A switching power supply, also known as a switching power supply or a switching converter, is a high-frequency power conversion device and a type of power supply. Its function is to convert a voltage level into the voltage or current required by the user through different forms of architecture.
The input of a switching power supply is mostly AC power (such as mains) or DC power, and the output is mostly a device that requires DC power, such as a personal computer, and the switching power supply performs voltage and current conversion between the two. The following are several problems encountered during switching power supply debugging and their solutions.
Transformer saturation phenomenon
When the power is turned on under high 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 the switch tube) increases nonlinearly. When this phenomenon occurs, the peak current cannot be predicted and controlled, which may cause current overstress and the resulting overvoltage of the switch tube and damage.
Situations that are prone to saturation:
1) The transformer inductance is too large;
2) The number of turns is too small;
3) The saturation current point of the transformer is smaller than the maximum current limit point of the IC;
4) There is no soft start.
Solution:
1) Reduce the current limit of IC;
2) Strengthen soft start to make the current envelope through the transformer rise more slowly.
Vds is too high
Vds stress requirements:
Under the worst conditions (highest input voltage, maximum load, highest ambient temperature, power supply startup or short circuit test), the maximum value of Vds should not exceed 90% of the rated specification.
Ways to reduce Vds:
1) Reduce the platform voltage: reduce the ratio of the number of turns of the primary and secondary sides of the transformer;
2) Reduce the peak voltage:
a. Reduce the leakage inductance. The leakage inductance of the transformer is the main reason for the generation of this peak voltage because it stores energy when the switch tube is turned on. Reducing the leakage inductance can reduce the peak voltage;
b. Adjust the absorption circuit:
① Use TVS tube;
② Use a slower diode, which can absorb a certain amount of energy (spike);
③ Inserting a damping resistor can make the waveform smoother and help reduce EMI.
IC temperature is too high
Reasons and solutions:
1) The 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 MOSFET's turn-on and turn-off currents and Vds. Solution: Increase the distance between the transformer windings to reduce the interlayer capacitance, just like when the winding is wound in multiple layers, add a layer of insulating tape between the layers (interlayer insulation).
2) Poor heat dissipation:
A large part of the heat of the IC is conducted to the PCB and the copper foil on it by the pins. The area of the copper foil should be increased as much as possible and more solder should be applied
3) The air temperature around the IC is too high:
The IC should be in a place where the air flow is smooth and should be away from parts with too high temperature.
No-load and light-load cannot start
Phenomenon: No-load and light-load cannot start, and Vcc repeatedly jumps back and forth from the start voltage and the turn-off voltage.
Reason: When no-load and light-load, the inductive voltage of the Vcc winding is too low, and it enters a repeated restart state.
Solution: Increase the number of Vcc winding turns, reduce the Vcc current limiting resistor, and add a dummy load appropriately. If the number of Vcc winding turns is increased and the Vcc current limiting resistor is reduced, Vcc becomes too high when overloaded. Please refer to the method of stabilizing Vcc.
Cannot add heavy load after startup
Reasons and solutions:
1) Vcc is too high when overloaded
When overloaded, the Vcc winding induction voltage is high. When Vcc is too high and reaches the OVP point of the IC, the overvoltage protection of the IC will be triggered, causing no output. If the voltage rises further and exceeds the IC's tolerance, the IC will be damaged.
2) Internal current limiting is triggered
a. The current limit point is too low
When overloaded or capacitively loaded, if the current limit point is too low, the current flowing through the MOSFET is limited and insufficient, resulting in insufficient output. The solution is to increase the current limit pin resistance and increase the current limit point.
b. The current rise slope is too large
If the rise slope is too large, the current peak will be larger, which is easy to trigger the internal current limit protection. The solution is to increase the inductance without saturating the transformer.
Standby input power is large
Phenomenon: Vcc is insufficient when no load or light load. This situation will cause the input power to be too high and the output ripple to be too large when no-load or light-load.
Reason: The reason for the excessive input power is that when Vcc is insufficient, the IC enters the repeated startup state, and high voltage is frequently required to charge the Vcc capacitor, causing the starting circuit loss.
If there is a resistor in series between the start pin and the high voltage, the power consumption of the resistor will be large at this time, so the power level of the startup resistor must be sufficient. The power IC has not entered Burst Mode or has entered Burst Mode, but the Burst frequency is too high, the number of switches is too many, and the switching loss is too large.
Solution: Adjust the feedback parameters to reduce the feedback speed.
Many engineers who have not debugged the switching power supply will have a certain fear of it, such as worrying about the interference problem of the switching power supply, various abnormal phenomena of the switching power supply, etc. In fact, as long as you understand it and eliminate the problem step by step, the debugging of the switching power supply is still very convenient.
