Implementing VRM Cooling in PCB Power Supply Design

January 13, 2021 Cadence PCB Solutions

Key Takeaways

  • Learn what the VRM is.

  • Understand why VRM cooling is important.

  • Explore ways to cool down the VRM.

The worst thing that could happen to a night of blissful sleep is an abrupt power breakdown—and that’s exactly what happened to me recently. The air conditioner shut down, the ceiling fan ground to a halt, and I was rudely woken up and drenched in sweat. There was nothing that I could do except hope and pray that the electricity would be promptly restored. 

It reminded me of how often we’ve taken cooling devices for granted, whether at home or when designing a PCB. If you happen to be working with the VRM, you’ll never want to miss out on the cooling part. That’s because things could turn awry and some parts could essentially melt down during operation, making VRM cooling essential in PCB power supply design.

What Is VRM? 

A CPU mounted on a motherboard

VRM provides clean and regulated voltage for the CPU.

If you’re not sure what a VRM is, here’s the lowdown. VRM stands for voltage regulator module, and in this case, the term specifically points to an array of components on a motherboard. Ask any PC enthusiast and they’ll gladly tell you that a VRM’s role is to provide a clean and efficient power supply to the CPU or GPU.

The setup of a modern PC involves the power supply, which produces 12V, 5V, and 3.3V. However, the CPU mounted on the motherboard requires a lower voltage, typically in the region of 1.0- 1.3V, depending on the chip manufacturer. The VRM’s role is to step down the voltage from the power supply to one that’s acceptable for the CPU.

For PCB designers, VRM sounds strikingly similar to a DC-DC buck converter. Well, it is a buck converter when you’ve inspected the elements of a VRM. It consists of a PWM IC, an array of MOSFETs, inductors, and capacitors. 

The PWM IC switches the MOSFET at a particular duty cycle to produce the lowered voltage at the output. Both the inductor and capacitor form an LC circuit to filter off any high-frequency noises. The result is a clean, efficient, and steady voltage for the CPU or GPU.

Why Does VRM Cooling Matter?

A 3D illustration of an isolated TO-220 electronic package with MOSFET pinout

MOSFET becomes inefficient as heat increases.

VRM is crucial to CPU and GPU getting clean power supplies. Both of these components are the power-hungriest ones on the motherboard. As the current rushes through the MOSFET, some of them are dissipated as heat. As the heat builds up, the MOSFETs can get pretty hot.

It is known that VRM for a CPU measures around 80°C- 100°C without cooling. For a GPU, the VRM’s temperature often increases up to 120°C. The whole idea of a VRM is to provide CPU and GPU with a reliable, efficient power source. However, an uncontrolled increase in temperature does the opposite.

MOSFET is known to have a positive temperature coefficient. In other words, the internal resistance of MOSFET increases with the temperature. When it heats up, the amount of current passing through decreases. This also means the heated MOSFET has a lower power efficiency.

Therefore, if you’re not taking a proactive approach to VRM cooling, the power delivered to the CPU and GPU will be compromised. It will affect the performance of those processors. Also, the uncontrolled rise in temperature can shorten the lifespan of the MOSFET and adjacent components.

How Do You Implement VRM Cooling?

A VRM on a CPU socket on a motherboard

Heatsink helps to dissipate heat from the VRM.

As processors increase in power, so does heat dissipated by the VRM. This makes VRM cooling more challenging than ever. The first step lies in the PCB design itself. Designers would have to turn the surrounding area of the MOSFET into an on-PCB heatsink. This is done by implementing thermal vias beneath the MOSFET to enhance heat dispersion.

However, thermal pads are not sufficient to disperse a large amount of heat generated. A single rectangular heatsink is often installed over the array of MOSFET to increase to better dissipate the heat. Cooling fans and ventilation outlets are also important in keeping the VRM temperature within the optimum level.

For some graphic cards, liquid cooling may be needed to prevent temperature overshoot. This is done by attaching a cooling block, which circulates water or other types of coolant to a radiator. 

It’s always wise to check for thermal hotspots on the VRM and surrounding areas with a PCB design tool, like OrCAD PCB Designer. In addition, Cadence’s Celsius Thermal Solver provides accurate analysis and simulation of thermal footprint for both solid structures and airflow.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

 

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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