Many cost and engineering improvements are realized by integrating the current pass element and currentsense function into a hot-plug controller integrated circuit (IC). Controlling the passelement power is the safest and fastest way to charge the load capacitance. Pass-element protection is maintained while boosting efficiency and reliability.
A circuit controlling current to a load usually consists of three components—an IC controller, a MOSFET switch through which current passes, and a current-sense element. A simple but powerful concept is to integrate these two parts into one monolithic IC.
At first glance, it may seem that the primary benefits are reducing circuit board space and some of the cost of the external components. This is all true. But if that integrated product also manages the load by controlling the power dissipated in its MOSFET, it then becomes a much more robust solution— and ideal for high-reliability systems.
Many high-reliability systems must stay powered on through equipment upgrades, failures, or repairs. A current-management solution, sometimes known as a “hot swap” or “hot plug” controller, keeps systems operating in two ways. First, it controls power turn-on to avoid excessive inrush current when applying power to a module with discharged capacitors. Second, it’s a fast-acting circuit breaker on an overcurrent fault. In both cases, module components are protected, and the local failure doesn’t cause the system voltage to drop out of specification and disrupt other operating modules on the same power bus.
SAFE OPERATING AREA
Some power systems use dv/dt control, which limits inrush current by slowly ramping the voltage to the output. The issue here is that the MOSFET must be oversized because operating at constant current may take the MOSFET out of its safe operating area (SOA).
Remember, the MOSFET SOA data is published for the device operating at 25°C. It must be de-rated for real-world conditions. For dv/dt control, changing the load capacitance from the designed value changes the inrush current. Increasing load capacitance increases the inrush current.
Other systems use di/dt controllers, which hold a constantcurrent ramp to charge the load capacitance. The current required to charge the load capacitor is unchanged with capacitance, but generally has a longer charge time than the dv/dt method.
Methods that charge the load capacitance by limiting the power dissipation in the MOSFET will protect the MOSFET and charge the load capacitance in the shortest possible time within the maximum current limit selected.
Figure 1 defines the general SOA curve, which shows the operating limits of a typical MOSFET. Sections 2 and 4 are the current and voltage limits of the MOSFET. The RDS(ON) for the MOSFET determines the Section 1 limit. Section 3 is the thermal limit, which is important because excessive heat is the most common reason for MOSFET destruction. Not surprisingly, the plot of the thermal limit is also a plot of constant power.
The typical MOSFET datasheet SOA curve shows the family of constant power curves for a fixed time (Fig. 2). The red line demonstrates the problem with current-limiting controllers. The MOSFET must be over-rated from its normal operating point to survive startup and fault when operating at point C.
MOSFET PROTECTI ON
In contrast, the blue line sets the constant-power operating point for reliable operation of the MOSFET under fault conditions. At startup, the controller operates at point B. By charging the capacitor and decreasing the voltage across the MOSFET, the control returns to current limit when the product of current demand and the voltage across the MOSFET is less than the power limit of the MOSFET.
The TPS2420 and TPS2421 from Texas Instruments incorporate this power-limiting feature. These power controllers with integrated MOSFET control the power turn-on and overcurrent events by limiting the power dissipated in the package. The power limiting is a controlled way to manage the load and guarantee that the MOSFET will always be operated within its SOA. (TPS2420 and TPS2421 datasheets and technical documents are available at www.ti.com/tps2420-ca and www.ti.com/tps2421-ca.)
A scope waveform shows the voltage and current starting up into an 800-µF, 60- load (Fig. 3). The MOSFET power dissipation is constant as calculated by the scope in the yellow math trace. The voltage across the MOSFET is the constant 12-V input (not shown) minus VOUT. The MOSFET power is the product of the MOSFET voltage and IOUT. Maximum power is fixed at 5 W. As VOUT increases, the MOSFET voltage decreases and IOUT can increase to maintain constant power dissipation.
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