[SOLVED] HEI primary current duration

[Zak28]

Can the dwell time and period on the primary side be any duration so long as it abides by the maximum peak current?

Manufacturer's Part Number:D3070
Primary Resistance:0.350 ohms
Coil Internal Construction:Epoxy
Maximum Voltage:50,000 V
Turns Ratio:85:1
Secondary Resistance:8.30K ohms
Inductance:2.80 mH
Peak Current:7.9 amps
Spark Duration:1.5 mS

https://www.summitracing.com/parts/pnx-d3070

 

[prairiedog]

Yes but you need to allow extra time- for the spark to finish, which can vary a bit due to plug gap and plug+cable resistance, misfires etc. Also, dwell varies with system voltage so assume 11V say, not 14V; and you have some wiring and IGBT resistance, so the current ramp up is never as fast as theoretical.
Say 2msec dwell max. and 1.5msec for spark duration, so 3.5msec cycle time around 300Hz. You can go faster if you compromise dwell time. This is another reason for coil-on-plug technology which allows higher engine speeds.
That gives a single HEI coil on a V8 at 300Hz is (I think) 4,500RPM. If you wanted 6,000 RPM you'd have to live with the 4.5A that you can ramp up to in the available time.

 

[Zak28]

Do ignition drivers typically adjust the falling edges during ordinary operation to prevent excessive back emf spikes?

 

[FvM]

Back emf is is causing the ignition spark (fly-back converter principle), you don't want to cut it.

 

[Zak28]

Would using straight square waves on the primary, but keeping within the rated peak current damage the HEI?

 

[FvM]

No damage, but also no high voltage output. Primary fly-back voltage needs to go up to 600V for 50000V secondary.

 

[Zak28]

Back emf is is causing the ignition spark (fly-back converter principle), you don't want to cut it.

What was meant is reducing the falling edge to keep it below the many thousands of volts it will rise to thus lowering the power dissipation of the RC snubber.

No damage, but also no high voltage output. Primary fly-back voltage needs to go up to 600V for 50000V secondary.

There has to be a large back emf on the primary to make the rated HV on the secondary.

Automotive ignition drivers use ramps with dwell time to prolong primary charging durations, not squares as the ramps raise the current into the primary gradually.

Wouldn't the very sharp rising edges of square waves destroy the primary winding?

 

[lggrover]

The High Energy Ignition System (HEI) has undergone many improvements .... A duration of 5 ms allows the primary current to build to 3.8 amperes.HEI primary current duration Say 2msec dwell max. and 1.5msec for spark duration, so 3.5msec cycle time around 300Hz. You can go faster if you compromise dwell time. This is another reason for coil-on-plug technology which allows higher engine speeds.

 

[Zak28]

For GM HEI coils are dwell times adjusted by an onboard processor?

 

[Easy peasy]

for a single coil the dwell time is limited by the rpm, lots of dwell avail at idle speeds - less so at 7000 rpm ( 3500 rpm on dizzy ) as the ignition system cannot accurately predict when the exact spark should be, the coil must be energised up until the spark is required = igbt off = flyback volts on LV side of coil ( up to 250V for a 350V igbt ) = 20,000V on HV side before plug strikes and V falls to lit (plasma) value ...

 

[Zak28]

for a single coil the dwell time is limited by the rpm, lots of dwell avail at idle speeds - less so at 7000 rpm ( 3500 rpm on dizzy ) as the ignition system cannot accurately predict when the exact spark should be, the coil must be energised up until the spark is required = igbt off = flyback volts on LV side of coil ( up to 250V for a 350V igbt ) = 20,000V on HV side before plug strikes and V falls to lit (plasma) value ...

Min automotive rpm = ~700rpm - 1000RPM max dwell for 700rpm = ~84mS long which is plenty to to burn up a low resistance primary HEI coil. According to attached document resistors on the primary used were used later replaced by varying dwell time with control module.

Ignition gate drivers can have long dwell durations.

https://www.onsemi.com/pub/Collateral/FAN1100_F085-D.PDF

Is there an industry standard maximum dwell time for HEI coils?

View attachment HEI.pdf

 

[Easy peasy]

my reply concerned a single coil system - you appear to have misread it ...

 

[prairiedog]

You have to shut off the IGBT at precisely the right (spark) time, so you are always calculating backwards when to start the next dwell period.

If you have reached max. dwell time before spark time, you have to shut off the IGBT to protect it and the coil from overcurrent. But if you then don't want a backfire, "soft-shutdown" involves gradually lowering gate voltage (Fig. 15).

MOSFET's are much faster, that is why I mentioned they can turn off too fast and cause insulation breakdown within the coil's leakage inductance. I don't see mosfets used in automotive ignition systems also because their on-resistance is too high for HV parts.
Ignition IGBT's are slow devices compared to MOSFETS. Example Fairchild FGB3040G2-F085 400V internal clamp, 25A, 5-15usec turnoff.

On-semi (Fairchild) AN-8208 Introduction to Automotive Ignition Systems with good research references listed at the end.

 

[Easy peasy]

certainly you will need soft turn off of the igbt to avoid a spark - if you are calculating dwell periods based on current rpm -

 

[Easy peasy]

interestingly that IGBT has 120 ohm internal gate resistor to guarantee a slower turn on / off - it also has 1.25V Von drop at 6A ... which will subtract from the 12V available