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Measuring Input Current of a Buck or Flyback Converter

Nov. 1, 2005
Input currents for non-boost converter topologies can be derived from current measurements in the primary switch.

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When designing converters, it is often desirable to measure the dc current coming into the converter. This is particularly true when batteries are being used and the amount of discharge current out of the batteries needs to be monitored and/or limited for the health and long life of the batteries.

In most converters, there is a circuit that monitors the current through the main switch. This circuit is necessary to prevent damage to the switch under transient conditions for voltage-mode control and is an input to the control loop for current-mode control. In addition, this circuit can be used to measure the dc current into the converter.

In the topologies described here, the current through the primary switch is either measured directly with a resistor or indirectly with a current-sense transformer. This measured signal then gives the instantaneous current through the power switch. With buck topologies, Sepic or Flyback configurations, the only path for the input current is through the power switch. This means that the current's integral through the switch is equal to the integral of the current into the converter over the same time frame. However, this method will not work for boost topologies because the total current into the converter is greater than the current through the switch and additional circuitry would be required to measure the input current (IIN).

A Flyback Topology Circuit

Fig. 1 illustrates a flyback topology circuit that uses the existing current-sense resistor (RSENSE) to measure, amplify and present the information in the form of a voltage that tracks the dc level of the input current.

The circuit's function can be examined by referring to Fig. 1. Let's take a specific case that allows for a simple explanation and shows how the input current information can be derived. Suppose that the input voltage is constant, that the pulse width of the switching converter is at a 50% duty cycle, and further that each cycle starts from zero. This is a realistic configuration for a discontinuous flyback circuit.

Let's assume that the line input current from the source is constant at 1 A. When the switch Q1 is off, all the current from the line goes into the bulk capacitor. But when the switch is on, the current is drawn from the bulk capacitor and the line. Because the voltage and current are fixed, the total charge into the supply through input terminal J1 over one cycle must equal the total charge through the switch during the on time.

Since the switch is on half the time and the current into the inductor is linearly increasing, the peak current through the switch is 4 A. Therefore, if you integrate the voltage developed across the current-sensing resistor over a single cycle and divide by the resistance, the average value of the input current can be determined. In the Fig. 1 circuit, this is accomplished by resistors R1 and C1, where:

and fS is the switching frequency.

But since RSENSE is small, that information is a small voltage and the signal would be easier to use if it was amplified. The operational amplifier U1 with resistors R2 and R3 provide this function. Resistors R2 and R3 set the operational amplifier's gains to:

C2 provides additional filtering if needed. By selecting the values of R2 and R3, it is possible to achieve any scaling factor desired.

A Forward Topology Circuit

Fig. 2 shows a circuit configuration using a current transformer for measurement. The difference is in the current-sense transformer's turns ratio. Here it demonstrates a forward instead of a flyback topology.

It should be noted in Fig. 2 that if a ramp is added into the current signal ISENSE via Q2 to provide slope compensation for control/stability reasons, it will be small (much less than 1%) compared to the desired switch-current voltage when measured across the current-sense resistor R6. As such, the ramp will have little effect on the input current monitor.

Both Figs. 1 and 2 show the addition of the stabilizing signal for the power converter's current-mode control. Current mode is a method of controlling the output voltage by using the current signal in the primary of the transformer as one of the inputs in the feedback loop.

Sometimes it is desirable to shut the converter down in the event of a sustained overcurrent but also allow for short-duration transients above the maximum steady-state levels. The components shown in red in Fig. 2 form a circuit that is designed to shut down the converter in the event of an overcurrent being drawn from the battery. The input current is measured by the current transformer T2 and a current-sense resistor R6.

The information from the current monitor is passed through a low-pass filter R7 and C8, which integrates it over several cycles and generates a dc voltage that is a function of the input current. The measured current level is then compared to a dc level set by the reference and R9 and R10. If this signal exceeds the reference level, a signal is generated that will shut down the converter. C7 is used to prevent noise from causing false tripping. An additional capacitor from the output of the comparator to the positive input (not shown) may be desirable to provide hysteresis and reset time. This connection would form a capacitive voltage divider, so the additional capacitor should generally be larger than C8.

After being filtered by R7 and C8, the dc voltage at the noninverting input of the comparator is equal to:

where N is the number of secondary turns (T2:B) on the current transformer. The primary (T2:A) is one turn. The resistors R9 and R10 are chosen so that the voltage at the R9-R10 junction is equal to the voltage on the comparator's noninverting input when IIN is at the maximum allowed level. R4 is chosen so that the voltage on the ISENSE pin of the converter is greater than the voltage necessary to shut down the converter if the output of the comparator becomes high.

Testing the Theory

Tests were performed to prepare documented results of the theory. A reference design using a UCC38083 was modified to verify this operation. The circuit consisted of the reference push-pull converter plus an operational amplifier and three resistors and a capacitor.

Fig. 3 shows the essential elements of the modified circuit. The input current was measured using a Tektronix current probe. The results were compared with the output of the operational amplifier at J5. Extremely good results were obtained at high- and low-current levels and in tracking dynamic changes.

Fig. 4 was taken with the current on the output set at 3 A and 10 A. All of these figures were taken with a dc input voltage of 24 V. The upper trace is the current probe and the lower trace is the output of the error amplifier. In these figures, the lower waveforms (showing voltages on J5) reflect the increase in load and track with the upper waveforms (showing the current probe signals).

Fig. 5 shows the effect of a constant load for different input voltages. Since the load is the same, the input current varies inversely with the input voltage variations. These two figures also show the effects of an input voltage change from 19 V to 36 V. Also of note in these figures is that the current sensed by the current probe and the sensed voltage at the operational amplifier output change by the same percentage.

In the input current waveform, there is a ripple due to the switching of the power switch that is causing a harmonic resonance with the power source. This is not sensed by the current-monitoring circuit. However, there is some noise on the error amplifier output. This can be taken care of by additional filtering or a good layout. The circuit used here was connected together by lengthy leads and was intended for demonstration only.

The next test was to see the effects of a dynamically changing load condition and its effect on the input current as well as the monitored current. For this measurement, the error amplifier output was sensed using a tip-and-barrel connection to eliminate the noise previously seen.

The circuit was set at 10 Adc, and an additional 10-A pulse was applied at 30 Hz with a 50% duty cycle. Fig. 6 shows the effect on the input current and the error amplifier monitored output.

The signal's ripple from the current probe is the result of resonant currents between the input capacitor and the power source. The low frequency seen is an aliasing effect. As seen in the lower waveform in Fig. 6, the error amplifier does not respond as quickly as the current probe. However, it would be easy to increase the monitor circuit's frequency response by changing the R/C values of the low-pass filter. In most cases, this low-frequency response is desirable. That's because the unit's transients are not shut off unless they would damage the unit. Indeed, the shutoff is taken care of by the pulse-by-pulse current limiting of the converter itself.

Positive Results

It is possible to monitor the input current with minimal additional circuitry using this method while getting an accurate reproduction of the current levels into a power converter. By adjusting the filter cutoff frequency, it would be possible to increase the signal's frequency content.

With the correct scaling, this circuit would be suitable for monitoring and limiting the current from a battery into a converter. It could also determine such things as power consumption, total-charge consumption and remaining battery life.

More on Buck Converters

Buck-Converter Design Demystified Optimizing Voltage Selection in Buck Converters Power Conversion Synthesis Part 1: Buck Converter Design Improving Efficiency in Synchronous Buck Converters

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