By David DeLeonardo, Analog Specialist AE, Future Electronics
In this Part II, we finish with an overview of the other 4 most common protection components; provide a set of application tips and finish with a Device Comparison and Characterization Chart. In part I, we looked at the most common damaging events that threaten electronic circuits and 4 of the 8 most common components used to address them.
Fuses: These are the most diverse type of protection devices. In the types used to protect most electronic circuits, they range in size from as small as 0402 SMT devices up to ¾” dia. cylinders 2” long. All fuses function in a similar manner. First, the fuse is placed in series with the load being protected such that the entire load current passes through a specially prepared conductive element. When the temperature of this conductive element exceeds a given value, the element will undergo a phase change from conductive solid to gas. (Metallic elements melt to a liquid state first.) This gas is further heated by the resulting arc into plasma which is then either harmlessly dissipated into the air around the fuse or “captured” within the fuse body. In any case, since there is no longer any material present to conduct current, the fuse becomes an open circuit and the load current is interrupted. The speed of this irreversible process can take less than a millisecond in the case of Fast Fuses to multiple seconds in the case of “slow blow” devices which are intentionally designed with a delayed response to allow for inrush and transient conditions.
Fuses are not re-usable once they have been blown opened due to an overcurrent event.
The parameters used to characterize fuses are not uniformly defined among various manufacturers but do consistently include the following:
- Ampere Rating: the load current the fuse can conduct indefinitely without tripping within a set of test conditions defined by the manufacturer.
- Voltage Rating: the maximum voltage at which the fuse can interrupt the rated short circuit current.
- Cold Resistance: the resistance of the fuse when conducting no more than 10% of the rated current.
- Hot Resistance: the resistance of the fuse when conducting the maximum rated current.
- Nominal Melting I2t: This is the amount of energy that is required to melt the fuse element under a particular set of test conditions.
Electronic/PTC Resettable Fuses: As in the case of traditional fuses, PTC Electronic fuses are also placed in series with the load to guard against overcurrent to the load. However, unlike traditional fuses, they can be activated or “tripped” many times. They are comprised of a conductive material (carbon black) mixed into a polymer binder that expands when heated. This thermally induced expansion greatly increases the resistivity of the overall device. This, in turn, limits the current into the protected circuit. Once conditions return to nominal, the device will cool down and it will return to its prior low resistance state. This allows the PTC to protect the load against multiple transient events without needing to be replaced. It should be noted however, that PTCs do exhibit some aging with repeated activation such that their nominal resistance slowly increases. This can lead to “thermal runaway” such that, even under non-fault conditions, the resistivity will rise to the point that the device will need to be replaced. However, this effect is usually not observed until a very large number of events have occurred (on the order of hundreds.)
PTC devices range in size from small 0402 SMT devices that will trip at 0.1A to radial disk type devices that cover the mid-range of voltage and currents to large “blade” types that measure over an 1.2” x 0.4” and will trip at 50A.
Inrush Current Limiters/NTC Devices: As in the case of the previously discussed fuses and PTCs, these devices are placed in series with the current into the load. However, since their resistance DECREASES with temperature, NTCs serve an opposite function. NTCs have HIGH resistance at lower temperatures, so when power is first applied to the load, they limit the initial input current to a safe value. Then, as load current passes through them, their temperature will increase due resistive heating. However, as they heat up, their resistance drops by orders of magnitude. This allows them to pass sufficient load current without excessive losses. They are commonly used in AC/DC power supplies, but are being displaced by more efficient FET-based solutions in order to meet increasingly tough efficiency standards. Their schematic symbol is the same as the PTC, but with a negative sign in front of the “t” indicating the negative temperature coefficient.
Thyristors: A thyristor is a four layer semiconductor device that can be thought of as a combination of an NPN and PNP transistor pair connected in the following manner:
The addition of a Gate-to-Cathode resistor (as shown in Figure 4) will allow the thyristor to be “self-triggering”. In this case, simply applying a voltage across the anode and cathode terminals (called the “Switching Voltage”) will cause the lower NPN structure to turn “ON” which turns on the upper PNP structure and thus “sets” or triggers the thyristor to remain on until the conducted current falls below the minimum “Holding Current” for the device. The value of the thyristor switching voltage should be at or below the peak voltage rating of the components being protected. This “self-triggering” characteristic is very desirable in circuit protection applications and is why most thyristors used for protection have a built in-resistor and are thus two-terminal devices.
Normally, the thyristor is triggered to conduct from anode to cathode by a current injected into the gate. Then, the voltage across the anode-cathode terminals will collapse to less than a few volts or so, depending on how much current is being conducted. The device will remain in this state until the current falls below a minimum “holding” current. Then, the device returns to its “off” state and will not conduct again until the next current pulse into the gate terminal.
Other critical device parameters include:
- On-State Voltage: This is the voltage across the anode and cathode when the full rated current is being conducted (typically below 5V).
- Surge and Peak Current Rating: These are the currents the device can conduct without degradation for a given set of test conditions/ time durations.
- Capacitance: Since thyristors are often used to protect high-bandwidth signal lines, the capacitance that is present between the anode and cathode terminals is an important consideration.
Thyristors used for circuit protection range in size from 3.3 x 3.3mm QFN devices with trigger voltages as low as 25V and peak pulse currents as low as 100A to TO-218-3 packaged devices with trigger voltages of around 200V and peak pulse currents of 5,000A.
Tips for Choosing a Protection Solution
When deciding on a circuit protection strategy, a good place to start would be with the relevant Safety Agency standards and certification requirements for your application. Once these are identified, copies of the standards can be obtained from UL for a fee. While these are often only available from UL, there are a number of NRTLs (Nationally Recognized Testing Laboratories) that can certify a given product’s compliance to nearly any UL Standard. That is, UL may write a given standard, but there are MANY NRTLs can certify compliance to that standard. Next, searching the sites of the leading protection device providers for application guides and design notes for your application can be a very helpful next step. Of course, any due diligence should include a review of “prior art” and how those established solutions have fared in field in terms of product failures, returns, warranty claims, etc. Finally, a cost-benefit analysis should be done to balance the usual design criteria of device cost reduction verses warranty and liability claim costs.
Device Key Specifications and Relative Comparisons
|Component||Relative Energy Absorbtion (1 to 10 scale) 1 = smallest||Relative Speed of Activation (1 to 10 scale) 1 = fastest||Device Impedance Prior to, During and After Activation||Activated Voltage Range||Activated Current Range||Cycle Endurance|
|ESD Diodes||1 to 3||1||Clamp to diode voltage during activation; open circuit when reverse biased||Activated upon biasing of diode junction, so 0.6V||Using 10/1000μs standard pulse; devices range from 0.15A to 3A||Limited by magnitude of applied test pulse. Else, undefined.|
|TVS Diodes||2 to 6||2 to 4||Depends on conduction mode; Diode when forward and Zener in reverse.||From 0.6V to 570V per single device. Multidevice assemblies used for higher voltages||Using 10/1000μs standard pulse; devices range from 0.25A to 15KA||Limited by magnitude of applied test pulse. Else, undefined.|
|MOVs||4 to 9||3 to 5||High impedence prior to activation. Clamp during transient, high and slightly reduced impedence, after.||5V to 4.7KV||1A to 100KA||Limited by magnitude of energy absorbed per pulse. Leakage current will increase with activation cycles.|
|GDTs||6 to 10||4 to 8||Open circuit prior to activation. Low impedance during transient. Returns to open circuit after transient.||55V to 8.5KV||Using 10/1000μs standard pulse; devices range from 500A to 100KA||Limited by magnitude of applied test pulse. Activations will “age” device.|
|Fuses||Not applicable||Varies widely by device type: 3 to 9||Short circuit prior to activation; open circuit thereafter.||Current activated, but can “break” voltages from 12VDC to 1000VDC||Fuses for circuit protection are rated from 2mA to 600A||Not resettable; single cycle only.|
|Electronic/ PTC Resettable Fuses||1 to 4||4 to 10||Low impedance when “cold” and high impedance when “hot”.||Current activated, but voltage ratings range from 12VDC to 600VDC||Trip current ratings range from 14mA to 32A; hybrid devices to 60A||Cycle endurance extremely high when used within ratings >1K|
|In-Rush Current Limiters/NTC Devices||2 to 4||7 to 10||Low impedance when “hot”, high impedance when “cold”.||Current activated, but can “break” voltages from 12VDC to 1000VDC||Resistance decreases exponentially with current. Max loads range from 0.1A to 50A||Cycle endurance extremely high when used within ratings >1K|
|Thyristors||2 to 5||5 to 9||Open circuit prior to activation. Low impedance during transient. Returns to open circuit after transient.||15V to 700V||24A to 5KA. NOTE: ON state voltages range from just 1.5V to 8V.||Limited by magnitude of applied test pulse. Else, undefined.|