Whitepaper

Almost all published synchronous rectifier designs use a voltage based zero-crossing detection method. This method has drawbacks. For instance: It is subject to voltage interferences which makes it unreliable, it is difficult to configure as a full wave bridge rectifier, and it is not scalable.

This new patented method uses a current detection method through a Hall-Effect Switch and an Iron Core (HES-IC). This method, unlike the voltage detection method, avoids the necessity of detecting the true zero-crossing every half cycle. The combined HES-IC current detection unit is only triggered by a predetermined current value.

If there is no current or the current is below the predetermined threshold, the rectifier circuit conducts by the MOSFET’s body diode. The trigger current can be custom designed to be as low as 1A or less, but the rectified current can be 100A or more.

Thus, when the HES-IC unit is used, the rectifier should not be described as a synchronous rectifier as with the Voltage triggered units.

Reliability is critical in any switching rectifier circuit. It is proven in theory and practice that using an iron core and Hall-Effect switch is the most reliable method of detecting current. Line voltage transients, interferences, or phase shifting don’t trigger the Hall-Effect Switch, only current can.

Since the current detection circuit is connected in series with the AC Input, it is very flexible in configuring all types of switching rectifier circuits, including positive/negative half wave, and single and 3 phase full wave H bridge types. It is also flexible in its ability to be operated in a large frequency range. For Active Rectifier Switching, the frequency (speed) of operation is dependent on the Hall-Effect switch. A typical Hall-Effect switch’s switching time is T= 1uS(switching) + 6uS(Hall plate time delay) . The frequency specifics of Hall-Effect switches range from 10kHz -30kHz.

This design is a very scalable solution. It is suitable for AC voltage ranges from a few volts to thousands of volts and Amperage ranges from a few amps to hundreds amps.

With a controllable Enable/Disable switch added to the three phase MOSFET H bridge design it can work in drive mode to power an AC motor (switch Disabled), OR with switch Enabled it can work as a switching rectifier for charging. The Same Power MOSFET module can be used in dual application.

AC to DC conversion has two parts: the AC voltage to DC voltage and the AC current to DC current. The diode is the most common device for the conversion. The diode is a P-N junction device, so it has a 0.7V junction voltage. It is not a linear device either and will generate harmonic distortion in switching power supply application. The high current and high voltage AC to DC rectification using diode devices needs a reliable, cost effective innovation to meet high industrial standards.

• Voltage based solutions: Most prior art designs use the voltage detection method at AC zero-crossing as the reference point for a voltage comparator. The output of the comparator controls the power MOSFET’s gate for the switching. Another design approach is to detect the changes of MOSFET’s Vds voltage.
• This types of switching rectifier is called a synchronous switching rectifier (SSR or SR). The drawbacks are obvious. The MOSFET switch ON/Off must be in sync with AC zero-crossing voltage detection at every single zero-crossing point regardless of the current flow. If it applies to the inductive load, the delay of the current flow will cause the zero-crossing voltage detection to be unreliable. As a result, the SSR rectification output must connect to a larger capacitor in order to bring the load current forward.
• It needs a dedicated controller.
• It has consistent issues working as a bridge rectifier(single phase and three phase) due to the floating zero-crossing on a bridge rectifier.
• It also has issues when working with higher AC input voltage or higher frequency.
• Applications such as arc welding, alternators, and DC motors are completely off-limits.

• This solution involves synchronized switching rectification in DC/DC power supply. The idea here is to use the detected secondary of transformer’s output frequency signal to sync up with primary switching frequency. Power MOSFET at primary of transformer for switching from DC to AC and MOSFET at secondary for switching from AC to DC are using the same frequency and phase. This solution requires a feedback loop. It runs into problems during the initial power-up cycle, and there is a phase loop issue when the load changes.
• The drawbacks of the above solution are obvious. It is complicated, requires dedicate controllers, and is not flexible.

• AC/DC rectification contains two parts: AC/DC voltage and AC/DC current. The new concept is to focus on current detection, in particular using a Hall effect switch placed inside of the iron core where an AC current conductor passes through the iron core. See below:
• The iron core concentrates the magnetic field produced by conductor current at the gap. The unipolar Hall effect switch can be triggered ON and OFF at small threshold current in real time. If there is no current or the current is under the threshold, MOSFET’s body diode forwards the conducting like a regular diode. When the current rises above the threshold, the Hall-effect switch outputs the control signal to turn the MOSFET ON. The current will flow through MOSFET’s Rds instead of body diode.(Note: body diode’s P-N junction collapses during Rds)
• Some capabilities of the patented novel technology include:
  • reliable operation under dynamic variable loads
  • low cost and simple fabrication
  • scalability for high/low voltage ranges;
  • suitability for variable frequency from DC up to 50KHz (as fast as Hall Effect switch switching frequency).
  • capable of single or three phase full bridge rectifications.
  • can be used as part of almost all electrical applications.