See a demonstration in the film below of how the ZPZ foil pointwise self-regulation works.
Each point on the foil surface automatically regulates its heating power to accomodate for varying thermal loads and ambient temperatures. Efficient cooling and/or low ambient temperature gives high power and vice versa.
Because of the ZPZ design, all points on the foil surface are independent. Each point can in fact be thought of as an individual PTC heater, wired in parallel with all other points, as illustrated in figure. Note, however, that all points have the same PTC characteristic, since the SIP compound is the same in all points.
In traditional etched foil heaters, one tries to compensate for different thermal loads across the heater surface by varying the width and density of the etched circuit, hence creating zones with different watt densities. But once the circuit is etched, there is no way to change it. To increase power in one zone, one has to increase power in all zones. This leaves no room for flexibility and requires a detailed analysis of the thermal properties of the system at hand.
In contrast, the ZPZ foil can automatically accomodate for varying thermal loads, to a large degree. Pointwise PTC characteristic means pointwise self-regulation – each point on the ZPZ foil surface will adapt to its surroundings and regulate its heating power accordingly. Lower ambient temperature and/or more efficient cooling gives more power, higher ambient temperatures and/or less cooling gives less power, as illustrated in figures below.
Pointwise self-regulation to varying ambient temperatures.
Pointwise self-regulation to varying cooling conditions. The heating power is higher beneath the metal cylinders.
The extent to which the ZPZ foil can compensate for different thermal loads depends on recipe and voltage.
Each point on the foil surface has a built-in temperature limiter, inherent to the material itself, which ensures that the foil will never overheat.
Pointwise self-regulation implies pointwise temperature limiting. The limiting temperature Tlim is defined as the equilibrium temperature of a ZPZ foil suspended horizontally in still air of room temperature, a set-up representing minimal cooling conditions. All realistic applications have more efficient cooling, hence the foil temperature will never exceed Tlim as long as the ambient temperature stays at 22°C. Should the ambient temperature increase, the maximum foil temperature will also rise, but always less.
There are two categories of heating applications based on the ZPZ foil; with and without electronic temperature regulation. In the first case, the foil is held at a constant operating temperature Top by an electronic circuit which regulates the supplied power. In the second case, the foil is allowed to reach a self-regulated equilibrium temperature Teq. The relationship between Top, Teq and Tlim is illustrated in figure.
Temperature limiting corresponds to a two-stage overheating protection system. Should the temperature regulating electronic circuitry fail, the ZPZ foil temperature will increase but it will stabilize at Teq and not go into thermal runaway. Even if the cooling conditions changes dramatically, the ZPZ foil temperature will never exceed Tlim.
There is no equivalent to Tlim in a traditional etched foil heater, since the temperature increase is practically independent of ambient temperature. In order to avoid dangerously high temperatures one therefore has to resort to external safety devices such as thermal fuses or electronics. The built-in safety of the ZPZ foil, inherent to the material itself, is a more elegant and reliable solution. Moreover, whereas external overheating protection devices monitor a single point on the heater surface, the ZPZ foil’s temperature limiting is pointwise. Every point on the ZPZ foil surface has a built-in temperature limiter.
Pointwise temperature limiting. Top is the foil temperature in an electronically regulated heating application, Tlim is the foil temperature in a self-regulated application, Tlim is the maximum foil temperature.
Heat is generated in the entire foil and the power density can therefore be kept at a minimum level.
In a traditional etched foil heater, the actual surface covered by the circuit is approximately 50% of the heater surface. In order to achieve a certain average power density or temperature, the power density and temperature of the circuit has to be roughly twice as high. Therefore, there will always be a temperature variation across an etched foil heater surface – it is part of the construction itself.
The ZPZ foil on the other hand, has almost 100% surface coverage (depending on design, isolation, soldering points etc), which allows for a smoother temperature distribution and hence that the power density and temperature can be kept lower than in the etched foil case. It is usually preferable to work with as low power densities and temperatures as possible.
The ZPZ foil’s powerful PTC effect allows for a rapid warm-up, since the initial heating power is much higher than the equilibrium one. The benefits compared to an etched circuit foil are illustrated in the figures for two different cases. In both cases, the heater is supplied with full power until the operating temperature is reached, after which an electronic regulation circuit sets in and cycles the power on and off in order to keep the desired temperature constant.
In the first case, the upper figure, the ZPZ foil is designed for the same operating power as the etched foil. Since the average initial power is higher, the operating temperature is reached faster.
In the second case, the lower figure, the ZPZ foil is designed to reach the operating temperature in the same time as the etched foil. Because of the PTC effect, the ZPZ foil’s operating power will then be lower than the etched foil’s. Maintaining a constant operating temperature with a lower power is in general less demanding for the regulatory circuitry, which may be more important than a rapid warm-up time in some applications.
Rapid warm-up: Both heaters are supplied with full power until the operating temperature is reached, after which an electronic regulation circuit sets in and cycles the power on and off in order to keep the desired temperature constant. The ZPZ foil is designed for the same operating power as the traditional heater.
Low power regulation: Both heaters are supplied with full power until the operating temperature is reached, after which an electronic regulation circuit sets in and cycles the power on and off in order to keep the desired temperature constant. The ZPZ foil is designed to reach operating temperature in the same time as the traditional heater.
The surface temperature and power density are independent of area, allowing for a simple and cost effective product design process.
Each foil comes with a power density and a surface temperature which is independent of area. Once a suitable SIP recipe has been found, the design process is therefore simple – simply cut out the desired shape and size. Should the same voltage, power density and temperature specifications be required in another project, no changes need to be made to the foil – simply cut out another shape and size.
