This article explains the difference between controlgear case temperature (tc) and rated ambient temperature (ta), and highlights the importance of the position of the tc point.
For compliance with the requirements of luminaire standard SANS 60598 Luminaires Part 1: General requirements and tests, no part of the luminaire shall attain a temperature which would impair safety when the luminaire is operated at its rated ambient temperature (ta).
The temperature limit for controlgear is defined as the rated maximum temperature of the case of the controlgear (tc) measured at the given reference point marked by the device manufacturer.
Definitions
Rated maximum ambient temperature
The controlgear standard SANS 61347 Lamp controlgear Part 1: General and safety requirements provides no definition of ta and its measurement for inbuilt luminaire controlgear.
The luminaire standard applies to independent controlgear and requires the marking of the rated maximum ambient temperature, where it is found not to be 25°C.
Rated maximum case temperature
SANS 61347 defines tc as the “highest permissible temperature which may occur on the outer surface (at the indicated place, if marked) under normal conditions and at the rated voltage or the maximum of the rated voltage range”.
| Type | ta | 40°C | 50°C | 60°C |
| LED driver type 1 | tc | 55°C | 65°C | x |
| Life-time | 50 000 h | 30 000 h | x | |
| LED driver type 2 | tc | 60°C | 70°C | x |
| Life-time | 50 000 h | 30 000 h | x |
Difference between ta and tc
Ambient temperature is the temperature of the air surrounding the electronic controlgear, as declared by the manufacturer, while case temperature indicates a reference point on the controlgear housing.
The importance of ta and tc
The ambient temperature has a direct influence on the lifetime of the electronic components. Should ta be too high, the device cannot radiate enough heat into the environment. This can result in so-called hotspots, points where the temperature “concentrates”.
Hotspots in the air close to the components can lead to premature ageing and failure of the devices. The lifetime of controlgear is determined by the ambient temperature in which it functions.
The case temperature can be defined as the highest permissible temperature (at which the controlgear operates safely) that may occur at the tc point on the controlgear housing.
Two temperature values
It can be very difficult to measure the ambient temperature of controlgear when it is situated inside luminaires. Measuring tc temperature is, however, an easier and more reliable approach.
Tridonic conducts measurements for both ta and tc as part of the design process, and ensures that both measurements meet requirements. With a reliable result for tc and ta, it is possible to provide exact correlations between these two parameters in the product datasheet.
| Specification item | Value | Unit | Condition |
| Ta-max | 50 | °C | |
| Tc-max | 90 | °C | Maximum temperature measured at Tc-point |
| Tc-life | 80 | °C | Measured at Tc-point |
| Driver lifetime | 50 000 | h | Measured temperature at Tc-point is Tc-life |
This allows the luminaire manufacturer to determine the expected lifetime of the controlgear based on the tc, which is easier to measure.
Position of the tc point
The tc point is typically placed directly above temperature and lifetime-critical components such as the capacitor or the coil.
There is, however, no definite “rule of thumb”. The tc point is defined and determined by the manufacturer. Due to this fact, tc is not suitable for comparing different controlgear in terms of their thermal performance.
Practical uses of ta and tc
Ambient temperature must be taken into consideration where a comparison is to be made between controlgear from different manufacturers to obtain reliable information about their lifetimes.
Case temperature is highly dependent on the exact position of the tc point and, as the manufacturer can define this position, it makes no sense to compare case temperatures from different manufacturers.
A lower or higher tc reading does not guarantee quality or lifetime. The tc could differ from product to product because different manufacturers use different, more (or less) favorable tc points.
In addition, controlgear from the same manufacturer can have different case temperatures but identical ambient temperatures. Ambient temperature is important for estimating lifetime. Only the ta can provide reliable information about the lifetime of the controlgear.
Practical examples
Table 1 shows two different Tridonic devices. Although the tc temperatures are different (55 and 60°C), the ta temperature (40°C) and the lifetime (50 000 h) are the same.
The LED driver type 1 has a ta of 40°C and a tc of 55°C for a lifetime of 50 000 h.
In the example in Table 2, the LED driver is compared to a device from a different manufacturer which uses different wording.
The information says the driver has a ta (max) of 50°C and a tc (max) of 90°C, but the tc temperature for a lifetime of 50 000 hours is 80°C, which means that, for this lifetime, the ta is not 50°C but 40°C.
This means that the comparable values for a lifetime of 50 000 hours are:
The result shows that the two controlgear types from different manufacturers have different tc temperatures
(80 and 55°C respectively) but the ta temperature (40°C) and the lifetime (50 000 h) is the same.
Contact Henk Rotman, Tridonic SA, Tel 011 894-3525, hendrik.rotman@tridonic.com
