Electronic Design

Semi-Precision Current Limiter Handles High-Power Loads

Implementing a current limit with a ground-referred load often requires a power transistor, a high-side current-sense resistor, and some form of level shifting to get the current-sense signal referred to a convenient potential, usually ground. The circuit shown in Figure 1 does away with all of that.

Key to the design is a "smart" high-side power switch (IC2), the Infineon BTS6144 (www.infineon.com). It implements an integral power MOSFET and a variety of protection functions, such as short-circuit current limiting and thermal-overload shutdown. Another feature exploited in this design is the current-sense output at pin 5, a low-value current proportional to the load current.

The BTS6144 supply voltage (VBB) can be as high as 30 V, and the device is able to switch load currents exceeding 30 A. Shorting the input (pin 3) to ground turns it on. Under normal conditions, sense current IS and load current ILOAD are related by current-sense ratio K (= ILOAD/IS), typically around 12,500. A current-sense voltage (VCS) derived from IS is compared to reference voltage (VREF) so that the load current shuts off when it exceeds a preset value. The TLC393, a micropower dual comparator with open-drain outputs, is the only other active device needed. C2 and R8 decouple IC1's supply voltage when the circuit is used in noisy environments.

Essentially, the circuit operates as a self-resetting circuit breaker. To understand how it works, assume that switch S1 is open. With the supply voltage, VS, at a nominal 12 V, the quiescent potentials at comparator IC1a's inverting and noninverting inputs are 9.2 and 9.9 V, respectively. Thus, IC1a's output transistor is off, and IC2's input is pulled high by R9, holding the power switch off.

Because no load current flows, VCS is zero. Therefore, provided VREF is a non-zero positive voltage, IC1b's output transistor is off and the potential at the R4-R5-R6 junction is unaffected by IC1b.

When S1 is closed, IC1a's noninverting input is pulled down to 8.4 V, and IC1a's output goes low. This turns on IC2, which sources current to the load and develops a proportional sense voltage across RCS. Provided VCS remains below VREF, IC1b has no effect on IC1a's inverting input, and IC2's power switch remains on.

But if ILOAD rises and causes VCS to exceed VREF, IC1b's output goes low, rapidly discharging C1 via R6, and IC1a's inverting input is pulled down to ground. IC1a's output transistor turns off, and IC2's input is pulled high via R9, turning off the power switch. Consequently, ILOAD and VCS now fall to zero, and IC1b's output transistor turns off.

IC1a's inverting input now rises as C1 charges via R4. However, IC2 remains off until C1's voltage exceeds the potential at IC1a's noninverting input, at which point the power switch turns on again and sources current to the load. If the overload is still present, the process repeats and the circuit breaker continually sets and resets at a rate determined by the R4-R5-C1 time constant.

The waveforms for the overcurrent state show how IC2's output remains off while C1's voltage rises toward the potential at IC1a's noninverting input (Fig. 2). With the values for R4, R5, and C1 as shown in Figure 1, the "off" time (tOFF) is typically 120 ms.

The circuit's "on" time is influenced by the comparators' response time, by IC2's turn-off time, and to a lesser extent, by the R6-C1 time constant. C3 also slightly delays IC2's turn-off and is necessary to allow IC1b sufficient time to discharge C1 fully. C4, recommended to decouple switching noise at IC1b's inverting input, forms a time constant with R11. This also affects the "on" time.

But at around 900 µs, the "on" time is much less than tOFF, resulting in a very low duty cycle (less than 1%) and minimal power dissipation in the load during overload. Resistors R6 and R7 limit the comparators' low-level output current to a safe level (around 20 mA for the TLC393). VCS and ILOAD are related by:

VCS = IS × RCS = (ILOAD/K) × RCS

Therefore, for a particular load current range, RCS can be chosen to produce a suitable range of VCS. Note that K isn't a rigid constant. For example, at ILOAD = 10 A, K can range from 10,800 to 13,800 at 25°C. The characteristic is also slightly nonlinear (K tends to be larger at low load currents).

Although Figure 1's circuit is shown with a typical automotive supply voltage of 12 V, other voltages can be accommodated with appropriate component changes. The minimum supply voltage depends on IC2's minimum operating voltage (5.5 V). The upper limit of VS is determined primarily by the maximum operating voltage of IC1 and IC2.

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