Zener diodes

JaredC79

Banned
2015-09-02 4:32 am
NJ
Finalizing a few different things I been working on for months with the help of other members and at this point making sure i have the best part for the job. While I need zeners in my circuit let's say 6.2 or 6.8 at 1.3w additional parameters I never paid attention to are available like zener impedance and zener ma. I spoke with tech support and got a text book answer however no one seems to be able to practically apply the differences. Common sense tells me to go for the easiet load (4ohm rather then 2) and higher current values. Is this flawed? Or is this not applicable to an audio circuit? Please assist
 

rayma

Member
2011-04-29 8:37 pm
While I need zeners in my circuit let's say 6.2 or 6.8 at 1.3w additional parameters
I never paid attention to are available like zener impedance and zener ma.

You have to take into account all the worst case operation extremes.
The Zener subcircuit must still operate properly within all these ranges
and their possible combinations.
Lowest input voltage/highest
Lowest Zener voltage of the tolerance range/highest
Highest load current draw/lowest
Highest Zener incremental impedance/lowest
Highest Zener bias current and Pd/lowest
etc.
 

infinia

Member
2005-05-15 9:51 am
SoCal
While I need zeners in my circuit let's say 6.2 or 6.8 at 1.3w

This high power rating cuts down your part choices drastically and also raises delta Temp. >use a buffer transistor!
Is this a voltage reference or an AC clamp or what?
Choice of 6.2V implies good V reference. most stable zeners use a reference test current ~ 7.5mA. E.g. Don't load the zener down, all it's specs go to caca. unless deltas I & V are small. Consider using a TL431 buffered bandgap.

zeners have lousy initial voltage tolerance unless you pay the factory big bux to cherry pick, so be prepared to have a circuit that buffers and allows in circuit trimming.
 
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JaredC79

Banned
2015-09-02 4:32 am
NJ
This high power rating cuts down your part choices drastically and also raises delta Temp. >use a buffer transistor!
Is this a voltage reference or an AC clamp or what?
Choice of 6.2V implies good V reference. most stable zeners use a reference test current ~ 7.5mA. E.g. Don't load the zener down, all it's specs go to caca. unless deltas I & V are small. Consider using a TL431 buffered bandgap.

zeners have lousy initial voltage tolerance unless you pay the factory big bux to cherry pick, so be prepared to have a circuit that buffers and allows in circuit trimming.

Extra protection for Lfets when using a second transformer for more voltage swing. Gates can only handle 14v. They will not be loaded. So the current listed is the test current, what about impedances? Final biasing is done by a germainium transistor. Once it's all done and a few people help w the finishing touches it should be a good design.
 

infinia

Member
2005-05-15 9:51 am
SoCal
So the current listed is the test current, what about impedances?

zener impedance? it's the slope (degree of sharpness = 1/Rz) of the VI curve around the test current.
if you are driving zeners hard E.g. clipping, i'm sure the heating will have an effect on Rz. Gate drive needs low impedance to handle fet capacitance, IDK Lfets
 
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JaredC79

Banned
2015-09-02 4:32 am
NJ
zener impedance? it's the slope (degree of sharpness = 1/Rz) of the VI curve around the test current.
if you are driving zeners hard E.g. clipping, i'm sure the heating will have an effect on Rz. Gate drive needs low impedance to handle fet capacitance, IDK Lfets

Lateral MOSFET - lfet. So am I correct picking the faster one think was actually 750ma at 4 ohms being 4 is easier load then 2? Can't imagine why I would want a slower one that has a heavier load
 

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
If I were deciding which Zener diodes to buy, so that I could place them across MOSFET gate-to-source connections, I would proceed roughly as follows:


  1. I would estimate (or simulate) the worst-case, largest value of MOSFET VGS when the circuit is operating, including the power-on transient. This would include assuming the datasheet max Vgate_threshold, the datasheet min transconductance, and the largest conceivable current drawn from the MOSFET. Which perhaps might be an inrush current or a driving-reactive-load-during-clipping current.
  2. I would look up the MOSFET's absolute max rating for VGS. Hopefully it is less than the number found in step 1 above!
  3. I would calculate TargetZenerValue = the average of VGS_step1 and VGS_step2.
  4. I would estimate (or simulate) the worst-case, largest value of current that could ever be forced through the zener during a protection event. Find the largest possible voltage that the gate drive circuitry might possibly try to shove in, perhaps during power-on. Find the lowest possible source voltage during this event. Then use Kirchoff's Voltage Law to calculate the current that flows in the Zener.
  5. I would estimate the required zener wattage rating: WattageRating >= 1.5 * (TargetZenerValue * Imax_step4)
  6. I would choose the Zener Dynamic Impedance such that (Imax_step4 * ZenerDynamicImpedance) is less than 0.5 volts.
  7. I would start searching distributor sales sites for a zener with a wattage rating as calculated in step 5, and with a dynamic impedance as calculated in step 6, and with a zener voltage as calculated in step 3.
 

JaredC79

Banned
2015-09-02 4:32 am
NJ
If I were deciding which Zener diodes to buy, so that I could place them across MOSFET gate-to-source connections, I would proceed roughly as follows:


  1. I would estimate (or simulate) the worst-case, largest value of MOSFET VGS when the circuit is operating, including the power-on transient. This would include assuming the datasheet max Vgate_threshold, the datasheet min transconductance, and the largest conceivable current drawn from the MOSFET. Which perhaps might be an inrush current or a driving-reactive-load-during-clipping current.
  2. I would look up the MOSFET's absolute max rating for VGS. Hopefully it is less than the number found in step 1 above!
  3. I would calculate TargetZenerValue = the average of VGS_step1 and VGS_step2.
  4. I would estimate (or simulate) the worst-case, largest value of current that could ever be forced through the zener during a protection event. Find the largest possible voltage that the gate drive circuitry might possibly try to shove in, perhaps during power-on. Find the lowest possible source voltage during this event. Then use Kirchoff's Voltage Law to calculate the current that flows in the Zener.
  5. I would estimate the required zener wattage rating: WattageRating >= 1.5 * (TargetZenerValue * Imax_step4)
  6. I would choose the Zener Dynamic Impedance such that (Imax_step4 * ZenerDynamicImpedance) is less than 0.5 volts.
  7. I would start searching distributor sales sites for a zener with a wattage rating as calculated in step 5, and with a dynamic impedance as calculated in step 6, and with a zener voltage as calculated in step 3.

the white pages for the transistor spec the use of a 6.2v 1.3w zener if using elevated drive rails of 4-5v however i have seen where someone used 6.8 - I would go with the lesser value as a precaution as it has a 14v+- gs max and not roll the dice since it seems to not make much of a difference and allows more margin of error. separate driver rails would not be needed if i can get the driver circuit to 6v or better. It would also only be worth while if the output transistors are carrying less than 4 amps each and the drive circuits peak output is more than 2 v below the supply rail. that is a lot of variables where just throwing another transformer into the mix makes things easier. zener impedances of that value are 2 or 4 ohms. 146, 170, 183ma are the current values...the 183 is only offered in 2 ohms, the 170 is only offered in 4 ohms, the 146 is only in 2 ohms. there is an outlier at 35MA and all the other offerings aren't showing this spec. all that being said, which would you recommend. lets say the elevated rails are 82v
 

JaredC79

Banned
2015-09-02 4:32 am
NJ
relying on back to back zeners on MOSFET gate drive circuits is crutch, or band-aid to reliable designs. spend your efforts more on them.

please see last post, its not back to back zener. to get the full potential of LFETs 6 to7v has to be seen from the driver circuit but there is also a lot of variables stated in the last reply. Holton's AB amps allow the use of 1 or 2 transformers and i feel he is a talented engineer/designer and not using band-aids. a second transformer improves the sonics and minimizes components and complexity that may hinder other areas.
 
I have seen the Gate-Source zeners used for current limiting with lateral mosfets. A 12 volt zener with 1 ohm mosfets gives 12 amps current limit.
This is the issue.
The mosfets are not failing because Vgs>14V and gate breakdown, but because Ids and therefore die power dissipation gets too high under fault conditions.
The use of 6V2 or 6V8 is limiting current to the point the amplifier can stand short to ground for a while

This method only works for laterals because of the low gm. There is no safe fixed value for verticals to be sure of limiting current sufficiently
 

JaredC79

Banned
2015-09-02 4:32 am
NJ
If you want to perform the calculations I suggested, please go right ahead. However I am not especially motivated to calculate them for you. Good luck with your project!

thanks however it seems like i may be aiming for a moving target, Since the LFets have their pros and cons, no thermal runaway, internal protection, easy to drive etc. i am basically looking to have and input and vas capable of driving 1 to 4 sets of the 125w lfets or 2 pairs of the 250s. so a lot can change as the voltage and current changing. having the elevated rails as a option would be nice but not mandatory so would like both options included. you don't have to do any math - educated guess of off experiences would be sufficient. Formulas are great but i like to understand the purpose of them and how the current and impedance of a zener effects the over all circuit. as most manufacturers omit that area, i would assume it isn't as critical as i am making it out to be.