Parasitic effects Snubbers and clamps Turn on and turn off effects - PDF document

Mor M. Peretz, Switch-Mode Power Supplies [5-1] Parasitic effects Snubbers and clamps • Turn on and turn off effects • Turn on - diode reverse recovery • Turn off – dI/dT effect on transistor • Turn off – diode forward recovery • Mosfet gating • Transistor protection - Clamp • Flyback leakage problem • Clamp configuration • Component selection • Diode protection – Snubber • Switching snubbers and lossless (ZVS) snubber [5-2] Mor M. Peretz, Switch-Mode Power Supplies Diodes reverse recovery L V x V o V in Diode on – bidirectional R C current flow 1

Mor M. Peretz, Switch-Mode Power Supplies [5-3] Diodes recovery – spike and oscillations V D Diode on L stray t L stray V O ESR L stray V Dmax I D t Diode off L stray L stray ESR [5-4] Mor M. Peretz, Switch-Mode Power Supplies Transistor turn off - parasitics L main Transistor L stray on L stray D L main L stray L D Transistor C o V o R G off G L stray ESR L S L stray S 2

Mor M. Peretz, Switch-Mode Power Supplies [5-5] Transistor turn off – diode forward recovery I I t V D V PK V F t [5-6] Mor M. Peretz, Switch-Mode Power Supplies Mosfef gating R L R G D V  (real) R L C GS L D gs R G G V gs L S L S S V gs L S C  gs Q V' gs Depend on Q  R R L G 3

Mor M. Peretz, Switch-Mode Power Supplies [5-7] Flyback leakage problem V O V in Voltage spike can develop HIGH voltage L lkg Vc - ? C dss V - ? [5-8] Mor M. Peretz, Switch-Mode Power Supplies Clamp V O V in Zener diode D – C - R L lkg C dss 4

Mor M. Peretz, Switch-Mode Power Supplies [5-9] Clamp configurations V in n   1 V V V c in o n 2 V in n  1 V V c o n 2 [5-10] Mor M. Peretz, Switch-Mode Power Supplies Parasitic inductance and physical placement L 1 V in V in R c C c L lkg C L 4 L 2 L 3 Connection of the clamp/snubber should be directly on the element 5

Mor M. Peretz, Switch-Mode Power Supplies [5-11] Clamp design Equivalent circuit Current waveform Assuming small ripple on the clamp capacitor (will be designed that way)  V V Cc V av T s [5-12] Mor M. Peretz, Switch-Mode Power Supplies Calculation of clamp components n   1 V V I R V I o cc av c cc _ av n pk 2  C R T L c c s Lkg I av n  1 V V o cc dI n  p 2  I t dt L   p Lkg pk p I f av s 2 6

Mor M. Peretz, Switch-Mode Power Supplies [5-13] Design procedure 1. Select V Cc av > Vo’  I t   pk p I f 2. Calculate I p av av s 2 V  cc _ av R 3. Select c I av 4. Select C c T > T s 5. Adjust values [5-14] Mor M. Peretz, Switch-Mode Power Supplies Diode turn off snubber V D t V o Diode on V O L stray V Dmax snubber I D t C D Diode off L stray 7

Mor M. Peretz, Switch-Mode Power Supplies [5-15] Diode turn off snubber V D no snubber bad snubber C S R S V o V O C D L stray V D C S is very l arg e good snubber C s > C D V O [5-16] Mor M. Peretz, Switch-Mode Power Supplies Snubber design C S R S Required values Ipk (reverse current) Stary inductance V O C D   L stray   2 2 L I C V V  stray pk s o 2 2  Resistor for damping C R T s s s Cs needs to discharge Use simulation to optimize the snubber 8

Mor M. Peretz, Switch-Mode Power Supplies [5-17] Switching snubber control control V GS V GS t t dV V V V S S S dI I I dt S S dt t t J d J p P switching t t The objective: avoid overlap of current and voltage Slow the rise of either current or voltage [5-18] Mor M. Peretz, Switch-Mode Power Supplies Switch snubber V o C dV/dT (at turn off) can be slow down by adding external snubber capacitor C V o dV I  Turn off  C C dt C C dss dss 9

Mor M. Peretz, Switch-Mode Power Supplies [5-19] Switch snubber Problem at turn on V o 2 2 CV CV    O O E P f C C d s 2 2 Dissipated through the transistor Solution V O C S R S [5-20] Mor M. Peretz, Switch-Mode Power Supplies Switch snubber design V O If R ds on < R s most energy will be lost to R s  Heat C S R DSon Selection of C s  R S Selection of R s  to ensure reset 1    T t t 4 R C on on s s R C s s Question: how VDS will look if Cs is not fully discharged? 10

Mor M. Peretz, Switch-Mode Power Supplies [5-21] Lossless (ZVS) snubber Q 1 V C t C 1 L 2 Q 2 delay t V C I C 2 L 2 t C 1 , C 2 of transistor V DS 1 plus external t V DS 2 t [5-22] Mor M. Peretz, Switch-Mode Power Supplies 11

Mor M. Peretz, Switch-Mode Power Supplies [5-23] [5-24] Mor M. Peretz, Switch-Mode Power Supplies 12