Active clamp circuits Can be viewed as a

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 vds = 2 Vg + 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 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 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 5 Lecture 37

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 6 Lecture 37

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 7 Lecture 37

Conventional case Peak vds = 2 Vg + 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 = Dn. I = Did-on with P = 200 W = Vg ig = DVg id-on So id-on = P/DVg = (200 W) / (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 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 9 Lecture 37

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 Q 1: id-on = P/DVg = (200 W) / (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 11 Lecture 37

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. ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 12 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 15 Lecture 37

Waveforms (including Ll) ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 16 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 Ll ECEN 5817 Resonant and Soft-Switching Techniques in Power Electronics 19 Lecture 37