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Using two-switch-quasi-resonant flyback converter

Posted: 13 Jul 2015     Print Version  Bookmark and Share

Keywords:flyback converter  quasi-resonant  MOSFET  controller 

A flyback converter is quite attractive in that it is typically the least expensive isolated topology because it uses the fewest number of components. For offline flyback designs a quasi-resonant (QR) controller achieves the best efficiency and the best EMI behaviour.

The controller monitors the resonant ringing of the switchnode voltage and switches directly at the valley. This results in a lower voltage during the turn on of the MOSFET. Unfortunately this popular topology has some drawbacks. The input and output ripple currents of a QR-flyback are high because they are discontinuous. The primary and secondary windings are not coupled ideal.

This results in leakage inductance that stores energy during each cycle. This energy causes an overshoot (spike) of the switchnode voltage. To limit the peak voltage that the primary switch sees a snubber network is needed, that burns the energy stored in the leakage inductance, which lowers the efficiency. Another disadvantage of the standard flyback is that the MOSFET must withstand a very high voltage. During the demagnetizing time the drain-to-source voltage VDS of the MOSFET is equal to:


VBULK = voltage of the input capacitor

VFLYBACK = reflected output secondary voltage

VLEAKAGE = voltage overshoot due to leakage inductance

VLEAKAGE is typically in the range between 50V and 100V for offline designs.

The Two-Switch-Quasi-Resonant (2S-QR) flyback topology has the potential to overcome some of the drawbacks. First of all it recycles the energy of the leakage inductance. Secondly the maximum drain-to-source voltage of the MOSFET's is equal or less the input voltage. The trade-off for using this topology is not only the higher design effort, but also the obvious cost of the extra components. Two MOSFET's (figure 1: Q1, Q2), two diodes (figure 1: D2, D3) and a driver-IC or gate-drive transformer are needed. It depends on the specific application if the 2S-QR topology is a serious alternative to the standard flyback.

Principle of operation
The operation of the 2S-QR flyback topology is very similar to a traditional flyback. The main difference is that there are two MOSFET's Q1 and Q2 (figure 1) which turn on and off at the same time. When both MOSFET's are on, the primary current starts at zero and rises to a peak value.

Figure 1: Two-Switch-Quasi-Resonant Flyback.

At turn off, the current transfers to the secondary (the energy is stored in the gab of the transformer) and the secondary current decreases to zero (called demagnetizing time), where it remains until the beginning of the next switching cycle. After the demagnetizing time no energy is left in the transformer but there is still energy in the parasitic switch node capacitance. The energy builds a resonant circuit with the primary inductance and the typically resonant ringing of the switch node voltage occurs. Modern QR controller monitors the ringing and switches directly at the valley. Diodes D3 and D2 (figure 1) clamp the voltage overshoot caused by the leakage inductance. The energy is returned back to the input capacitor and recycled.

Advantages of the 2S-QR vs. standard single switch topology

 • Voltage rating requirements of the MOSFET's (figure 1: Q1, Q2) are lower. The drain-to-source voltage is equal or less than the input voltage.
 • The energy stored in the leakage inductance is recycled.

Design hints
Please note that the overall voltage stress is not always divided equally over both MOSFET's. The system is not always balanced. During the demagnetizing time the diode D3, the primary inductance and the diode D2 build a series connection to the input voltage. The voltage across the primary inductance is fixed, it is VFLYBACK. If there is an imbalance, the voltages across the diodes D2, D3 must not be equal.

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