Switching Power Supply Design Tips

The characteristic of switching power supplies is that they generate strong electromagnetic noise, which will cause great interference if not strictly controlled. The following technology can help reduce switching power supply noise and can be used for highly sensitive analog circuits.

1. Circuit and device selection

A key point is to keep dv/dt and di/dt at a low level. There are many circuits that reduce radiation by reducing dv/dt and/or di/dt, which also reduces the stress on the switch tube. These circuits include ZVS (zero voltage switching), ZCS (zero current switching), resonant mode (a type of ZCS), SEPIC (single-ended primary inductor converter), CK (a set of magnetic structures named after its inventor), etc.

Reducing the switching time does not necessarily lead to an increase in efficiency, because the RF oscillations of the magnetic components need to be buffered with strong losses, and eventually a weakening return path can be observed. Using soft switching technology, although it will slightly reduce efficiency, has greater benefits in terms of cost savings and space occupied by filtering/shielding.

2 Damping

In order to protect the switch tube from the impact of oscillation spike voltage caused by parasitic parameters and other factors, damping is often required, as shown in Figure 5. The damper is connected to the problematic coil, which can also reduce the emission.

There are several types of dampers: From an EMC perspective, RC dampers are usually the best in terms of EMC, but they generate more heat than the others. Weighing the pros and cons of each, inductive resistors should be used with caution in snubbers.

3 Radiator

There is 50pF capacitance between the heat sink and the collector or drain of a TO247 power device, so it can generate strong emissions. Simply connecting the heat sink directly to the chassis will only lead the noise to ground and will probably not reduce the overall emission level.

A better approach is to connect them to an appropriate circuit node - the primary rectifier output, but pay attention to safety requirements. Insulating spacers with shielding effects can be connected to the switch tubes, and their shielded inner layer is connected to the primary rectifier end. The heat sink is either suspended or connected to the casing.

Heat sinks can also be connected to lines with dangerous voltages through capacitors. The inductance formed by the capacitor leads and PCB tracks may "resonate" with the capacitor, which can be particularly effective in solving problems at certain special frequencies. You should test it many times on the prototype to find the best way to install the heat sink.

4 Rectifier

The rectifier and secondary rectifier used on the primary power supply can cause a lot of noise due to their reverse current, so it is best to use fast soft switching devices, as shown in Figure 6.

5. Problems and solutions related to magnetic components

It is particularly important to note that the magnetic circuit of the inductor and transformer must be closed. For example, a toroidal or seamless magnetic core is used. The toroidal iron powder core is suitable for storing magnetic energy. If there is a gap in the magnetic ring, a complete short-circuit ring is required to reduce the parasitic leakage magnetic field.

The primary switching noise will be injected into the secondary through the inter-turn capacitance of the isolation transformer, generating common-mode noise in the secondary. These noise currents are difficult to filter out, and since they flow through a long path, they will cause emission.

An effective technique is to connect the secondary ground to the primary power line with a small capacitor to provide a return path for these common-mode currents, but be careful not to exceed the total leakage current indicated by the safety standard. This capacitor also helps the secondary filter work better.

The inter-turn shielding (inside the isolation transformer) can more effectively suppress the primary switching noise induced on the secondary. Although more than five layers of shielding have been used, three layers of shielding are more common. The shield near the primary coil is usually connected to the primary power line, the shield near the secondary coil is often connected to the common output ground (if any), and the middle shield is usually connected to the chassis. It is best to repeatedly experiment during the prototype stage to find the best way to connect the inter-turn shielding.

The above two techniques can also reduce the secondary switching noise induced on the input. The properly sized output inductor can transform the secondary AC waveform into a half-sine wave (upper part of Figure 7), thus significantly reducing the transformer inter-winding noise (DC ripple)