current mode.pdf

VENABLE TECHNICAL PAPER # 5

Current Mode Control

Abstract:

Current mode control, one of the hot new subjects of power electronics, has actually

been in use for many years under another name — the discontinuous mode fly back

converter. This paper details the principle of current mode control, how it is supposed

to work, what topologies come closest to truly representing the concept, and how to

test a current mode converter to see if it is actually working as advertised. Description

of an implementation of the concept in a commercially available integrated circuit is

included, together with a description of the principles of operation of a little-used

topology that is one of the few true implementations of the concept, the continuous-

mode buck converter with constant off time.

Introduction

Switching regulators have been with us for many years, but it took Silicon General and the advent of

the SG1524 Pulse Width Integrated Circuit to make them really take off in popularity. The 1524 drives

a pair of switching transistors with a duty cycle which is proportional to a control voltage, and by

using the switching transistors to switch a voltage source on and off into an L-C low pass filter, a

relatively efficient voltage regulator can be produced. This technique has come to be known as

"voltage-mode programming", since the duty cycle is proportional to the control voltage.

Another technique, which has also been around for a long time, senses the peak current in the power

switch and turns the switch off at a programmed level of current. By pretending that the average

current from the low-pass filter is proportional to the peak current in the power switch (a good

assumption in most cases), a functional block is formed which uses local feedback to create what is

essentially a voltage-to-current converter. By using this voltage-to-current converter block inside an

overall voltage feedback loop, a voltage regulator can be produced where the control voltage sets the

load current rather that the switch duty cycle. Figure 1 is a block diagram of the concept. The net result

of the two approaches is the same, to regulate voltage, but this latter approach is called "current-mode

programming" since the load current is the directly controlled variable and the output voltage is

controlled only indirectly. Like switching regulators in general, this approach languished until an

integrated circuit was developed to make the job easy. In this case Unitrode developed the 1842 and

1846 chips for single-ended and push-pull applications respectively, and the technique was off and

running.

Current Mode Control

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Figure 1. Ideal Current Mode Converter

Why Current-Mode Control?

Why bother with such a roundabout way to regulate output voltage anyway? Well, it turns out that the

current-mode approach offers some advantages.

For one thing, since the output current is proportional to the control voltage, the output current can be

limited simply by clamping the control voltage. In fact, since the current is controlled by sensing the

peak current in the power switch (es), the current can be limited on a cycle-by-cycle basis. The two

sides of a push-pull circuit can be forced to share, even if there are significant imbalances in circuit

component values.

Another advantage is that the energy storage inductor is effectively absorbed into the current source.

Provided the bandwidth of the local feedback in the voltage-to-current converter is sufficiently high,

the transfer function from control voltage to output voltage has a single- pole rolloff, due to the current

source driving the filter capacitor. With voltage-mode control, the inductor does not disappear and the

control to output transfer function has a two-pole rolloff. Systems with a single-pole rolloff of the

control-to-output transfer function can be stabilized with a simpler compensation network around the

error amplifier.

For higher power applications, power stages can be connected in parallel. Since the output currents are

proportional to the control voltages, the power stages can be forced to share equally by simply

connecting the control voltages to a common bus.

A final advantage, which is touted, is automatic feed forward from the line voltage. This particular

feature is actually more readily attained in voltage-mode converters by a technique known as "ramp

compensation". In fact, in current-mode converters perfect feed forward is obtained only by a

particular value of slope compensation (a concept which will be explained more fully later in the

paper) and this value of compensation is not practical in actual practice.

Current Mode Control

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Is Current-Mode Control Perfect?

Like almost everything, there is some bitter with the sweet. The hardest part of current-mode control is

measuring the current accurately and with the required bandwidth.

There are also open-loop instabilities that have to be dealt with. The local current feedback

loop is open-loop unstable for duty cycles over 50% unless some form of "slope

compensation" is used. In the half- bridge topology, the circuit will unbalance the two filter

capacitors. The transfer function of the voltage-to-current converter (essentially the

transconductance) is not as wide-band as previously thought, and has recently been proven to

have a bandwidth of at best 1/6 to 2/3 of the switching frequency. And finally, the familiar

right-half-plane zeros of the buck and buck-boost topologies are still present, even with

current-mode control. Figure 2 shows some of the problems associated with measuring the

current. The sense resistor has parasitic inductance. The various capacitances around the

power switch must charge and discharge through the current sense resistor. The R and C of

the current signal filter could be chosen to match the L/R time constant of the sense resistor, if

it were not for the parasitic capacitance currents. These currents are significant, and in most

applications the C of the current signal filter is chosen large enough to filter out the spikes

caused by these currents flowing through the R and L of the sense resistor. This causes the C

of the filter to be much larger than required to match the L/R time constant, and causes

another pole in the feedback loop, which must be accounted for in the compensation calculations.

Figure 2. Parasitics Affect Accurate Current Measurement

Time delays cause the switch to turn off at a time later than indicated by the current crossing

the threshold. Figure 3 shows this effect. Although a relatively minor effect, the present trend

toward higher switching frequencies makes this parasitic increasingly important.

Current Mode Control

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Figure 3. Time Delays Affect Accuracy of Peak Current Sensing

As the line voltage and duty cycle changes, the peak-to-peak and average current values of the inductor

change, as indicated in Figure 4. This contributes to inaccuracies in the transconductance, since the

average current does not track the peak and the peak is what is being controlled.

Figure 4. Average and Peak-to-Peak Values Change with Duty Ratio

4 . Open Loop Instability

An early discovery in the development of current-mode control was that the current feedback loop

became open loop unstable when the duty cycle was increased past 50%. This phenomenon has been

thoroughly studied and analyzed. Disturbances in the operating point gradually die out when the duty

Figure 5. For Duty Ratio Less Than 0.5, Disturbances Die Out

Current Mode Control

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For duty cycles greater than 50% however, a disturbance from the nominal operating point grows larger

with each cycle. This leads to large deviations from the nominal operating point and a phenomenon

known as "sub-cycle oscillation." Figure 6 shows the beginning of this process.

Figure 6. For Duty Ratio Greater Than 0.5, Disturbances Grow

By adding "slope compensation" to the trip level (or to the sensed current signal), the duty cycle at which

a disturbance begins to grow can be increased. Figure 7 shows the effects of slope compensation.

Figure 7. Slope Compensation Can Cause Disturbances to Die Out for Any Duty Ratio

If the slope of the falling current in the energy storage inductor is called m2, then a negative slope equal to

half the slope of m2 will in theory cause a disturbance to die out for any duty cycle up to 100%. Two

other advantages that occur with this particular amount of slope compensation are that the average

current is no longer a function of duty cycle, and that line voltage changes are perfectly rejected without

requiring action by the voltage loop. Figure 8 shows slope compensation where -m = m2/2, and the effect

on average current. Although the advantages of having -m = m2/2 are significant, they are difficult to

achieve in practice. In practice, it is better to have more compensation than -m = m2/2 to assure freedom

from oscillation at high duty cycles.

Current Mode Control

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