Figure 1. Unprotected CPU on Fire
Protection circuits are the unsung heroes of modern electronics. No matter the application, the long electric chain, from the AC line to the digital load, is interspersed with fuses and transient voltage suppressors of all sizes and shapes. Along the electrical path, electrical stressors – such as inrush currents due to storage capacitors, reverse currents due to wiring errors or power outages, overvoltages, and undervoltages induced by inductive load switching or lightning – can damage precious electronic loads. This is true for microprocessors and memories, built with fragile sub-micron and low-voltage technologies. Like soldiers building a fort wall, it is necessary to build a perimeter of protection around the load to handle these potentially catastrophic events (Figure 1).
Protection electronics must handle fault conditions such as overvoltage/undervoltage, overcurrent, and reverse-current flow within limits of their voltage and current rating. If the expected voltage surge exceeds the protection electronics rating 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 the smart load, for example, a microprocessor. A DC-DC converter—complete with control (IC2), synchronous rectification MOSFETs (T3, T4), and associated intrinsic diodes (D3, D4), as well as input and output filter capacitors (CIN, COUT)—powers the microprocessor or PLC. A voltage surge that comes from the 24V power bus (VBUS), if directly connected to VIN, would have catastrophic consequences for the DC-DC converter and its load (Figure 3). For this reason, front-end electronic protection is necessary. Here, the protection is implemented with a controller (IC1) that drives two discrete MOSFETs, T1 and T2.
Figure 2. Typical Electronic System and Protection
Based on the DC-DC converter's maximum operating voltage (CONTROL IC2 in Figure 2), the protector IC essentially consists of a MOSFET switch (T2) that is close within this operating range and is open above it. The associated intrinsic diode D2 is reverse-biased in case of an overvoltage and does not play any role. The presence of T1/D1 is also inconsequential in this case, with T1 fully 'on.'
Figure 3. Hot Plug-in Causes Voltage Surges
Even when the incoming voltage is confined within the allowed operating range, problems can persist. Upward voltage fluctuations generate high CdV/dt inrush currents that can blow a fuse (Figure 4), burn up a PCB trace, or overheat the system, reducing its reliability. Accordingly, the protection IC must be equipped with a current-limiting mechanism.
A MOSFET's intrinsic diode between drain and source is reverse-biased when the MOSFET is 'on' and forward-biased when the MOSFET voltage polarity reverses. It follows that T2 by itself cannot block negative-input voltages. These can happen accidentally, for example, during a negative transient or a power outage, when the input voltage (VBUS in Figure 2) is low or absent and the DC-DC converter input capacitor (CIN) feeds the power BUS via the intrinsic diode D2. To block the reverse current, it's necessary to have the transistor T1, placed with its intrinsic diode D1 opposing the negative current flow. The result, however, is a costly back-to-back configuration of two MOSFETs with their intrinsic diodes oppositely biased.
Integrated Back-to-Back MOSFETs
The need for a back-to-back configuration is obvious if discrete MOSFETs are utilized, like in Figure 2, and less obvious if the protection is monolithic, namely when the control circuit and MOSFET are integrated in a single IC. Many integrated protection ICs equipped with reverse-current protection utilize a single MOSFET, with the additional precaution of switching the device body-diode to reverse-bias no matter the MOSFET polarization. This implementation works well with 5V MOSFETs, which have a symmetrical structure with respect to source and drain. Source-body and drain-body maximum operating voltage are the same. High-voltage MOSFETs, in our case, are not symmetrical and only the drain is designed to withstand high voltage with respect to body. The layout of high-voltage MOSFETs is more critical and HV MOSFETs with optimized RDS(ON) only come with the source shorted to the body. Bottom line, a high-voltage (> 5V) integrated solution will have to utilize a back-to-back configuration as well.
In motor driver applications, the DC motor current is PWM-controlled with a MOSFET bridge driver. During the OFF-portion of the PWM control cycle, the current recirculates back to the input capacitor, effectively implementing an energy recovery scheme. In this case, reverse-current protection is not called for.
Traditional Discrete Solution
Figure 5 illustrates the high costs, in terms of PC board area and bill of materials (BOM), of utilizing a discrete implementation like the one in Figure 2 (24VIN, -60V to +60V protection). The PCB area is a hefty 70mm2.
Figure 5. Traditional Discrete Protection with Higher PCB Area (70mm2)
Figure 6 shows the advantage of integrating the control and power MOSFETs within the same IC, which is packaged in a 3mm x 3mm TDFN-EP package. In this case, the PCB area occupation is reduced to roughly 40% of the discrete solution (28mm2).
Figure 6. Integrated Protection with Reduced PCB Area (28mm2)
Integrated Protection Family
The MAX17608–MAX17610 family of adjustable overvoltage and overcurrent protection devices provides an example of such an integrated solution. It features a low 210mΩ, on-resistance integrated FET pair as shown in Figure 7.
Figure 7. MAX17608/MAX17609 Overvoltage/Overcurrent Protection Device Block Diagram
The devices protect downstream circuitry from positive and negative input voltage faults up to ±60V.The overvoltage-lockout threshold (OVLO) is adjusted with optional external resistors to any voltage between 5.5V and 60V (Figure 8). They feature programmable current-limit protection up to 1A. The MAX17608 and MAX17610 block current flows in reverse direction, whereas the MAX17609 allows current to flow in the reverse direction. The devices also feature thermal shutdown protection against internal overheat. 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 integrated features, this solution has precise current sensing at ±3% compared to ±40%, which is typical with a discrete solution. The IC also reports the load instantaneous current value on the SETI pin (Figure 8). This is a great feature, helping the system to monitor current consumption of each circuit board.
The devices can be programmed to behave in three different ways under current-limit condition: Auto-retry, Continuous, or Latch-off modes. This is a great way for the system designer to decide how to manage load transient to minimize system downtime and service cost.
Figure 8. MAX17608/MAX17609 Application Diagram
Electronic loads require protection from the effects of power outages and fluctuations, inductive load switching, and lightning. We reviewed a typical protection solution, with a low level of integration that not only leads to inefficiencies in PC board space and high BOM but has high tolerance and poses circuit qualification challenges. We presented a rich family of highly integrated, highly flexible, low-RDS(ON) protection ICs that provide direct and reverse-voltage and current protection. They are extremely easy to use and provide the necessary features with minimal BOM and PC board space occupation. With these ICs, you can design a tight perimeter of protection around your system for enhanced safety and reliability.
Nazzareno (Reno) Rossetti is an analog and power management expert at Maxim Integrated. He is a published author and holds several patents in this field. Reno holds a doctorate in Electrical Engineering from Politecnico di Torino, Italy.