PPTC devices protect automotive electronics from reverse polarity damage

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PPTC devices protect automotive electronics from reverse polarity damage [Copy link]

Automotive electronic devices must have protection against reverse polarity power failures. Reverse polarity occurs when jumper cables are connected to the wrong polarity terminals, or when connected to an over-discharged battery, or when a new battery is installed in the wrong position. If no protective measures are taken, excessive heat can cause electronic modules to fail, or load devices such as solenoid valves and motors on the car to fail, causing unsafe hazards. Traditional protection technologies are expensive and can cause excessive voltage drops, which can affect the performance of some systems. New technologies using polymer positive temperature coefficient (PPTC) devices, such as Raychem's PolySwitch products, can solve these shortcomings at the same time and have other advantages.

Traditional diode protection
--- In order to protect the electronic module from damage due to the reverse polarity of the battery, the common solution is to use a forward-conducting (rectifying) diode to prevent the current from flowing in the opposite direction (see Figure 1).
--- The most fundamental disadvantage of using a forward-conducting diode is the inherent voltage loss (0.7~1.0V) and the reduction of the actual supply voltage of the electronic module. For some automotive electronic modules in the system (such as the engine control unit), the operating voltage is critical, and reducing any form of voltage drop (such as the voltage across the forward-conducting diode) is important to ensure that the vehicle starts normally when the battery voltage is low. In other cases, such as audio systems, the system voltage has a direct impact on the output power (Po=V*I=V2/R). In other words, it will directly affect the audio performance. In order to minimize voltage loss, some electronic modules use Schottky diodes to reduce the voltage drop, which can generally be controlled below 0.5V.
--- If a standard rectifier diode or Schottky diode is used for reverse battery protection, the current carrying capacity (current rating) of the diode depends on the size of the load that will be connected to the diode. When the current through the electronic module is less than 1A, the cost of a standard rectifier forward-conducting diode is relatively low (less than $0.05). However, if a Schottky diode is used, or the current exceeds 1A, its cost will increase.
--- Another factor to consider when selecting the size of the forward-conducting diode is the size of the surge current and the ability of the device to absorb and dissipate the surge current that occurs during a "load drop". Disconnecting the car's battery while the alternator is supplying current will cause a load drop event. Generally, this load drop waveform will reach its peak voltage within a few milliseconds. For silicon devices, the worst-case rating is usually considered.

Polymer PTC protection
--- Polymer PTC devices, such as PolySwitch products, consist of a composite of semi-crystalline polymers and conductive particles. Under normal operating conditions, the conductive particles in the device form a low-resistance path that allows current to flow. Under fault conditions that cause excessive temperatures, such as overcurrent or high ambient temperature, the crystals in the polymer begin to melt and form an amorphous material, causing the conductive particles to separate, resulting in a very large nonlinear increase in the resistance of the device. This increase in resistance is typically more than three orders of magnitude, reducing the current to a relatively low and safe level. PolySwitch polymer PTC devices reset after the fault is cleared and the circuit power is disconnected.
--- Using PolySwitch devices to replace forward-conducting diodes in the above applications (see Figure 2) provides multiple advantages, including reduced voltage drop, because the voltage drop across the polymer PPTC device is generally above 0.1V. Second, polymer PPTC devices can provide additional protection for other types of electronic components (heating, wires, relays, and solid-state components).

Protection of High-Power MOSFET Circuits
--- For electronic modules that use high-power MOSFETs for solid-state switching of various loads, other reverse polarity conditions are mainly concentrated on the high-voltage side or low-voltage side of the drive configuration (see Figure 3a). In the reverse polarity condition, the internal diode of the high-power MOSFET becomes forward biased and allows current to flow to the motor, lamp or solenoid load connected to it (see Figure 3b).
--- This does not form a fault condition that will cause instantaneous destruction. However, the power dissipation of the FET will generally increase by about 5 times, because the voltage drop across the device in this case is about 1V (current flows through the PN junction of the internal diode) instead of the rated forward VDS voltage of 0.2V (measured from the drain to the source of the FET). Unless proper thermal control practices are adopted, such as using a heat sink large enough to dissipate the heat generated during the duration of the reverse polarity condition, continued operation in this condition will burn out the MOSFET.
--- This additional heat sink will increase the cost, weight and size of this application, which are areas that automotive manufacturers and suppliers want to reduce. Even with thermally protected field effect transistors, such as TEMPFET, it is still not possible to prevent burning under such conditions because the gate of the field effect transistor cannot control the current flowing through the internal diode.
--- Adding a polymer positive temperature coefficient device in series with the load and coupling it with a bypass diode (see Figure 4d) can provide protection against battery polarity reverse connection faults and allow the use of a smaller heat sink. More importantly, it can prevent the reverse flow of current, thereby avoiding abnormal operation of the solenoid valve or motor.

Inductive load and battery polarity are reversed

--- For inductive loads, the most common approach is to use a freewheeling diode connected across the load to suppress the voltage spike generated when the load is disconnected. Figure 4a shows a high-power MOS field-effect transistor with an inductive load used as a high-side and low-side switch. In the reverse polarity state, current will flow through the forward-biased internal diode in the field-effect transistor and the freewheeling diode connected in parallel across the load, establishing a direct short-circuit path between the positive and negative terminals of the power supply (Figure 4b). One way to stop this current flow is to use a forward-conducting diode as shown in Figure 4c. However, for high-current loads, as mentioned earlier, this solution may be too costly to adopt. Another alternative is to use a polymer positive temperature coefficient device and couple it with a smaller rectifier diode, which only needs to withstand the surge current required for the polymer positive temperature coefficient device to "break off", unlike the forward-conducting diode that must continuously support the full load current.

Motor Protection
--- Most low-power motors used to provide greater comfort and convenience to vehicle occupants are brushed DC motors. Bidirectional motors (such as power windows, power seats, and power locks) are driven using an "H-bridge" configuration consisting of four high-power MOS field-effect transistors connected as shown in Figure 5a.
--- To rotate the motor in the forward direction, field-effect transistors 1 and 4 are turned on simultaneously; to rotate the motor in the reverse direction, field-effect transistors 2 and 3 are turned on simultaneously. In the case of reverse polarity connection, the equivalent circuit produced for the H-bridge circuit is two series internal diodes connected in parallel between the positive and negative terminals of the power supply (see Figure 5b), so a short circuit path is actually established.
--- For the same reasons mentioned above, the use of a series forward-conducting diode may not be economically feasible. However, by using a series polymer positive temperature coefficient device, it helps to provide an economically feasible reverse polarity protection method while minimizing voltage losses in the system (see Figure 5c). The equivalent circuit under reverse polarity conditions is shown in Figure 5d. In general, the internal diode of the field effect tube facilitates the provision of the temporary surge current required to cause the polymer positive temperature coefficient device to break within milliseconds.
--- For the circuits shown in Figures 2, 3b, 4d, and 5d, the diode that establishes the current path in the reverse polarity state must have a certain surge capacity rating to cause the PPTC device to break within the safe operating area (SOA) of the diode. In other words, the "break time" of the polymer positive temperature coefficient device must never exceed the time limit of the surge current of the diode. Polymer positive temperature coefficient devices can be selected in a range of current and maximum break time ratings to meet the needs of most applications.

Reducing Power Losses in Vehicles
--- As vehicle loads continue to increase, automakers and their electronic system suppliers are planning the next generation of vehicle power systems to replace the 12V battery systems that have been used in vehicles since the 1950s. The PowerNet technology specifies a voltage limit of three times that of today's conventional systems.
--- This 42V system includes tighter specifications for 12V products that can still be used in a dual-voltage configuration. Therefore, today's lower-power products can continue to be used for many years and combined with higher-power products on a 42V bus. Because costs are closely associated with the process of migrating to 42V power, automakers are trying to slow the process and look for any opportunity to reduce power consumption.
--- One way to reduce power consumption is to use brushless DC current instead of brushed motors, especially for higher-power applications. Brushless DC motors have the advantage of not wearing out and reducing electromagnetic interference because they do not have brushes that are prone to arcing. In a three-phase brushless motor, the topology of the FET bridge is three branches, similar to the two branches of a brushed DC motor. Battery polarity reversal has the same effect on BLDC motors, but fortunately, the PTC reverse battery protection configuration suggested in Figure 5c can also be used for BLDC motors.
--- Using PTC devices to replace series diodes can provide additional benefits for vehicles that are already close to the full capacity of the available power system. Since the power loss of the series diode is proportional to the voltage, reducing the 0.7V diode voltage to about 0.1V in a 20A circuit can reduce (0.7-0.1) × 20 = 12W. In the dozens of motors used in a typical vehicle, this technology can save more than 100W of power in typical conditions.
--- These three savings have the potential to delay the transition to higher voltage systems by 1 to 2 years. Some vehicles, such as GM's GMC Sierra and Chevrolet Silverado, will use limited 42V systems in the 2004 model year. The technical specifications for 42V vehicles do not allow for reverse battery connection. The methods discussed can help automakers ensure that these specifications are met.

A safe path for future development
--- As automakers add more and more electronic devices to cars, the possibility of single failures is also increasing, such as reverse battery connection failure. It is becoming increasingly important for automakers to use the lowest power solution and keep costs within the target range. Polymer positive temperature coefficient devices help manufacturers reduce component costs and improve the efficiency and reliability of electronic systems.