Theory of ESBT and quasi-resonant operation

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Theory of ESBT and quasi-resonant operation

As mentioned in Introduction, the application studied in this Application Note implements

zerovoltage switching (ZVS). This principle of operation is also known as quasi-resonant or

valley switching.

These names come from the waveform shape of the voltage across the primary side switch

during or just before switch turn-on. Figure 1. shows the switch voltage, which is the sum of

winding.

The winding voltage depends on the state of the switch and the amount of magnetizing

energy stored in the magnetic circuit of the transformer. One switching period can be divided

into three basic areas determined by the state of the primary switch and the output diode

conduction: the ON time, the OFF time and the DEAD time areas.

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The “ON time” area corresponds to the time during which the primary switch is on and

the transformerfs magnetizing inductance stores energy.

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During the “OFF time”, the primary switch is off and the magnetizing inductance energy

is discharged through the conducting output diode to the output capacitor. A ringing

related track inductance and by the leakage inductance created by the imperfect

magnetic field coupling between the transformer primary and secondary windings. The

ringing voltage amplitude is controlled and limited by a clamp circuit.

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The “DEAD time” starts once all the stored magnetizing inductance energy has been

discharged to the output capacitor. It is called “DEAD time” because neither the primary

switch nor the output diode is conducting. So there is no energy transfer between the

primary side and the secondary side. The primary winding voltage during this phase is

resonating and has a cosine waveform starting from a voltage equal to the OFF-time

plateau voltage. The DEAD time is used for the initiation of the next switching cycle,

only there is no energy conversion, which is why this concept is called QUASI resonant,

in comparison with pure resonant converters where the resonance of the primary

current or voltage is the means of energy conversion. The voltage waveform has a

negative slope and approaches or can even cross zero. The suitable moment to turn

the primary switch on again is when the voltage across the primary switch is lowest.

The shape of the switch voltage waveform at this point evokes a valley. This is why

quasi-resonant or zero-voltage switching is also called valley switching. The resonance

frequency during the DEAD time is determined by the magnetizing inductance and

parasitic capacitance. The parasitic capacitance consists of the primary switch

capacitance, the transformer winding capacitance, the inter-winding capacitance, the

capacitances of the diodes located in the secondary, auxiliary and clamp circuits

transformed to the primary side. The PCB tracks also generate some parasitic

capacitance depending on the layout.