Various techniques for switching power supplies to reduce losses

Various techniques for switching power supplies to reduce losses

The efficiency of PWM switching power supply without special loss control technology is lower than that of switching power supply with loss control technology. For switching power supplies that have no problem with heat dissipation, such as some off-line power supplies, the efficiency may be satisfactory. But for portable power supplies and devices requiring small size, greater efficiency must be sought. To improve the overall efficiency of the power supply, several techniques can be used.

The main loss types of switching power supply are on-off and on-off loss. Choosing better power switch or rectifier with lower on-voltage can reduce on-loss. The use of synchronous rectifier can reduce the on-off loss of the rectifier, but it can only be used in the forward circuit topology and does not include the intermittent mode boost converter. Synchronous rectifiers increase the efficiency of the power supply by 1% to 6%, depending on the power supply's average operating duty cycle. Further improvements will require other techniques.

When the input or output voltage inside the switching power supply is high (greater than DC20V), the switching loss will be larger, and the forward voltage drop of the rectifier tube is small relative to the input and output voltage. Switching instantaneous voltage and current and product are proportional to the voltage and current.

Switching losses occur mainly at two equivalent nodes of the switching power supply: the drain (or collector) of the power switch and the anode of the output rectifier tube. They are the main AC nodes inside the switching power supply. In a non-transformer isolated topology, the collector (or drain) of the power switch is directly connected to the anode of the output rectifier, and thus has only one node. In the transformer isolation topology, the two nodes are separated by a transformer, and they are treated slightly differently.

During the switching period, the voltage and current values are very large, and the voltage and current peak appears at the same time, which makes the loss greater. There are four goals to be achieved with these two nodes:

1. Reduce the voltage and current flowing at all on-off and off-off moments.

2. Minimize the reflection recovery effect of all PN rectifier tubes.

3. Eliminate all spikes produced by parasitic components.

4. Return as much of this "lost" energy to the power stream as possible.

The designers may not achieve all of these goals, but improving these conditions could improve the overall efficiency of the power supply by another 3-9%.Another consideration when modifying these circuits is to limit the bandwidth of the waveform as much as possible to reduce EMI. Most EMI energy is generated during the switching process of the power supply and radiates around it. Generally, EMI performance can be greatly improved by adding a small inductor to the current branch of the energy return power supply.

For this purpose, additional resistive elements and diodes or MOSDET are often used to control this action. There are three main types of improvements to the standard PEM topology:

1. Lossless absorption circuit.

2. Active clamp circuit.

3. Quasi-resonant improved circuit. Lossless absorption and active clamp circuits give the PWM waveform a "soft" edge.

For power-switched AC nodes, there is a delay in the voltage at the moment of switching off, which provides the output rectifier with a progressive loading process of the magnetic element during the forward recovery. For the output rectifier AC node, it is hoped that the current will be delayed during the shutdown, which limits the reflected current spike caused by the rectifier during reverse recovery.