开云体育

Another wrinkle to this power supply story.

As the Zener diodes at the PWM outputs are heroically trying to keep the output voltage near the set point of the control loop, they deny the creation of a feedback error signal that proportionally reflects how much the PWM pulse width needs to be reduced. In the worst case with the Zener voltage at its spec minimum (e.g. a 3.6V Zener showing an actual Vz of 3.3 volts) there may be no feedback error signal at all. Ugh!

JZ

On Sun, Jul 30, 2023, 5:43 PM John Zbrozek <jdzbrozek@...> wrote:

Imagine your QMX is connected to a power supply. You flip the switch on the supply and its output begins to rise.?

At some point when the supply voltage is high enough and the QMX linear regulator Vdd supply is high enough the processor fires up and the PWM regulators start to operate. When the processor sees that ( internally to the PWM circuits) Vdd and Vcc are ready, the processor throws a big switch and now the PWM regulators are in charge of delivering Vdd and Vcc to QMX 's circuits.

Here is what I worry about.

If the external supply voltage continues to rise after the handoff occurs, and does so at a rate faster than the slew rate of the processor PWM control loop, output spikes will take place at Vdd and Vcc. The Zeners there may be inadequate to safely absorb that blow and downstream damage may occur.

I believe this is what happened when Gunnar switched away from his 6V power supply and onto a 12V supply.

Gunnar gets kudos for understanding this first.

I would love to be convinced that this cannot happen.

JZ KJ4A?

On Sun, Jul 30, 2023, 5:00 PM William Smith <w_smith@...> wrote:

Depending on how the regulator circuit was designed, there could well have been a brief interruption in an SPDT switch that let the voltage rise to maximum for an instant, which is all it takes.

Yes, we’d all design it for 12V and switch in another shunt for 6V, but who knows what the actual designer did or what his control loop parameters look like.

73, Willie N1JBJ

> On Jul 30, 2023, at 2:06 PM, Ugglekatten@... wrote:
>
> when I switched from 6V to 12V, the QMX just died.