The circuit in the figure is a current commutated dc

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The circuit in the figure is a current commutated dc – dc chopper where, Th_M is the main SCR and Th_AUX is the auxiliary SCR. The load current is constant at 10 A. Th_M is ON. Th_AUX is triggered at t = 0. Th_M is turned OFF between

1. 0 μs < t ≤ 25 μs
2. 25 μs < t ≤ 50 μs
3. 50 μs < t ≤ 75 μs
4. 75 μs < t ≤ 100 μs

Correct Option: B

For t < 0, V_c = V_s, i_c = 0 and i_T1 = I₀
At t = 0, TH_AUX is trigered, and a resonant Current i c designs to flow from C through TH_AUX, L and back to c.
The resonant current is given by,

At t₁ =	π	, i_c = 0 and V_c = -V_s
	ω₀

Since i_C gets reverses, TH_AUX is off.
For V_c = – V_s , resonant current i_c flows through C, L, D and TH_m. Because this current i_c flows and builds up in opposite direction to forward current of TH_m Then,
i_m = I_o – i_c = I_o – I_p sin ω∆t
when i_m = 0, TH_m gets turned off

i.e. , ∆t =	1	sin^-1		I_o
	ω₀			I_p

Hence, TH_m is off between t₁ < t < t₁ + ∆t

t₁ =	π	= π√LC = π√10 × 25.28 sec = 50 sec.
	ω₀

i.e., commutation time given by 50 µs < t < 75s