Series RLC circuit Energy Calculation

[vaibhavwaman]

Hello Team,

we have below circuit in actual product where we got the oscillation at the input the voltage is oscillating double its value but the device at the input is of 200V only this oscillating voltage is damaging the device. so to damp this waveform we placed one TVS diode SMCJ130CA breakdown start at 144V. we need to validate the TVS stability with calcualtion so we calcualted the voltage and current considering underdamp case.

is the provided calculation is in correct way ?
also how should we calculate the Energy formed by this LC tank which will be latter on dessipated in TVS ?

below calculation:-

Simulation snap:-

 

[BradtheRad]

At resonance the series LC is capable of boosting an AC waveform to alarming levels. Your configuration is the same that's cautioned by more than one amplifier designer who advises always putting a load on the output stage. Usually it's a speaker parallel to the capacitor since you have a second-order filter and a resistance across the capacitor damps the filter. When the frequency is non-resonant there's a formula having to do with Q and reactive impedance.

There's more than one way to express power in the loop. There's real power and there's elevated power. I'm not sure whether we should use the term Watts or Volt-Amperes. I'm not sure about phase changes depending on applied frequency and how close you are to a harmonic of resonant frequency.

The 1Ω resistor does little to limit current around the loop.

 

[KlausST]

Hi,

A boosted voltage ... caused by a resonance .. is only across the capacitor and the inductor.
The max total voltage is the input voltage
The max resistor voltage is the input voltage

Where did you connect the TVS diode?

this oscillating voltage is damaging the device.

which voltage ... and which device?

Klaus

 

[FvM]

Unclear which real technical problem is modelled here. What is pulse source respectively switching device? Actual dv/dt?

 

[vaibhavwaman]

Hi,

A boosted voltage ... caused by a resonance .. is only across the capacitor and the inductor.
The max total voltage is the input voltage
The max resistor voltage is the input voltage

Where did you connect the TVS diode?

which voltage ... and which device?

Klaus

Hello Klaus,
The TVS is connected after the capacitor

The voltage which is generated after the capacitor when TVS was not there is damaging the MOSFET which is there after the TVS, MOSFET is of 200V VDS and I am getting oscillated voltage of 304V which is damaging the MOSFET that's why I have placed the TVS diode before MOSFET.
the series Inductor and resistor I have modelled as cable inductance in actual the inductor is not there in circuit.

Vaibhav

 

[KlausST]

The TVS is connected after the capacitor

The voltage which is generated after the capacitor when TVS was not there is damaging the MOSFET which is there after the TVS, MOSFET is of 200V VDS and I am getting oscillated voltage of 304V which is damaging the MOSFET that's why I have placed the TVS diode before MOSFET.
the series Inductor and resistor I have modelled as cable inductance in actual the inductor is not there in circuit.

a textual description is alyways ambiguous.
A schematic is way better for this.

--> please post a complete schematic..

Klaus

 

[vaibhavwaman]

a textual description is alyways ambiguous.
A schematic is way better for this.

--> please post a complete schematic..

Klaus

Hello Klaus,

as per your feedback please find the attached schematic block we are not allowed to put complete schematic so i just placed a part of it.we are monitoring 10V input from outside in the board when the voltage increased more than 10.6V which is set by the TL431 the gate of the depletion mosfet will be hold by that and source voltage will be Vg+Vgs so it will be controlling or protecting the further section for Higher voltages more than 10.6V.
we need biderectional flow in the circuit for some of application, we are measuring the outside resistance by flowing the current though the depeltion mosfet from inside the board. for that purpose we used Depletion mosfet.

now coming back to the original question i am applying 154V using external relay for testing the worst case scenario so i have added the cable inductance as modelled inductor in series and the capacitor is present actual in circuit. i am interested to know whether the TVS wattage is sufficient to absorb the energy created by LC oscillation.

 

[FvM]

Pulse energy is several orders of magnitude below TVS diode rating, but 130V TVS isn't safe with 154V DC voltage, it will most likely die by overheating.
A RC snubber (e.g. 51R/22n) can dampen oscillation effectively.

 

[vaibhavwaman]

Pulse energy is several orders of magnitude below TVS diode rating, but 130V TVS isn't safe with 154V DC voltage, it will most likely die by overheating.
A RC snubber (e.g. 51R/22n) can dampen oscillation effectively.

Hello FvM
Thanks for the quick reply !
i assume that your asking RC snubber to place parellel of 1nf capacitor i already tested that senario with RC snubber parellel to existing 1nF capacitor yes it will reduce the oscillation but the first peak most likely satys there itself
Also how should i calcualte the Current going into the TVS from the LC tank i need that for calculation of Max clamping voltage of TVS diode.
actually i have tested TVS diode with 154V its taking 2.5mA constant current where as it is heated upto 40 degree only is that normal operation we could consider ?
below are the TVS parameter :-
min temperature (-40 degree) Breakdown voltage start at 136V
maximum breakdown voltage at temp 85 160V
maximum clamping 209 V
my normal operating volatge is of 143V it shoots to 154V for short duration

 

[KlausST]

my normal operating volatge is of 143V it shoots to 154V for short duration

Your input voltage is 154V (as per your sketch) .. so don´t be surprised if the capacitor voltage also goes up to 154V.

And the FET should widthstand 200V..

As FvM states .. don´t use a TVS on a voltage that is higher than the TVS´ standoff voltage.
And a TVS is not a very accurate device ... so read te datasheet carefully about how to operati it safely (worst case conditions).
The voltage may vary from piece to piece, with tempertature, with aging....


Klaus