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large fall time in high voltage pulses

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adhil.iqbal1986
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large fall time in high voltage pulses

Post by adhil.iqbal1986 » June 15th, 2017, 6:26 pm

I am using a trigger pulse model to create a -75V (1 A compliance pulse @ 350us pulse width, nplc 0.01). the pulse shows a good risetime but when it falls, it falls initially at a decent angle but then tapers off which to me seems as if some internal mechanism kicks in to stop a large fall, is there a work around this?
I am using an open source load (because our devices have very high impedances), and my keithley version is 2612A.
is it possible to give me any code that can pulse a clean -75v (1A compliance, pulse width 350us) on an open source circuit?

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brian.d.smith
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Re: large fall time in high voltage pulses

Post by brian.d.smith » June 20th, 2017, 6:22 am

I would try using TSP Express to create the pulse and have a look with the oscilloscope. Leave the DUT unconnected to see the instrument response open circuit.

adhil.iqbal1986
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Joined: October 23rd, 2016, 8:35 pm
Country: Singapore

Re: large fall time in high voltage pulses

Post by adhil.iqbal1986 » June 20th, 2017, 9:59 am

Hi brian,

What is tsp express, im not aware of this new script, please can you give me some sample scripts to pulse -75 volts,

Andrea C
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Re: large fall time in high voltage pulses

Post by Andrea C » June 20th, 2017, 11:52 am

TSP Express is the Java applet from the internal web page of the 2600A or 2600B.

Even so, the pulse width behavior is range and load dependent:
2600B_PW_loads.PNG
2600B_PW_loads.PNG (16.91 KiB) Viewed 25192 times
Here is scope shot when using 100Ω resistor as the load and longer requested pulse width. From the settling time, we can see it takes nearly 700usec to reach the -75V level:
2636B_-75V_Pulse_1msec_duration.png
2636B_-75V_Pulse_1msec_duration.png (16.85 KiB) Viewed 25192 times

Your desire for a 350usec pulse of -75V is beyond slew rate capabilities of the instrument.

adhil.iqbal1986
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Joined: October 23rd, 2016, 8:35 pm
Country: Singapore

Re: large fall time in high voltage pulses

Post by adhil.iqbal1986 » June 20th, 2017, 12:22 pm

Understood

A slew rate of about 400us is totally acceptable, however my post pulse rise time is about 3ms(as seen in my -) which is not acceptable. Please share the code which you used to generate that pulse in the diagram you presented. I can verify where my code went wrong. Furthermore, the devices that i am testing have very high impedances, therefore, i am using an open source load, so a 100ohm resistor wont cut it.Thanks brian and andrea

Andrea C
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Re: large fall time in high voltage pulses

Post by Andrea C » June 22nd, 2017, 10:34 am

It is not my code that allows the faster fall time, it is the 100Ω load that is permitting the faster discharge to occur.

Another factor is the different current compliance limits used by the SMU.
The -75V amplitude uses the 200V range. You have a 1Amp current limit.
This Wattage level is pulse width and duty cycle restricted.
The 1Amp limit is used only during the pulse: Between the source.stimulus occurring and the PULSE_COMPLETE_EVENT_ID.
Before and after these, a lower compliance limit is in use such as 100mA.

smua.source.limiti = the limit when not pulsing, e.g., the idle level
smua.trigger.source.limiti = the limit during the pulse

As a test with your open circuit/scope probe only conditions, try with a compliance limit of 100mA during pulse and 100mA during idle level. Then try again with the 1Amp during pulse. There is your different outcome.

You said your DUT is very high impedance. Is it a diode type device? And sometimes you need high watts, but as the SMU turns off your DUT changes from low R to high R? If not, why do you need 1Amp limit for high R device?

adhil.iqbal1986
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Joined: October 23rd, 2016, 8:35 pm
Country: Singapore

Re: large fall time in high voltage pulses

Post by adhil.iqbal1986 » July 10th, 2017, 3:48 am

Hi Andrea,

sorry for the late reply, I was on vacation. I tried with the 100mA limit, works fine, but when I switch the pulse and dc mode limit to 1A I get the 5005 out of the operating limits of the smu error. this because I had set the source.rangev to -75V. how can I get the pulse current limit to be 1A and thereby pulse a clean waveform?

I am using this test routine to stress a diode. therefore I need to pulse as much voltage as I can without worrying the smu will hit current compliance.

Andrea C
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Re: large fall time in high voltage pulses

Post by Andrea C » July 13th, 2017, 5:00 am

The current limit is controlled with two different commands.

Command 1:
smua.source.limiti = the limit when not pulsing, e.g., the idle level

The combination of voltage sourcing range and current limit will need to be within the Continuous Operating capability of the SMU. If using the 200V source range, then 100mA is max current limit.

Command 2:
smua.trigger.source.limiti = the limit during the pulse

This limit will be applied only during the trigger model controlled pulse and permits the SMU operation into the higher watt operating points such as -75V and 1Amp limit.

In my previous post, I was indicating that for your open circuit condition (or scope probe only condition), you will see very different fall time if both of the current limit settings are at 100mA.
The longer settling time when using the 1Amp limit during the pulse is in part due to the SMU having to change the current limit as the pulse comes to an end.
The other part of the long settling time for the -75V to 0V discharge is due to the high load value and consequently small current that can flow. The load and the actual current is also part of the system dynamics.

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