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prevent nmos from overvoltage damage

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Zak28

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Trying to use an ignition coil for high voltage and the arrangement is with an irfz34n nmos chopping DC into the coil. The trouble is Vds is overvolting Im not sure the ideal remedy to this issue.

Is a TVS of some sort the ideal way to suppress the overvolting Vds in this arrangement?

diagram.png

traces.png

The irfz34n has 55Vds max and the simulator is right at that voltage due to that.

http://www.irf.com/product-info/datasheets/data/irfz34npbf.pdf
 
Last edited:

Hi,

The series diode won't help.

The zener will do. But it adds capacitance ... and you need to care about impedance and speed.

But first you need to decide what is the peak operating voltage at this node?
I mean: you could use a free wheeling diode across the coil .... but this also limits the positive output voltage....and increases decay time.
I don't know what output voltage (waveform, polarity, timing ...) you want to use...thus I can't give a recommendation for now.

Klaus
 
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The required maximum voltage rating of an electronic ignition primary switch is in the several 100 V range.
 
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KlausST said:
Hi,

The series diode won't help.

The zener will do. But it adds capacitance ... and you need to care about impedance and speed.

But first you need to decide what is the peak operating voltage at this node?
I mean: you could use a free wheeling diode across the coil .... but this also limits the positive output voltage....and increases decay time.
I don't know what output voltage (waveform, polarity, timing ...) you want to use...thus I can't give a recommendation for now.

Klaus
Click to expand...

I switched to an irfp260 since more Vds yeilds more high voltage and a 150v zener seems to have solved the over voltage issue dissipating ~75mW.

Can TVS zeners be used instead or ordinary zeners? Im not sure if TVS diodes can be used for switching.
 
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An "avalanche rated" FET is one option. A FET whose BVdss
exceeds the flyback overshoot is another (and this is what will
maximize the output voltage; zener clamping or FET breakdown,
survivable or not, will clamp the primary hence the secondary).

NE555 is not that strong a driver and you may be seeing other
issues relating to weak drive.
 
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dick_freebird said:
An "avalanche rated" FET is one option. A FET whose BVdss
exceeds the flyback overshoot is another (and this is what will
maximize the output voltage; zener clamping or FET breakdown,
survivable or not, will clamp the primary hence the secondary).

NE555 is not that strong a driver and you may be seeing other
issues relating to weak drive.
Click to expand...

The 555 can output more current than the gate needs.
 

Hi,

The 555 can output more current than the gate needs.
Click to expand...
Only true for DC currents..

Nowadays gate drivers can deliver >1A peak for fast switching.
Fast switching means low switching_loss and enables high switching frequency...

Klaus
 
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KlausST said:
Hi,


Only true for DC currents..

Nowadays gate drivers can deliver >1A peak for fast switching.
Fast switching means low switching_loss and enables high switching frequency...

Klaus
Click to expand...

The simulator shows the MOSFET dissipates less than 180mW with 12v 50% duty square wave to the gate from a 555 - is this not a realistic simulation?

I tried the same frequency with a voltage source which has extremely fast switching speeds the mosfet had identical dissipation from 555 signal.
 
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Gate drive current and switching speed are almost meaningless in this application due to the very low switching frequency.
 
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No, switching speed at turnoff goes directly to how much
magnetizing energy is left in the inductor to produce the
flyback pulse, vs being scrubbed off by a leisurely drain
current transition. The slower the turnoff, the more switching
loss (the drain component, of; gate component is roughly
fixed Q per cycle, given constant Vgs(on)-Vgs(off)).

OP could interpose a vcvs between 555 block and FET gate
(av=1) and observe what (if any) impact there is to the
care-abouts. If it makes a difference then a gate driver
stage might be a win. Or, using a nearly-as-cheap PWM
IC which would also give you (if current mode controlled)
some authority beyond open loop volt-seconds over the
primary current.
 
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dick_freebird said:
No, switching speed at turnoff goes directly to how much
magnetizing energy is left in the inductor to produce the
flyback pulse, vs being scrubbed off by a leisurely drain
current transition. The slower the turnoff, the more switching
loss (the drain component, of; gate component is roughly
fixed Q per cycle, given constant Vgs(on)-Vgs(off)).

OP could interpose a vcvs between 555 block and FET gate
(av=1) and observe what (if any) impact there is to the
care-abouts. If it makes a difference then a gate driver
stage might be a win. Or, using a nearly-as-cheap PWM
IC which would also give you (if current mode controlled)
some authority beyond open loop volt-seconds over the
primary current.
Click to expand...

The turn off drain current waveform is very sharp with the 555 signal into the gate.

Is the primary coil capacitor mandatory to have the secondary ring at high voltage?

Im not sure if ignition coils have them built in.

img.png
 

Kettering ignition systems with points, used the condenser to
hold down the voltage "just long enough" to keep points from
arcing and wearing out prematurely and from depressing the
ultimate coil voltage (arcing steals energy, just like a leisurely
turning-off FET; same difference). Not sure if this is also
present inside HEI modules or coil-per-plug ignitions, these
are hard to see inside of and I don't see condensers hung off
the distributor body like on older cars. Maybe the power switch
just has enough breakdown headroom to not need them.
 
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Not sure how to remove the on switch delay.

Drain current waveform:

Screenshot_20181230_202821.png

Im quite sure the inexpensive ignition coil Im going to use has nothing other than a coil inside an oil filled canister. The capacitor depicted in the diagram is likely required to sustain the ringing on the high voltage.

**broken link removed**
 
Last edited:

No, switching speed at turnoff goes directly to how much magnetizing energy is left in the inductor to produce the flyback pulse
Click to expand...
Theoretically, yes. In practice, the ignition coil SRF (plus an connected external capacitor) sets a limit to current and respective voltage speed of change.

As long as the switch is acting faster, there is about no effect of switching speed. Unlike for a fast inverter, where some 10 ns switching speed matter, the ignition switch speed range is µs.

The simulation circuit parameters seem to be however far from real ignition coils. According to internet sources, typical primary coil parameters are around 5mH inductance and 1 - 2 ohm DC resistance. There's no built-in intentional capacitance but considerable secondary winding capacitance. And an external capacitor in parallel to the breaker contact.
 
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I recently ordered an LCR meter and an ignition coil
**broken link removed**

I dontt know what what values to input for parallel resistance for each winding.

These 2 specific values are apparently extremely important to obtaining a valid simulation output - and I am trying to achieve a voltage under the RMS AC capacitor voltage rating.
 

Im also giong to order very high voltage/resistance resistors - giga and mega ranges rated to 20kv to measure the ignition coil voltage with a dmm.

But I would like an accurate ignition coil simulation also just not sure how to obtain parallel resistances and parallel capacitances of the transformer windings.
 

Is your ignition coil the same as found in an automobile? Car repair manuals tell how to test a coil's DC resistance with an ohmmeter, and what range of readings indicates a normal or faulty winding.

Remember that an inductor/transformer can generate high voltage spikes when you apply current and then suddenly remove it. So look for special instructions about taking ohm readings.
 
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BradtheRad said:
Is your ignition coil the same as found in an automobile? Car repair manuals tell how to test a coil's DC resistance with an ohmmeter, and what range of readings indicates a normal or faulty winding.

Remember that an inductor/transformer can generate high voltage spikes when you apply current and then suddenly remove it. So look for special instructions about taking ohm readings.
Click to expand...

There shouldn't be any high voltage kickback from an ordinary ohm meter since it passes very small and precise amount of current.
 

Zak28 said:
I recently ordered an LCR meter and an ignition coil
**broken link removed**

I dontt know what what values to input for parallel resistance for each winding.

These 2 specific values are apparently extremely important to obtaining a valid simulation output - and I am trying to achieve a voltage under the RMS AC capacitor voltage rating.
Click to expand...

The item description at eBay gives 0.310 ohms resistance on primary and 9.2k ohms on secondary winding.
 

Kajunbee said:
The item description at eBay gives 0.310 ohms resistance on primary and 9.2k ohms on secondary winding.
Click to expand...

Im going to measure for more accurate simulation.
 

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