9.2.2.2 Current Sensing and Limiting

The UC184xA-SP current sense input is configured as shown in Figure 18. Current-to-voltage conversion is done externally with ground-referenced resistor R_S. Under normal operation, the peak voltage across R_S is controlled by the E/A according to the following relation:

Equation 2.

where

• V_C = Control voltage = E/A output voltage

R_S can be connected to the power circuit directly or through a current transformer, as Figure 18 shows. While a direct connection is simpler, a transformer can reduce power dissipation in R_S, reduce errors caused by the base current, and provide level shifting to eliminate the restraint of ground-referenced sensing. The relation between V_C and peak current in the power stage is given by:

Equation 3.

where

• N = Current sense transformer turns ratio = 1 when transformer not used.

For purposes of small-signal analysis, the control-to-sensed-current gain is:

Equation 4.

When sensing current in series with the power transistor, as shown in Figure 18, the current waveform often has a large spike at its leading edge. This spike is due to rectifier recovery and/or inter-winding capacitance in the power transformer. If unattenuated, this transient can prematurely terminate the output pulse. As shown, a simple RC filter is usually adequate to suppress this spike. The RC time constant should be approximately equal to the current spike duration (usually a few hundred nanoseconds).

The inverting input to the UC184xA-SP current-sense comparator is internally clamped to 1 V (Figure 18). Current limiting occurs if the voltage at pin 3 reaches this threshold value, that is, the current limit is defined by:

Equation 5.

Figure 18. Transformer-Coupled Current Sensing