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UC3902 Application Circuit

Warm hints: The word in this article is about 1627 and  reading time is about 12 minutes.  
Guidance: There are many parallel connection schemes for DC modules, but there are some drawbacks when they are used in power operation power supply: for example, the current sharing accuracy of output impedance method is too low; neither master-slave setting method nor average current method can realize redundancy technology, so that the reliability of parallel power supply module system can not be guaranteed well; and the independent current sharing method is based on its own characteristics. Performance, such as "high current sharing accuracy, good dynamic response, redundancy technology can be achieved", is increasingly favored by the majority of product developers.

According to the characteristics of the current sharing method, UC company has developed the integrated chip UC3902, which has eight pins in shape, pin number and function as shown in Figure 1. The module with the largest current is automatically identified as the main module, which drives the current-sharing bus voltage to be proportional to its output current. The voltage of current-sharing bus is used as the reference of slave module to achieve the goal of current-sharing for each module.

2UC3902 application circuit
2UC3902 application circuit

The peripheral circuit of UC3902 is shown in Figure 2. The chip requires only a few external components. Before the values of these components are calculated, the three parameters of the module converter must be known:

  • (1) VONOM is the rated output voltage;
  • (2) IOmax is the maximum output current.
  • (3) Maximum output voltage regulation range of VOmax.

For accurate current sharing between modules, the output current of each module must be detected. The current detection resistance RSENSE detects a negative signal input to the reverse end of the current detection amplifier. The selection of the detection resistance is based on the following two factors: the maximum power consumption and the maximum voltage drop through the detection resistance. Power consumption is limited by efficiency and rated power of devices. The maximum voltage drop must correspond to the signal limitation inside the chip. It is very important to prevent the saturation of the current detection amplifier. The maximum output voltage VCSAO of the amplifier is the function of VCC. According to the information provided by the chip and the experience of actual debugging, it is appropriate to take 5V-10V. The corresponding results can be obtained as follows:

(1) The gain ACSA = 40 of the current detection amplifier.

(2) IADJmax, according to experience, should work between 5 mA and 10 mA, because a lower value may cause the system to be sensitive to noise, but not more than 10 mA. Its actual current is determined by the possible maximum voltage (2.6V) on the ADJR pin and the resistance RG connected between the ADJR pin and the ground.

(3) RADJ is the impedance of the front end of the power supply detection line. Its value is a function of VOmax and IADJmax. Because the detection resistance reduces the adjustment range of the output voltage, so:

(4) The current sharing loop of all parallel units is a negative feedback control loop. In order to work reliably, the negative feedback control loop must obey the stability principle. The current sharing loop is added to the existing single module power supply, so the interference between the control loops must be avoided. In order to ensure the stability of the voltage loop, the crossover frequency of the current sharing loop is at least 10 times lower than that of the voltage loop, so that the current sharing loop is minimized at the crossover frequency of the voltage loop.

The following transfer functions can be used to analyze the network: APWR: transfer function of voltage loop.

AVo Vis: This gain term describes the relationship between the output voltage and the voltage on the detection resistance, which varies with the load impedance:

AVo Vis=

(5) ACSA: Current detection amplifier gain, size 40.

AEA = GM. XCOMP

(6) GM is a transconductor and XCOMP is a complex frequency function to compensate the impedance of the device.

AADJ: Adjust circuit gain.

(7) So the current sharing loop gain:

ASH = APWRAVo VisACSAASHAAEAAADJ (8)

ASHA: Current sharing driver and current sharing detection amplifier gain 1.

AEA: Gain of error amplifier.

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