This can be achieved by operating the power modules in parallel.
The following objectives can be achieved by operating DC regulated power supplies in parallel:
Expanded capacity: Enables high-power power supply systems.
Redundancy and fault tolerance: Fault tolerance is achieved through N+1 and N+2 redundancy, and hot-swappable functionality facilitates maintenance of the power system without affecting normal system operation, ensuring uninterrupted power supply.
After N+m (where m represents the power system redundancy) power modules are connected in parallel for expansion, the source voltage effect, load effect, transient response and other technical indicators of the total power system should all remain within the range required by the system.
Each DC regulated power supply module has an automatic output current sharing function.
Redundancy technology is employed so that when a power module fails, the normal operation of the entire power system is not affected, and the power system should have sufficient load capacity.
To ensure high reliability of the power system, the internal circuit structure of the power module unit should be kept as unchanged as possible.
The bandwidth of the common current sharing bus should be small to reduce power system noise.
Ensure that each power supply unit shares the load current. That is, by using parallel current sharing, the entire power supply system should work as a whole, and the performance of the entire power supply system should be optimized through parallel current sharing technology.
Current feedback is used to adjust the output impedance of the power module unit to achieve current sharing. When the output current of the power module increases, feedback adjustment increases the slope of the output voltage drop, i.e., increases the output internal resistance, thereby reducing the output current of the module and achieving current sharing. This method is simple to implement, but the current sharing accuracy is relatively low, and the power supply output voltage will fluctuate significantly with changes in load current.
In a parallel-operated power module unit, one unit is selected as the master power module, and the remaining modules act as slave power modules. The master module operates as a voltage source, while the slave modules operate as current sources, with their current values independently adjustable. The master module controls the output voltage of the entire system, and the slave modules adjust their output current according to the master module's instructions to achieve current sharing. However, in this approach, if the master module fails, the entire system collapses, clearly lacking redundancy. Therefore, it is rarely used in applications with high reliability requirements.
This method does not require an external current sharing controller; instead, a common current sharing bus (CSB) is connected between each power module unit. The voltage Ub of the current sharing bus is the average of the voltage signals Ui representing the output current of each of the N power modules (i.e., representing the average current of the power system). Ub is compared with the sampled voltage signal of each power module, and an error voltage is output through an adjusting amplifier to adjust the output current of the module unit, thereby achieving current sharing. The average current method can accurately achieve current sharing, but when the common bus (CSB) is short-circuited or any power module unit connected to the bus fails, the CSB voltage will drop, causing the output voltage of each power module to decrease, even reaching the lower limit, leading to a power system failure. Therefore, the reliability of the current sharing bus is critical.
An external current sharing controller is used to compare the current of all modules and adjust the corresponding feedback signal to achieve current sharing. The current sharing controller collects the output current of each power module in real time, calculates the current value that each module should output through an internal algorithm, and feeds back the adjustment signal to each module. This control method is effective, but it requires an external current sharing controller and additional wiring, increasing system cost and complexity, and may also affect system reliability.
Current sharing is achieved by monitoring the temperature of each power module unit in the power supply system, resulting in higher-temperature modules having lower output current and lower-temperature modules having higher output current. This is because higher output current in a power module leads to higher power consumption and consequently higher temperature. This method can achieve a more uniform distribution of thermal stress among the modules, extending their lifespan. However, this method has a slow response time, and the accuracy and stability of temperature detection can affect the current sharing effect.
This method employs a maximum value comparator. The module with the largest output current at any given time acts as the master module, and its output current is converted into a voltage signal Ui, which is sent to the current sharing bus CSB. The voltage Ub on CSB reflects the maximum value of Ui in each power module unit, i.e., the maximum current. The Ui and Ub of each slave module are compared to automatically adjust the output current to achieve current sharing. The UC3907 is a current sharing control chip that uses this working principle, and such current sharing chips are currently widely used. It enables parallel-operating power module units to operate at the set current value, achieving a high level of current sharing accuracy.
The UC3907 current sharing control chip enables parallel power module units to operate at a set current value, with a current sharing accuracy of up to 2.5%. Its internal block diagram includes multiple functional modules working collaboratively. Its working principle is as follows: The UC3907 detects the output current of the corresponding power module unit. The output current signal of each power module unit is amplified and fed back to the current sharing bus CSB. The output current of each unit module is adjusted according to the maximum current sharing control principle, thereby achieving the purpose of current sharing.
The pin functions of UC3907 are briefly described below:
Pin 4: System ground. This is a high-impedance pin used to monitor system ground by measuring the voltage drop on the power return line.
Pin 6: Dummy ground, this is a low impedance terminal, 250mV higher than pin 4.
Pin 5: Power return terminal, the most negative terminal, should be connected as close as possible to the power supply.
Pin 7: Vref terminal, internal reference voltage is 2V relative to pin 4 and 1.75V relative to pin 6.
Pin 11: The inverting input of the voltage amplifier. The load voltage feedback signal (around 2V) is led to this pin for comparison with the non-inverting input signal. In practical applications, a compensation capacitor should be added between pins 11 and 12.
Pins 8, 9, and 12 form a buffer amplifier with a fixed gain of 2.5. Pin 8 is connected to a current-setting resistor. For the master module, this pin outputs a high voltage (2.5V - 3.5V), and current flows into pin 9 (up to 10mA). For the slave module, the voltage at pin 8 is close to zero volts, and the current at pin 9 is also zero.
Pins 1, 2, and 3: These form a current amplifier, sampling the output current signal from the parallel power supply module and amplifying it by 20 times.
1. Pin 15: Forms the maximum value comparator driver, used to drive the current sharing bus CSB.
Pins 13, 14, and 15: Compare the output current of each module unit with the output current of the main module, and use the output value to adjust the given reference voltage.
Pin 16: Status indicator, an open collector output terminal used to indicate the main power module. When it is low, it indicates that the module is the main module.
Pin 10: Ucc power supply terminal, supply voltage range is 4.5V - 35V.
In the offline load current sharing circuit using UC3907, UC3844 is the power controller, with a switching frequency Fs = 1.72/Rt・Ct. Resistor R5 is used to detect the primary-side inductor current; the maximum peak current of UC3844 is determined by Ismax = 1.0V/R5. R1 and C5 form the UC3844 startup circuit, while D3, R2, and C4 form an RCD snubber circuit to protect the power MOSFET. The UC3844 soft-start circuit consists of Q1, R9, and C10. Note that resistor Rset and the adjustment compensation in the circuit are connected to the dummy ground terminal (pin 6). The "dummy" ground (pin 6) is a mapping of the voltage to the "real" ground (pin 4), which is the negative detection terminal voltage plus a potential bias of 0.25V. The connection to ground-related components is a low-impedance terminal. The main control indicator circuit is used to indicate the current of the module unit with the largest load current and can detect the output voltage. It can be used to detect power module units with overcurrent/overvoltage.
The DC-DC non-isolated converter using UC3907 is widely used in parallel current sharing systems. In parallel current sharing systems employing UC3524A and UC3907 buck PWM converters, for non-isolated parallel power supply current sharing systems, the current sensing resistor cannot be connected to the ground terminal of the power module, but only to the non-ground terminal of the power output. Otherwise, the current sensing resistors of several parallel power supply systems will be connected in parallel, causing the system to fail to correctly share current, and even if there is a faulty module unit, it will be difficult to detect. The only limitation of this connection is that the current amplifier of UC3907 has a common-mode voltage range of 0 to -2V, so some form of level bias or average current sensing is required. Since an optocoupler is not needed, the circuit is simplified. The error amplifier of UC3524A is an inverting amplifier, and no driver amplifier is used to ensure that the phase of the signal sent from UC3907 to UC3524A meets the requirements. The voltage range of Iset (pin 8) is 0 to 3.8V. By outputting a signal from the current amplifier of the UC3907 and sending it to the UC3524A, the output current of the power module can be regulated, thereby achieving the purpose of current sharing.
In practical applications, the UC3907 exhibits the best parallel current sharing performance and is the most widely used. Different current sharing methods and application circuits are suitable for different occasions. When designing high-power power supply systems, it is necessary to comprehensively consider factors such as system performance requirements, cost, and reliability, and select appropriate current sharing schemes and chips to achieve efficient and stable power supply.
