Protection circuits are the unsung heroes of modern electronics. Whatever the application, the long electrical chain from AC lines to digital loads is interspersed with fuses and transient voltage suppressors of all sizes and shapes. Along the electrical path, electrical stressors (such as inrush currents caused by storage capacitors, reverse currents caused by wiring errors or power failures, over- and under-voltage caused by inductive load switching or lightning) can damage valuable electronic loads. This is true for microprocessors and memories built with fragile submicron and low-voltage technologies. Just as a soldier builds a fortress wall, it is necessary to build a protective perimeter around the load to handle these potentially catastrophic events.

Protected electronics must handle fault conditions such as over/under voltage, overcurrent, and reverse current within their voltage and current ratings. If expected voltage surges exceed the ratings of the protection electronics discussed here, additional layers of protection can be added in the form of filters and transient voltage suppression (TVS) devices.

Figure 1 shows a typical system protection scheme around an intelligent load such as a microprocessor. DC-DC converter - with control (IC 2 ), synchronous rectifier MOSFETs (T 3, T 4 ) and associated intrinsic diodes (D 3, D 4 ), and input and output filter capacitors (C IN , C OUT ) -Power is supplied to the microprocessor or PLC. Voltage surges from the 24V power bus (V BUS ), if connected directly to V IN , can have catastrophic consequences for the DC-DC converter and its load. For this reason, front-end electronic protection is necessary. Here, the protection is realised by means of a controller (IC 1 ), which drives two discrete MOSFETs, T 1 and T 2.

Figure 1. Typical electronic system and protection

Overvoltage protection

Depending on the maximum operating voltage of the DC-DC converter, the protector IC essentially consists of a MOSFET switch (T 2 ), which approaches and opens above this operating range. The associated intrinsic diode D 2 is reverse biased in case of overvoltage and does not have any effect. In this case, the presence of T 1 /D 1 is irrelevant and T 1 is fully ‘on’.

Overcurrent protection

Even if the input voltage is limited to the permissible operating range, the problem remains. Upward voltage fluctuations generate high CdV/dt inrush currents that can blow fuses, burn PCB traces or overheat the system, thus reducing its reliability. Therefore, the protection IC must be equipped with a current limiting mechanism.

Reverse Current Protection

The MOSFET's intrinsic diode between drain and source is reverse biased when the MOSFET is ‘on’ and forward biased when the MOSFET voltage polarity is reversed. It follows that T 2 itself cannot block negative input voltages. These can occur unexpectedly, for example, during negative transients or brownouts, when the input voltage (V BUS in Figure 1) is low or absent, and the DC-DC converter input capacitor (C IN) passes through the intrinsic diode D 2. In order to block reverse currents, it is necessary to place the transistor T 1 together with its intrinsic diode D 1 to oppose the negative currents. However, the result is a costly back-to-back configuration of two MOSFETs with their intrinsic diodes reverse biased.

Integrated Back-to-Back MOSFETs

The need for a back-to-back configuration is obvious if discrete MOSFETs are used (as shown in Figure 1), and less obvious if the protection is monolithic, i.e., when the control circuitry and MOSFETs are integrated in a single IC. Many integrated protection ICs equipped with reverse current protection use a single MOSFET and take extra care to switch the device body diode to reverse bias, regardless of MOSFET polarisation. This implementation applies to 5V MOSFETs with symmetrical source and drain structures. The source and drain have the same maximum operating voltage. In our example, the high voltage MOSFET is not symmetrical and only the drain is designed to withstand high voltages relative to the body. The layout of the HV MOSFET is even more critical, as HV MOSFETDS(ON) with optimised R are only available with the source and body shorted. Ultimately, high voltage (> 5V) integrated solutions must also be in a back-to-back configuration.

In motor drive applications, the DC motor current is PWM controlled by the MOSFET bridge driver. During the off portion of the PWM control cycle, the current is recirculated back to the input capacitor, effectively enabling an energy recovery scheme. In this case, reverse current protection is not required.

Conventional Discrete Solutions

Figure 3 illustrates this in terms of PC board area and bill of materials (BOM). the PCB area is up to 70 mm² .

Figure 3. Conventional discrete protection with larger PCB area (70mm ² )

Integrated Solution

Figure 4 shows the advantage of integrating the control and power MOSFETs in the same IC in a 3mm x 3mm TDFN-EP package. In this case, the PCB footprint is reduced to approximately 40% (28mm ² ) of the discrete solution.

Fig. 4 Integrated protection with reduced PCB area (28mm ² )

Integrated Protection Family

The MAX17608 -MAX17610 family of adjustable overvoltage and overcurrent protection devices provides an example of this integrated solution. It has a low 210mΩ on-resistance integrated FET pair, as shown in Figure 5.

Figure 5. MAX17608/MAX17609 Overvoltage/Overcurrent Protection Device Block Diagrams

 These devices protect downstream circuits from positive and negative input voltage faults up to ±60V. The overvoltage lockout threshold (OVLO) can be adjusted to any voltage from 5.5V to 60V with an optional external resistor (Figure 6). They have programmable current-limit protection up to 1 A. The MAX17608 and MAX17610 prevent reverse current flow, while the MAX17609 allows reverse current flow. These devices also feature thermal shutdown protection against internal overheating. They are available in a small 12-pin (3mm x 3mm) TDFN-EP package. The devices operate over the -40°C to +125°C extended temperature range.

In addition to the desirable integration features, the solution features accurate current sensing of ±3%, compared to ±40% typical for discrete solutions.IC also reports the value of the load transient current on the SEti pin (Figure 6). This is a great feature that helps the system monitor the current consumption of each board.

The device can be programmed to operate under current-limited conditions in three different ways: auto-retry, continuous, or latching mode. This is a great way for system designers to determine how to manage load transients to minimise system downtime and service costs.

Figure 6. MAX17608/MAX17609 Application Block Diagrams

 Conclusion

Electronic loads need to be protected from power outages and fluctuations, inductive load switching, and lightning. We reviewed a typical protection solution with low integration, which not only results in inefficient PC board space and high BOM, but also high tolerances and circuit certification challenges. We demonstrate a range of highly integrated, highly flexible, low R DS(ON) protection ICs that provide direct and reverse voltage and current protection. They are very easy to use and provide the necessary functionality with minimal BOM and PC board footprint. With these ICs, you can design a tight protection envelope around your system for increased safety and reliability.

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