Application of hot-swap power modules and safety control during hot-swap operation

1 Introduction
The hot-swap function is also called the hot-plug function, which is very important in power supply design. In application systems using fault-tolerant power architecture, hot-swap function is required to meet the requirement of zero downtime. In modern analog communication and data communication systems, this requirement must be met.
In fact, many large telecommunications and data communication systems are built with multiple circuit boards or blades inserted into a common backplane in the rack. As modern blades have more advanced functions, they need to consume more power, such as the power consumed by the Advanced Telecommunications Computing Architecture (ATCA) blade is about ≥200W. The backplane provides power (such as +48V, -48V, 12V) for the blades and the communication between them. Since the backplane power is always on, it is called a "hot" or "running" backplane. The blade must be inserted into the rack without affecting the operation of the other blades on the backplane. The most recently inserted blade will work using the power of the backplane. If a blade failure is detected, the blade must be unplugged from the slot and a new blade must be inserted into the same slot to restore service. The process of inserting a blade into a running backplane or removing it from a slot is called "hot swapping". Blades that can support this function are called "hot-swappable" blades.
Therefore, this article will introduce the following two aspects: ① Design issues that should be considered when using power modules with hot-swappable functions to form a 48V distributed power structure; ② Using hot-swappable controller circuits to solve safety issues in hot-swappable operation of multiple circuit boards or blades.

2 Design rules for hot-swap power modules (such as IAM type) to form a 48V distributed power structure
The hot-swap function is particularly important to ensure the safety of hot-swap components. In addition, during the hot-swap process, the hot-swap function must avoid significant fluctuations in the input and output power line voltages. Any significant, even momentary fluctuations in the bus voltage may cause the system to malfunction. In commonly used connectors, the various connectors are not connected or disconnected at the same time, but are connected or disconnected one by one in a regular manner. Therefore, it is necessary to ensure that the power supply is connected or disconnected in sequence in steps.
In order to meet the above requirements, the design rules of the hot-swappable power supply module (such as IAM type) that forms a 48V distributed power supply structure should pay attention to the following points:
① When hot-plugging, various parameters must not exceed the limit value or absolute maximum rating of each component;
② When hot-plugging the power module (IAM), the surge current must be limited to an acceptable value to avoid interruption or drop of the 48V input bus voltage, and at the same time reduce the sparks generated between the contacts;
③ When hot-plugging the DC-DC converter, the load current of the converter must be limited to the rated value to ensure that the output bus voltage Vout is stable and does not produce sudden changes that affect the regulation rate;
④ When the load is disconnected or the sampling line is not connected, the DC-DC converter is never allowed to transmit energy, that is, a power mutation occurs.

[page]2.1 Functions that a live plug-in module should have
In order to implement the above design rules, a simple protection and sequencing circuit should be added to each live plug-in module (IAM), so that when the power module is unplugged, the DC-DC converter is turned off before the load is cut off or the sampling line is interrupted. At the same time, when the power module (IAM) is plugged in with power, the DC-DC converter is temporarily kept in the off state before all its input and output terminals are connected. Otherwise, when plugging and unplugging with power, the connector contacts are irregularly powered on and off, which may damage the DC-DC converter module or even the entire system.

2.2 Some current sharing control circuits should be added to the power module
As we all know, the first and most important requirement for fault tolerance is redundancy, that is, in the power system, there should be at least one additional converter or a redundant converter. This system is usually called an N+M configuration, in which N converters can meet the power required by the load and M converter modules are used as backup. During the application process, when a converter module is turned off or fails, although the load current of each module suddenly increases, the other modules can still ensure that the system output power is not affected. Similarly, when an additional converter module is connected to the power system, although the load current of each power module suddenly decreases, the output power of the system is not affected. For this reason, each converter should have current sharing capability, and the dynamic response time required for each module to restore normal power supply should be minimized. Therefore, in order to automatically share the load, some current sharing control circuits should be added to the power module. The operating temperature of the power module has a great influence on reliability. The mean time between failures can be doubled for every 10℃ reduction in operating temperature. Practice has shown that in a power supply system, when the output current of one module is twice that of another module, the temperature rise of the power supply module will double.

3 48V distributed power solution using the second-generation DC-DC converter and hot-swappable power module (IAM48)

3.1 Second Generation DC-DC Converter Module Characteristics

The second-generation converter module used today is Vicor, which has some excellent features that greatly simplify its application in parallel redundant systems. The important features of the second-generation converter module include: enable and shutdown, unique master-slave current sharing control and autonomous command function, in which one converter module is always in the master position in the entire system; it also has some common features, such as undervoltage lockout, soft start, output current limiting and remote sampling. In particular, the Vicor converter uses zero-current resonant switching. That is, by controlling the switching frequency and the energy pulse rate transmitted from the primary to the secondary of the isolation converter, the required power regulation rate and load regulation rate can be achieved. At any given input voltage, the pulse width is constant, so the energy of each pulse is also constant. While maintaining the output voltage stable, in order to meet the load current requirements, the pulse repetition rate (i.e., switching frequency) can be controlled. Therefore, if the switching frequencies of each module are completely synchronized, the same module can achieve automatic current sharing.
[page] As shown in Figure 1, the PR pin on the second-generation DC-DC converter module is a bidirectional port, which can be connected to the parallel current sharing bus. This port can receive or transmit synchronization pulse signals, and can control the converter module to transmit synchronous operation. All other modules receive synchronization pulses to ensure that all modules work at the same frequency. The PC (primary control) pin is also a bidirectional port. This port is used as a module status output. During the operation of the converter, the DC voltage of this pin is 6Vdc. Under fault conditions, such as overheating or output overvoltage, the PC pin will become a low level (the voltage of the negative input pin -Vin is close to 0V). If the fault continues to exist, the PC pin will periodically turn to a high level and try to restart the converter module. Only after the fault condition is eliminated can the PC pin remain at a high level. The PC pin can also be used as an enable/shutdown input pin. If the PC pin is connected to a low level, the converter is shut down. When the PC pin is maintained at a low level, the output current is close to 2mA. In order to complete the enable/shutdown function, an open collector or drain transistor switch can be used (see Q3 shown in Figure 1).
The sampling pin S is used to improve the stability accuracy of the output terminal power bus voltage. Usually, the load of the power supply system is connected to the output terminal power bus. The terminal sampling closes the adjustment control loop and adjusts the converter output voltage Vout to compensate for the voltage drop on the output bus Vout (on the left end of Figure 1). The sampling pin terminal connection is necessary to maintain output voltage control. In the fault-tolerant parallel redundant system, a diode must be connected in series from the output terminal Vout of each converter module to the power bus (on the left end of Figure 1). The total output current is the sum of the DC-DC converters at the common cathode of the diode on the output bus. In this way, when any module has any fault state including output short circuit, the bus and power system can be ensured to work reliably. When the output voltage of the module decreases, the series diode withstands the reverse voltage, so the power bus and the converter can be simply isolated. The sampling line of each module must be connected in front of the series diode, and it is best to be connected in front of the hot-swappable plug to ensure that there will be no instantaneous open circuit in the converter control circuit during the plug-in and unplug process of the power module. The optimal resistance value of this resistor is 24Ω/V, that is, the resistance value of this resistor is determined by the output voltage. For example, when the output voltage is 5V, it is best to use a 120Ω resistor.
In short, a power module with hot-swap function should have the following characteristics: the power module should be turned off before unplugging; when plugging in, the power module should be temporarily turned off; the power module should be able to limit the inrush current.

3.2 Application of IAM48 Module
The IAM48 input power adjustment module Vin is 36V-76V, 10A, and Vout is +75V～-75V, with an efficiency of 97%.
The IAM48 module contains a series FET switch that can realize the on-off control of the 48V bus to the DC-DC converter input. The on/off control pin on/off (see the pin of the IAM48 module in Figure 1) has an internal pull-up circuit, and in order to connect the 48V bus to the DC-DC converter module, the on-off controller must be pulled to a low level. There is also a parallel switch between the two output terminals (+Vout and -Vout) in the module. When the on/off control pin is high (disconnected) to the negative pole of the 48V bus, the parallel switch is in the conducting state. When the 48V bus is turned off, the holding capacitor on the bus can be quickly discharged through the parallel switch. In addition to the on-off control function, the IAM48 module also has the function of limiting surge current, and in conjunction with the Filt Mod module or EMI filter module, it can also provide transient overvoltage protection. In order to meet EMC (electromagnetic compatibility) standards, communication equipment usually uses IAM48 and Filt Mod modules (see the connection between the Filt Mod module and the IAM48 module in Figure 1). In communication equipment, the power module is required to have hot-swap function, so the lAM48 power module or other modules that can limit surge current should be selected.

3.3 Filt Mod Module Features
The VI-IAM (Filt Mod Module) input attenuation module is a component-level DC input front-end filter. It takes up very little space while providing maximum protection, making it suitable for precision electronic systems. The VI-IAM can be used with Vicor's 24V, 48V or 300V input modules to form a high-efficiency, high-power density power supply system. The system's output voltage ranges from 1V to 95V, with a power of 400W (expandable to 800W). The VI-IAM can be used to form a compact, efficient and reliable power supply system that meets the highest requirements of telecommunications and industrial applications.

3.4 Startup sequence when the power module is plugged into the power bus
First, except for the short pin, all the pins of the connector are connected in an irregular order, and the converter cannot be started. Because the on-off control short pin is not connected, this pin keeps the IAM48 module in the off state through transistor Ql. At the same time, transistor Q3 also pulls the PC pin of the DC-DC converter module to a low level, so the converter module is in the off state. After all other pins are connected, the short pin is connected. The on-off pin of the IAM48 module is pulled to a low level, so the IAM48 module is turned on, the capacitor on the 48V power bus begins to charge at a controllable rate, and the bus voltage begins to rise along the ramp, which can limit the inrush current to a safe value. After the 1AM48 module is turned on, the DC-DC converter module receives an enable signal, but before the bus voltage reaches the undervoltage lockout threshold (about 34V), the DC-DC converter module cannot be started. After the bus voltage reaches the undervoltage lockout value, due to the soft start characteristics of the DC-DC converter module, it will take at least 100ms before the converter module starts to absorb current and the output voltage starts to gradually rise. Finally, when the output voltage of the converter module rises to the point where the diode connected in series at the output end is forward biased, the converter module outputs a balanced load current.
[page] The working order of the power module IAM48 when it is unplugged from the bus is roughly the opposite of the order when it is inserted. The short pin is disconnected before the other pins of the lAM48 module turn off the 48V power supply, and at the same time, the converter module is turned off. The bus capacitor is rapidly discharged through the parallel switch at the output end of the IAM48 module, and the discharge time is less than 50ms. At this time, capacitor C2 continues to provide the current required to keep transistor Q3 conducting. This ensures that the PC pin remains at a low level until the 48V bus voltage drops to the undervoltage lockout value. In this way, it can be ensured that the DC-DC converter module does not generate power conversion pulses during the irregular disconnection of all other contacts.
The above hot-swap technology has been successfully applied to many products, and during the plug-in and pull-out process, the input and output bus voltages fluctuate very little. During the plug-in and pull-out process, it should be ensured that the pin voltage of all modules does not exceed the maximum rated voltage. When inserting the power module IAM, the short pin must be disconnected after the other pins are completely disconnected.

4 Use hot-swap controller circuit to solve the safety problem of hot-swap operation of multiple circuit boards or blades
Although the power module with hot-swap function (such as IAM type) can form a 48V distributed power structure, how to ensure the safety during hot-swap operation is a very important control technology. Therefore, the hot-swap controller circuit technology for -48V or +48V suitable for high-power blades was put on the agenda for discussion.

4.1 Basic Architecture of Hot Swap Controller Circuit
When a blade is plugged into the backplane, all the capacitors on the blade connected to the backplane begin to charge, drawing a large amount of current from the backplane. The inrush current causes the backplane voltage to drop momentarily and creates arcs on the connector. Excessive inrush current can overload the backplane power supply, shutting down the power supply completely and affecting the operation of the remaining blades in the rack.
In order to minimize the impact of hot-swapping of the circuit board on the remaining blades in the rack, the inrush current of the blade needs to be limited during hot-swapping. The circuit that limits the inrush current is called a "hot-swap controller circuit". Figure 2 shows the basic architecture of the hot-swap controller circuit implemented in a high-power blade -48V.

Starting from the top left of Figure 2, the GND terminal sends power to the DC/DC converter through a Schottky diode. The DC/DC module is an independent power supply that generates the payload supply voltage (12V, 5.6V, etc.). The negative terminal of the DC-DC converter is connected to the -48V power supply through a MOSFET switch and a current sensing resistor. The isolation capacitors across the DC/DC converter retain enough charge to ensure that the board remains operational during a backplane voltage drop. The hot-swap controller monitors the MOSFET current and voltage using the current sensing resistor Rsense and the VMOSFET signal to control the power dissipated by the MOSFET during a surge.

4.2 Safety Operation of Hot Swap Controller Circuitry
When a card is inserted into the backplane, a brief inrush current pulse (typically a few milliseconds) is seen due to the parasitic capacitance of the MOSFET. In addition, power is applied to the blade in a pulsed manner due to the contact bounce of the connector. The hot swap controller keeps the MOSFET and DC/DC converter off until the contact bounce stops. The MOSFET is then slowly turned on using the voltage across R sense as a feedback voltage. This is done to limit the inrush current value to below the maximum given value of the blade power current. This current will charge the isolation capacitor until the voltage at the VMOSFET pin approaches -48V. At this point the DC/DC converter is turned on to power the payload portion of the blade. The
isolation capacitor serves to keep the board operational when the backplane voltage drops due to the insertion of another card. The size of the isolation capacitor is directly proportional to the total power consumed by the blade and the need to prevent brownouts. When the pulse width of the brownout condition exceeds a preset time limit, it is classified as a "supply brownout" condition and the undervoltage lockout process begins. The undervoltage lockout process turns off the MOSFET until the backplane voltage returns to normal. In the event of an undervoltage condition, a Schottky diode in series with GND blocks reverse current from the isolation capacitor from flowing into the backplane. The hot-swap controller can also detect power faults such as undervoltage and overcurrent. In both cases, the hot-swap controller will re-power the blade after the fault is removed. ■