Why You Should Consider Ideal Diodes in Your Designs - ELE Times

By Jim Harrison, Maxim Guest Blogger, Lincoln Technology Communications

Although ideal diode ICs can have a positive impact on many designs, engineers often forget to use them. The ideal diode IC only uses MOSFET to work like a diode, but can greatly reduce voltage drop and leakage.

The ideal diode has three main applications. One is simple reverse voltage protection for battery-powered equipment. This is just a diode in series from the battery to the application circuit. The second is used as a diode OR for high-reliability redundant power supplies. Finally, the same diode "OR" circuit is only used for the selection circuit between a car rechargeable battery and a wall charger, just like your mobile phone and many portable devices. Ideal diodes (such as MAX16915) can also be used for power input overvoltage protection.

In all three applications mentioned above, designers can move from standard diodes to Schottky diodes. This will greatly help the forward voltage drop, because the forward voltage drop will rise from 1.1V for standard diodes to about 0.45V at 1A forward current. However, the ideal diode is very cheap and much smaller, and it will drop you to 85mV at this current. In addition, the ideal diode IC can reduce power consumption, reduce voltage loss (important when coming from a low-voltage battery), and can reduce the area of ​​the PC board. In addition, they solved a big Schottky problem. Schottky diodes have a high reverse leakage current, -1A devices are about 1mA. This kind of leakage is particularly bad for primary batteries. An ideal diode usually has a reverse leakage of less than 1µA over the entire temperature range.

There are three ways to obtain

. The use of driver IC external FET or integrated FET device makes self-manufacturing an option. However, driving a FET is not as easy as you think. In order to smoothly transfer current from one path to another without oscillation, it is very important for the drive circuit to control the forward voltage drop on the MOSFET. If the power supply fails or short-circuits, fast shutdown can minimize reverse current transients. A properly implemented ideal diode can provide front-end protection against battery reverse conditions, overvoltage transients and surge currents. Ideal diode controller ICs using external FETs have rated currents up to 5A and rated voltages up to 80V.

A good example of a complete ideal diode device is Maxim's MAX40200, which has an operating voltage range of 1.5V to 5.5V, a maximum operating current of 1A, and a tiny 0.73mm square 4-ball WLP or SOT23-5 package. With thermal self-protection function, it can work in the temperature range of -40° to 125°C. When disabled, the MAX40200 can block voltages up to 6V in either direction.

The function diagram in Figure 1 shows the unique symbol of the internal FET. The p-channel FET adds a circuit that detects the voltage from the drain to the source of the MOSFET. In addition to driving the gate, it also reverse-bias the body diode. The difference with ordinary diodes is that this "ideal diode" is not suitable for rectifying alternating current. In applications where the power supply is inductively coupled 60Hz AC, conventional diodes should be used as the rectifier part of the circuit. The MAX40200 is designed for applications that switch between different DC power supplies. The chip has a stable ~20mV voltage drop and can provide up to 100mA of forward current.

On top of this, when the maximum rated forward current is 1A, the forward voltage drop increases to approximately 90mV. This small voltage drop will improve efficiency and significantly increase battery operating time. This application note "Static and Dynamic Behavior of MAX40200 in Diode ORing Application" details the dynamic response of the IC. You can also use the evaluation board MAX40200EVKIT to test drive the device. For example, consider an AAA battery with a capacity of 1Ah. The battery capacity is about 3V, which is suitable for two batteries. If the Schottky diode drops 0.36V at 1A, and our MAX40200 only drops 0.09V, the difference of 0.27V can save 0.27Wh. Therefore, your device will run for another 25 minutes with a maximum load of 1A.

The thermal shutdown temperature of this IC is approximately +154°C with a hysteresis of 12°C. If the current exceeds ~500mA, please note that your design does not exceed this temperature. The thermal performance of the WLP package actually exceeds the thermal performance of the SOT package.

The operation of the MAX16141 ideal diode controller is quite different from our first example. The chip can provide the system with protection against reverse current, overcurrent, input overvoltage and undervoltage, and overheating conditions. It will cut off and isolate the faulty input power. The MAX16141 has a wide operating voltage range of 3.5V to 36V and a low shutdown current as low as 5μA (typical), making it ideal for automotive applications (by the way, the IC is also compliant with automotive standards).

In the circuit of Figure 3, the IC's charge pump drives the gates of back-to-back external nFETs to 9V above the source connection. The fast-acting comparator prevents reverse current within 1μs (maximum) of the input voltage lower than the output voltage. An external current sense resistor between RS and OUT prevents current overload. The maximum current is adjustable and is determined by the FET you choose. The OVSET and UVSET inputs provide set points for input overvoltage and undervoltage events. The MAX16141 is available in a 4 x 4 x 0.75mm, 16-pin TQFN package and operates over a temperature range of -40° to 125°C.

All in all, it is clear that ideal diodes can bring benefits to many designs. They may be underutilized, because perhaps no one seems to know exactly where to put them. Their classification is also different, sometimes classified as amplifiers, sometimes classified as power management, and sometimes classified as circuit protection. In any case, it does bring benefits for engineers to understand what these devices can provide.

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