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Boost converter

+ v. –. L. Boost converter. i. I. i. L. out. D. i. in. +. L. V. V. C. out. in. i. C. –. Output voltage is larger than Vin, but output current is smaller Pin is approximately equal to Pout.

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Boost converter

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  1. + v – L Boost converter i I i L out D i in + L V V C out in i C – • Output voltage is larger than Vin, but output current is smaller • Pin is approximately equal to Pout • If the MOSFET gate driver sticks in the “on” position, then there is a short circuit through the MOSFET • If the load is disconnected during operation, so that Iout = 0, then L continues to push power to the right and very quickly charges C up to a high value

  2. I I out out Switch closed for DT seconds: Charging phase + Vin− i L i in + L V V C out in – Reverse biased, thus the diode is open for DT seconds Note – if the switch stays closed for very long time, the current continues increasing until the limit of switch and/or battery!

  3. I out Switch open for (1 − D)T seconds + (Vin− Vout ) − i L i in + L V V C out in (iL – Iout) – Diode closed. Assume continuous conduction. for (1−D)T seconds

  4. Examine the inductor current Switch closed, Switch open, Iavg = Iin is half way between Imax and Imin iL Imax Iavg = Iin ΔI Imin DT (1 − D)T T

  5. Inductor current rating Max impact of ΔI on the rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iin iL 2Iin Iavg = Iin ΔI 0 Use max

  6. + v – L MOSFET and diode currents and current ratings i I i L out D i in + L V V C out in i C – 2Iin 0 2Iin 0 Use max Take worst case D for each

  7. Capacitor current and current rating i I i L out D i in + L V V C out in i C – iC = (iD – Iout) 2Iin −Iout 0 −Iout Max rms current occurs at the boundary of continuous/discontinuous conduction, where ΔI =2Iout Use max

  8. Worst-case load ripple voltage iC = (iD – Iout) 0 −Iout The worst case is where C provides Iout for most of the period. Then, D~ 1 and

  9. I I out out Voltage ratings Diode sees Vout i L i C sees Vout in + L V V C out in – i L i in + L V V C out in – MOSFET sees Vout • Diode and MOSFET, sees (worst case) Vout + Vpk • Capacitor also sees a large voltage

  10. Continuous current in L; worst case iL 2Iin Iavg = Iin 0 (1 − D)T Then, considering the worst case (i.e., D → 1), use max guarantees continuous conduction use min

  11. Stability and “AC” Response http://www.ti.com/lit/an/slva061/slva061.pdf • AC analysis is tricky! • System is very non-linear! • AC is for small signal sinewave waveforms! • This system Is not even close to that!

  12. Stability and “AC” Response http://www.ti.com/lit/an/slva061/slva061.pdf • Similarly to any AC analysis, we have to assume small signal variations, and define an operating point! • Output of the PWM is duty cycle! • This chart includes resistive losses, otherwise, D=0.7 should result in V-D convertion ratio of approximately 4.

  13. Stability and “AC” Response http://www.ti.com/lit/an/slva061/slva061.pdf

  14. Stability and “AC” Response

  15. Impedance matching Iin DC−DC Boost Converter + − + Vin − Source Iin + Vin − Equivalent from source perspective

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