Innovative bridge design eliminates technical gaps

Today, low-power solutions have become a hot topic in the industry, especially energy-saving solutions for battery-powered portable devices. Generally speaking, semiconductor devices that operate at lower voltages can extend the use time of portable devices such as mobile phones and multimedia players after a single charge. However, while paying attention to portable devices, it is easy to overlook other less portable devices that actually require higher efficiency. Environmental issues and rising electricity prices have made the market pay more attention to the overall power requirements of everyday electronic devices.

Although we tend to think of energy conservation from the perspective of low voltage, in fact, most electronic devices are operated by high voltage electricity supplied by the country, some of which are directly powered, and some are charged by adapters. There is no doubt that high-voltage power distribution is the most efficient way to distribute electricity to a wide area, but such a high voltage is too high for the power consumption area, and an inefficient voltage reduction process must be carried out.

Therefore, the purpose of improving this efficiency gap is obvious, that is, how to maximize the efficiency in the process of converting high AC voltage to more practical DC voltage. This efficiency maximization is especially important for electrical devices that are directly connected to the AC mains power supply.

Diode Bridge Circuit

The diode bridge circuit is the most basic element in the field of electronic engineering. Engineers widely use the diode bridge circuit to full-wave rectify the AC voltage so that it begins to resemble the characteristics of a DC power supply. Then, a network of resistors, capacitors, and inductors is used to filter the output DC voltage to make it smoother.

The diode is an active device formed by a PN junction of semiconductor materials. However, compared with more advanced active devices such as silicon-controlled rectifiers and transistors, it lacks controllable functions. Therefore, a standard diode will have a forward conduction voltage drop of about 0.7V when it is forward-conducting, resulting in a reduction in full-wave rectification efficiency. This also reflects that in high-current applications, this power dissipation can cause very large heat loss and power loss. Another feature of the traditional full-wave input bridge rectifier circuit is that no matter what time, two diodes will be turned on at the same time, which further increases the power consumption.

Despite the above disadvantages, this topology is still a cost-effective solution for low-load applications. At the same time, it is also widely used in high-power applications and is becoming more and more popular. For example, DC motors are gradually replacing AC induction motors, and such designs often use diode bridge circuits to provide voltage conversion. At this time, the power cost is more obvious.

Synchronous Rectification

To address this problem, International Rectifier (IR) has developed a new solution that makes better use of semiconductor technology by using the parasitic diode of MOSFET. This solution uses four FETs to build a bridge structure (as shown in Figure 1), avoiding the power loss caused by using diodes.

Figure 1: Using four FETs to create a bridge circuit

Synchronous rectification technology keeps the transistor in the same state as much as possible within half a cycle, thereby reducing the conduction time of the parasitic diode in the transistor body. When the transistor is turned on, the current does not pass through the body diode, which greatly reduces the power loss compared to crossing the entire half cycle.

In operation, at the beginning of the AC power half cycle, current begins to flow through the body diode of the FET, and a negative voltage is generated between the drain and source of the transistor. Detecting this negative voltage, the control circuit turns on the FET, allowing current to flow through the FET body instead of the parasitic diode, thereby reducing the power consumption of the device. The lower the RDS(on) of the transistor, the more efficient the solution.

The efficiency of this technology depends mainly on two factors: the FETS used and the accuracy of the control circuit. As shown in Figure 2, IR's two synchronous rectification control chips IRF1166 and IRF1167 provide a simple discrete solution for voltages below 200V. This is similar to the circuit structure using 4 FETs to drive a brushless DC motor. The correct FET switching time must be ensured to avoid short circuits. When the AC voltage starts to rise from 0V, current will also begin to flow through the FET, generating a negative voltage. At this time, the selected FET determines whether the control circuit can effectively sense this negative voltage.

Figure 2: IR's two synchronous rectifier control chips, IRF1166 and IRF1167, provide a simple discrete solution for voltages below 200V.

Another challenge in this design is to ensure that the control IC's comparator can withstand the high supply voltage while also detecting the small reverse bias of the body diode. IR's advanced Gen 5 HVIC technology successfully overcomes this challenge by integrating precision low voltage functions with high voltage devices using high voltage isolation.

To achieve maximum benefit, the FET must be fully turned on in half a cycle until the input voltage reaches 0V, and of course it cannot be turned on in the alternating current. However, the control circuit may mistake these slowly changing voltage/current signals as the current leading or trailing edge of the next cycle. When the voltage drop caused by the rising current is high enough, the circuit may repeatedly turn the FET on and off for a short time when the input voltage crosses zero. This situation is most likely to occur in a circuit with a resistive load because the rate of change of current in this circuit is slower than other circuits such as capacitive loads.

The solution to this problem is to add an RC network, two bootstrap diodes and bootstrap capacitors to the control circuit. This will inject more current into the 0V voltage range, ensuring that the FET source-drain voltage is higher than the diode threshold voltage in this uncertain voltage range.

For voltages up to 600V, a single IC solution can integrate the bootstrap diode, and each driver section can also replace the RC network with a dedicated configurable blanking time module, allowing the design to accommodate different FETs. The design can also integrate the FET, bootstrap capacitor and control functions into the same device, becoming a direct replacement for existing diode full-wave bridge rectifiers. This not only saves significant power, but also greatly reduces the required PCB space. Figure 3 shows an integrated dynamic bridge device that can achieve this function.

Figure 3: Integrated dynamic bridge device that integrates FET, bootstrap capacitor and control functions into one device