Innovative Baseplate Cooling Design from RECOM Provides a Better Option than Fans in AC/DC Converter Applications
2020-4-27
The power density of AC/DC converter modules is increasing, but often requires high airflow rates to achieve their stated performance. However, forced cooling using mechanical fans brings reliability, noise and dust pollution issues. Baseplate cooling solves all of these problems, but only if the converter is specially designed from the outset to utilise conducted cooling measures, such as the new RACM230-G and RACM550-G series from RECOM, which delivers high power in a small form-factor along with a long list of other useful features.
Be careful with power density comparisons
The power density specification of an AC/DC converter is a good comparative measure of size as long as the environmental conditions are comparable and realistic - some high-end (read: expensive) modules in custom housings can boast impressive figures of over 100W/cubic inch (6W/cm3), but these often need excessively large or water-cooled heatsinks and sometimes achieve their compact size by omitting large external components such as the input filter and the rectifier electrolytic capacitor. For typical applications where cost and ease of use are important, a realistic comparison is really between modules in industry-standard footprints such as 5” x 3” or 4” x 2”, which are fully integrated without the need for external components.
These AC/DC converters will normally headline their data sheets with the highest possible power output under the most favourable operating conditions. To be fair, there might be many applications that have the optimum AC input voltage and can provide the necessary ambient cooling airflow. In some product datasheets, airflow figures of over 35 m3/hr (20 CFM) are specified for cooling to achieve full rated power, which typically requires a 60mm x 60mm x 25mm axial fan placed directly next to the converter, with access to inlet air at ambient room temperature and a direct route to exhaust the hot air without any obstruction. Figure 1 shows how this is calculated. The fan could easily be a quarter of the size of the power supply in volume, costing several euros and taking a precious 1.5 Watts itself.
Figure 1: Calculating air flow given power dissipated and target temperature difference
A built-in fan just for the power supply is not always practical and airflow from larger system fans may not be able to provide sufficient localised high flow-rate air, which could anyway be pre-warmed by other hot components, diminishing the cooling effect.
Fans additionally present other problems; they have limited lifetime, typically 30,000 hours at 50°C for sleeve types (sensitive to orientation) and about double that for the more expensive ball-bearing versions. At higher temperatures, the expected lifetime drops dramatically, Figure 2 shows a typical life expectancy curve for a 90% survival rate in a population of installed ball bearing type fans.
Figure 2: Typical fan life expectancy curve (survival rate of 90%)
Acoustic noise can also be an issue, increasing as the components age and in some applications, such as medical and broadcasting, any background noise at all is undesirable. Fans also introduce dust and dirt contamination into electronics unless the input filtering is very effective, in which case the air flow is reduced anyway. More expensive temperature-controlled variable-flow fans mitigate these problems to an extent, but at the very least, applications using fans need a program of performance monitoring and regular replacement of filters and the fan itself with all of the attendant costs involved.
Practical cooling solutions
One solution to mechanical fan cooling problems is to specify a part with a higher power rating under forced air conditions, then use it at lower power with just convection cooling. Unfortunately, products designed for fan cooling often derate heavily without any forced airflow. Figure 3 is a typical example which needs 21 CFM for its rated 500W power but in natural convection delivers only 125W up to 50°C.
Figure 3: Typical derating curves for a 3” x 5” AC/DC converter designed for forced air applications
An obvious disadvantage is the extra cost for an over-specified converter, but there are other considerations; a 500W converter run at 125W is often not optimised for efficiency and the application may need additional protection to withstand much higher fault currents under overload and short circuit conditions that the power supply can deliver, again adding to cost.
Conduction cooling can be effective
Figure 3 shows that if that particular converter is attached to a cold wall, heavy duty equipment housing or large heatsink, it can supply a little more power – 200W up to 50°C. The only minimal improvement over convection cooling is because for conduction cooling to be effective, the product has to be designed for that purpose with a good heat transfer path from the hot components to the converter baseplate. A good example of a product series that achieves this is the new RACM230-G and RACM550-G from RECOM which employs advanced techniques to minimise the thermal resistance between critical components, such as the power switches and transformer, to their baseplates. The resulting performance of the RACM550 is shown in Figure 4, allowing a full 300W up to 50°C, 50% extra compared with the example in Figure 3 and more than double the power, 225W, at 70°C ambient.
Interestingly, the convection cooling performance is also much better as the closely thermally-coupled baseplate forms a good surface for dissipation in still air, compared with the localised component hot spots in the Figure 3 example.
Figure 4: The RECOM RACM550-G series thermal performance
In some applications, there may be a high peak load or start-up current requirement and there may be an opportunity to draw more instantaneous power and then use conduction or natural convection cooling for the average load, depending on the duty cycle of the peak load. In the RACM550-G datasheet, RECOM provides a useful guide to calculating the maximum average load, depending on the cooling conditions, given peak load and duty cycle. An example calculation in the product shows that a load with 550W peak and 81W ‘recovery’ value with duty cycle 10 seconds peak to 40 seconds recovery, sets the average output power to 245W with some safety margin. From Figure 4, this is handled by the product with natural convection up to 40°C ambient or up to nearly 65°C with baseplate conduction cooling.
The RACM230-G offers peak output power of 230W in a compact 4”x2” footprint and can also supply up to 135W of continuous power up to 50°C ambient with just baseplate and natural convection cooling. A further important specification is the maximum output power at lower input voltage; all AC power supplies can deliver more power with 230VaAC mains than with 115VAC supplies, supply because for the same load the input current is halved for the higher input voltage. Both the RACM230-G and the RACM550-G are universal input power supplies. With forced air cooling, the load rating is independent of the supply voltage as long as the input voltage does not fall below 110VAC. However, with baseplate cooling only, there is no difference in the load rating. For the RACM230-G, for example, it is up to 160W @ 230VAC and @ 115VAC as well.
EMC and medical specifications can be met with conduction cooling
The design techniques used to achieve low thermal resistance to the baseplate for effective conduction cooling can pose problems with conducted electrical interference levels from the converter. EMC Standards such as EN55032:2015 specify levels of so-called ‘common mode’ noise which increases with close physical proximity of switching devices to the baseplate ground. When conduction-cooling products are designed primarily for forced air, complex and costly shielding and additional filtering may be necessary to achieve regulatory compliance. Heavy filtering in turn adds AC mains leakage current, which can prevent the use of the power supply in many sensitive applications such as medical devices. The RECOM RACM550-G series utilises an inherently low-noise resonant ‘LLC’ circuit which avoids the need for heavy input filtering so that the product meets the stricter EN55032:2015 Class B limits while showing a leakage current of just 0.25mA, suitable for Body (B) and Body-Floating (BF) applied medical part applications.
To facilitate the use of the RACM230-G and RACM550-G in medical applications, these series have been certified to medical standards ANSI/AAMI 60601-1 and EN60601-1 (safety) and EN60601-1-2 (EMC). The certification level is to the stringent 250VAC/2MOPP (Measures Of Patient Protection) level, making the product suitable for a wide range of hospital, clinical and other healthcare applications such as dental. Additional certifications include household, industrial and I.T.E. applications, making these products all-rounders that can be used in many different environments and applications.
A range of certifications and features is available
The RACM230-G and RACM550-G series hold general worldwide safety certifications including the latest IEC/EN/UL62368 requirements and is compliant with European Commission ErP Ecodesign directive Lot 6, along with the US DoE Level VI specification for standby losses. Low-load losses are minimised with efficiency at 230VAC input remaining greater than 90% down to 20% load.
Other features of the series include:
Protections for the two series are comprehensive, with short circuit, over-voltage, over-current and over-temperature included, with auto-recovery after a fault. The products are available open frame or with an optional metal cover.
Austria-based RECOM has set the standard again for performance at an affordable cost with the new RACM230-G and RACM550-G series with its high power density and flexible cooling arrangement. Further products using their innovative baseplate cooling technique are planned.
RECOM: We Power your Products
The power density specification of an AC/DC converter is a good comparative measure of size as long as the environmental conditions are comparable and realistic - some high-end (read: expensive) modules in custom housings can boast impressive figures of over 100W/cubic inch (6W/cm3), but these often need excessively large or water-cooled heatsinks and sometimes achieve their compact size by omitting large external components such as the input filter and the rectifier electrolytic capacitor. For typical applications where cost and ease of use are important, a realistic comparison is really between modules in industry-standard footprints such as 5” x 3” or 4” x 2”, which are fully integrated without the need for external components.
These AC/DC converters will normally headline their data sheets with the highest possible power output under the most favourable operating conditions. To be fair, there might be many applications that have the optimum AC input voltage and can provide the necessary ambient cooling airflow. In some product datasheets, airflow figures of over 35 m3/hr (20 CFM) are specified for cooling to achieve full rated power, which typically requires a 60mm x 60mm x 25mm axial fan placed directly next to the converter, with access to inlet air at ambient room temperature and a direct route to exhaust the hot air without any obstruction. Figure 1 shows how this is calculated. The fan could easily be a quarter of the size of the power supply in volume, costing several euros and taking a precious 1.5 Watts itself.
Figure 1: Calculating air flow given power dissipated and target temperature difference
A built-in fan just for the power supply is not always practical and airflow from larger system fans may not be able to provide sufficient localised high flow-rate air, which could anyway be pre-warmed by other hot components, diminishing the cooling effect.
Fans additionally present other problems; they have limited lifetime, typically 30,000 hours at 50°C for sleeve types (sensitive to orientation) and about double that for the more expensive ball-bearing versions. At higher temperatures, the expected lifetime drops dramatically, Figure 2 shows a typical life expectancy curve for a 90% survival rate in a population of installed ball bearing type fans.
Figure 2: Typical fan life expectancy curve (survival rate of 90%)
Acoustic noise can also be an issue, increasing as the components age and in some applications, such as medical and broadcasting, any background noise at all is undesirable. Fans also introduce dust and dirt contamination into electronics unless the input filtering is very effective, in which case the air flow is reduced anyway. More expensive temperature-controlled variable-flow fans mitigate these problems to an extent, but at the very least, applications using fans need a program of performance monitoring and regular replacement of filters and the fan itself with all of the attendant costs involved.
Practical cooling solutions
One solution to mechanical fan cooling problems is to specify a part with a higher power rating under forced air conditions, then use it at lower power with just convection cooling. Unfortunately, products designed for fan cooling often derate heavily without any forced airflow. Figure 3 is a typical example which needs 21 CFM for its rated 500W power but in natural convection delivers only 125W up to 50°C.
Figure 3: Typical derating curves for a 3” x 5” AC/DC converter designed for forced air applications
An obvious disadvantage is the extra cost for an over-specified converter, but there are other considerations; a 500W converter run at 125W is often not optimised for efficiency and the application may need additional protection to withstand much higher fault currents under overload and short circuit conditions that the power supply can deliver, again adding to cost.
Conduction cooling can be effective
Figure 3 shows that if that particular converter is attached to a cold wall, heavy duty equipment housing or large heatsink, it can supply a little more power – 200W up to 50°C. The only minimal improvement over convection cooling is because for conduction cooling to be effective, the product has to be designed for that purpose with a good heat transfer path from the hot components to the converter baseplate. A good example of a product series that achieves this is the new RACM230-G and RACM550-G from RECOM which employs advanced techniques to minimise the thermal resistance between critical components, such as the power switches and transformer, to their baseplates. The resulting performance of the RACM550 is shown in Figure 4, allowing a full 300W up to 50°C, 50% extra compared with the example in Figure 3 and more than double the power, 225W, at 70°C ambient.
Interestingly, the convection cooling performance is also much better as the closely thermally-coupled baseplate forms a good surface for dissipation in still air, compared with the localised component hot spots in the Figure 3 example.
Figure 4: The RECOM RACM550-G series thermal performance
In some applications, there may be a high peak load or start-up current requirement and there may be an opportunity to draw more instantaneous power and then use conduction or natural convection cooling for the average load, depending on the duty cycle of the peak load. In the RACM550-G datasheet, RECOM provides a useful guide to calculating the maximum average load, depending on the cooling conditions, given peak load and duty cycle. An example calculation in the product shows that a load with 550W peak and 81W ‘recovery’ value with duty cycle 10 seconds peak to 40 seconds recovery, sets the average output power to 245W with some safety margin. From Figure 4, this is handled by the product with natural convection up to 40°C ambient or up to nearly 65°C with baseplate conduction cooling.
The RACM230-G offers peak output power of 230W in a compact 4”x2” footprint and can also supply up to 135W of continuous power up to 50°C ambient with just baseplate and natural convection cooling. A further important specification is the maximum output power at lower input voltage; all AC power supplies can deliver more power with 230VaAC mains than with 115VAC supplies, supply because for the same load the input current is halved for the higher input voltage. Both the RACM230-G and the RACM550-G are universal input power supplies. With forced air cooling, the load rating is independent of the supply voltage as long as the input voltage does not fall below 110VAC. However, with baseplate cooling only, there is no difference in the load rating. For the RACM230-G, for example, it is up to 160W @ 230VAC and @ 115VAC as well.
EMC and medical specifications can be met with conduction cooling
The design techniques used to achieve low thermal resistance to the baseplate for effective conduction cooling can pose problems with conducted electrical interference levels from the converter. EMC Standards such as EN55032:2015 specify levels of so-called ‘common mode’ noise which increases with close physical proximity of switching devices to the baseplate ground. When conduction-cooling products are designed primarily for forced air, complex and costly shielding and additional filtering may be necessary to achieve regulatory compliance. Heavy filtering in turn adds AC mains leakage current, which can prevent the use of the power supply in many sensitive applications such as medical devices. The RECOM RACM550-G series utilises an inherently low-noise resonant ‘LLC’ circuit which avoids the need for heavy input filtering so that the product meets the stricter EN55032:2015 Class B limits while showing a leakage current of just 0.25mA, suitable for Body (B) and Body-Floating (BF) applied medical part applications.
To facilitate the use of the RACM230-G and RACM550-G in medical applications, these series have been certified to medical standards ANSI/AAMI 60601-1 and EN60601-1 (safety) and EN60601-1-2 (EMC). The certification level is to the stringent 250VAC/2MOPP (Measures Of Patient Protection) level, making the product suitable for a wide range of hospital, clinical and other healthcare applications such as dental. Additional certifications include household, industrial and I.T.E. applications, making these products all-rounders that can be used in many different environments and applications.
A range of certifications and features is available
The RACM230-G and RACM550-G series hold general worldwide safety certifications including the latest IEC/EN/UL62368 requirements and is compliant with European Commission ErP Ecodesign directive Lot 6, along with the US DoE Level VI specification for standby losses. Low-load losses are minimised with efficiency at 230VAC input remaining greater than 90% down to 20% load.
Other features of the series include:
- Wide input range 80-264VAC
- Operating temperature range -40°C to +70°C (+80°C for the RACM230-G)
- Up to 5000m operating altitude
- Smart fan output for temperature controlled forced-air cooling applications
- Standby output 5V/1A which is ‘always on’ (RACM550-G)
- Compact dimensions 4” x 2” x 1.5” (RACM230-G) or 3” x 5” x 1.5” (RACM550-G)
- On/off control included (RACM550-G)
- Remote sensing included (RACM550-G)
- Cable drop compensation
Protections for the two series are comprehensive, with short circuit, over-voltage, over-current and over-temperature included, with auto-recovery after a fault. The products are available open frame or with an optional metal cover.
Austria-based RECOM has set the standard again for performance at an affordable cost with the new RACM230-G and RACM550-G series with its high power density and flexible cooling arrangement. Further products using their innovative baseplate cooling technique are planned.
RECOM: We Power your Products
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