Metal Core Board For A High Voltage Linear Amplifier

A linear power amplifier that has to deliver ten amps at two hundred volts is a thermal design problem before it is anything else. Unlike a switching amplifier, it does not convert the surplus energy into a waveform with high efficiency; it dissipates it as heat, continuously, and the dissipation is highest when the output is at half the supply rather than at the extremes. Ten amps and a hundred watts of dissipation in a linear stage is a normal operating condition, not a fault.

This article explains why a metal core board is used for that kind of amplifier, what the substrate has to provide beyond heat spreading, and which specifications have to be settled before the design is released.

The electrical performance of such an amplifier is usually good: a slew rate of tens of volts per microsecond is achievable with a linear output stage, and the distortion is low. The difficulty is keeping the devices cool enough to deliver it.

Why A Linear Amplifier Is Hard To Cool

Efficiency in a linear stage is the ratio of output power to input power, and it falls as the output voltage drops toward zero, because the voltage not delivered to the load is dropped across the output devices. At the worst point in the signal cycle, half of the supply voltage is across the transistor while the full load current flows through it, and the product of those two numbers is heat.

That heat has to leave through the package into the board and out to a heatsink. On a conventional laminate the thermal path is poor, because the dielectric under the device is a thermal insulator, and the devices run hotter than they need to. The metal core board exists to shorten and widen that path.

What The Substrate Provides

A metal core board replaces most of the laminate with an aluminium base, with only a thin insulating layer between it and the copper circuit. Heat crosses that layer and spreads in the aluminium, which has a thermal conductivity several hundred times that of the laminate it replaces. The result is a much lower thermal resistance from the device to the base, and a base that can be bolted to a heatsink or to the chassis with a large contact area.

The mechanical benefit is worth noting too. An aluminium base stiffens the assembly and gives a mounting surface that can be machined and fastened, which matters for an amplifier that dissipates a hundred watts and needs a real thermal interface. The mounting and outline of the board is part of the thermal design for exactly that reason.

Dielectric Breakdown And Layer Thickness

The insulating layer is the component that makes the substrate possible and the one that limits it. It has to hold the working voltage between the circuit and the earthed aluminium base, and at two hundred volts that requirement is not trivial. Thicker layers give a higher dielectric breakdown voltage and a higher thermal resistance; thinner layers do the opposite.

Choosing a dielectric thickness is therefore a decision that has to be made with both numbers in view. It is not enough to specify a board as rated for high voltage, because the breakdown voltage depends on the layer thickness, on the material and on the quality of the coating, including its freedom from pinholes and voids. Where the supply is above a couple of hundred volts, it is worth asking for the test method as well as the figure, and for a sample of the layer to be inspected for defects rather than assumed uniform.

Creepage And Clearance At Two Hundred Volts

Voltage does not only stress the dielectric under the circuit; it also has to be held across the surface. Creepage, the distance along the surface between two conductors at different potentials, is what prevents a tracking failure as contamination and humidity accumulate. On a board that switches a few hundred volts and carries current in the same region, the creepage and clearance distances are a primary design constraint rather than a detail.

In practice that means the high voltage nodes have to be kept apart, and the copper around them designed so that the distances are predictable. A copper pour connected to ground beside a high voltage trace has to be held back by the creepage requirement, which may override the usual pour clearance. The dimensions should be stated on the fabrication drawing, because the design rules that make the board manufacturable are not the same as the rules that make it safe.

Thermal Protection And Shutdown

An amplifier rated for a hundred watts of dissipation will be damaged if that figure is exceeded, which happens when the load is wrong, the heatsink is blocked or the ambient is higher than assumed. The usual protection is a temperature sensor mounted on the board close to the output devices, connected to a circuit that reduces the drive or shuts the output down above a threshold. A limit somewhere around a hundred degrees on the substrate is a common choice, since it leaves margin below the point where the solder and the devices are stressed.

The sensor has to be placed where it sees the temperature of the devices rather than the temperature of the room, and the threshold has to be set from the thermal resistance of the assembly and the safe operating area of the output stage. Adding the sensor to the layout is a design decision, because the location and the routing both matter, and a protection circuit that trips too late protects nothing.

What To Specify

Five specifications carry most of the risk. The first is the dielectric layer, its thickness, its breakdown voltage and its thermal conductivity. The second is the copper thickness, which sets the current capacity of the output path and the spreading of heat at the same time. The third is the creepage and clearance distances for the highest voltage node, stated explicitly. The fourth is the mounting interface, including flatness and the thermal interface material. The fifth is the temperature sensor location and its threshold.

The copper weight is worth a moment because it does double duty here. The output path has to carry ten amps, and sizing a trace for current tells you the width and thickness needed; using heavier copper also improves heat spreading under the devices. The two requirements point the same way, which is convenient. gopcb builds metal core boards with controlled dielectric thickness and heavy copper, and reviews the voltage clearances and the thermal path with the customer before the panel is released.

FAQ

Why not use an aluminium board for a switching amplifier instead? A switching stage dissipates far less heat for the same output, so a laminate or a lighter metal core construction is usually enough. The linear stage is what forces the heavier solution.

Can the insulating layer be made thin to improve cooling? Only up to the voltage it has to hold. At two hundred volts the dielectric breakdown requirement sets a floor on the thickness.

Is the isolation between the circuit and the base an electrical safety requirement? It can be, depending on the application. Where the base is earthed and the circuit is at a hazardous voltage, the isolation is part of the safety design and needs to be documented.