2. Vds too high

Stress requirements of 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

Ways 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 leakage:

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

B. Adjust the absorption circuit:

① Use TVS pipe;

② Using a slower diode, it can absorb a certain amount of energy (spike);

③ 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 opening and closing current of the MOSFET and Vds. Solution: increase the distance between transformer windings to reduce the interlayer capacitance, just as a layer of insulating tape (interlayer insulation) is added between layers when the winding is wound in multiple layers.

2) Poor heat dissipation:

A large part of IC heat is transferred to PCB and copper foil on it by pins. The area of copper foil should be increased as much as possible and more solder should be applied

3) The air temperature around IC is too high:

IC should be in a place with smooth air flow, and should be away from parts with too high temperature.

4. Unable to start with no load or light load

Phenomenon:

No load and light load can not be started, and Vcc repeatedly bounces back and forth from the starting voltage and shutdown voltage.

reason:

During no-load and light load, the induced voltage of Vcc winding is too low, and it enters into repeated restart state.

terms of settlement:

Increase the number of coils of Vcc winding, reduce the current limiting resistance of Vcc, and properly add false load. If the number of Vcc windings is increased and the current limiting resistance of Vcc is reduced, the Vcc becomes too high under heavy load, please refer to the method of stabilizing Vcc.

5. Cannot load after starting

Causes and solutions:

1) Vcc is too high under heavy load

In case of heavy load, the induced voltage of Vcc winding is high. When the Vcc is too high and reaches the OVP point of IC, the overvoltage protection of IC will be triggered, causing no output. If the voltage rises further, the IC will be damaged if it exceeds its bearing capacity.

2) Internal current limiting is triggered

A. Current limiting point is too low

Under heavy load and capacitive load, if the current limiting point is too low, the current flowing through the MOSFET will be 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. The current rise slope is too large

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

6. High standby input power

Phenomenon:

Vcc is insufficient at no load and light load. This situation will cause the input power to be too high and the output ripple to be too large at no load and light load.

reason:

The reason why the input power is too high is that when Vcc is insufficient, the IC enters into the repeated starting state, and the high voltage is frequently required to charge the Vcc capacitor, resulting in the loss of starting circuit. If there is a resistance between the starting pin and the high voltage, the power consumption on the resistance will be large, so the power level of the starting resistance should be sufficient.

The power IC has not entered 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:

In case of output short circuit, the input power is too large and Vds is too high.

reason:

When the output is short-circuited, there are many repetitive pulses, and the peak current of the switch tube is large, which causes the switch tube current with too large input power to store too much energy on the leakage inductance. When the switch tube is turned off, it causes high Vds.

There are two possible reasons for the switch tube to stop working in case of output short circuit:

1) Triggering OCP can stop the switch action immediately

A. Trigger OCP of feedback pin;

B. Switch action stops;

C. Vcc drops to IC closing voltage;

D. The Vcc rises to the IC starting voltage again and then restarts.

2) Trigger internal current limiting

When this mode occurs, limit the available duty cycle and stop the switch action depending on the Vcc falling to the lower limit of UVLO, while the Vcc falling time is longer, that is, the switch action is maintained for a longer time, and the input power will be larger.

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

B. Vcc drops to IC closing voltage;

C. Switch action stops;

D. The Vcc rises to the IC starting voltage again and then restarts.

terms of settlement:

1) Reduce the number of current pulses to trigger the OCP of the feedback pin when the output is short-circuited, which can quickly stop the switch action and reduce the number of current pulses. This means that when the short circuit occurs, the voltage of the feedback pin should rise faster. So the capacitance of the feedback pin should not be too large;

2) Reduce the peak current.

8. No load, light load output ripple is too large

Phenomenon:

Vcc is insufficient at no load or light load.

reason:

When Vcc is insufficient, the oscillating IC works intermittently between the starting voltage (such as 12V) and the turn-off voltage (such as 8V) for a long period of time, providing energy to the output for a short time, and then stopping working for a long time, which makes the energy stored by the capacitor insufficient to maintain the output stability, and the output voltage will drop.

resolvent:

Ensure that Vcc can supply stably under any load condition.

Phenomenon:

In Burst Mode, the frequency of intermittent operation is too low. This frequency is too low, and the energy of output capacitance cannot be maintained stable.

terms of settlement:

Under the condition of meeting the requirements of standby power consumption, slightly increase the frequency of intermittent operation and increase the output capacitance.

9. Heavy load and capacitive load cannot be started

Phenomenon:

Light load can be started, and heavy load can be added after starting, but it cannot be started under heavy load or large capacity load.

General design requirements:

Regardless of heavy load or capacitive load (such as 10000uF), the input voltage is the lowest or the lowest, and the output voltage must rise to a stable value within 20mS.

Causes and solutions (on the premise that Vcc is within the normal working range):

Let's take the capacitive load C=10000uF as an example for analysis,

According to the specification, there must be enough energy to make the output rise to a stable output voltage (such as 5V) within 20mS.

E=0.5*C*V^2

The larger the capacitance C, the greater the energy that needs to be transferred from input to output within 20mS.