Active clamp circuits Can be viewed as a lossless voltage-clamp snubber that employs a current-bidirectional switch See Vinciarelli patent (1982) for use in forward converter Related to other half-bridge ZVS circuits Can be added to the transistor in any PWM converter Not only adds ZVS to forward converter, but also resets transformer better, leading to better transistor utilization than conventional reset circuit

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

1

Lecture 37

The conventional forward converter • Max

v ds

= 2

V g

+ ringing • Limited to

D

< 0.5

• On-state transistor current is

P

/

DV g

• 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

2

Lecture 37

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

3

Lecture 37

Approximate analysis: ignore resonant transitions, dead times, and resonant elements

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

4

Lecture 37

Charge balance

V b

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

L M

operates in CCM.

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

5

Lecture 37

Peak transistor voltage Max

v ds

=

V g

+

V b

=

V g

/

D’

which is less than the conventional value of 2

V g

when

D

< 0.5

This can be used to considerable advantage in practical applications where there is a specified range of

V g ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

6

Lecture 37

Design example 270 V ≤

V g

≤ 350 V max

P load

=

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

7

Lecture 37

Conventional case Peak

v ds

= 2

V g

+ ringing = 700 V + ringing Let’s let max

D

= 0.5 (at

V g

which is optimistic = 270 V), Then min

D

(at

V g

= 350 V) is (0.5)(270)/(350) = 0.3857 The on-state transistor current, neglecting ripple, is given by 

i g

 =

DnI

=

Di d-on

with P = 200 W =

V g



i g

 =

DV g i d-on

So

i d-on

=

P/DV g

= (200W) / (0.5)(270 V) = 1.5 A

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

8

Lecture 37

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

V g / D’

, and is reduced: At

V g

= 270 V: At

V g

= 350 V:

D D

= 0.5

= 0.3857

peak peak which is considerably less than 700 V

v v ds ds

= 540 V = 570 V

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

9

Lecture 37

Active clamp case: scenario #2 Suppose we operate at a higher duty cycle, say,

D

= 0.5 at

V g

= 350 V. Then the transistor voltage is equal to

V g / D’

, and is similar to the conventional design under worst-case conditions: At

V g

= 270 V: At

V g

= 350 V:

D D

= 0.648

= 0.5

peak

v ds

= 767 V peak

v ds

= 700 V But we can use a lower turns ratio that leads to lower reflected current in Q1:

i d-on

=

P/DV g

= (200W) / (0.5)(350 V) = 1.15 A

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

10

Lecture 37

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

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Lecture 37

Example: addition of active clamp circuit to the boost converter The upper transistor, capacitor

C b

, and tank inductor are added to the hard switched PWM boost converter. Semiconductor output capacitances

C ds

are explicitly included in the basic operation.

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Lecture 37

Active clamp circuit on the primary side of the flyback converter

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

13

Lecture 37

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

14

Lecture 37

Active clamp forward converter

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Lecture 37

Waveforms (including

L l

)

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

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Lecture 37

Details: different modes

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

17

Lecture 37

ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

18

Lecture 37

About

L l ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics

19

Lecture 37