DC-DC Buck Converter in Inner Detector Environment

J Kierstead BNL Environment

Radiation 75 Mrad, 2 x 1015 1 MeV neutron equivalentMagnetic field 2 TeslaLimited volume

• ImplicationsNo magnetic materialsSimple 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 = 2V, Ipeak= 1A Lmin = (1.8/fswitch) henries

• If fswitch = 1MHz Lmin = 1.8 x 10-6 henries

Pulse Wid thM od ula to r

V Vin o ut

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 N2r henries, r is radius in

cm To get L = 1.8 x 10-6 henries for 1 MHz switching• If 0.2cm < r < 0.4cm• 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 0.2cm < r < 0.4cm 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

•

Vicor VTM module1.5 Volt

0

5

10

15

0 20 40 60

Dose (krad)

Po

wer

(W

atts

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Input

Output

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