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Zener diode snubber question

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boylesg

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Is there anything that I should consider in changing a 4 x 75V zener diode snubber to a 2 x 150V zener diode snubber?

It is just that 2 series zener diodes is a little more convenient on a circuit board than 4 series zener dioides.

From the data sheet the main difference seems to be the amount of current the diodes pass in reverse with the 150V zener passing half as much current as the 75V zener.

They are the 1N53.. series by the way.
 

zorro

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You should take care to:
*) power dissipation
*) current at the operation point (all diodes must operate in the low resistance region)
*) zener impedance at the operating point

Regards

Z
 

boylesg

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You should take care to:
*) power dissipation
*) current at the operation point (all diodes must operate in the low resistance region)
*) zener impedance at the operating point

Regards

Z

4 x 75V/5W zener seems to work OK with a 12V car battery through a tv flyback transformer.

But clearly 3 x series car baterries (36V) is producing a kick back current that is exceeding the power rating of the zeners and frying them.

Possibly 2 x series car batteries (24V) is doing the same thing but perhaps frying them more slowly as 24V was operating my mosfet based jacobs ladder circuit OK, at least for the short time I have been trying it.

Any suggestions on more appropriate zener diodes than 1N5374s?



Also I have been reading that MOV varistors are often used to suppress voltage spikes. From what I have read these would be a far more robust way of doing so than zener diodes in that it would be far less sensitive to increases in the input voltage that I might like to try with my mosfet flyback transformer driver.

My research has revealed this:

Ordering code*****VRMS(V)****VDC(V)****max(8/20 μs)A****Wmax(2 ms)J****Pmax W****Vv(1 mA)V****ΔVv(1 mA)%***vc, max(ic)V**icA*****Ctyp(1 kHz)pF

B72220S0111K101**115********150******6500************60.0**********1.00*******180*********±10*********300********100.0***1520

Would this device for example be a suitable replacement for 4 x 75V/5W zener diodes?
 
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zorro

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Another option for transient supression is to use gas discharge tubes.They have very low capacitance and huge parallel R.
Sometimes, gas dischargers are combined with MOV and other devices in order to have several stages of protection.
Gas dischargers and MOV have negative-resistance characteristic. Zeners don't.
Speed, parallel capacity, parallel R and surge rating are important considerations.
Regards

Z
 

FvM

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From the data sheet the main difference seems to be the amount of current the diodes pass in reverse with the 150V zener passing half as much current as the 75V zener.
No. The zener current numbers in the data sheet just reflects an arbitrary operation point chosen for specification. The diodes can pass any current between leakage current and maximum rating.

It's easy to operate a flyback circuit in a way, that nearly all input power is transferred to "snubber" diodes (perhaps we should name them load diodes in this case). This happens, if no suitable secondary load exists or the transformer parameters don't fit the application.

A snubber is normally assumed to consume a small amount of energy stored in the transformer leakage inductance. Apparently your circuit is different. Then there's no other means than further increasing the total diode power rating, either using diodes with higher individual rating, or more diodes of lower zener voltage in series connection.

P.S.: You have quoted some data of transorb diodes with high pulse power rating. But your application seems to involve continuous power (repeated pulses), so the continuous power rating matters, and transorb diodes aren't better in this regard than regular zener diodes. The case size allows a rough estimation of power handling.
 
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