Page 63 – Ultra Hard Mixed DC-DC Numerical Test
Subject: Power Electronics
Level: GATE / PSU Advanced
🔹 Problem 1 – Identify Mode + Output Voltage
Buck-Boost converter:
Vin = 15 V
D = 0.5
L = 50 μH
fs = 40 kHz
R = 5 Ω
Determine:
1) CCM or DCM
2) Output voltage
Step 1 – Find Lcritical
Lcritical = R(1−D)² / (2fs) = 5(0.5)² / (2×40000) = 5(0.25) / 80000 = 1.25 / 80000 = 15.6 μH
Given L = 50 μH > Lcritical → CCMStep 2 – Output Voltage
Vo/Vin = −D/(1−D) = −0.5/0.5 = −1 Vo = −15 V
Final Answer: CCM, Vo = −15 V🔹 Problem 2 – Boost Converter Ripple
Vin = 20 V D = 0.6 L = 200 μH fs = 25 kHz Find inductor ripple current.
ΔIL = Vin D / (L fs) = 20×0.6 / (200×10⁻⁶ × 25000) = 12 / 5 = 2.4 A
Answer: 2.4 A🔹 Problem 3 – SEPIC Duty Cycle
Vin varies 10–20 V Vo = 15 V Find duty cycle range.
Vo/Vin = D/(1−D) Case 1: Vin = 10 15/10 = 1.5 1.5 = D/(1−D) D = 0.6 Case 2: Vin = 20 15/20 = 0.75 0.75 = D/(1−D) D = 0.43
Duty cycle range: 0.43 – 0.6🔹 Problem 4 – Ćuk Converter Power
Vin = 12 V D = 0.5 R = 8 Ω Find output power.
Vo/Vin = −D/(1−D) = −0.5/0.5 = −1 Vo = −12 V Io = 12/8 = 1.5 A P = Vo × Io = 12 × 1.5 = 18 W
Answer: 18 W🔹 Problem 5 – DCM Output
Vin = 24 V D = 0.4 R = 20 Ω L = 100 μH fs = 50 kHz Find Vo (DCM).
Vo/Vin = − (D² R) / (2Lfs) = − (0.16 × 20) / (2×100×10⁻⁶ × 50000) = − (3.2) / (10) = −0.32 Vo = −0.32 × 24 = −7.68 V
Final Answer: −7.68 VMaster Concept Summary
✔ CCM → Gain depends only on D ✔ DCM → Gain depends on R, L, fs ✔ Always check Lcritical ✔ SEPIC = Non-inverting ✔ Ćuk = Low ripple ✔ Buck-Boost = Inverting
Ultra Hard DC-DC Series – Shaktimatha Learning
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