“Protection circuits are the unsung heroes of modern electronics. Regardless of the application, long electrical chains from AC lines to digital loads are interspersed with fuses and transient voltage suppressors of all sizes and shapes. Along the electrical path, sources of electrical stress, such as inrush currents from storage capacitors, reverse currents from miswiring or outages, over- and under-voltages from inductive load switching, or lightning, can damage valuable Electronic loads.

“

Author: Reno Rossetti

Protection circuits are the unsung heroes of modern electronics. Regardless of the application, long electrical chains from AC lines to digital loads are interspersed with fuses and transient voltage suppressors of all sizes and shapes. Along the electrical path, sources of electrical stress, such as inrush currents from storage capacitors, reverse currents from miswiring or outages, over- and under-voltages from inductive load switching, or lightning, can damage valuable electronic loads. This is the case for microprocessors and memories built with fragile sub-micron and low-voltage technologies. Just as soldiers build fort walls, it is necessary to build a protective perimeter around the load to handle these potentially catastrophic events.

Protection electronics must handle fault conditions such as overvoltage/undervoltage, overcurrent, and reverse current within their voltage and current ratings. If the expected voltage surge exceeds the protection electronics ratings discussed here, additional layers of protection can be added, in the form of filters and transient voltage suppression (TVS) devices.

Figure 2 shows a typical system protection scheme around a smart load such as a microprocessor. DC-DC converter – complete 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 supply to the microprocessor or PLC. A voltage surge 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 (Figure 3). For this reason, front-end electronic protection is necessary. Here, protection is achieved by a controller (IC 1 ) that drives two discrete MOSFETs, T 1 and T 2 .

Figure 2. Typical Electronic System and Protection

Overvoltage protection

Based on the maximum operating voltage of the DC-DC converter (CONTROL IC 2 in Figure 2), the protector IC essentially consists of a MOSFET switch (T 2 ) that closes over this operating range and opens above it. The associated intrinsic diode D2 is reverse biased in the event of overvoltage and has no effect. In this case, the presence of T1/D1 also doesn’t matter, T1 is completely “on”.

Overcurrent Protection

Even if the input voltage is limited within the allowable operating range, the problem remains. Upward voltage fluctuations generate high CdV/dt inrush currents that can blow fuses (Figure 4), burn PCB traces, or overheat the system, 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. From this it can be seen that T2 itself cannot block negative input voltages. These can happen unexpectedly, for example, during negative transients or power outages, when the input voltage (V BUS in Figure 2) is low or absent, the DC-DC converter input capacitor (C IN ) passes through the intrinsic diode D 2. To block reverse current flow, it is necessary to place transistor T1 with its inherent diode D1 against negative current flow. However, the result is a costly back-to-back configuration of two MOSFETs with inherent diode reverse bias.

Integrated back-to-back MOSFETs

The need for a back-to-back configuration is obvious if discrete MOSFETs are used (as shown in Figure 2), but less so if the protection is monolithic, ie when the control circuit and MOSFET are integrated in a single IC. Many integrated protection ICs with reverse current protection use a single MOSFET and take extra care to switch the device body diode to reverse bias regardless of MOSFET polarization. This implementation is suitable for 5V MOSFETs with symmetrical source and drain structures. The source body and drain body have the same maximum operating voltage. In our case, the high voltage MOSFET is not symmetrical, only the drain is designed to withstand the high voltage relative to the body. The layout of the high voltage MOSFET is more critical, the HV MOSFET DS(ON) with optimized R is only provided in the case of a source-to-body short. At the end of the day, high voltage (>5V) integrated solutions must also be in a back-to-back configuration.

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

Traditional discrete solutions

Figure 5 illustrates the high cost of using a discrete implementation similar to Figure 2 in terms of PC board area and bill of materials (BOM) (24V IN, -60V to +60V protection). PCB area up to 70mm².

Figure 5. Traditional discrete protection with larger PCB area (70mm 2 )

Comprehensive solution

Figure 6 shows the advantages of integrating the control and power MOSFETs in the same IC packaged in a 3mm x 3mm TDFN-EP. In this case, the PCB area footprint is reduced to approximately 40% (28mm 2 ) of the discrete solution.

Figure 6. Integrated protection to reduce PCB area (28mm 2 )

Comprehensive protection series

The MAX17608 CMAX17610 family of adjustable overvoltage and overcurrent protection devices provides an example of this integrated solution. It features a low 210mΩ on-resistance integrated FET pair as shown in Figure 7.

Figure 7. Block diagram of the MAX17608/MAX17609 overvoltage/overcurrent protection device

These devices protect downstream circuits from faults with positive and negative input voltages up to ±60V. The Overvoltage Lockout Threshold (OVLO) is adjustable to any voltage between 5.5V and 60V with an optional external resistor (Figure 8). They feature programmable current limit protection up to 1A. 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. These devices operate over the -40°C to +125°C extended temperature range.

In addition to the ideal integrated characteristics, the solution features accurate current sensing of ±3% compared to ±40% typical for discrete solutions. The IC also reports the instantaneous load current value on the SETI pin (Figure 8). This is a great feature that helps the system monitor the current draw of each board.

The device can be programmed to operate in three different modes under current-limit conditions: auto-retry, continuous, or latched mode. This is a great way for system designers to decide how to manage load transients to minimize system downtime and service costs.

Figure 8. MAX17608/MAX17609 Application Block Diagram

in conclusion

Electronic loads require protection from power outages and fluctuations, inductive load switching, and lightning. We reviewed a typical protection solution with low integration, which not only results in PC board space inefficiency and high BOM, but also large tolerances and circuit certification challenges. We present a family 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 space footprint. Using these ICs, you can design a tight protection envelope around your system for increased safety and reliability.