Power Tip: What's the best way to drive a synchronous rectifier in a flyback power supply?

There are several techniques for driving synchronous rectifiers (SRs) in a flyback topology: using a gate drive transformer, having the power transformer self-drive the SR, or using a dedicated driver. Gate drive transformers and self-driving techniques result in less than ideal efficiency due to shoot-through or reverse recovery losses, but specialized SR drivers have been developed over the years. Driving the SR with a driver that achieves volt-second balance minimizes shoot-through and reverse recovery losses and maximizes efficiency.

    There are several ways to implement synchronous rectification:

    Self-driving. Including voltage self-driving and current self-driving.

    External drive

There are two points to note when using SR drivers:

The dead time of the rectifier and freewheeling tubes must be long enough, otherwise the transformer and SR tube will be burned if the two tubes are turned on at the same time.

    The dead time cannot be too long, otherwise it will pass through the body diode and increase the loss.

    Dead time: When the synchronous rectifier should be turned on, there is no gate drive voltage and the current can only flow to the body diode. This time can be called dead time.

    Different topologies and synchronous rectification drivers solve different problems, including dead zone problems and commutation problems.

    When multiple synchronous rectifier tubes are connected in parallel, pay attention to the problem of current sharing for each tube.

With volt-second balancing, the driver monitors the volt-seconds on the secondary transformer winding during the primary FET on-time. When the secondary winding voltage collapses due to the primary FET turning off, the driver opens the SR and applies the same number of volt-seconds in the opposite direction.

This drive technique is very simple and requires only a few resistors: a voltage divider to sense the transformer winding voltage, a voltage divider to sense the output voltage, and a resistor to program the blanking time. Figure 1 shows an example simplified schematic.

Transformer drive techniques typically force continuous conduction operation. However, volt-second balance allows for discontinuous operation, making it ideal for quasi-resonant operation. Consequently, more and more AC/DC adapter designers are adopting this technique.

Figure 1: A synchronous rectifier driver with volt-second balancing offers a simple and very effective solution

    If we have been using a transformer-driven SR in a flyback design, next time consider using a dedicated SR driver such as the UCC24630 SR controller.

    The UCC24630 SR controller is a high-performance controller and driver for N-channel MOSFET power devices used for secondary-side synchronous rectification.
The controller and MOSFET combination form a near-ideal diode rectifier. This solution not only directly reduces rectifier power dissipation but also reduces primary-side losses through improved efficiency.
The UCC24630 utilizes a volt-second balance control scheme and is not directly connected to the MOSFET drain, making it ideal for flyback power supplies over a wide output voltage range. The SR driver turn-off threshold is independent of the MOSFET R_DS(on), optimizing for maximum on-time. Furthermore, secondary-side current ringing caused by device and wiring inductance does not affect the SR turn-off threshold.
The UCC24630 controller provides a programmable false trigger filter, a frequency detector (which automatically switches to standby mode during low-power states), and pin fault protection. The UCC24630 is compatible with discontinuous conduction mode (DCM), transition mode (TM), and continuous conduction mode (CCM) operation.
With a wide VDD operating range and a wide range of programmable VPC voltage and blanking time, the device can be applied to various flyback converter designs.

    ● Features:
■ Secondary-side synchronous rectification (SR) controller optimized for 5V to 24V flyback systems
■ Operating frequency up to 200kHz
■ Volt-second balanced SR on-time control
■ Minimizes the effects of metal-oxide semiconductor field-effect transistor (MOSFET) devices and wiring inductance
■ Continuous conduction mode (CCM) compatible
■ Compatible with primary-side synchronous rectification (PSR) and secondary-side synchronous rectification (SSR) control
■ Automatic low-power detection and 110µA standby mode current
■ Wide VDD range: 3.6V to 28V
■ Rail-to-rail gate driver with 13V clamp voltage
■ Open and short pin fault protection