Difference between current & voltage mode circuits. Advantages & disadvantages.

1. CURRENT MODE
&
VOLTAGE MODE
CIRCUITS

2. INTRODUCTION
Voltage-Mode:Information is represented by voltage at the nodes of the circuit.
Current-Mode:Information is represented by current flowing in the branches of the circuit.
However, none of the definitions used in the literature are precise. For
example, some authors write that signals are represented by currents in currentmode circuits and by voltages in voltage-mode circuits. This is not a precise
definition, because every circuit node has an associated voltage and every branch an
associated current. Therefore, current-mode and voltage-mode do not actually
divide circuits into two categories, they are just alternate ways of looking at a
circuit.

3. EXAMPLES

4. EXAMPLES (CURRENT MODE)
Bipolar Junction Transistor
Current Mirror

5. INTERCONNECTS IN VLSI DESIGN
Depending on the signal carriers of data links, wire channels can be classified as
voltage mode or current mode signalling.
In voltage mode signalling, receiver provides high input impedance (ideally infinity).
The information is conveyed in the form of voltage. The output voltage is a function of input
signal and is varied according to supply voltage. Fig 1 shows the theoretical model of
conventional voltage mode interconnect implementation. The output is terminated by an
open circuit.
This high input impedance of the receiver gives
rise to high input capacitance which leads to high charging
and discharging time for RC interconnect chain. Hence
voltage mode signalling has large delay. Due to high input
impedance at the receiver, the charge accumulated at the
input of the receiver does not get effective discharge path
to ground as a result this may cause electrostatic
induced gate oxide break down.

6. INTERCONNECTS IN VLSI DESIGN

7. DIFFERENCES
In integrated circuits, current-mode offers some advantages over voltage-mode :
o Performances improvement




low power consumption at high frequency
less affected by voltage fluctuations
low cross-talk & switching noise
high speed
o Structural advantages
 controlled gain without feedback components
 current summing without components
 schematic simplicity
o Specific features
 well suited for low voltage, low power applications
 pseudo conductance networks
 current switching technique

8. WHY WE SWITCH TO THE CURRENT MODE
CIRCUITS?
#1: Easy Compensation
• With voltage-mode, the sharp phase drop after the filter resonant frequency requires a
type three compensator to stabilize the system.
• Current-mode control looks like a single-pole system at low frequencies, since the
inductor has been controlled by the current loop.
• This improves the phase margin, and makes the converter much easier to control.
• A type two compensator is adequate, which greatly simplifies the design process.

9. WHY WE SWITCH TO THE CURRENT MODE
CIRCUITS?
#2: RHP Zero Converters
• Contrary to some papers on the topic, current-mode control does NOT eliminate the right-half
plane (RHP) zero of boost, flyback, and other converters.
• It does make compensation of such converters easier, though.
• With voltage-mode control, crossover has to be well above the resonant frequency, or the
filter will ring.
• In a converter where the crossover frequency is restricted by the presence of an RHP zero, this
could be impossible.
• It's not a problem with current-mode control to have a control loop crossover at or below the
filter resonant frequency.

10. WHY WE SWITCH TO THE CURRENT MODE
CIRCUITS?
#3: CCM and DCM Operation
• When moving from continuous-conduction
mode (CCM) to discontinuous-conduction
mode (DCM), the characteristics with voltagemode control are drastically different as shown
in Fig 4.
• It is not possible to design a compensator with
voltage-mode that can provide good
performance in both regions. With currentmode, crossing the boundary between the two
types of operation is not a problem. The
characteristics are almost constant in the region
of crossover, as shown in Figure 5.
• Having optimal response in both modes is a
major advantage, allowing the power stage to
operate much more efficiently. Keeping a
converter in DCM for all changes of load, line,
temperature, transients, and other parameter
variations can lead to severe component
stresses.

11. WHY WE SWITCH TO THE CURRENT MODE
CIRCUITS?
#4: Line Rejection
• Closing the current loop gives a lot of attenuation of input noise. For the buck, it
can even be nulled under some special conditions, with the proper compensating
saw-tooth ramp.
• Even with only a moderate gain in the voltage feedback loop, the attenuation of
input ripple is usually adequate with current-mode control.
• With voltage-mode control, far more gain is needed in the main feedback loop to
achieve the same performance.

12. DISADVANTAGES OF CURRENT MODE
#1: Current Sensing
• Either the switch current or inductor current must be sensed accurately.
• This requires additional circuitry, and some power loss.
• In most isolated power supplies, the switch current is sensed either with a resistor or
current transformer.
• The current sensing must be very wideband to accurately reconstruct the current signal.
• A current transformer needs a bandwidth several orders of magnitude above the
switching frequency to work dependably.

13. DISADVANTAGES OF CURRENT MODE
#2: Sub harmonic Oscillations Instability
• Current-mode control can be unstable when the duty cycle of the converter
approaches 50%.
• This does not occur abruptly at 50%, as some data books claim, but can manifest
the problem even at lower duty cycles.
• A compensating ramp is needed to fix the problem, and this too can introduce
complications.

14. DISADVANTAGES OF CURRENT MODE
#3: Signal-to-Noise Ratio
• The biggest problem in almost every currentmode supply is noise on the current sense signal.
• In many power supplies there is simply not enough
signal to control the converter smoothly over
the full range of operation.
• Even with the ideal current waveform of Figure 6a,
the signal available for control is small.
• The peak of the current signal is limited by the
PWM controller, usually to less than 1 V.
• Much of the available signal range can be taken by
the DC value of the switch current. When the real
current waveform of Figure with spikes and
ringing is considered, the problem becomes even
worse.

15. BJT CURRENT MIRROR
If a voltage is applied to the BJT base-emitter junction as an input quantity and the collector
current is taken as an output quantity, the transistor will act as an exponential voltage-to-current
converter. By applying a negative feedback (simply joining the base and collector) the transistor can be
"reversed" and it will begin acting as the opposite logarithmic current-to-voltage converter; now it
will adjust the "output" base-emitter voltage so as to pass the applied "input" collector current.
The simplest bipolar current mirror implements this idea. It
consists of two cascaded transistor stages acting accordingly as a
reversed and direct voltage-to-current converters. Transistor Q1
is connected to ground. Its collector-base voltage is zero as
shown. Consequently, the voltage drop across Q1 is VBE, that is, this
voltage is set by the diode law and Q1 is said to be diode
connected. It is important to have Q1 in the circuit instead of a
simple diode, because Q1 sets VBE for transistor Q2. If Q1 and Q2 are
matched, that is, have substantially the same device properties,
and if the mirror output voltage is chosen so the collector-base
voltage of Q2 is also zero, then the VBE-value set by Q1 results in
an emitter current in the matched Q2 that is the same as the
emitter current in Q1. Because Q1 and Q2 are matched, their β0values also agree, making the mirror output current the same as
the collector current of Q1. The current delivered by the mirror
for arbitrary collector-base reverse bias VCB of the output

16. BJT CURRENT MIRROR
where IS = reverse saturation current or scale current, VT = thermal voltage and VA = Early
voltage. This current is related to the reference current IREF when the output transistor VCB = 0 V
by:
as found using Kirchhoff's current law at the collector node of Q1:
The reference current supplies the collector current to Q1 and the
base currents to both transistors — when both transistors
have zero base-collector bias, the two base currents are equal,
IB1=IB2=IB.
Parameter β0 is the transistor β-value for VCB = 0 V.

17. IIIrd Semester Self Study Project
by:-
Gajera Kevin, EC/066
Gautam Rathee, EC/069
Shaleen Rathode, EC/161
Nag Mani, EC/102