Reverse polarity solutions are seen as a necessary evil. It is essential in automotive systems to safeguard against hooking up the battery in reverse or reversing cables during a jumpstart for example, however the system designer has to live with the power losses that it incurs. Typically diodes are the first thing that comes to an engineer’s mind when he thinks of to protect against reverse polarity conditions.
Have you ever wondered why your child’s toy does not work after installing batteries, but when you reverse the direction of the batteries, it suddenly works? Well, that is a reverse polarity circuit at work and a simple diode that kept your child happy.
Now why can’t we use a diode for all applications requiring reverse polarity protection? Conventional diodes have a 0.7V drop across them and the power loss across the diode is V x I. Consider an application that has a 5A power demand. If you are using a Schottky diode, the power loss is approximately 3.5W. Apart from the power dissipation, the voltage available to the circuitry is a diode drop below the supply voltage.
In industrial and automotive applications, most front-end interfaces need reverse-polarity protection, traditionally provided by diodes or MOSFETs. A p-channel MOSFET has been conventionally used for high current applications since it does not need a charge pump. However, the Rds(on) of the p-channel MOSFET gets much higher at low input voltages and it does not prevent reverse current from flowing back into the input. It also needs additional circuitry and signals to turn it off to reduce quiescent current. More about the woes of using a p-channel MOSFET later.
How can we use a simple n-channel MOSFET and make sure we do not need any additional circuitry and make it behave exactly like a diode minus the power losses?
In comes a smart diode controller, the LM74610-Q1. This device is gaining a lot of traction in automotive applications because many electronic control modules in cars connect to the car battery directly. Any module connected to the battery needs to be protected against reverse voltage, a common problem related to erroneous jump-start procedures. A typical application circuit for automotive front end systems is shown in Figure 1. The LM74610-Q1 smart diode controller along with an n-channel MOSFET and the charge pump capacitor make up the smart diode solution.
Figure 1: Typical use case of the LM74610-Q1 smart diode controller and n-channel MOSFET.
For modules with low current requirements, it may be practical to use diodes, but for current needs greater than 2-3A, most designers will use a p-channel MOSFET to protect during reverse-voltage conditions. The control circuitry is complicated, however, and high-current p-channel MOSFETs are also expensive and increase overall system cost. The Rds(on) values common to p-channel MOSFETs go up drastically at the low input voltages typical in start-stop applications. Lab tests have verified that p-channel MOSFETs have lower thermal performance than Schottky diodes at low input voltages, as seen in Figure 2. p-channel MOSFETs also do not have reverse-current shut off, and thus rob the bulk capacitor voltage during any dips in input caused by typical automotive conditions like voltage interruption, warm start, cold start and start-stop scenarios.
Figure 2: Smart diode controller (plus n-channel MOSFET) performance compared to p-channel MOSFET performance.
ORing applications also use diodes or MOSFETs. One recent trend in automotive is to use redundant battery connections – typically two fused power paths – into modules for safety-critical applications. E-call boxes incorporate the car battery’s redundant power sources for normal operation and an auxiliary emergency battery in case the connection to the main battery is severed.
Low-current modules typically use diodes for ORing. High-current ORing applications require more complicated circuits, with many associated discrete components and large multipin packages. Automotive and industrial applications focus on reliability, causing designers to minimize part and pin counts to reduce failure rates.
In applications requiring low quiescent current consumption, ground-referenced schemes for input protection are not desirable. Automotive emissions standards and the increasing number of electronic modules in vehicles have pushed strict budget requirements for current in the off and on states. Typically, off states for each electronic module can be as low as 100µA. This is what makes sure that we can start our cars after parking at the airport for two weeks.
The LM74610-Q1 along with an n-channel MOSFET can better address the requirement for low quiescent current. It provides diode-like reverse-polarity protection and MOSFET-like performance for normal polarity conditions. Because the device does not need any control signals, the LM76410-Q1 mimics a two-terminal device and is not ground-referenced.
The key advantage of not being ground-referenced is that the LM76410-Q1 consumes zero quiescent current. When applying reverse voltage, the body diode of the MOSFET is not turned on, so it does not turn on the LM74610-Q1. When applying a normal polarity voltage, the body diode conducts, and internal charge-pump circuitry starts up with the diode voltage and generates voltage for the MOSFET to turn on. Periodically (at a 1% duty cycle), the MOSFET turns off to replenish the charge pump. A protected circuit would see a 0.6V drop at periodic intervals at that 98% duty cycle. With a 2.2µF capacitor used as the charge-pump capacitor, the MOSFET turns off for approximately 50ms once every 2.6 seconds. Figure 3 shows the block diagram of the LM74610-Q1.
Figure 3: LM74610-Q1 block diagram.
One of the inherent properties of a diode is that it blocks reverse voltage and does not allow reverse current to flow. The smart diode controller mimics this behavior and has very fast turnoff during reverse currents (typically 2µs). Blocking reverse voltage is an important feature to pass automotive testing as per ISO7637. The ISO7637 specification calls for electronic modules to be subjected to negative voltage pulses dynamically while operating at 12V.
A slow response to the reverse voltage can cause the output to go negative or discharge significantly during the pulse. If the output went negative or the capacitors discharged significantly, the downstream electronics could get damaged. To prevent significant discharge, bigger bulk capacitors may be used but this requires more board space and can be expensive. Lab tests have also verified that the smart diode controller is much faster than a p-channel MOSFET scheme. Figure 4 shows the fast acting response to reverse polarity and hence being able to meet an ISO7637 pulse 1 with a small 4.7uF output capacitor as seen in Figure 5.
Figure 4: Response time of the LM74610-Q1 to reverse voltage.
Figure 5: Smart diode controller solution – ISO pulse 1 with a 4.7µF output capacitor.
Figure 6: Small form factor for a smart diode implementation (8mm x 12mm).
The LM74610-Q1 smart diode controller and an n-channel MOSFET combine to form an effective automotive and industrial front-end reverse-polarity scheme, scalable from low to very high currents. Figure 6 shows the small form factor (117mm2) that can be achieved for a 100W solution, which is almost 60% the size of a D2PAK diode (180mm2).
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