Technical Seminar Series: Circuit Protection Q&A
Bourns
Q1: I am using a GDT for lightning protection on an RF line because I need low capacitance. However, the line is also used for AC power, so I have put a varistor in series with the GDT to force the GDT to stop conducting after the surge has ended. I am assuming that the varistor capacitance will be sufficient to allow the GDT to switch on when the surge appears (1us rise time). Is this assumption correct?
Craig Shipley: The MOV needs to be above the AC voltage (with additional tolerances etc) as this will govern where the circuit will operate. The insulation resistance of the GDT will be a much higher than the MOV, so voltage across the GDT first which will cause it to switch. Remember, the voltage of the MOV and GDT must be added to arrive at the total turn on voltage. An example of this is if you have a 150V MOV and a 150V GDT, the protection will turn on at 300V. After the GDT begins conduction, there will be about a 160V drop across the protection (150V clamping voltage of MOV plus ~10V drop across GDT) until the transient dissipates. The capacitance of the varistor should not impact the turn on characteristics. The benefit of this solution is the total capacitance of the protection should be around 1pF, thus limiting loss.
Q2: How fast does the gas tube conduct?
Craig Shipley: This really depends upon how fast the transient is, meaning the rise time or dv/dt of the voltage front. The faster the transient passed the rated voltage of the GDT, the faster it will turn on. As an example, a 250V GDT would fire in .5 to .6 usec on a 1000v/usec surge.
Littelfuse
Q1: What are the differences between GDT and solid state TVS (such as thyristor)?
Phillip Havens:
a) GDT is based on a gas plasma technology which exhibits a very low off-state capacitance, while the solid-state TVS is based on silicon technology. The solid-state devices will have higher off-state capacitance than the GDT counterparts.
b) Solid-state will exhibit much faster turn-on characteristics with lower overshoot values than the GDT.
c) The GDT will have much higher peak pulse current rating; 5kA to 20 kA as compared to 500 A type ratings for solid-state.
d) The solid-state devices have turn on voltages ranging from 6 volts to 400 volts, while the GDT has turn on voltages ranging from 100 volts to 1000 volts.
e) GDT historically has been used for primary protection while solid-state has been used for secondary. There are many differences between these two technologies, I have only highlighted a few here.
Q2: Are there “fail safe” varistors?
Phillip Havens: We have fail safe options for GDT devices and for solid-state SIDACtor devices in T0220 through-hole type packages. I am not aware of any fail safe options for varistors.
Raychem
Q1: In some cases, is it necessary to use a resistor in series with PPTCs to insure the maximum interrupt current isn't exceeded? If the maximum interrupt current is exceeded, what is the failure mode of the PPTC?
Jim Toth: PPTC’s are rated at the maximum interrupt current that is used for UL testing. They are usually insensitive to higher interrupt current, as long as the maximum voltage is not exceeded, especially for a small number of cycles. However, we cannot recommend that they be used to interrupt currents above the maximum rating. Please contact the factory or one of our sales reps for additional information. http://www.circuitprotection.com/contactus.asp
Q2: What is the specific solution for USB port?
Jim Toth: Please see our application note at: http://www.circuitprotection.com/04Databook/C16_USB_(129-132).pdf
Q3: PPTC question - Are these suitable for use on boards that normally operate in an environment that covers large temperature range such as automotive underhood?
Jim Toth: PPTC’s are widely used in the automotive industry. Raychem Circuit Protection has released a line of PPTC products that are specifically designed for automotive applications, including under hood. Please see the following link for more specific information: http://www.circuitprotection.com/auto.asp
General Questions:
Q1: Is this a marketing presentation?
TTI: All of the webcasts in the TTI Technical Seminar Series are technical in nature; however, each webcast begins with about a 10-minute market industry overview by a market industry expert who is proficient in the webcast topic. The remainder of the webcast is quite technical in nature. We, at TTI, hope you take full advantage of all of our free technical seminars by registering at www.ttiinc.com/object/tech_seminars.html.
Q2: Do any of your companies have standard components that are used in aircraft installations?
Craig Shipley: No we don’t. Bourns made the decision to no longer offer these tubes after the purchase of Joslyn in 2001 due to the compliance requirements and special manufacturing requirements of these special tubes.
Phillip Havens: Littelfuse has ESD protection devices used in aircraft electronics and we are investigating other possible applications for aircraft lightning protection. We would invite further conversations on this topic.
Q3: Which circuit protection is best for static discharge, like touching an electronic device after walking across carpet?
Craig Shipley: We have a range of chip diode products that are more aligned to ESD protection. The GDT and thyristor are not really suited to protecting against these types of transients as the energy content is relatively low (high voltage, no current in <60ns impulse). I have attached the URL of the products to help identify something that is suitable. http://www.bourns.com/component.aspx?cmsphid=7631383|7163299|9243093
Phillip Havens: Our PulseGuard technology (PGB 0603, PGB 0805, OR PGB 10603 and PGB 10805 RoHS compliant series) is used extensively in handheld devices such as cell phones. Our SIDACtor family of parts is used in telcom equipment. The best solution depends on the technology being protected and the type of environment exposure. For carpet walking, the PulseGuard series is the best choice.
Jim Toth: A variety of components can be used to protect circuits from ESD discharge. They best solution depends on the specific circuit requirement. PESD devices are especially applicable for ESD protection of high speed data lines (USB 2.0, IEE 1394, HDMI etc) Please see the following link for additional information: http://www.circuitprotection.com/esd/