Peak transistor voltage occurs during transformer reset
Could reset the transformer with less voltage if interval 3 were reduced
2 The active-clamp forward converter
Better transistor/transformer utilization
Not limited to D lt 0.5
Transistors are driven in usual half-bridge manner 3 Approximate analysisignore resonant transitions dead times and resonant elements 4 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. 5 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 6 Design example
270 V Vg 350 V
max Pload P 200 W
Compare designs using conventional 11 reset winding and using active clamp circuit
7 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 8 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
9 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.
10 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) 11 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. 12 Active clamp circuit on the primary sideof the flyback converter 13 Active clamp to snub the secondary-side diodes of the ZVT phase-shifted full bridge converter 14 Active clampforward converter 15 Waveforms(including Ll) 16 Details different modes 17 (No Transcript) 18 About Ll 19 Definitions 20 Subinterval 1 21 Subinterval 2 22 Subinterval 2 23 State plane subinterval 2 24 Subinterval 3 25 Subinterval 3 state plane trajectory 26 Subinterval 4 27 Subinterval 5 28 Subinterval 6 29 State plane trajectoryincluding intervals 5 and 6 30 Averaging 31 Averaging 32 Averaging 33 Averageoutput voltage 34 The system of equationsthat describes this converterpage 1 35 The equations that describe this converter page 2 36 Results
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