Re: QDX FDT86256 Mosfet PA Modification
|
Hi Steven, yes very true.? I do that frequently, shoot at the hip only to get shot down in the details!
Steve KY4GX
Glory to God in the highest, and on earth peace, good will toward men. Luke 2:14
On Saturday, September 23, 2023 at 06:23:04 PM EDT, Steven Dick, K1RF <sbdick@...> wrote:
Steve, Good thought but you will likely have asymmetrical rise and fall times with an active pull-down and passive pullups which gets worse as you go up in frequency.? In this application, symmetric rise and fall times are very important. In addition, the resistors would have to be fairly small, dissipating a lot of power as well as causing additional power dissipation in the driver compared to a driver with an active pull-up and pull-down.
-Steve K1RF??
------ Original Message ------
Date 9/23/2023 5:59:05 PM
Subject Re: [QRPLabs] QDX FDT86256 Mosfet PA Modification
If you run open collector AND gates you could tie the pull up resistor to a larger voltage.? You'd need to wire pull up resistors on all gate outputs though.
Steve KY4GX
Glory to God in the highest, and on earth peace, good will toward men. Luke 2:14
On Saturday, September 23, 2023 at 05:38:36 PM EDT, Tony Scaminaci < tonyscam@...> wrote:
I agree with John regarding the Vgs threshold issues. Microchip¡¯s TN0110 (100V) is the device I found to be most suitable for use as a replacement for the BS170. Unfortunately, last I checked, they were unavailable but there was plenty of stock for the 60V version, TN0160. TN0110 should be back in stock no later than November.
I strongly believe that any of the devices we¡¯ve been discussing, including the BS170, would perform much better being driven from a 7V square wave. Too bad we¡¯re tied to 5V logic for the gate drive. We need a good 100mA drive pulse into each of the gates for good switching times but the ACT08 driver can¡¯t source anywhere near enough current, even with the outputs paralleled. As a result, John showed that we end up with about 4.5V at the gate due to drop in the ACT08¡¯s drivers. This makes things worse because we¡¯re only 1.5V above the worst-case BS170 gate threshold of 3V. The TN devices provide 2.5V of threshold margin at their worst-case which turns them on harder and increases power output.?
I ran a few sims with various bipolar drivers in emitter follower configurations but achieved only marginal improvement. I¡¯m still looking for a driver that can switch at 30MHz and pump out a hundred mils to charge up the gate capacitance at turn on and also discharge it at turn off. Anyone? Buehler?
Tony - AC9QY On Sat, Sep 23, 2023 at 3:02 PM Ryuji Suzuki AB1WX < ab1wx@...> wrote: JZ, that's basically my conclusion when I was considering QCX fitted with FTD86256 a few years ago. Not exactly a show stopper because I was ok with a small annex board, but effort-benefit tradeoff was poor in my view. It is a very good switching transistor, though.
On Sat, Sep 23, 2023 at 01:57 PM, John Z wrote:
While it is a superb
choice for a clean sheet project, it would not be my choice for an
upgrade to QDX or QMX. The Vth spec is a show-stopper.
|
