What are the difficulties in debugging switching power supplies?

Switching power supply is a type of power supply that utilizes modern power technology to control the transistor switching time ratio of the circuit and stabilize the output voltage by adjusting the pulse width. (PWM) consists of an operation (MOSFET). The development trend of contemporary power electronics technology is to learn from traditional low-frequency technology electronics to modern power electronics. But in the application of power electronics technology and various electrical systems, switching power supply technology is at the core. The following is a brief analysis of the issue of switching power supply regulation for your reference.

1. Transformer saturation

Starting under high or low voltage input (including light load, light load, capacitive load), resulting in short circuit, dynamic load, high temperature, and nonlinear growth of transformer (and switch tube) current. In this phenomenon, the peak current cannot be predicted and controlled, which may lead to current overstress and switch tube overvoltage damage.

Saturation phenomenon is prone to occur:

1) Transformer induction is too high;

2) Too few turns;

3) The saturation current point of the transformer is less than the main current limiting point of the IC;

4) There is no soft start.

Solution: 1) Reduce the current limiting point of the IC;

2) Strengthen soft start to slowly increase the current envelope of the transformer.

2. Vds too high

Stress requirements for VDS:

The maximum value of VDS shall not exceed 90% of the rated specification (maximum input voltage, high load, high operating temperature, power start or short circuit test).

Method for VDS reduction:

1) Reduce platform voltage: reduce the ratio of primary and secondary slopes of transformers;

2) Reduce peak voltage:

a. Reduce leakage, transformer leakage is an important reason for storing kinetic energy, and reducing leakage can reduce peak voltage;

b. Adjusting the absorption circuit:

① Application of TVS tubes;

② Using a slow diode itself can absorb a certain amount of energy (peak);

③ Inserting a damping resistor can make the waveform smoother and help reduce EMI.

3. IC temperature too high

Reason and solution:

1) Excessive internal MOSFET consumption:

The switching loss is too large, and the parasitic capacitance of the transformer is too large, resulting in a large cross range between the switching current of MOSFET and VDS. Solution: Increase the distance between transformer windings, reduce solid capacitance, such as double-layer windings, and add a layer of insulation tape (interlayer insulation) to the solid layer.

2) Poor heat removal:

With the help of pins, most of the heat in the IC leads to copper deposits on the PCB and above. The copper deposit area should be increased as much as possible, and soldering should be increased

3) Excessive air temperature around IC:

The IC should be located in an area with smooth air flow to avoid parts with excessively high temperatures.

4. Unable to start full and light loads

Condition: Unable to start full load and light load, VCC bounces repeatedly from start voltage to stop voltage.

Reason: The induced voltage of VCC winding is too low at full load and light load, leading to repeated restarts.

Solution: Increase the number of VCC winding turns, reduce the VCC current limiting resistance, and moderately increase the false load. If the number of VCC winding turns increases and VCC becomes too high under light load, please refer to the method of stabilizing VCC.

5. Cannot increase load during startup

Reason and solution:

1) VCC too high under light load

When the VCC winding resistance induction voltage is high, and the VC reaches the ICOVP point too high, the overvoltage protection of the IC will be turned on, resulting in no output. If the voltage rises further and exceeds the carrying capacity of the IC, the IC will be damaged.

2) Open internal current limiting

a. Current limiting point too low

When carrying light and capacitive loads, if the current limiting point is too low, the current flowing through the MOSFET is limited and insufficient, resulting in insufficient output. The solution is to increase the resistance of the current limiting pin and increase the current limiting point.

b. Excessive slope of current rise

If the rising slope is too large, the peak current will be greater, and internal current limiting maintenance is easy to open. The solution should increase induction while avoiding transformer saturation.

6. High power sleep input

Condition: VCC is insufficient at full and light loads. This situation can lead to high input power and excessive output ripple at full and light loads.

Reason: The reason for the high input power is that when the VC is insufficient, the IC enters repeated startup, and the VCC capacitor must be frequently charged with high voltage, resulting in the consumption of the startup circuit. If there is a resistor connected between the starting pin and the high voltage, the power consumption on the resistor will be greater, so the power level of the starting resistor is sufficient. The power IC did not enter Bursttt Mode or has entered Burstttte Mode, but the burst frequency is too high, the switching frequency is too high, and the switching loss is too large.

Solution: Adjust feedback parameters and reduce feedback rate.

7. Excessive short-circuit power

Condition: When exporting a short circuit, the input power is too high and the VDS is too high.

Reason: When exporting a short circuit, there are many repeated pulses, resulting in a large peak current in the switch tube, resulting in excessive input power. The switch tube current stores too much kinetic energy in the leakage sense, and the VDS is high when the switch tube is closed. When exporting a short circuit, the switch tube may stop running:

1) Opening OCP can immediately stop the switch action

a. Open the OCP of the feedback foot;

b. Terminate the switch action;

c. VCC drops to IC shutdown voltage;

d. The VCC rises again to the IC startup voltage and restarts.

2) Open internal current limiting

When this method occurs, the duty cycle is limited and the switch action is stopped with the help of VCC dropping to the lower limit of UVLO. However, VCC has a longer descent time, which means the switch action is held for a longer time and the input power is higher.

a. Open internal current limiting, with limited duty cycle;

b. VCC drops to IC shutdown voltage;

c. Terminate the switch action;

d. The VCC rises again to the IC startup voltage and restarts.

8. Full load and light load export rebound

Condition: When exporting full load or light load, turn off the input voltage, and the export (such as 5V) may show a voltage rebound waveform as shown in the figure.

Reason: When the input is turned off, the 5V output may decrease and the VCC may also decrease. The IC stops running, but when fully loaded or lightly loaded, the large capacitance voltage of the huge PC power supply cannot rapidly decrease, and can still provide greater current for the high-voltage starting pin, promoting IC restart, and 5V exports and rebounds again.

Solution: When the starting pin is connected to a large current limiting resistor and the voltage of the large capacitor is reduced to a relatively high level, it is not sufficient to provide sufficient starting current to the IC. Before receiving the rectifier bridge, the startup is not affected by the large capacitor voltage. When the input voltage is turned off, the starting pin voltage can quickly decrease.

The above is an introduction to issues related to switching power supply regulation, hoping to provide you with some reference!

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