DCDC Buck Converter in Inner Detector Environment J

DC-DC Buck Converter in Inner Detector Environment J Kierstead BNL Environment Radiation 75 Mrad, 2 x 1015 1 Me. V neutron equivalent Magnetic field 2 Tesla Limited volume • Implications No magnetic materials Simple Design (limited number of parts) Buck Converter?

Buck Converter Strengths and Weaknesses • Efficiency • Failure modes are well understood and easy to isolate failures • Requires higher voltage rated components making more susceptible to radiation damage and other failures • Noise

Buck Converter • Simple Buck Converter • Magnetic field requires that we use air core inductors • Vin = 20 V, Vout = 2 V, Ipeak= 1 A Lmin = (1. 8/fswitch) henries • If fswitch = 1 MHz Lmin = 1. 8 x 10 -6 henries Lmin = (Vin – Vout)ton / Ipeak ton = Vout/ (fswitch*Vin) fswitch = switching frequency

Practical Air Core Inductor • • Within a factor of 2 or so L = 10 -6 N 2 r henries, r is radius in cm To get L = 1. 8 x 10 -6 henries for 1 MHz switching If 0. 2 cm < r < 0. 4 cm Then 30 > N > 22 Length of Cu wire between 35 – 55 cm with diameter of 0. 3 – 0. 4 mm. Length of coil = 1 – 2 cm. COIL IS TOO BIG DC resistance about 0. 1 ohm giving voltage drop of 0. 1 volt

Practical Air Core Inductor cont. • Solution: Increase switching frequency to > 10 MHz • Lmin = 1. 8 x 10 -7 henries • Then If 0. 2 cm < r < 0. 4 cm 10 > N >7 • Length of Cu wire = 13 -18 cm, diameter 0. 18 – 0. 2 mm wire, Length of coil = 1. 5 – 2 mm MUCH BETTER • But now we are moving into frequencies where we will have more trouble with the switch(es).

Mosfet Switch Issues • Mosfets suitable for switching 20 volts are not available in rad-hard technologies (at least not to 75 Mrad) ? • Switching losses are proportional to switching frequency. • Mosfets and pulse width modulators for switching at 10 MHz and greater are not readily available

Gamma Irradiation Test 1. 5 Volt VTM • • • Conditions: Dose rate 1. 4 x 104 rad/hr 1. 5 Volt, 90 Amp VTM powered with commercial supply Fan cooled Voltage and current monitored on input and output Device failed between 50 and 60 krad Switching MOSFET(30 volt) tested independently on this device and failed between 3 -4 Mrad. Control ASIC is probably what failed in the VTM at 50 krad

Conclusions • Switching regulators have significant technological challenges when used in this environment • No magnetic materials allowed implying air coil inductors/transformers • At low frequencies (<1 MHz) coil is too large • At high frequencies(> 10 MHz) efficiency decreases due to switching losses and parts not readily available • No known commercial switches for radiation environment