Analysis of grounding methods for heat sinks in switching power supplies

There are mainly the following grounds: primary ground, secondary ground. Their function is to provide their own reference zero point. There is also a real earth PE. It has a protective function and also has an EMC function.

The original border area and the secondary border area are strictly isolated.

No electrical connections may be made.

They can all be connected to the earth PE, but Y2 must be added.

All heat sinks can be placed in the system in four ways:

1.Floating, which means floating on the ground.

2. The respective reference zero point, such as the primary ground or the secondary ground

3. The real earth, that is, PE, the yellow-green line when input

4. There is a special case where the main MOSFET is connected to the drain, which is the high voltage end after rectification. There is also one connected to the source, which is the S end.

If it is not handled properly, the machine will explode. I have experienced it before. The heat sink of the main MOSFET of the power supply hits the outer shell, which is metal and connected to PE.

In addition, when these components are attached to the heat sink, there is a small capacitance coupling, which generates common-mode noise.

If not handled properly, it will just be a big antenna.

At this time, add Faraday shield.

There is a trench between the analog ground and the digital ground, which makes it easy to divide them.

There can't be any traces in between, but you can put something across, like a 0 ohm resistor, a small SMD bead, or even a Y capacitor.

My understanding comes from Motorola's article on electromagnetic compatibility, which talks about the division of the ground.

That is, PE is the protective ground or functional ground, and all other grounds plus Y capacitors can be connected to this.

All functional modules, digital belongs to the digital part, analog belongs to the analog part.

Whether numbers need to be isolated depends on the system requirements.

Similarly, the difference between simulations also depends on the system requirements. They can be shared or separated.

For the entire system, the power supply should be distributed first, and then the ground should be divided.

The power supply of many systems comes from isolated DCDC, which is 12V, ±5. As for 3.3 and 1.8, many of them are provided by LDO.

There is no specific requirement for grounding the heat sink. You can do whatever you want. But please note:

1. If connected to the ground, then these components need to be equipped with gaskets and thermal paste. The surrounding components must maintain sufficient creepage distance from PE. The heat sink also has stress testing.

2. If you connect to the ground, pay attention to the creepage distance with PE. I had a machine exploded before because I connected to a certain ground, but after the shell was covered, the shell accidentally touched the heat sink. But the shell was connected to PE. The result was a terrible explosion. The heat sink is conductive, even if it is painted, it will often wear off.

3. Floating land is often a good choice.