LED Driver with minimum input current and large LED current(peak)
Given: 


  Power source - 18V-42V (I can draw max of 6mA-7mA from source) LED string - 4 series LEDs of 6 strings in parallel ( total 24 LEDs)
  I need min of 200mA of peak/instantaneous current through LEDs with 20ms or 30ms ON every 1sec duration. Vf= 2.65V to 3.5V


Translation:

Input:  

Vi = 30V nom. ±12V (±40%)
Ii = 6mA to 7mA (nom.)
Pi = 180 to 210 mW (±40%) (nom.) , 108mW (min.)
Ri = 30V/6mA= 5kΩ (nom.), 18V/6mA= 3kΩ min. (est. for MPT)   

Output:   

24 W. LEDs in Array , Pulsed at 1Hz rate 2~3% duty cycle  

Vf= 3.1 V ± 0.4V (depends on quality of parts and 
ratings)
If= 200 mA, 20ms min.  to  250 mA  (est.), 30ms max.

LED Array = 4S6P white (CCT, CRI, LPW unknown)
Vo = Vf * 4 + Io * Rs
Io = 6 * 200 mA (min.) (Rs=0 if buying binned parts  ±0.1V)
Po= Vo * Io = 12.4V (nom) * 1.2 A (min) ~ 15 W nom.
Po avg.= Po * duty cycle , d.c. = 2~3%   


Po avg. = 300 mW (nom) = 15 W (nom) * 20ms/sec   


d.c.= 2% min)



Output Power, Po avg exceeds input Pi: Problem.

Conclusion   


change power source or requirements or efficacy of LEDs