# 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
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Details: different modes

ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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Definitions

ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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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
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Subinterval 2

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

ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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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
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Averaging

ECEN 5817 Resonant and Soft-Switching
Techniques in Power Electronics
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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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