Tony,
My guess is that when you were running the BS170s and seeing a
flat-topped 60V you were observing avalanche breakdown of the BS170
drain. Drain-gate oxide breakdown can also happen at that voltage.
The TN0106 and VN0606 also have 60V breakdown specs, but the spec is a
minimum and actual breakdown may occur at substantially higher
voltage.
In any case, you have seen first-hand what the inductive kick looks
like and what damage it is capable of inflicting.
JZ KJ4A
On Tue, Aug 8, 2023 at 8:01?AM mux_folder2001 <
canthony15@...> wrote:
>
> The diode is a good idea I think. This is based not only on simulation but on measurements I have made with my scope.
>
> In my case, the loss of two or three sets of BS170 in my two QDX units led me to try different transistors. I ran the other transistors without any failures for months. But I was curious why the BS170s would fail and the TN0106 and VN0606 would not. Yesterday
I got a new scope that allows me to see transients and I decided to look at the QDX with VN0606 transistors in it. To my surprise, the glitch at the end of a transmission was over 120 volts measured at the center tap. The transistor drains were similar. The
spike was 100ns wide at the 60 volt level. And this is into a dummy load. The transistors must just be capable of dealing with this.
>
> I then replaced the transistors with BS170s and discovered another surprise. The spike was below 60 volts and had a broad flat (but ragged looking) top.
>
> Then I added the commutating diode and the spikes were gone.
>
> Right now my theory is that the BS170s are operating in a "make before break" mode which minimizes the di/dt. The VN0606 (and TN0106) might be in the "break before make" mode. The specific operating conditions might possibly allow the circuit to move between
these two cases and the BS170s are just not capable of handling the over-voltage spike while the other transistors can. Just a theory mind you. I have not tried yet to make measurements to verify this. Prior to these measurements, I was thinking that the opposite
would be the case, thus explaining why the VN0606/TN0106 did not fail.
>
> In any case, I am now running with BS170s again but with the diodes installed. We shall see how long this lasts.
>
> Tony
> AD0VC
>
> ________________________________
> From:
[email protected] <
[email protected]> on behalf of John Z <
jdzbrozek@...>
> Sent: Monday, August 7, 2023 6:14 AM
> To:
[email protected] <
[email protected]>
> Subject: Re: [QRPLabs] QDX Success story
>
> Simulations are here:
>
>
/g/QRPLabs/message/105254
>
> JZ
>
> On Mon, Aug 7, 2023 at 8:07?AM John Zbrozek <
jdzbrozek@...> wrote:
> >
> > Hi Pierre,
> >
> > The diode simply is in parallel with the 47uH inductor L14, with the
> > orientation shown in the photo provided by Paul W9AC. Cathode towards
> > the power supply connection, anode to the transformer center tap.
> > Any of the diodes he mentioned? (1N4448, 1N4148, 1N914) will do a fine
> > job of absorbing the end-of-transmission voltage spike it produces.
> > I have used a 1N914.
> >
> >
/g/QRPLabs/message/105256
> >
> > Here you can see simulations with and without the diode present.
> >
> > On another thread, Evan, Hans and I explore the ability of QDX to
> > withstand SWR. It seems to have a reputation for SWR intolerance but I
> > think there is a case that the problem may actually be as described
> > above.
> >
> >
/g/QRPLabs/message/106759
> >
> > Regards, JZ KJ4A
> >
> > On Mon, Aug 7, 2023 at 7:46?AM Pierre FK8IH <
jb.gallauziaux@...> wrote:
> > >
> > > Could you please elaborate: what diode, how did you connect it? A handmade schematic and a photo would be nice.
> > > 73
> > > Pierre
> > > FK8IH
> > >
>
>
>
>
>
>
