What causes a mosfet to oscillate? I have a high power h-bridge,
the circuit was set up so that the turn-on turn-off of the fet’s was
fast. However, since I am switching into a reactive load, to limit the
inrush current, I was told to slow down the rise time of the lower half of
the h-bridge (in effect current limit the h-bridge on turn on).

Now that I have slowed down the rising edge, the fets oscillate during
the transition. I am not sure what to do about this, or even where to
begin. I did replace the reactive load with a resistive load, and the
slowed down h-bridge does not oscillate. One person recommended placing a
small capacitor from gate to source… does this make sense? Will this
extra capacitance just make the mosfet switch even slower?

My switching speed is under 1kHz, and the mosfets oscillate around 16MHZ.
I have a fet driver driving the mosfets, there is a gate resistor (with a
diode across it so that turn off time is faster then rise time).

To me, this is the start of where electronics becomes voodoo.

It is a transformer, which to the output of the H-Bridge looks like a
capacitive load. there is a very large turns ratio in the transformer, and
the few picofarads on the secondary of the transformer end up looking like
tens of microfarads at the input. before we slowed down the h-bridge we
were pulling ~40A inrush over a couple hundred nanoseconds.

I managed to 'solve' the problem today, although it left me with other
questions. On the prototype, we used through-hole parts, and surface mount
parts on the PCB. it turns out that there are some differences between the
surface mount and through hole versions of the same diode even though all
the data sheets specs are identical.

As i was pulling my hair out today, (looking over the layout and dreading
having to start to cut traces and run wires) i decided to try putting the
through hole diodes inplace of the surface mount diodes and it works like
a charm now..

I am not quite sure what the differences between the two packages could
be. some sort of parasitics?? In all the reading I did over the past few
days, everything pointed to reducing the stray inductances and
capacitances around the fets, which is why I am guessing that maybe the
surface mount diodes have larger stray capacitances?

Eric,

Is your reactive load inductive or capacitive? An inductive load does not
have an inrush current.

Also, is your switching (1 kHz) to control the load with a PWM? Any inrush
may be seen over a number of cycles, not on individual cycle.

What is the load?

John Musselman


"eric" wrote in message
news

What causes a mosfet to oscillate? I have a high power h-bridge,
the circuit was set up so that the turn-on turn-off of the fet's was
fast. However, since I am switching into a reactive load, to limit the
inrush current, I was told to slow down the rise time of the lower half of
the h-bridge (in effect current limit the h-bridge on turn on).

Now that I have slowed down the rising edge, the fets oscillate during
the transition. I am not sure what to do about this, or even where to
begin. I did replace the reactive load with a resistive load, and the
slowed down h-bridge does not oscillate. One person recommended placing a
small capacitor from gate to source. does this make sense? Will this
extra capacitance just make the mosfet switch even slower?

My switching speed is under 1kHz, and the mosfets oscillate around 16MHZ.
I have a fet driver driving the mosfets, there is a gate resistor (with a
diode across it so that turn off time is faster then rise time).

To me, this is the start of where electronics becomes voodoo.

Eric,

There should be no difference in the diode itself. Having the through-hole
lead adds a (very) small inductance to the circuit.

When you say the circuit runs OK with the TH diodes, do you mean the
oscillation goes away? I will guess it's not really oscillating, but
ringing. In other words, it would decay away if you looked at it over a
long enough time, but the 1 kHz drive is starting it up again.

By my calculation, if your few pF is transformed into tens of uF, you have a
transformation of over 1,000,000 and therefore a transformer turns ratio of
over 1,000. Are you generating some super high voltage and the diodes are
rectifiers?

John Musselman

"eric" wrote in message
news
It is a transformer, which to the output of the H-Bridge looks like a
capacitive load. there is a very large turns ratio in the transformer, and
the few picofarads on the secondary of the transformer end up looking like
tens of microfarads at the input. before we slowed down the h-bridge we
were pulling ~40A inrush over a couple hundred nanoseconds.

I managed to 'solve' the problem today, although it left me with other
questions. On the prototype, we used through-hole parts, and surface mount
parts on the PCB. it turns out that there are some differences between the
surface mount and through hole versions of the same diode even though all
the data sheets specs are identical.

As i was pulling my hair out today, (looking over the layout and dreading
having to start to cut traces and run wires) i decided to try putting the
through hole diodes inplace of the surface mount diodes and it works like
a charm now..

I am not quite sure what the differences between the two packages could
be. some sort of parasitics?? In all the reading I did over the past few
days, everything pointed to reducing the stray inductances and
capacitances around the fets, which is why I am guessing that maybe the
surface mount diodes have larger stray capacitances?