Re: LF: W1VD amp help

["jrusgrove@comcast.net" <jrusgrove@comcast.net>]

Paul

 

Pressed for time so just a quick response.

 

Notice the small 'bobble' (around 3 volts) on the gate waveform on the way up ... that's where the FET is turning on. Turn off is about the same level ... maybe a bit lower. As long as the gate waveforms don't overlap at that point it should be okay. A slight 'dead zone' (both FETs off) isn't a bad idea.

 

IIRC, the heating coil in an electric hot water heater (240 volt / 2500 watt?) is roughly 50 ohms. One can run a coax to it and use it as a water cooled dummy load. At higher frequencies one might want to resonate out the inductance. Probably not much of an issue at 137 kHz.

 

Jay W1VD

 

 

 

 

----- Original Message -----

From: N1BUG <paul@n1bug.com>

Reply-To: <rsgb_lf_group@blacksheep.org>

To: <rsgb_lf_group@blacksheep.org>

Sent: 4/8/2018 7:53:27 AM

Subject: Re: LF: W1VD amp help

---

Jay,

Thanks for the detailed response! See below, threaded in.

> HP3325A ... set frequency x2, square wave, amplitude 5 volts, dc
> offset 2.5 volts. Have used the HP3325A here for countless numbers
> of years.

Done, verified that it looks OK with scope. (Previously I had not
been testing with the DC offset!)
 
> Always start out on the dummy load.

I've reduced drain voltage to 13V so that I can do that. I have no
dummy load which can handle the next step up (25V). No one near me
does either.

> Test in stages. Test the driver
> into the FETs with the FET drain supply off. Verify the waveforms
> with a dual trace scope to see that there's a small 'dead zone'
> between the two phases.

There is a delay between one phase beginning to switch off and the
other phase beginning to switch on, but I am not sure I would quite
call it a 'dead zone'.

Sorry about the quality of images. Scope 5V/div.

Gate waveforms:

http://www.n1bug.com/gates1.jpg

With the traces overlaid and time stretched out so you can better
see the crossing between phases:

http://www.n1bug.com/gates2.jpg

It appears one has not fallen to zero by the time the other one is
coming up. Should it?

> Once the gate waveforms are confirmed to be correct,

I've paused testing at this point pending any comment on my gate
waveforms or request to repeat with different settings.

Paul