Q7

Question 1

General operation of converter

Answer 1

• Switching bridge generates a square wave voltage of frequency equal to switching frequency.
• The resonant tank filters harmonics and converts square wave signal into sinusoidal I that feeds the transformer.
• The sinusoidal I is transferred to the secondary and its magnitude depends on turns ratio of the transformer
• On the secondary side diodes rectify the AC voltage and I to supply the load.

Question 2

Q7. slide 2 steps to find resonant tank gain

Answer 2

• Replace primary of transformer by equivalent R.
• approximate square wave voltages Vin_ac and Vo_ac.
• Find fundamental component of Vin_ac by Fourier analysis
• Find fundamental component of Vo_ac by Fourier analysis and reflected output voltage.
• Find fundamental component of I through equivalent R as a function of output I.
• Calculate equivalent R.

Question 3

Q7. slide 3 steps to find voltage gain

Answer 3

• Calculate voltage gain Vout/Vin using voltage divider rule.
• Define the quality factor, M, and normalized switching frequency.
• Find normalized form of voltage-gain.

Question 4

Q7. slide 4 explain effect of Q in gain curve of resonant tank

Answer 4

• As Q increases resonant peak moves towards resonance frequency due to CrLr (Rac decreases)
• As Q increases curve becomes narrower and maximum gain approaches 1
• When Q decreases the curve expands and higher boost gains can be achieved.

Question 5

Explain effect of M in resonant tank gain

Answer 5

• As M decreases the curves are narrower and resonant frequencues fr1 and fr2 get closer, resulting in higher frequency modulation.

Question 6

Advantages/disadvantages of higher Q

Answer 6

• Higher boost gains can be achieved
• Frequency modulation is poor above resonant frequency

Question 7

Disadvantage of small M

Answer 7

• Usually means smaller Lm
• If Lm is small magnetizing current ripple would increase, increasing conduction losses

Question 8

Q7. slide 5 explain ZVS

Answer 8

• ZVS is achieved when resonant I lags input voltage.
• i.e. when Q2 and Q3 are turned off, there is still I circulating in the negative direction.
• This I flows through drain-source capacitances of MOSFETs charging Q2 and Q3, and discharging Q1 and Q4 output capacitances.
• When Q1 and Q4 are discharged body diodes will conduct
• At this point we can apply gate signal achieving ZVS.