AC/DC converters in general have become more efficient over the years enabling a reduction in size, matching the overall trend for miniaturisation of electronics. Board-mount AC/DC’s have followed the same path, but some specifications such as safety and thermal dissipation have become problematic as size reduces and the power density increases.
Board-mount AC/DC converter design challenges
2019. 5. 29
A board-mount AC/DC converter is often seen as ‘just another’ commodity component, which is expected to reduce in size and cost while improving in performance as technology advances. However, these miniaturized converters have difficult design constraints with ever-wider input voltages, operating temperature ranges and more stringent safety and energy efficiency standards. This article examines how these specifications trade-off against each other and how latest generation products are achieving a combination of performance, size and cost that makes them widely applicable.
Table of contents
Flyback designs are commonly used for low power
The reduction in cost of a board-mount AC/DC naturally leads to a topology that has a minimum possible component count. Below about 10W, this will invariably be a ‘flyback’ converter with an integrated power transistor and transformer (or more accurately coupled inductor) with a single diode and capacitor for each output. A control IC samples the output voltage directly (primary side regulation) or indirectly (secondary side regulation via an opto-coupler) and applies feedback to the power stage in the form of pulse-width or frequency modulation or sometimes both. There are usually additional filter components to reduce EMI to statutory levels and a ‘bulk’ capacitor to provide energy for hold-up time on mains interruptions, Figure 1.
There will be a ‘barrier’ between the high voltage AC and the output, formed by solid insulating material or separation distance through air and across surfaces. Components that cross the barrier, such as a transformer or opto-isolator, will be constructed to provide sufficient isolation to satisfy international safety agencies.
There’s a downside though to the simplicity of flybacks – internal voltages can be high, typically 600V or more on the switch and some components are stressed with high ripple currents. The output capacitors for example, have to cope with peak ripple currents which can be 1.5 to 2 times the DC output current value. The high ripple current dissipates power in the capacitor’s ESR and raises its temperature. Depending on the mode of operation, the peak and rms values of the primary switch current can also be surprisingly large and as high-voltage switches have relatively high on-resistances, efficiency is reduced and power is dissipated both in the switch and input capacitor which is sourcing the AC component of input current.
However, there is plenty of design data in books and IC application notes to enable even a novice to select component values for a simple flyback converter and given sufficient knowledge of high-frequency layout rules and transformer construction, producing a working product is not so difficult. If size is not so important, achieving safety clearances is easy and there is plenty of room to allow the free flow of air to dissipate heat. Components can also be large and conservatively rated to guarantee functional and EMI performance. A version of the flyback topology is seen in just about every cell-phone charger, so shrinking the electronics is clearly possible but these products only have to operate within limited temperature ranges, perhaps 0 – 40°C in home or office environments.
On the other hand, when space and cost is limited, or the AC/DC has to operate inside other equipment, there are formidable challenges. If the application is industry-grade, there may be additional clearance requirements to cope with damp or dirty environments and significantly higher transient over-voltages to withstand with much wider temperature variations from as low as -40°C to +75°C and higher. Input voltage ranges may be very wide and reliability and lifetime may also be expected to be very high compared with commercial products. Additionally, an extended altitude performance may be necessary affecting safety clearances.
There will be a ‘barrier’ between the high voltage AC and the output, formed by solid insulating material or separation distance through air and across surfaces. Components that cross the barrier, such as a transformer or opto-isolator, will be constructed to provide sufficient isolation to satisfy international safety agencies.
There’s a downside though to the simplicity of flybacks – internal voltages can be high, typically 600V or more on the switch and some components are stressed with high ripple currents. The output capacitors for example, have to cope with peak ripple currents which can be 1.5 to 2 times the DC output current value. The high ripple current dissipates power in the capacitor’s ESR and raises its temperature. Depending on the mode of operation, the peak and rms values of the primary switch current can also be surprisingly large and as high-voltage switches have relatively high on-resistances, efficiency is reduced and power is dissipated both in the switch and input capacitor which is sourcing the AC component of input current.
However, there is plenty of design data in books and IC application notes to enable even a novice to select component values for a simple flyback converter and given sufficient knowledge of high-frequency layout rules and transformer construction, producing a working product is not so difficult. If size is not so important, achieving safety clearances is easy and there is plenty of room to allow the free flow of air to dissipate heat. Components can also be large and conservatively rated to guarantee functional and EMI performance. A version of the flyback topology is seen in just about every cell-phone charger, so shrinking the electronics is clearly possible but these products only have to operate within limited temperature ranges, perhaps 0 – 40°C in home or office environments.
On the other hand, when space and cost is limited, or the AC/DC has to operate inside other equipment, there are formidable challenges. If the application is industry-grade, there may be additional clearance requirements to cope with damp or dirty environments and significantly higher transient over-voltages to withstand with much wider temperature variations from as low as -40°C to +75°C and higher. Input voltage ranges may be very wide and reliability and lifetime may also be expected to be very high compared with commercial products. Additionally, an extended altitude performance may be necessary affecting safety clearances.
Practical design constraints
A practical limit to size reduction are the minimum safety separations, with standards such as EN 62368-1 for IT/media and EN 60335-1 for household requiring a minimum of 9mm clearance between input and output and 4mm separation across surfaces in 250VAC systems. This is for the worst pollution degree and material groups and can be relaxed with encapsulated devices, coated PCBs and with materials with high Comparative Tracking Index (CTI) but it illustrates how a ‘safe’ default creepage cannot practically be used in a converter that may only be around 25mm wide anyway. Careful design is therefore necessary to use specification concessions to the full and to ensure the minimum regulatory distance requirements by cutting slots in the PCB or adding separators or caps over critical components.
The creepage and clearance requirements are also problematic in the transformer. High power large sized transformers can use standard enamelled wire wound inside insulating tape ‘margins’ to guarantee typically 6mm creepage distance from primary to secondary windings. At low power where the winding width of a bobbin may only be a few millimeters, this clearly doesn’t work. A solution is to use safety-rated ‘triple insulated wire’ (TIW) which has an overlapping helical wrap of insulation, guaranteeing at least three layers at any point. Forming an EMI screen is still a problem though, with some designs using a half-turn layer of TIW with one end non-terminated but carefully insulated.
A component that also resists miniaturisation is the bulk storage capacitor in the input. It provides DC bus voltage smoothing and provides ‘hold-up’ energy during a mains cycle drop-out interruptions. A typical professional requirement is to maintain operation for 20ms (one mains cycle at 50Hz) from the nominal input voltage of 115V or 230VAC. The voltage that the capacitor sees with rectified 115VAC for example, is about 150V average with perhaps 20V of mains frequency ripple. If the supply is interrupted, the capacitor discharges and the converter must still operate for 20ms with the reducing voltage. Practically the converter can operate down to about 70V, so with a 75% efficient, 5W output converter for example, you can equate loss of energy in the capacitor with energy required in 20ms:
Equation 1
This would require a 18µF capacitor which would need to be rated at 400V for the highest AC input. The smallest type is an aluminium electrolytic and even this is about 3cm3, more if a high temperature version is chosen. A ‘rule of thumb’ applicable to many flyback circuits for 20ms hold-up is to require a minimum of 2µF/Watt for wide input range and 1µF/Watt for just 230V nominal. Any smaller capacitance values than this would make the problem worse as the ripple voltage would be anyway larger, reducing the voltage headroom and leading to output voltage dips on single mains cycle failures.
Board-mounted AC/DC converters can produce conducted EMI levels close to those of high-power products because common-mode noise doesn’t scale directly with power – it is mainly generated by high internal dV/dt levels coupled through stray capacitances which occur in every design. Simple low power designs therefore might need EMI filters bigger than the converter itself to meet emissions standards. Control IC designers address the problem by controlling dV/dt with resonant or semi-resonant topologies and techniques such as ‘frequency dithering’ which reduces the average EMI seen in the bandwidth of standard measuring receivers. Board-mounted AC/DCs are often ‘Class II’ devices with no primary ground connection, but their outputs are often grounded in real applications providing a path for common-mode conducted noise. ‘Y’ rated capacitors from primary to secondary help reduce high frequency interference but cannot be made large in value in case the output is not grounded and dangerous AC ‘touch current’ could flow through the user. Some household standards (e.g. EN 60335) require very low capacitance values and two Y-caps in series in case of failure of one component. Fitting two series capacitors, each with the full safety creepage distances between pins inside a miniature package is extremely difficult and is often left to the user to add externally so that EMC standards can be met.
The creepage and clearance requirements are also problematic in the transformer. High power large sized transformers can use standard enamelled wire wound inside insulating tape ‘margins’ to guarantee typically 6mm creepage distance from primary to secondary windings. At low power where the winding width of a bobbin may only be a few millimeters, this clearly doesn’t work. A solution is to use safety-rated ‘triple insulated wire’ (TIW) which has an overlapping helical wrap of insulation, guaranteeing at least three layers at any point. Forming an EMI screen is still a problem though, with some designs using a half-turn layer of TIW with one end non-terminated but carefully insulated.
A component that also resists miniaturisation is the bulk storage capacitor in the input. It provides DC bus voltage smoothing and provides ‘hold-up’ energy during a mains cycle drop-out interruptions. A typical professional requirement is to maintain operation for 20ms (one mains cycle at 50Hz) from the nominal input voltage of 115V or 230VAC. The voltage that the capacitor sees with rectified 115VAC for example, is about 150V average with perhaps 20V of mains frequency ripple. If the supply is interrupted, the capacitor discharges and the converter must still operate for 20ms with the reducing voltage. Practically the converter can operate down to about 70V, so with a 75% efficient, 5W output converter for example, you can equate loss of energy in the capacitor with energy required in 20ms:
Equation 1
This would require a 18µF capacitor which would need to be rated at 400V for the highest AC input. The smallest type is an aluminium electrolytic and even this is about 3cm3, more if a high temperature version is chosen. A ‘rule of thumb’ applicable to many flyback circuits for 20ms hold-up is to require a minimum of 2µF/Watt for wide input range and 1µF/Watt for just 230V nominal. Any smaller capacitance values than this would make the problem worse as the ripple voltage would be anyway larger, reducing the voltage headroom and leading to output voltage dips on single mains cycle failures.
Board-mounted AC/DC converters can produce conducted EMI levels close to those of high-power products because common-mode noise doesn’t scale directly with power – it is mainly generated by high internal dV/dt levels coupled through stray capacitances which occur in every design. Simple low power designs therefore might need EMI filters bigger than the converter itself to meet emissions standards. Control IC designers address the problem by controlling dV/dt with resonant or semi-resonant topologies and techniques such as ‘frequency dithering’ which reduces the average EMI seen in the bandwidth of standard measuring receivers. Board-mounted AC/DCs are often ‘Class II’ devices with no primary ground connection, but their outputs are often grounded in real applications providing a path for common-mode conducted noise. ‘Y’ rated capacitors from primary to secondary help reduce high frequency interference but cannot be made large in value in case the output is not grounded and dangerous AC ‘touch current’ could flow through the user. Some household standards (e.g. EN 60335) require very low capacitance values and two Y-caps in series in case of failure of one component. Fitting two series capacitors, each with the full safety creepage distances between pins inside a miniature package is extremely difficult and is often left to the user to add externally so that EMC standards can be met.
An increasingly common application is for low powered AC/DC converters to be powered from nominal 277VAC, 305VAC peak supplies. This is the line to neutral voltage in three-phase 115VAC systems, often used in large buildings in the US and Asia. The higher voltage requires associated wider separations and higher voltage-rated parts than in 230VAC systems and beefier components; the bulk capacitor for example needs to be 450VDC rated minimum. For small converters, this adds to the space problem. Some requirements are even tougher with operation from 480VAC, (525VAC peak) directly with the same part required to function down to 85VAC, the minimum tolerance of 100VAC systems. The high voltage and extreme input range compounds the difficulties of component stresses and safety separations especially when small size, low cost and high efficiency are part of the specifications!
A factor sometimes overlooked is operational altitude; standard tables for air clearances within power converters are typically for up to 2000m. At higher altitude, they increase quite substantially. For example, according to EN 62368-1 (Figure 3), clearances are multiplied by a factor of 1.48 at 5000m (this may seem extreme but there are eight capital cities worldwide over 2000m altitude). In addition, there are many lung clinics located at high altitude. Allowing for higher than 2000m is therefore common and adds to the design difficulty in a small space.
Fig. 3: Clearance and test voltage multipliers with altitude (Source EN 62368-1)
AC/DC’s for PCB mounting are expected to operate in the same thermal environment as the other board components, normally without any special heatsinking arrangements. We have seen how high ripple currents make high efficiency difficult, so a major part of the product design is to control internal temperatures so that maximum power can be taken to as high an ambient temperature as possible. As case sizes reduce, effective surface area for dissipation gets less, compounding the problem.
A factor sometimes overlooked is operational altitude; standard tables for air clearances within power converters are typically for up to 2000m. At higher altitude, they increase quite substantially. For example, according to EN 62368-1 (Figure 3), clearances are multiplied by a factor of 1.48 at 5000m (this may seem extreme but there are eight capital cities worldwide over 2000m altitude). In addition, there are many lung clinics located at high altitude. Allowing for higher than 2000m is therefore common and adds to the design difficulty in a small space.
Fig. 3: Clearance and test voltage multipliers with altitude (Source EN 62368-1)
AC/DC’s for PCB mounting are expected to operate in the same thermal environment as the other board components, normally without any special heatsinking arrangements. We have seen how high ripple currents make high efficiency difficult, so a major part of the product design is to control internal temperatures so that maximum power can be taken to as high an ambient temperature as possible. As case sizes reduce, effective surface area for dissipation gets less, compounding the problem.
Modular solutions
One manufacturer that has risen to the challenge of designing low cost, board-mount AC/DC converters with industrial grade specifications in small sizes is RECOM [1]. Their range spans 1W to 30W with all products operating from at least 85VAC to 264VAC. Some include 305VAC and one 5W product, the RA05-K/480 operates up to 525VAC. Parts are available with up to 5000m altitude rating and with operating temperatures as low as -40°C and up to +90°C with derating. Sizes range from an industry-leading 22.5mm x 27.94mm x 18mm for a new 3W part with a 20W part still only 25.4mm x 50.8mm x 23mm. Uniquely, the range includes 3W, 18W and 30W parts in circular footprint packages for inclusion in standard, flush, wall-mount installations with the 3W part featuring an ultra-low 11mm profile. All parts in the range feature EN 60950 or EN 62368 ITE safety certifications with most also holding EN 60335 household certification as well, while the 18W and 30W circular parts hold medical certification and have wired connections. Figure 4 summarises the range.
Fig. 4: The range of board-mount AC/DC products from RECOM
Fig. 4: The range of board-mount AC/DC products from RECOM
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