Ideal Rectifier Bridge

Introduction

Over the past few decades, increasing energy efficiency has become a key challenge in electronics design, particularly in the realm of battery-powered devices and power supplies. Although commonly employed, traditional voltage rectification methods and reverse polarity protection are far from ideal due to significant power losses, which increase thermal demands and impose design constraints.

This article focuses on an innovative approach to this issue, namely the use of MOSFETs as substitute rectifying diodes. These transistors, used as ideal diodes, offer significant reductions in power losses and eliminate the need for complex and costly cooling systems. In the first part, we will focus on using MOSFETs instead of diodes in input circuits aimed at protecting systems against reverse polarity. In the second part, we will also analyze how further advancements in MOSFET control techniques can revolutionize power supply design, leading to systems with even greater energy efficiency and smaller dimensions.

Classical Approach to Reverse Polarity Protection

From the onset of developing mobile battery-powered devices, a challenge for designers has been ensuring effective reverse polarity protection while minimizing power loss. The classical approach to reverse polarity protection involves the use of a rectifying diode in series with the power supply as presented on Figure 1. These diodes, placed in the power supply circuit, allow current to flow in only one direction, thereby preventing device damage caused by reverse polarity. The first step towards optimization, improving efficiency by about 50%, involved replacing the rectifier diode with a Schottky diode, which reduced the voltage drop from 0.6-0.7V to about 0.3-0.4V. Although this is a commonly used method, it has its drawbacks, such as voltage drops and power losses. Despite the development of specialized diodes for battery applications with a voltage drop of 250-300mV (at low currents), the classical solution is still far from optimal.

Figure 1: Classical Reverse Polarity Protection

The approach presented in Figure 1 was acceptable for a long time in energy-efficient battery-powered devices, with power losses being somewhat "factored into the costs" of such devices. However, this solution was entirely unsuitable for more power-hungry devices. Examples of such devices include various automotive equipment designed for self-installation, such as CB radios, car audio systems and multimedia systems. In these cases, it was common to use an input diode in parallel with the powered receiver, as shown in Figure 2. Unfortunately, this configuration did not provide 100% protection against circuit damage in the event of incorrect polarity.

Figure 2: Reverse polarity protection used in high-current devices

Reverse Polarity Protection Using a MOSFET Transistor

With the popularization and availability of MOSFET transistors, an effective solution emerged in the form of a MOSFET used in a diode configuration, as shown in Figure 3.

Figure 3: MOSFET as reverse polarity protection:

A) Using a P-channel MOSFET B) Using a N-channel MOSFET

The ideal diode configuration provides a low voltage drop, determined by the RDS(ON) value of the transistor and the load current. For example, with a current of 1 A and RDS(ON)=10 mΩ, the voltage drop across the transistor is only 10 mV. This value is negligible compared to the voltage drop across a regular diode (600 mV) or a Schottky diode (350 mV).

The configuration shown in Figure 3, using a MOSFET transistor, has a drawback that is not significant from the perspective of reverse polarity protection of devices, but makes it impossible to call the above configuration an ideal diode. If a voltage that can open the MOSFET appears on the load side, then a voltage will appear at the input. Therefore, when using a battery or large capacities on the load side (as shown in Figure 4), an additional circuit or a dedicated driver available in the market is required.

Figure 4: The circuit stops working when large capacitance or a voltage that can open the transistor appears on the load side

In the market, we can find many ready-made solutions that act as controllers for ideal diodes, such as:

• DZDH0401DW from Diodes Incorporated provides MOSFET shutoff at a voltage difference between input and load of approximately 34mV.
• MAX16171 from Maxim Integrated is an advanced ideal diode controller that includes reverse-current protection.
• LM66100 from Texas Instruments is a complete ideal diode with an integrated MOSFET, offering a ready-to-use solution for systems powered by a 5V supply.

Conclusion

Traditional methods of reverse polarity protection have served their purpose, the use of MOSFETs presents a more efficient and effective alternative, paving the way for advancements in power supply design and energy efficiency. For classical reverse polarity protection, such as in battery-powered devices or those powered by an external supply, a simple circuit using a single MOSFET transistor is sufficient. However, to increase reliability and to maintain the properties of a diode placed at the input, it is necessary to use more advanced circuits available on the market from many manufacturers at very low prices.