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CPUs need heatsinks to keep at a safe temperature, but VRMs, one of the supporting components, they are not paid attention they deserve. Your VRM, whether single-stage or multi-stage, will likely need a heatsink to stay within the safe operating temperature range.
In a VRM consist of discrete components, if you decide to use a VRM heatsink for temperature regulation, you'll run into some difficulties. In a VRM ASIC, the design of the chip itself determines whether a VRM heat sink is required.
A VRM is usually implemented as a buck converter, so it uses a PWM signal with a low duty cycle to switch the power MOSFETs. The output voltage is regulated between the two levels with a large capacitor. A single-stage VRM contains two MOSFETs, while a multi-level VRM contains two MOSFETs per stage. The MOSFETs in each stage are switched continuously, which then modulates the regulation level in the LC circuit at the output of the VRM.
If you look at a typical power MOSFET, you will see a large metal pad on the back of the component. This is the heat sink for the MOSFET. I've seen some designers attach custom heatsinks directly to these pads to dissipate the heat.
Once it is placed on the board, the power of MOSFET will slope down, which means the MOSFET heat sink is directly against the board. Typically, these boards are soldered to copper heatsink, which allow heat to flow through the heatsink to the back of the board. Most of the heat will gather on the pads, and having the MOSFET pads against the board means the board itself acts like a heat sink. If the computer motherboard is exposed and you find the VRM MOSFET, try powering on the computer and wait for the system to heat up. You will notice that the back of the board is very hot.
The top surface of each MOSFET is insulated, so very little heat flows off the board, so no additional heatsinks are required on top of these components. Some manufacturers still add heatsinks to the surface of components for decoration. Some modern VRMs are designed so that the thermal pad of the MOSFET is on the top surface, which is mostly for GPUs. Heatsinks are essential on these servers, as these VRMs don't use the motherboard as a heatsink.
Where to put the extra VRM heatsink
If an extra heatsink is really needed, where should it be placed? If the MOSFET pads are placed against the board and the backside of the board reaches an unacceptable temperature, you will
need to place the heat sink on the backside of the board. The most widely used heatsink for VRM is black anodized aluminum extruded heatsink.
Black anodized aluminum extruded VRM heatsink is the most popular heatsink type for VRM, the heatsink body is made of aluminum alloy 6063-T5, the process is extruding the aluminum ingot from the die, then cut the length as required, if the heatsink has some specific features, usually we use CNC to machine it. For aluminum extruded heatsink, the surface finish is often black anodizing, which can provide the anti-corrosion and extend the service life.
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Black Anodized Aluminum Extruded VRM Radiator Images