EMI of a heater regulator

HEF4093BP

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Add a series resistor between driver gate and MOSFET. Maybe 100 to 1k. Check if the power supply for the IC is correct and stabilize it with an capacitor.

What is the value of the R-C ?

- Henry

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"Bernhard Kuemel" schrieb im Newsbeitrag news:45158$4582cf58$d429f848$ snipped-for-privacy@news.inode.at...

Microwave Oven Transformer choke? That's gotta be gross overkill.

Too much inductance and too much stored energy - lucky to keep the Mosfet alive when dealing with the spikes that could produce if allowed to saturate.

anyhow . . . most EMI will respond to just a little bit of attention to just the right point in the circuit - not MOT's - and you shorted the HV winding? I can see the temptation - but would expect parts to fry doing that.

The advice on using a gate resistor? Good. A small ceramic disk cap from the gate ground may also help. Or a snubber around the mosfet (.01 uf cap in series with a 100 ohm resistor).

If you have a scope - look at the signal to the load - it should be on-off with little or no ringing or glitches in it.

If you are touching parts of the circuit and causing changes - that is probably a switching glitch/ringing - I saw that a lot when designing a 30 amp 24 volt pwm battery discharger - a snubber around the mostet and cap on the gate made it smooth right out. Problem was the leads out to the battery were radiating signal back to the gate of the mostet.

You don't say what frequency the EMI is occurring. If it is affecting FM radio, for instance, soft iron wire is probably not going to work.

Suspect layout - long leads carrying heavy current with fast rise times will radiate EMI - likewise the power supply can carry noise back into the power lines. Lots of inexpensive surplus IEC type power line filters on the market - which will work as well on the load side if the leads to heater are radiating. - but first try to get a smooth on/off transition with no ringing - since ringing is bad for the mostet wastes power and creates RFI.

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You need both L and C to make a good filter. Try your coil again but put 0.01uf from either side of the coil to ground.

Hmm, that will slow down the switching. I was using an extra NAND to minimize switching time to get mimimal switching losses/heat in the MOSFET. I'm not sure, though, whether the on resistance or the switching losses are the dominant heat source.

It is rated 10-11 V and measures 10.3 V, well within the recommended

Ok. There must be a C in the SMPS, but I guess the additional C shall stabilize against incoming EMI.

The oscillator C is 470 nF, the charging R is 47K, the discharging R is 0-18K. The hysteresis voltage of the Schmitttrigger is 1 V (5.2-4.2 V).

Bernhard

Sure, much too big, too. I just tried it because it was lying around.

Hmm, I have a 100 W light bulb in series, and a diode that allows the current to keep running when the transistor closes.

I thought, maybe when I arced some sort of puls made it through the gate capacitance to the NAND. It's just strange that it happened when I arced to the metal structure, while the top board is plastic covered wood and survived quite a while of direct short circuit arcing.

Ok. Even slower switching then. Although I don't plan on playing with the MOT any more and it worked quite long (at least about an hour) before I did that.

No scope. It's on my whishlist for about a year now. I use a piezo speaker from an old digital watch which showed me that the oscillator is still working.

I first noticed it in the headphone connected to my sound card.

I also get it in a small battery powered radio. So, is ferrite ring better then? I'll try some.

Thanks, Bernhard

The cap is used in conjunction with a resistor and if the resistor is small ~100 ohms and the cap in the 100 pf range it shouldn't slow the mosfet switching substantially.

If you are encountering ringing or glitches in the transitions they usually happen at relatively high frequencies 100 KHZ - MHZ range so it doesn't take much to kill them..

The snubber works similar to the way the condenser across points in old ignition systems used to. Without the cap you get a lot of arcing at the points and a weak spark at the plug. With the cap the spark is hotter and the points don't arc.

In the ignition you use a cap to increase the ringing (while saving the points). The snubber absorbs the pulse of energy caused by inductance in the heater and wiring to the heater when the mosfet opens. The 100 ohm series resistance with the cap, keeps the ringing down by lowering the Q of the LC circuit. Snubbers can work wonders.

How is the heater arranged? If it is coiled wire it may have a high inductance - that and parasitic capacitance is a tuned circuit - guaranteed to ring for a time after the power switches off diodes or not.

Diodes? They have to be very high speed to deal with transients effectively. 1N4000 series are useless in that application. An ordinary silicon rectifier takes a long time to turn off once it conducts.

Sorry about the scope - that would probably lead you right to it.

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Another candidate for a Darwin-Award, there doesn't seem to be even a GFI in the distribution panel. The microwave transformer will do the rest. Bravo!

The capacitor slows the rare of rise, allowing the points to develop some clearance. With fast solid state switch the self capacitance of the coil is enough. But to the topic, an integral cycle controller will reduce the EMI

Huh? Slows the rate of rise? Current begins falling the instant the points open.

The cap serves two functions. It appears as a dead short (that instant only) across the points and keeps the current flowing while the points open - suppressing the arc. It forms a tank circuit with the primary of the ignition coil and gives a damped sine wave at its resonant frequency - increasing the spark. That's the Kettering ignition system from the first auto's that had points and distributors

The first transistor ignitions still had points but they only switched the non-inductive current to the base of the transistor. The first systems still had caps across the transistor to take advantage of the resonant circuit.

Coil pickups replaced points and the ignition coils changed from induction coils to pulse transformers. Entirely different systems even if they use some of the same parts - the operation isn't the same.

I don't understand where the "self capacity of the coil is enough" comes in - the coil isn't across the points in any ignition system (that I'm aware of) and inter winding capacity is generally minimized in pulse transformers - slows the rise time.

Integral Cycle Controller usually means letting a string of pulses from a sine wave into the heater - gating them at the zero crossing - no EMI. That's my understanding of ICC - did you mean something else?

The op has a 40 volt DC supply with a NAND gate pwm switching DC through a heater modulating it for 30 watts average dissipation - using a single mosfet.

That is perfectly reasonable and can be done with no EMI to speak of. In that application he doesn't even need a particularly fast switch. I didn't question his using DC - it can be done and it is what he has chosen to do.

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instant the

No

increasing the spark No . It saves enrgy being lost at the points . That's the Kettering

The self capacitance referred to above is sufficient to leave the cap out

Cycle Controller usually means letting a string of pulses

No

Perhaps a triac and one of these ( or similar) will help