Improving the Efficiency of Dynamic Substrate Biasing through Charge Recovery

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Improving the Efficiency of Dynamic Substrate
Biasing through Charge Recovery
Liang Teck Pang, Professor Borivoje Nikolic
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Motivation

• Dynamic substrate biasing(DSB) involves charging
  and discharging the substrate/well (huge
  capacitance). Much energy is used in this
  process.
• Energy savings using DSB is possible only for
  long duty cycles otherwise, energy dissipated in
  charging/discharging the substrate will offset
  energy savings in leakage reduction

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Proposal

• Use a resonant circuit to recycle charge
• Less energy dissipated per cycle of changing the
  substrate bias
• Shorter duty cycle means more overall savings in
  energy

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RLC circuit Analysis
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Energy Savings
ER0 energy recovered without using
inductors ER1 energy recovered with the use of
an inductor minus energy required to charge the
switch capacitor k fraction of initial voltage
recovered
6 times more efficient
No gain in efficiency
Csw 4.37E-13F for w1mm
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RLC circuit SPICE
Rmos 38ohm (simulation matching) Rothers
20ohm (estimated) C1C260nF (estimated large
chip) Vo1.2V
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Simulation Results 1
V2 swings to around 0.8x initial voltage of V1
during discharging of V1. V1 recovers to around
0.8x initial voltage of V2 during charging of
V1 However, strong overshoots and oscillations
at switching node.
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Circuit 2
To suppress the oscillations, a second switch is
used to alternate between the resonant network
and a heavily damped network
To have no oscillations we require
Cd is chosen to be small in order not to affect
the voltage on C2
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Simulation Results 2
Spikes but no oscillations
The voltages settle immediately after an initial
spike. The spike is caused by non-zero current
in the wire during switching. The spike can be
reduced with good control of switching time
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On-Going Work

• Good control of switching time required to
  minimize spikes obtained during switching
• Automatic switch control circuitry to be designed
  and tested
• Large external inductor with low resistance
  required for improved efficiency