Document Sample

    Problems In Circuits Design
           Ronen Gabai
    Presentation Outlines

 The Need for Current Sensing
 Current Sensing Techniques – Overview
 The Chosen Technique – SENSEFET

   Article Overview
   Measurements Results
   Circuit Efficiency and Performance
   Conclusions & Summary

  The Need for Current-Sensing Techniques

 In Switch-mode power converters (SMPC), current-mode
  pulsewidth-modulation (PWM) control and current-limited
  pulse-frequency-modulation (PFM) control schemes are widely
  used in industries due to their fast dynamic response and
  automatic over-current protection.
 Both control make use of the Inductor current (of the
  Buck/Boost power stage) to modify the pulse width in PWM or
  oscillation frequency in PFM for voltage regulation.
 The Inductor current is particularly important for PWM, as the
  signal sensed from the inductor current is mandatory to combine
  with the artificial ramp signal in order to avoid sub harmonic
  oscillation in current-mode control PWM converter.
PWM Converters with Switch Model Inserted

 (a) Buck Converter 

 (b) Boost Converter 

 (c) Flyback Converter 

    Current-Sensing Techniques for DC-DC Converters

 Current sensing is one of the most important functions on a smart
     power chip.
    Regardless of the type of feedback control, almost all DC-DC
    converters and linear regulators sense the inductor current for
    over-current (over-load) protection.
    Additionally, the sensed current is used in current-mode control
     DC-DC converters for loop control.
    Conventional current sensing methods insert a resistor in the path of
     the current to be sensed; This method incurs significant power losses,
     especially when the current to be sensed is high.
    Lossless current-sensing methods address this issue by sensing the
     current without dissipating the power that passive resistors do.
    We’ll now review Six available lossless current sensing techniques. 5
1. Series Sense Resistor
 This technique is the conventional way of
     sensing current.
   It simply inserts a Sense Resistor in series with
    the inductor.
   If the value of the resistor is known, the current
    flowing through the inductor is determined by sensing the voltage across it.
   This method obviously incurs a power loss in Rsense, and therefore reduces
    the efficiency of the DC-DC converter.
   For accuracy, the voltage across the sense resistor should be roughly 100mV
    at full load because of input-inferred offsets and other practical limitations.
   If full-load current is 1A, 0.1W is dissipated in the sense resistor.
   Main Disadvantage: For an output voltage of 3.3V, the output power
    is 3.3W at full-load and hence the Sense Resistor reduces the system
    efficiency by 3.3%.
2. MOSFET RDS Current Sensing
 MOSFETs act as resistors when they
  are “ON” and they are biased in the ohmic
  (non-saturated) region.
 Assuming small VDS, as is the case for
  MOSFETs when used as switches:
               I D    COX       (VGS  VT ) VDS   ;   for : VDS  (VGS  VT )
 The equivalent resistance of the device is:                        RDS 
                                                                             W    COX  (VGS  VT )
 RDS in this case should be known ; VDS is measured.
 Main Disadvantage: The RDS of the MOSFET is inherently
  nonlinear: It usually has significant variation because of:   COX ,
  VT, and exponential variations across temperature
  (35% variation from 27°C to 100°C).
3. Filter-Sense the Inductor
 A simple low-pass RC network filter the voltage across the
  Inductor and sense the current through the equivalent series
  resistance (ESR) of the inductor.
 The Voltage across the Inductor is:   v L  ( RL  sL ) I L
 The Voltage across the feedback Capacitor is:
                            ( RL  sL ) I L       1  sL / RL  
        vc 
                                            RL                  I L  RL 1  sT I L
             1  sR f C f    1  sR f C f         1  sR C                 1  sT1
                                                          f  f 

 Forcing:   T  T1    vc  RL iL    Vc  iL

 Main Disadvantage:
  L and RL need to be known 
  Not appropriate for IC, but it is a proper
  design for a discrete/custom solution                                                   8
4. Sensorless (Observer) Approach
 This method uses the Inductor voltage to measure the Inductor
 Since the Voltage-Current relation of the Inductor is: vL  L , the
  Inductor current can be calculated by integrating the voltage over

 Main Disadvantage: As at the previous method, The value of L
  also should be known for this technique.                      9
5. Average Current
 This technique uses RC LPF at the junction of the switches of the
 Since the average current through the resistor is zero, the Output
  averaged–current is: I o  I L  Vout  Vc

 If RL is known (not the case of IC designers), the output Average
  current can be determined.
 Main Disadvantage: Only average current can be measured.      10
6. Current Transformers

 The use of this technique is common in high power systems.
 The idea is to sense a fraction of the high Inductor current by
  using the mutual inductor properties of a transformer.
 Main Disadvantages:
    Increased cost and size and non-integrablity.
    The transformer also cannot transfer the DC portion of
     current, which make this method inappropriate for
     over current protection.

 This method is the practical technique for Current sensing in new
  power MOSFET applications.
 The idea is to build a current sensing FET in parallel with the
  power MOSFET (Current Mirror).
 The effective width (W) of the sense MOSFET (SENSEFET) is
  significantly smaller than the power FET, and therefore linearly
  reduces ID per the MOSFET known equation: I    C  W  (V  V ) V
                                                    D    OX       GS   T   DS

  The voltage of nodes M and S
  should be equal to eliminate the
  Current-Mirror non-ideality
  resulting from channel length modulation

 The width (W) of the Power MOSFET is X100-1000 times the
  width of SENSEFET to guarantee it’s low power consumption.
  Complete Current Sensing -
  SENSEFETs circuit
 The Op amplifier is used to force VDS
   of M1 and M3 to be equal.
 As the width of the main MOSFET and
  SENSEFET increases, the accuracy of
  the circuit decreases.
 Main Disadvantage:
      Relatively low Bandwidth
      Proper layout scheme should be designed to minimize coupling between
       the transistor (which can induce significant error).
 Advantages:
      Lossless
      Integrable
      Practical
      Relatively good accuracy                                        13
Comparative overview of Current-Sensing Techniques

 For Voltage mode control, which current sensing is only needed
  for over-load current protection, RDS method can be used.
 In current-mode control for
  desktops (no power
  dissipation constraint), we
  can use RSENSE technique.
 RDS and SENSEFET are
  the dominant techniques
  were power consumption is
  critical (portable applications);
  SENSEFET is much more
  accurate.                                                   14
  The Current-mode Buck Converter
                                                                         = Vg*DON
 The next few slides
  focus on few design issues
  of the Current–Mode
  Buck converter:
    Pole-Zero
      cancellation.                                              1
    On-Chip Inductor
      current sensing.
    Subharmonic
      oscillations.                                              2
    Pulsewidth Generator

 The output of the Compensator, Compensation ramp, and Sensed Inductor
  current pass through the modulator and the digital control block to define 15
1. Pole-Zero Cancellation Compensator
 The Power stage of Current-mode
    Converters has Control-to-output
    Transfer function of two separated
    real poles.
    The Pole from the output filtering
    capacitor is heavily dependent
    on the equivalent resistance of the output load – RL.
    For dynamic response consideration, Pole-Zero cancellation is
     preferable as the bandwidth can be extended with Pole-Zero
     cancellation  Speed up the response time.
                                                          v        1  sC R
    The Transfer Function of the Compensator is: A( s)  bv  g R 1  sC R ; R  R
                                                                  m   0
                                                                          c   z
                                                                                  0    z
                                                              0           c   0

    R0 is the Output resistance of the Operational Transconductance Amp
  Calculation of the two frequency
  compensation components – RZ , CC
 The general purpose of introducing Zeros and Poles in the
  compensator is to cancel Poles and Zeros in the control-to-output
  function, respectively.
 This will yield an average -20 dB/decade closed-loop gain
  response with sufficient phase margin below the unity gain freq.
 When determining the unity gain frequency, it should not be too
  close to the converter’s switching frequency as the amplifier
  would amplify the output ripple voltage; A safe value of unity
  gain frequency is below 20% of the switching frequency.
 Since the dominant pole shifts inversely proportional to the load
  resistance, the lowest frequency occurs at the highest load
  resistance, two frequency compensation components - RZ and CC
  can be calculated using the corresponding Transfer Function.
2. Subharmonic oscillation are eliminated
   by Compensation Ramp - mc
 Subharmonic oscillation is a well-known problem for current-
  mode switching converters with the duty ratio – D > 0.5.
 To avoid Subharmonic oscillation, the slope of the compensation
  ramp must be larger than half of the slope of inductor current
  during the second subinterval D'T.
 The Compensation ramp (Ramp signal) and the Inductor current
  signal (Sensed signal) are summed together.

3. On-Chip Current Sensing Circuit

 Op Amp enforces: VA=VB
 L & C are off-chip
 I1,I2 are small and equal,
  pull current from VA,VB
 'ON' state: M1 – ON ,
  IL is mirrored to M2
 VDS and current density of
   M1, M2 are almost the same.
 Due to different Aspect ratios
   of M1, M2 (WM2:WM1 = 1:1000)  IS is much smaller and proportional to IL:
                          VSENSE  I SENSE RSENSE         RSENSE          19
On-Chip Current Sensing Circuit - Continue
 I2 (Small Biasing current) << IS                              PMOS

      I SENSE  I S  I L
 For Current mode DC-DC
  Converters, only VSENSE is
  needed in the control feedback
  loop during the ON-State
  (ramp up of the inductor current)
 MS2 tie VA to Vg during the
  OFF-State  ISENSE~0.
 The output of the Op Amp should be able to go up to Vg in order to make the
  Transistors – Mrs & MCS5 operate in Saturation region.
    Current Sensing - Design Issues
 The accuracy of the sensed Inductor current depends on the
    Current Mirror of transistors M1 and M2 and in the on-chip
    Poly Resistor - RSENSE.
   The matching of transistors M1 and M2 depends on the process
    parameters such as Mobility, Oxide Capacitance (COX) and
    Threshold voltage (VT).
   Therefore, proper layout technique should be well considered,
    especially the location of the transistor M2, to minimize error.
   In the suggested design, M2 is surrounded by 500 fingers of M1.
   Of course, This on-chip current-sensing circuit can be extended
    to sense power NMOS transistor by simply building a
    complement circuit for other topologies (as Boost, Buck-Boost).
  Pulsewidth Generator
 This Implementation deals with the Startup situation in which
  both inputs are high  In this situation, the Latch is SET.

  Measurements Results - 1
 The Converter is supplied with an Input voltage of 3.6V,
  and Switching frequency of 500KHz.
 Attached Steady-state measurements with:
   Maximum Loading current = 300mA ; RSENSE = 400 Ohm
Output voltage = 2.1V and Duty Ratio > 0.5 (Subharmonic oscillation zone)
           Inductor Current                Inductor Current

            Sensing Voltage               Inductor Voltage (Vx)

Measurements Results - 1
     DC Output Voltage = 2.12V   Output Ripple Voltage = 6.4mV

Measurements Results - 2
    Output voltage = 1.4V and Duty Ratio < 0.5
      Inductor Current             Inductor Current

      Sensing Voltage             Inductor Voltage (Vx)

Measurements Results - 2
    DC Output Voltage = 1.4V   Output Ripple Voltage = 3.17mV

    Circuit Efficiency and Performance
 The Current Sensing circuit performs accurately and the absolute
    error between the sensing signal and the scaled inductor current
    is less than 4% (10mA with load current of 300 mA);
   This absolute error is mainly due to
    the mismatch of transistors M1 and M2
     in the sensing circuit.
   The efficiency is shown with the
                                                         Conduction Loss
     Input voltage of 3.6 V and the
    Output voltage of 2.0 V.
   The maximum efficiency is 89.5%
    at loading current 300 mA.
   There are two major power dissipations:
    Conduction loss and switching loss
    Conclusions & Summary
 Experimental results show that the converter regulates properly
     with duty ratio – D larger and smaller than 0.5.
   Using the internal Current-Sensing technique, it not only reduces
    the external pins for the monolithic controller, but also reduces
    the complexity of the design.
   Due to the accurate Sensing performance, a compensation ramp can be added
    to the sensing signal without any consideration
    on the variation of the sensing performance.
   The accurately sensed Inductor current can
    also be used for over-current protection and
    Load-dependent mode-hopping schemes for
    optimizing power efficiency.
   In addition, this current-mode DC–DC buck
    converter with internal current sensor can
    operate from 300 kHz to 1MHz with the input voltage range from 3 to 5.2 V,
    which is suitable for lithium-ion battery supply applications.        28
 A Monolithic Current-Mode CMOS DC–DC Converter With
  On-Chip Current-Sensing Technique.
  Cheung Fai Lee and Philip K. T. Mok, Senior Member, IEEE,
  NO. 1, JANUARY 2004.
 On-Chip Current Sensing Technique for CMOS Monolithic
  Switch-Mode Power Converters.
  Cheung Fai Lee and Philip K. T. Mok,
  In IEEE Int. Symp. Circuits and Systems, vol. 5, Scottsdale, AZ,
  May 2002, pp. 265–268.
 Current-Sensing Techniques for DC-DC Converters.
  Hassan Pooya Forghani-zadeh, Student member, IEEE, and
  Gabriel A. Rincón-Mora, Senior member, IEEE,                 29

  Georgia Tech Analog Consortium.

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