For inspiration i link to old type BOSCH CDI ignition box schematic, and layout can be studied here

There is a preferred spark polarity (maybe for the ION current too). Anyone remembers ? If you find it while reviewing these (or other) papers, please answer here.

link by MembersPage/SteenAndersen


John Clarke's article on the Multi-Spark CDI (Same circuit as above but more readable copy. Looks like the entire article from Silicon Chip)

Some info..

Some more information including schematics of the MSD 6A ignition box. (posted by MembersPage/RichardChwiendacz )

CDI requires an appr. 400V DC supply

We want to be safe, so 100W power should be targetted (0.25A output current).

Input is 14V as usual.

I was thinking about CDI supplies. I built a transformer based supply earlier (somewhat lower wattage).

The best I can come up with is a simple flyback design. This has the lower part-count, beautiful simplicity (including testing!) If power was >300W a transformer based might become more attractive.

We need to use many thruhole parts in any case.

Flyback design consist of only 4 major parts:

There is a concern with the inductor:

Inductor - (note: not transformer)

For experiments we can use 4 of these in parallel:

The same barrel type/size as the 1.5uH 5.4 A we use on v3.x.

Finally we'll wind our own toroid or whatever type.

(12V / 12uH) < (16A / 16usec)

16A peak is a bit high (for the IGBT too, and definitely for the diode). 16A peak means about 16A / 2 * 12V=100W.


similar to what we have now, but to make 400V possible, clamping at 370V is not desired.

FETdriver is needed to drive the gate at 64 usec (15625 Hz).


To get 10A diode from 5 pieces of 2 A diodes is slightly more complex than simple parallel connection because of the -2mV/K temperature coefficient, the one with lowest voltage will overheat itself. Series resistors (for each diode, maybe trace is enough) can fix that.

Maybe 6..7 SMB in parallel and 0.022 Ohm in series with each.


Electrolitic cap is not the best idea at 16 usec period.

It might even explode. Some testing is needed in any case. Other cap that might work:

0.25 A * 16 usec = 4uC

4uC / 2uF = 2V (ripple)

Output arrangement

There are 2 possible types:

Push-pull type DC/DC

Push-pull use better the transformer, it can get more power with smaller transformer than forward, it needs two transistors.

There are some questions about CDI:

Are fired with Thyristors or IGBT? If it use Thyristor the supply must be shutdown when thyristor is actived until capacitor was discharged.

if IGBT are used, it is not necessary to shutdown supply but IGBT have to be fired with a precise pulse wide. Is it possible to configure ignition pulse wide in CDI mode?


It works fine!, At the moment i have meassured 100w at 350v, (input=13 volt). I calculated 10 W losses on the mosfets (it drops near 1 volt) but I'm not sure about losses on transformer.

I was trying several transformer configurations, and I finally got a good performing.

I realized that if the driver IC tries to down the voltage under the right voltage for the transformer, the efficiency of the circuit drops a lot and the mosfets drops more voltage. But if I set the voltage limit to a value a bit higher than the current one, the circuit works better.

Can you elaborate on this? Isn't it nonsymmetric pulse that causes DC inductance (B) field bias in the core ?

Yes, I did a fully simetrical winding (I placed the two windings in parallel in order they have exactly the same lenght and turns)

What happens is that this transformer has a fixed ratio (not as flyback) then if you made a x26 transformer and you fix the output to 300v at output, it will force the input to 11.5. If your supply has 14 volt, the difference (2.5) will fall into the mosfets. This is the reason why the supply works better at its "natural" voltage.

current transformer is about 30 x 30 mm size, but i think it could fit a smaller one, with more frequency. It could be really small!!!

I added an output stage based on MOSFET or IGBT to drive the coil. I placed one wire of the coil to 350 v in order to have always high voltage on the spark to do the current sense during the cumbustion.

This may be dangerous because the coil will have always high voltage, and touching it or the spark cable, can be dangerous.

I measure the ION current by using an optocoupler on the 350 volt line. This should work, but for the production circuit, the current mirror sounds more appealing.

uC controlled / standalone

For the CDI / Ion, uC controlled supply might be simpler and cheaper (and better logistics).

For uC main processor supply for the big display (with near-PC-performance, and a few Watts of dissipation at91rm9200+256 Mbyte SDRAM) power must be standalone.

Standalone is usually made from SG3525 , SG3524 SOIC16 or similar chip (compatible chips available from other manufacturers).

Standalone advantages

Standalone disadvantages

simple CDI implementation with v3.x

Rather for experiment than production.

When CDI is active, the tip electrode of the sparkplug is either at +300V or +30000V (during spark) potential, so take care.

See also