Design and Application of Hot Swap Controller in DC Boost Circuit

Hot-swap protection circuits are often used in high-availability systems such as servers, network switches, and other forms of communication infrastructure. Such systems often require replacing a faulty circuit board or module while the system is powered on, and the system remains operational. This process is called hot swapping. This article will describe another use of hot-swap controllers, using the overcurrent and short-circuit protection features of hot-swap protection circuits to solve the output protection problem of switching DC boost circuits.

1 Basic Principles of Switching DC Boost Circuit

The Boost Converter or Step-up Converter is a switching DC boost circuit. The output voltage is higher than the input voltage, and the output voltage polarity remains unchanged. The basic circuit diagram is shown in Figure 1.

When the switch tube is turned on, the power supply forms a loop through the inductor-switch tube, and the current is converted into magnetic energy storage in the inductor; when the switch tube is turned off, the magnetic energy in the inductor is converted into electrical energy at the left side of the inductor, which is negative and the right side is positive. This voltage is superimposed on the positive end of the power supply, forming a loop through the diode-load, completing the boost function.

When the output is overcurrent, the circuit will sample the peak current of the switch tube, reduce the duty cycle, and cause the output voltage to drop. When the output voltage drops to the input voltage, the overcurrent protection is no longer controlled and the protection fails. In addition, the output overcurrent point will increase as the input voltage increases. When the output is short-circuited, the input power will form a short-circuit loop through the inductor and boost diode, causing power failure. Another drawback of the BOOST circuit is that it is not convenient to control the output to turn off. When the control chip is turned off and the switch tube is turned off, the output still has voltage, unlike the BUCK circuit, which can easily reduce the output voltage to 0 V.

2 Basic Principles of Hot Swap Controller

Hot-Plugging or Hot Swap is live plugging and unplugging. The hot-plugging function allows users to remove and replace damaged power supplies or boards without shutting down the system or cutting off the power supply, thereby improving the system's ability to recover from disasters in a timely manner, scalability, and flexibility. If there is no hot-plug controller, when the module at the load end is plugged in or out, it will cause a surge current impact on the power supply, affecting the voltage stability and reliability of the power supply. This problem can be solved by a hot-plug controller, which can reasonably control the surge current and ensure a safe power-on interval. After power-on, the hot-plug controller can also continuously monitor the power supply current to avoid short circuits and overcurrents during normal operation.

3 Key Circuit Design and Examples

3.1 Power Requirements

An example of a power supply is shown in Figure 2, where the power supply input is 9~18 V, the rated output is 28 V/1.2 A, and the overcurrent protection is 1.5 A.

3.2 Circuit Introduction

This is a boost circuit using the TPS2491 hot-swap control chip with output over-current short-circuit protection. When the remote control terminal CTL is grounded, the power supply enters standby mode and the output is zero.

The hot-swap controller includes three main components: an N-channel MOSFET used as the main switch for power control, a sense resistor for measuring current, and a hot-swap controller TPS2491, as shown in Figure 2 above. The hot-swap controller is used to implement a loop that controls the on-current of the MOSFET, which contains a current detection comparator. The current detection comparator is used to monitor the voltage drop across the external sense resistor. When the current flowing through the sense resistor generates a voltage of more than 50 mV, the comparator will indicate an overcurrent and turn off the MOSFET. The TPS2491 has a soft-start function, in which the overcurrent reference voltage rises linearly instead of turning on suddenly, which allows the load current to change in a similar manner.

The TPS2491 integrates a comparator and a reference voltage to form an enable circuit for output enablement. The comparator has a turn-on voltage of 1.35 V and a turn-off voltage of 1.25 V, with a hysteresis of 0.1 V to ensure stable operation. The power supply voltage that must be reached to enable the controller is accurately set by the voltage divider resistor. Once the device is enabled, the MOSFET gate begins to charge, and the gate voltage of the N-channel MOSFET used in this circuit must be higher than the source. In order to achieve this condition over the entire power supply voltage (VCC) range, the hot-swap controller integrates a charge pump that can maintain the voltage of the GATE pin at 10 V higher than VCC. When necessary, the GATE pin requires a charge pump pull-up current to enable the MOSFET and a pull-down current to disable the MOSFET. The weaker pull-down current is used for regulation, and the stronger pull-down current is used to quickly turn off the MOSFET in a short-circuit condition.

Another module of the hot-swap controller is a timer, which limits the current regulation time in an overcurrent condition. The MOSFET selected can withstand a certain power for a specified maximum time. MOSFET manufacturers use Figure 3 to mark this range, or safe operating area (SOA).

The timer also determines the time for the controller to automatically restart. If a fault causes the MOSFET to be turned off, the chip will re-enable the output after 16 oscillation cycles.

3.3 Design Process

Select a suitable capacitor to ensure that the output capacitor can be fully charged when the output starts and does not cause fault protection.

(5) Select the enable start voltage

The start-up voltage of EN is 1.35 V, and the shut-down voltage is 1.25 V. This pin can be used for input undervoltage protection; the voltage divider resistors are designed to be 240 kΩ and 13 kΩ, the start-up voltage is 26.3 V, and it is shut down at 24.3 V.

(6) Other parameters

The GATE drive resistor is usually 10 Ω to suppress high-frequency oscillation; the PG pull-up resistor ensures that the absorption current is less than 2 mA, which is not required in this design and is left floating; the Vcc bypass capacitor is 0.1 μF.

A diode BAV70 is connected in series to the power enable terminal, which can turn off the boost circuit and power output when the level is low.

4 Test results and working waveforms of each test point

The test results are as follows: overcurrent protection action point: 1.45 A; the output is not damaged by long-term short circuit, and the output is restored after the short circuit is removed; the remote control end enables work normally.

The waveforms of various test points when powered on are shown in Figure 4.

In Figure 4, CH2 is the boosted voltage. When the input is powered on, the boost circuit works immediately and quickly reaches 28 V. In order to prevent the surge current of the rear load from impacting the MOSFET, it can be seen that the drive voltage (CH1) rises slowly, and the output voltage (CH3) also rises slowly. During the startup process, it is obvious that the drive voltage of the MOSFET is not high. The MOSFET works in the linear region, which can also suppress the increase of the output current and effectively protect the MOSFET from overloading during the startup process.

The voltages at various points during normal operation are shown in Figure 5. As can be seen from Figure 5, during normal operation, the output voltage (CH3) is equal to the boosted voltage (CH2), and the MOSFET drive voltage (CH1) is 14 V higher than the output voltage, which can ensure that the MOSFET is well turned on and reduce heat loss and voltage difference.

The waveform when the load is overcurrent or short-circuited is shown in Figure 6. As can be seen from Figure 6, when the output is overcurrent or short-circuited, the MOSFET drive voltage (CH1) drops rapidly, causing the output voltage (CH3) to drop accordingly, effectively protecting the safety of the power supply. After a 2 s restart cycle, the drive voltage has a small test voltage. If the fault still exists and the restart is unsuccessful, the drive voltage returns to zero. Otherwise, the restart is successful and the output is normal. As shown in Figure 7.

5 Conclusion

Practice has proved that the protection control circuit based on TPS2491 hot-swap controller has the characteristics of simple and reliable circuit and convenient application. This circuit is applied to the switching DC boost circuit, which perfectly solves the defects of no output over-current short-circuit protection and inability to remotely control the output, and has achieved good results.