Download Lectures 39-40

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
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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:
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Techniques in Power Electronics
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Approximate analysis:
ignore resonant transitions, dead times, and resonant elements
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Techniques in Power Electronics
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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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Techniques in Power Electronics
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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
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Techniques in Power Electronics
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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
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Techniques in Power Electronics
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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
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Techniques in Power Electronics
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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
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Techniques in Power Electronics
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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:
At Vg = 350 V:
D = 0.648
D = 0.5
peak vds = 767 V
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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Techniques in Power Electronics
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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)
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Techniques in Power Electronics
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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 hardswitched PWM boost converter. Semiconductor output capacitances Cds are
explicitly included in the basic operation.
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Techniques in Power Electronics
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Lectures 39-40
Active clamp circuit on the primary side
of the flyback converter
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Techniques in Power Electronics
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Active clamp to snub the secondary-side diodes of
the ZVT phase-shifted full bridge converter
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Techniques in Power Electronics
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Active clamp
forward converter
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Waveforms
(including Ll)
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Details: different modes
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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 trajectory
including intervals 5 and 6
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Averaging
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Averaging
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Averaging
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Average
output voltage
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The system of equations
that describes this converter
page 1
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The equations that
describe this converter
page 2
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Results
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