Zener Power Rating in circuit

Thread Starter

Sherri

Joined Mar 31, 2018
29
Hello all!
I am in need of help in understanding what the power rating should be for the 15v zener in the circuit shown below. What I am looking to do is to clamp the flyback voltage to 15v (with the zener) and send it to a uC. The inductor current is clamped at 5A. The rough schematic is my initial thought on this but I am not sure how to rate the zener.

Thank you for your help and as always please be kind as I am a hobbyist trying to learn.

Sherry
 

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sghioto

Joined Dec 31, 2017
3,288
I would think a 1/2 watt zener should be sufficient from the component values shown.
Will the circuit actually work? Will the back EMF exceed 12 volts?
SG
 
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Thread Starter

Sherri

Joined Mar 31, 2018
29
I would think a 1/2 watt zener should be sufficient from the component values shown.
SG
Thank you sghioto for your reply. Could you tell me how you came to the conclusion that a 1/2 watt zener would be the correct power rating?

Sherry
 

sghioto

Joined Dec 31, 2017
3,288
From the current. The current flowing through the zener is limited by the 220K resistor.
Will need to re-think this circuit. You have no back EMF protection for the transistor driving the coil. BTW what type is that?
SG
 
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Thread Starter

Sherri

Joined Mar 31, 2018
29
From the current. The current flowing through the zener is limited by the 220K resistor.
Will need to re-think this circuit. You have no back EMF protection for the transistor driving the coil. BTW what type is that?
SG
This is an IGBT (Part #: ISL9V3040D3ST) The datasheet can be found here: https://www.onsemi.com/pub/Collateral/ISL9V3040D-F085-D.pdf
In the datasheet it does show Back EMF protection in the symbol detail.

Thanks for your help.

Sherry
 

sghioto

Joined Dec 31, 2017
3,288
OK, got it.
Still not quite sold on this circuit. The resistor values are a concern.
Do you have a value for the flyback voltage generated?
SG
 
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ScottWang

Joined Aug 23, 2012
7,206
The Zener diode needs >=5 mA to stabilize the Vz and R2, R3 need <=1 mA, I assumed that the current flows through R1 is 8 mA (I_Zener = (300V-15V)/36K = 285V/36K = 7.92 mA.) when the Vc output voltage of IGBT is 300V, and the R1 as 36K, if the voltage is 350V then the current will be flows through Zener diode as I_Zener = (350V-15V)/36K = 335V/36K = 9.3 mA, the values of R3 will be affected by the frequency, so you may change it to suit your needs.

IGBT_300-350V to 15V to uC__Sherri_ScottWang.png
 

BobaMosfet

Joined Jul 1, 2009
1,979
ScottWang's schematic is the correct way to use the zener in this case. A zener is a current device- it dumps current in order to maintain voltage (knee breakdown), like a pressure escape valve in parallel with the sensing circuit, not in series with it.
 

Thread Starter

Sherri

Joined Mar 31, 2018
29
Thank you all for your help with this. I am trying to learn and not just look for answers to make a circuit work. It is nice to see detailed answers to my questions and seeing a better way to solve a problem than I had originally thought would be the best way.

Thank you!

Sherry
 

Thread Starter

Sherri

Joined Mar 31, 2018
29
In thinking more on this, am I correct to say that I would need the zener to be rated at 2.257W for 300V or 3.116W for 350V. So, I would be safe to use a 3W or 4W Zener respectively? Please correct my thinking here if it is incorrect.

Sherry
 

sghioto

Joined Dec 31, 2017
3,288
So, I would be safe to use a 3W or 4W Zener respectively?
Short answer yes, but a .5 watt zener would suffice in either circuit.
Max current through the zener in post #11 is about 10ma at 350 volts. Power dissipation is equal to voltage times the current. 15 volts X .010 amps = .150 watts
SG
 

Thread Starter

Sherri

Joined Mar 31, 2018
29
@Sherri
They seem weren't that much, how did you calculated the wattage?
I was incorrect. I was calculating the power as: 285V x .00792A =2.2572W and 335V x .0093A = 3.1155W respectively. Where I believe it should only be for the zener which would be 15V x .00792A = .1182W and I could use a 1/4W rated zener and 15V x .0093A = .1395W and I could use a 1/4 watt zener also. Should I derate this and use a 1/2 watt Zener?

Sherry
 

ScottWang

Joined Aug 23, 2012
7,206
I was incorrect. I was calculating the power as: 285V x .00792A =2.2572W and 335V x .0093A = 3.1155W respectively. Where I believe it should only be for the zener which would be 15V x .00792A = .1182W and I could use a 1/4W rated zener and 15V x .0093A = .1395W and I could use a 1/4 watt zener also. Should I derate this and use a 1/2 watt Zener?

Sherry
Normally I will use the real values as 5 times of the calculated values, so you can try 0.5W and touch the Zener diode to sense the temperature, sometimes a little warm is ok, the better is no heat and it can't be hot.
 
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