Common Problems and Solutions of Switching Power Supply Commissioning

Switching power supply, also known as switching power supply and switching converter, is a high-frequency electric energy conversion device and a kind of power supply. Its function is to convert a standard voltage into the voltage or current required by the user through different forms of architecture. The input of switching power supply is mostly AC power supply (such as mains power) or DC power supply, while the output is mostly equipment requiring DC power supply, such as personal computer, and switching power supply converts the voltage and current between the two. Let me introduce some problems and solutions that will be encountered in the commissioning of switching power supply.

1. Transformer saturation

When starting up under high voltage or low voltage input (including light load, heavy load, capacitive load), output short circuit, dynamic load, high temperature, etc., the current through the transformer (and switch tube) increases nonlinearly. When this phenomenon occurs, the peak value of the current cannot be predicted and controlled, which may lead to current stress and resulting switch tube overvoltage and damage.

Situations prone to saturation:

1) Transformer inductance is too large;

2) Too few turns;

3) The saturation current point of transformer is smaller than the maximum current limiting point of IC;

4) No soft start.

terms of settlement:

1) Reduce the current limiting point of IC;

2) Strengthen the soft start to make the current envelope through the transformer rise more slowly.

2. VDS too high

Stress requirements for VDS:

Under the worst conditions (maximum input voltage, maximum load, maximum ambient temperature, power start or short circuit test), the maximum value of VDS shall not exceed 90% of the rated specification.

Methods to reduce VDS:

1) Reduce the platform voltage: reduce the ratio of primary and secondary side turns of the transformer;

2) Reduce peak voltage:

a. Reduce the leakage inductance. The main reason for this peak voltage is that the transformer leakage inductance stores energy when the switch is turned on. Reducing the leakage inductance can reduce the peak voltage;

b. Adjust the absorption circuit:

â‘  Use TVs tube;

â‘¡ Using a slower diode, it can absorb a certain amount of energy (peak);

â‘¢ Inserting damping resistance can make the waveform smoother and reduce EMI.

3. IC temperature too high

Causes and solutions:

1) Internal MOSFET loss is too large:

The switching loss is too large and the parasitic capacitance of the transformer is too large, resulting in a large cross area between the on and off current of MOSFET and VDS. Solution: increase the distance between the transformer windings to reduce the interlayer capacitance. Just as when the windings are wound in multiple layers, add a layer of insulating tape (interlayer insulation) between the layers.

2) Poor heat dissipation:

A large part of the heat of IC depends on the pins to lead to the PCB and its copper foil. Try to increase the area of copper foil and solder more

3) Air temperature around IC is too high:

The IC shall be in a place with smooth air flow and shall be far away from parts with too high temperature.

4. No load and light load cannot be started

Phenomenon:

No load and light load cannot be started, and VCC repeatedly jumps back and forth from the starting voltage and shutdown voltage.

reason:

Under no-load and light load, the induced voltage of VCC winding is too low and enters the repeated restart state.

terms of settlement:

Increase the number of VCC winding turns, reduce the VCC current limiting resistance, and add false load appropriately. If VCC becomes too high under heavy load after increasing the number of VCC windings and reducing VCC current limiting resistance, please refer to the method of stabilizing VCC.

5. Overload is not allowed after startup

Causes and solutions:

1) VCC is too high during heavy load

Under heavy load, the induced voltage of VCC winding is high, which makes VCC too high and reaches the OVP point of IC, which will trigger the overvoltage protection of IC and cause no output. If the voltage rises further and exceeds the bearing capacity of the IC, the IC will be damaged.

2) Internal current limit triggered

a. Current limiting point too low

Under heavy load and capacitive load, 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 improve the current limiting point.

b. Current rise slope too large

If the rising slope is too large, the peak value of current will be larger, which is easy to trigger internal current limiting protection. The solution is to increase the inductance without saturating the transformer.

6. High standby input power

Phenomenon:

VCC is insufficient under no-load and light load. This situation will cause too high input power and too large output ripple under no-load and light load.

reason:

The reason why the input power is too high is that when the VCC is insufficient, the IC enters the repeated startup state and frequently needs high voltage to charge the VCC capacitor, resulting in the loss of the starting circuit. If there is a resistor in series between the starting pin and the high voltage, the power consumption on the resistor will be large, so the power level of the starting resistor should be sufficient. The power IC does not enter burst mode or has entered burst mode, but the burst frequency is too high, the switching times are too many, and the switching loss is too large.

terms of settlement:

Adjust the feedback parameters to reduce the feedback speed.

7. Excessive short circuit power

Phenomenon:

When the output is short circuited, the input power is too large and VDS is too high.

reason:

When the output is short circuited, there are many repeated pulses, and the peak current of the switch is very large, resulting in too much input power. The excessive switch current stores too much energy on the leakage inductance, and the VDS is high when the switch is turned off. In case of output short circuit, there are two possible causes for switch tube to stop working:

1) Triggering OCP can stop the switch action immediately

a. Trigger OCP of feedback pin;

b. Switch action stops;

c. VCC drops to IC off voltage;

d. VCC rises again to the IC start voltage and restarts.

2) Trigger internal current limit

When this mode occurs, limit the available duty cycle and stop the switching action by reducing the VCC to the lower limit of UVLO. However, the VCC decreases for a long time, that is, the switching action lasts for a long time and the input power will be large.

a. Internal current limiting is triggered and the duty cycle is limited;

b. VCC drops to IC off voltage;

c. Switch action stops;

d. VCC rises again to the IC start voltage and restarts.

8. No load and light load output bounce

Phenomenon:

When no-load or light load is output, turn off the input voltage, and the output (e.g. 5V) may have a voltage rebound waveform as shown in the figure below.

Cause: when the input is turned off, the 5V output will drop, VCC will also drop, and the IC will stop working. However, when the PC power supply is under no-load or light load, the large capacitance voltage cannot drop rapidly, and it can still provide large current to the high-voltage starting pin, so that the IC will restart, and the 5V output will be renewed and bounce back.

resolvent:

When the starting pin is connected in series with a large current limiting resistor, so that the voltage of the large capacitor is still relatively high, it is not enough to provide sufficient starting current to the IC. Before connecting the start-up to the rectifier bridge, the start-up is not affected by the large capacitor voltage. When the input voltage is turned off, the starting pin voltage can drop rapidly. (some deleted above)

Common Problems and Solutions of Switching Power Supply Commissioning 1

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