HVLED007 datasheet(10/33 Pages) STMICROELECTRONICS

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Application information

HVLED007

10/33

DS12866 Rev 1

Equation 1

Figure 4. Hi-PF QR flyback converter with the traditional TM control: current

waveforms

With the traditional TM control method, the term Ippk (θ), the peak envelope of the primary

current, is sinusoidal (Ippk(θ) = Ippk sin θ). The distortion stems from the term TON / T(θ),

due to the primary current being chopped, which is not constant (TON is constant, T(θ) is

not).

This distortion is prevented if the flyback converter is operated with a fixed switching

frequency in the Discontinuous Conduction Mode (DCM). However, there are a few benefits

in using TM operation that are lost when operating with a fixed frequency (FF): lower

conducted EMI emissions, safer operation under short-circuit conditions, valley-switching or

even true soft-switching (zero-voltage switching, ZVS).

The idea behind the novel TM method is to distort the current reference Vcsref(θ) that

determines Ippk(θ) (Ippk(θ) = Vcsref (θ) / Rs) by a term T(θ) / TON : this cancels out the term

TON / T(θ) introduced by averaging and results in a sinusoidal average primary current, i.e.

in a sinusoidal input current. Then, the control objective can be expressed in the following

terms:

Equation 2

Iin 

= 1

--- Ip pk 



TON





------------

TON

Vcsref 

 = Vcs

x sin 





T ON 



-------------------

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