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 Mor M. Peretz, Switch-Mode Power Supplies [5-2] Diodes reverse recovery L V x V o V in Diode on – bidirectional R C current flow Mor M. Peretz, Switch-Mode Power Supplies [5-3] Diodes recovery – spike and oscillations Diode on L stray L stray ESR L stray Diode off L stray L stray ESR 1
Mor M. Peretz, Switch-Mode Power Supplies [5-4] 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 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 Mor M. Peretz, Switch-Mode Power Supplies [5-6] Mosfef gating D R L R G 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 2
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 - ? Mor M. Peretz, Switch-Mode Power Supplies [5-8] Clamp V O V in Zener diode D – C - R L lkg C dss Mor M. Peretz, Switch-Mode Power Supplies [5-9] Clamp configurations V in n V V 1 V c in o n 2 V in n V 1 V c o n 2 3
Mor M. Peretz, Switch-Mode Power Supplies [5-10] 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 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 Mor M. Peretz, Switch-Mode Power Supplies [5-12] 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 V 1 V dI o cc n p 2 I t dt L pk p p Lkg I f av s 2 4
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 Mor M. Peretz, Switch-Mode Power Supplies [5-14] Diode turn off snubber V o Diode on L stray snubber C D Diode off L stray 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
Mor M. Peretz, Switch-Mode Power Supplies [5-16] Snubber design R S Required values C S Ipk (reverse current) V O Stary inductance 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 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 dt I I 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 Mor M. Peretz, Switch-Mode Power Supplies [5-18] 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 6
Mor M. Peretz, Switch-Mode Power Supplies [5-19] Switch snubber Problem at turn on V o 2 2 CV CV E O P O f C C d s 2 2 Dissipated through the transistor Solution V O C S R S Mor M. Peretz, Switch-Mode Power Supplies [5-20] 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 R C 4 on on s s R C s s Question: how VDS will look if Cs is not fully discharged? 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 7
Mor M. Peretz, Switch-Mode Power Supplies [5-22] Mor M. Peretz, Switch-Mode Power Supplies [5-23] Mor M. Peretz, Switch-Mode Power Supplies [5-24] 8
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