The conventional forward converter

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The conventional forward converter

• Max vds 2Vg ringing
• Limited to D lt 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

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The active-clamp forward converter

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

Transistors are driven in usual half-bridge
manner
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Approximate analysisignore resonant
transitions dead times and resonant elements
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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.
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Peak transistor voltage
Max vds Vg Vb Vg /D which is less than the
conventional value of 2 Vg when D gt 0.5 This can
be used to considerable advantage in practical
applications where there is a specified range of
Vg
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Design example

• 270 V Vg 350 V
• max Pload P 200 W
• Compare designs using conventional 11 reset
  winding and using active clamp circuit

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Conventional case
Peak vds 2Vg ringing 700 V
ringing Lets 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
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Active clamp casescenario 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

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Active clamp casescenario 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.

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Active clamp circuits some examples
Basic switch network reduces to
(if the blocking capacitor is an ac short
circuit then we obtain alternately switching
transistorsoriginal MOSFET plus the auxiliary
transistor in parallel. The tank L and C ring
only during the resonant transitions)
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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.
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Active clamp circuit on the primary sideof the
flyback converter
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Active clamp to snub the secondary-side diodes of
the ZVT phase-shifted full bridge converter
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Active clampforward converter
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Waveforms(including Ll)
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Details different modes
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(No Transcript)
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About Ll
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Definitions
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Subinterval 1
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Subinterval 2
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Subinterval 2
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State plane subinterval 2
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Subinterval 3
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Subinterval 3 state plane trajectory
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Subinterval 4
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Subinterval 5
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Subinterval 6
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State plane trajectoryincluding intervals 5 and 6
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Averaging
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Averaging
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Averaging
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Averageoutput voltage
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The system of equationsthat describes this
converterpage 1
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The equations that describe this converter page 2
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Results