One of the goals in modern power supply design is to minimize the losses respectively the heat dissipation. The biggest source of power loss is still the electronic switches - transistors and diodes, and while the transistors are experiencing progressive improvements of their properties, the diodes still have limitations, imposed by their structure. To avoid power losses in diodes they are often replaced with MOSFET transistors, as is the case with the synchronous rectifiers. The proposed design offers a simplified solution for improving the efficiency in the bridge rectifiers by replacing the diodes with N-channel MOSFET transistors. The modified scheme is shown in Fig. 1.
The gate voltage for T1 and T2 is provided by the charge accumulated in the capacitors C1 and C2. When the transistor T1 is not used, C1 is charged through D1 and R1, the zener diode ZD1 clamps the charge of C1 to the maximum allowed voltage for the gate of T1. At the same time, T2 is ON thanks to C2. The charging / discharging process of C1 and C2 is controlled by changing the polarity of the AC source.
The J-FET transistors T3 and T4 have a dual role, they invert the control logic of T1 and T2 because the J-FETs are a type of depletion mode transistor (they are ON in the absence of control voltage), and at the same time they clamp the T1 and T2 gates when the capacitors C1 and C2 are charging.
The semiconductor manufacturers can benefit by turning the proposed scheme into next generation highly efficient rectifiers. In modern semiconductors manufacturing process all components from the proposed design could be implemented into one single chip, even the capacitors – Murata recently reported an improved process of implementing semiconductor capacitors. The rectifiers are essential part of every power supply unit and there is always a demand for this type of components.
The proposed scheme has both advantages and disadvantages:
Advantage - offers high efficiency.
Advantage - reduces heat dissipation (consequence of 1).
Advantage - Safety critical compatibility - the circuit will continue to rectify through the flywheel diodes if the transistors are not operating due to a fault in the control loops.
Advantage - can be realized on a small area using standard components, hybrid technology or built into semiconductor device.
Advantage - improvability – as the MOSFET transistors are constantly improving this will lead to improvement of the rectifier efficiency and provide field for releasing new, improved products on the market.
Disadvantage - at DC voltage the transistors will not conduct since the process of charging the capacitors will be interrupted, but the circuit will continue to rectify through the diodes, i.e. it will continue working with reduced efficiency and lower current capacity.
Disadvantage - high complexity - the circuit is composed of more components compared to a standard diode rectifier where we have only four devices.