- Home
- Design & Build
- Parts
- Zener diodes

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter JaredC79
- Start date

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.

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.

Last edited:

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.

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

Last edited:

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

- 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.
- I would look up the MOSFET's absolute max rating for VGS. Hopefully it is less than the number found in step 1 above!
- I would calculate TargetZenerValue = the average of VGS_step1 and VGS_step2.
- 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.
- I would estimate the required zener wattage rating: WattageRating >= 1.5 * (TargetZenerValue * Imax_step4)
- I would choose the Zener Dynamic Impedance such that (Imax_step4 * ZenerDynamicImpedance) is less than 0.5 volts.
- 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.

- 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.
- I would look up the MOSFET's absolute max rating for VGS. Hopefully it is less than the number found in step 1 above!
- I would calculate TargetZenerValue = the average of VGS_step1 and VGS_step2.
- 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.
- I would estimate the required zener wattage rating: WattageRating >= 1.5 * (TargetZenerValue * Imax_step4)
- I would choose the Zener Dynamic Impedance such that (Imax_step4 * ZenerDynamicImpedance) is less than 0.5 volts.
- 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

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.

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.

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.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.

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

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

Bingo

you need a current sensing loop

its not easy integrating this with an existing voltage feedback loop. hence band-aids.

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.

Bingo

you need a current sensing loop

its not easy integrating this with an existing voltage feedback loop. hence band-aids.

isn't that with the .1 ohm resistors are for? sensing current?

- Home
- Design & Build
- Parts
- Zener diodes