A quasi resonant flyback converter is a variant of flyback converter wherein it makes use of the parasitic elements to partially resemble a resonance action. According to dictionary, the word quasi could mean partly, partially, comparatively or relatively. So, a quasiresonance means a partial resonance, not a total resonance. At light load, Quasi resonant flyback operation will be in DCM. However, it will come closer to the boundary or transition mode while in full load. There is a difference on how quasi resonant flyback works compared to the flyback converter. Read also “How Flyback Converter Works – Operation and Principle” to compare both operations.
Basic Flyback Converter Operation
The very basic operation of a quasi resonant flyback is just like that of the the flyback converter. It will store energy in the primary during switch on cycle. It will transfer the stored energy to the secondary during switch off cycle. To explain in details the operation of a flyback converter, read the article “How Flyback Converter Works – Operation and Principle”.
Difference Between Flyback Converter and Quasi Resonant Flyback Operation
In terms of circuit diagram, they are the same. They have the same parts counts too. The differences are on its operation and the controller type. A flyback converter is can be operated in three operating modes the same with the quasi resonant. For detailed discussion about Flyback converter operating modes, read “Quasi Resonant Flyback Operating Modes”.
However, the quasi resonant flyback can switch on the power switch in the lowest drain voltage. The flyback converter cannot do this and instead purely offering hard switching. Flyback converter can have fixed frequency while quasi resonant flyback can have variable frequency. The quasi resonant flyback is badly needing a properly selected primary inductance than the flyback converter needs.
Advantages of Quasi Resonant Flyback Converter Compared to a Flyback Converter
1. It will offer lower switching losses especially when switching on the lowest valley point
2. Since it has the ability to switch on the lowest valley, it can behave as partial resonance and with this EMI will be better
3. The partial resonance action will be performed by the parasitic elements thus no more added parts counts.
4. It can handle multiple outputs
5. Wide input range
6. It has a better transient response
7. Easier to compensate
Drawbacks of Quasi Resonant flyback
1. If not able to switch on the lowest valley, its advantage of having low switching losses will be compensated or even surpassed by the effect of higher peak currents as it will operates in the DCM region
2. It may require a higher capacitance in the output as it is operating in DCM region
3. Not usually recommended for applications more than 100 Watts
Comparison between True Resonant Converter and Quasi Resonant Flyback Converter
In true resonant converter, the current is sinusoidal. The switching actions take place when the drain voltage is zero. A true resonant converter will have a resonant inductor and capacitor (though sometimes the resonant inductor is can be derived from the leakage of the transformer).
For a quasi resonant flyback converter on the other hand, the current is not sinusoidal but, triangular in shape. Switching will occur at the minimum drain voltage (valley switching). In quasi resonant flyback also, the parasitic elements (parasitic inductance and capacitance of the circuit) will do the resonance action.
Quasi Resonant Flyback Converter Equivalent Circuit Diagram
Below is the detailed circuit for a quasi resonant flyback converter.
Lm – this is the magnetizing inductance for energy storing to be transferred to the load
Llk – this is the leakage inductance of the primary. This will store energy that cannot be transferred to the secondary side.
Cd – this is the drain capacitance which consists of Coss, stray capacitance, intra winding capacitance and others
Resonant Frequencies
Quasi resonant flyback converter has two resonant frequencies.
1. The combination of Llk and Cd. This is happening at higher frequencies.
2. The combination of Lm+Llk and Cd. This will happen in the lower frequency.
How Quasi Resonant Flyback Works – Quasi Resonant Flyback Operation
Quasi resonant flyback operation is design in DCM region. During the dead time period, there is no more energy. This result to a natural oscillation like a second order system. This oscillation is caused by the combination of Lp and Cd on the above figure.
Where;
Ton – ON time when the primary is charging
Tfb – flyback time or the time the primary energy is transferred to the secondary
Toff – the total time that the switch is not conducting
Td – dead time or idle time
T – period (1/Fsw)
Vref – reflected voltageVin – input voltage
The quasi resonant action is happening on the area in orange on the above waveform. The responsible for this are Lp and Cd. They are the primary inductance and parasitic capacitance. The converter is can be switched on again anytime during these oscillations. However, the best time to switch on is when the VDS is at the minimum level. This is called minimum valley switching. At this point, the switching losses will be low.
The Important Role of the Controller on How Quasi Resonant Flyback Works
The very important part on how quasi resonant flyback works is the controller. The best result when using a quasi resonant flyback is when able to operate in the lowest valley point. In this point, the switching losses are low. However, this is not possible if the controller is not doing its job. Quasi resonant flyback operation will not materialize if not selecting the right controller.
Thus, the controller must able to detect the minimum voltage before switching ON the power MOSFET. Switching in the minimum valley introduces lower loss and helps improve EMI. In usual cases, the controller will allow more than one valley points before it switches on again the power switch or MOSFET in above circuits. This will not be a problem because the load is small. However, at higher loads, the controller should switch on the power switch in the first valley to achieve optimum performance.
Types of Quasi Resonant Flyback Controllers
Free running QR Controller
This control type will force the power switch to always turn on in the first valley. This will result to a frequency increase when the load decreases. However, at some point, a fixed frequency operation will happen (reaching the maximum limit).
Ideal QR Controller
This control type will switch on the power MOSFET at different valley points like first, second, third, etc., depending on the load. The first valley switching is happening only at a certain load (usually designed at the higher loads). The frequency will increase when the load increases and will decreases when the load decreases.
How Quasi Resonant Flyback Works – Switching on the First Valley
The best point to switch the power switch on is at the first valley. There are several advantages in doing so. Like,

 Lower switching losses

 Better EMI response
The following derivations will formulate equations that can be used in selecting the key parameters of a quasi resonant flyback converter.
Where;
Ton – time when switch is ON
Tfb – time when the energy is transferred to secondary
Tv – valley time (or dead time)
Toff – total time the switch is OFF
T – switching period
Substitute Eq. 2, 3 and 4 to eq. 1 and combine with eq. 5 will give
The above equation relates the switching frequency to input power and the inductance. When using this equation, the switching element is set to turn ON in the minimum of the first valley.
Where;
Lp – Primary inductance
Pin – Input power
Vin – Input voltage
Vref – reflected voltage
Fr – Resonance frequency of Lp and Cd