CURRENT MODE&

VOLTAGE MODECIRCUITS

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 current-mode 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.

EXAMPLES

EXAMPLES (CURRENT MODE)

Bipolar Junction Transistor

Current Mirror

INTERCONNECTS IN VLSI DESIGNDepending 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.

INTERCONNECTS IN VLSI DESIGNIn current mode signalling, information is represented as current signal. The receiver

provides low impedance (ideally zero) at its input. In current mode signalling line is terminated by shorting the wire. The theoretical model of current mode signalling is as shown in Fig. 3.

It is found that current mode interconnects give lesser delay as compared to voltage mode interconnects.

. C = capacitance; = voltage swing; = average charging/discharging current.

It can be seen that either by reducing voltage swing

or by increasing charging and discharging current, delay

can be reduced. In voltage mode circuits, voltage swing

cannot be reduced since it is limited by signal to noise

Ratio requirements. However charging and discharging

current in current mode circuits can be increased. This

will give lesser delay and hence current mode circuits

can give faster transient response.

DIFFERENCESIn 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

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.

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.

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 voltage-mode 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 current-mode, 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.

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.

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.

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.

DISADVANTAGES OF CURRENT MODE#3: Signal-to-Noise Ratio

• The biggest problem in almost every current-mode 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.

BJT CURRENT MIRRORIf 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 β0-values 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 transistor is given by :-

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.

IIIrd Semester Self Study Project

by:-

Gajera Kevin, EC/066

Gautam Rathee, EC/069

Shaleen Rathode, EC/161

Nag Mani, EC/102