Slides, chapter 20 by KevenMealamu

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									                        The conventional forward converter




     • Max vds = 2Vg + ringing
     • Limited to D < 0.5
     • On-state transistor current is P/DVg
     • Magnetizing current must operate in DCM
     • Peak transistor voltage occurs during
       transformer reset
     • Could reset the transformer with less voltage
       if interval 3 were reduced

ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                               1             Lectures 39-40
                        The active-clamp forward converter

                                                              • Better transistor/transformer
                                                                utilization
                                                              • ZVS
                                                              • Not limited to D < 0.5


                                        Transistors are driven in usual half-bridge manner:




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                           2                                      Lectures 39-40
                                        Approximate analysis:
         ignore resonant transitions, dead times, and resonant elements




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                                3                Lectures 39-40
                     Charge balance




      Vb can be viewed as a flyback converter output. By use of a current-bidirectional switch,
      there is no DCM, and LM operates in CCM.




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                               4                                      Lectures 39-40
                                        Peak transistor voltage

            Max vds = Vg + Vb = Vg /D’
            which is less than the conventional value of 2 Vg when D > 0.5
            This can be used to considerable advantage in practical applications where
            there is a specified range of Vg




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                                 5                                  Lectures 39-40
                                        Design example


        270 V ≤ Vg ≤ 350 V
        max Pload = P = 200 W

        Compare designs using conventional 1:1 reset winding and using active
          clamp circuit




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                             6                            Lectures 39-40
                                        Conventional case

    Peak vds = 2Vg + ringing
       = 700 V + ringing


    Let’s let max D = 0.5 (at Vg = 270 V),
       which is optimistic
    Then min D (at Vg = 350 V) is
      (0.5)(270)/(350) = 0.3857


     The on-state transistor current, neglecting ripple, is given by
        ig  = DnI = Did-on
     with P = 200 W = Vg  ig  = DVg id-on
     So id-on = P/DVg = (200W) / (0.5)(270 V) = 1.5 A




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                                7                      Lectures 39-40
                                        Active clamp case:
                                            scenario #1



        Suppose we choose the same turns ratio as in the conventional design.
           Then the converter operates with the same range of duty cycles, and
           the on-state transistor current is the same. But the transistor voltage is
           equal to Vg / D’, and is reduced:

        At Vg = 270 V:    D = 0.5           peak vds = 540 V
        At Vg = 350 V:    D = 0.3857        peak vds = 570 V
           which is considerably less than 700 V




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                               8                               Lectures 39-40
                                        Active clamp case:
                                            scenario #2


        Suppose we operate at a higher duty cycle, say, D = 0.5 at Vg = 350 V.
           Then the transistor voltage is equal to Vg / D’, and is similar to the
           conventional design under worst-case conditions:

        At Vg = 270 V:          D = 0.648          peak vds = 767 V
        At Vg = 350 V:          D = 0.5            peak vds = 700 V

        But we can use a lower turns ratio that leads to lower reflected current in
           Q1:
        id-on = P/DVg = (200W) / (0.5)(350 V) = 1.15 A


        Conclusion: the active clamp circuit resets the forward converter
          transformer better. The designer can use this fact to better optimize the
          converter, by reducing the transistor blocking voltage or on-state
          current.
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                               9                               Lectures 39-40
                 Active clamp circuits: some examples

      Basic switch network reduces to:
                                              (if the blocking capacitor is
                                              an ac short circuit, then we
                                              obtain alternately switching
                                              transistors—original
                                              MOSFET plus the auxiliary
                                              transistor, in parallel. The
                                              tank L and C ring only
                                              during the resonant
                                              transitions)




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                         10                      Lectures 39-40
              Example: addition of active clamp circuit to the
                             boost converter




          The upper transistor, capacitor Cb, and tank inductor are added to the hard-
          switched PWM boost converter. Semiconductor output capacitances Cds are
          explicitly included in the basic operation.


ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                              11                                     Lectures 39-40
                    Active clamp circuit on the primary side
                            of the flyback converter




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        12                     Lectures 39-40
           Active clamp to snub the secondary-side diodes of
              the ZVT phase-shifted full bridge converter




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        13              Lectures 39-40
    Active clamp
  forward converter




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        14   Lectures 39-40
         Waveforms
        (including Ll)




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        15   Lectures 39-40
                                   Details: different modes




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                            16                Lectures 39-40
ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        17   Lectures 39-40
                                        About Ll




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                         18        Lectures 39-40
                        Definitions




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        19   Lectures 39-40
                     Subinterval 1




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        20   Lectures 39-40
                 Subinterval 2




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        21   Lectures 39-40
                                        Subinterval 2




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                           22           Lectures 39-40
                                  State plane, subinterval 2




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                            23                 Lectures 39-40
                    Subinterval 3




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        24   Lectures 39-40
                         Subinterval 3: state plane trajectory




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        25                       Lectures 39-40
                    Subinterval 4




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        26   Lectures 39-40
                    Subinterval 5




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        27   Lectures 39-40
                       Subinterval 6




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        28   Lectures 39-40
                                        State plane trajectory
                                         including intervals 5 and 6




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                                  29                   Lectures 39-40
                      Averaging




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        30   Lectures 39-40
                       Averaging




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        31   Lectures 39-40
                       Averaging




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        32   Lectures 39-40
                Average
             output voltage




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        33   Lectures 39-40
                                  The system of equations
                                that describes this converter
                                            page 1




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                           34                   Lectures 39-40
The equations that
describe this converter
page 2




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                        35   Lectures 39-40
                                        Results




ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
                                         36       Lectures 39-40

								
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