing to
four volts will reduce power dissipated in the keying transistor
a bit.
The gate-to-drain capacitor is a nice trick to get linear rise & fall
5396 2010-10-29 12:44:13 kb1gmx Re: PMOS keyclick-removing PA switch