Power supply specifications & some common types of SMPS circuits
Power supply specifications:
- Power supplies may have several specifications to be met, including their voltage and current ratings. There may be short time ratings of higher magnitudes of current and continuous ratings of somewhat lower magnitudes.
- One needs to specify the tolerable limits on the ripple voltages, short-circuit protection level of current (if any) and the nature of output volt-current curve during over-current or short circuit (the output voltage magnitude should reduce or fold back towards zero, gradually, depending on the severity of over-current).
- The fuse requirement (if any) on the input and the output side may need to be specified. One needs to specify the type of input supply (whether ac or dc) or whether the power supply can work both from ac or dc input voltages.
- Acceptable range of variation in input voltage magnitude, supply frequency (in case of ac input) are also to be specified.
- Efficiency, weight and volume are some other important specifications.
- Some applications require the electro-magnetic compatibility standards to be met. By electromagnetic compatibility it is meant that the level of EMI generation by power supply should be within tolerable limits and at the same time the power supply should have the ability to work satisfactorily in a limited noisy environment.
- It is quite common to have output voltage isolation and it is specified in terms of isolation breakdown voltage. In case of multiple power supplies it needs to be specified whether all the outputs need to be isolated or not and what should be the acceptable ripple voltage range for each.
- In majority of the cases the available source of input power is the alternating type utility voltage of 50 or 60 Hz. The voltage levels commonly used are 115V (common in countries like, USA) and 230 volts (common in India and many of the European countries).
- Most utility (mains) power supplies are expected to have ± 10% voltage regulation but for additional precaution the SMPS circuits must work even if input voltages have ± 20% variation.
- Now-a-days universal power supplies that work satisfactorily and efficiently both on 115 V and 230 V input are quite popular.
- These power supplies are very convenient for international travelers who can simply plug-on their equipments, like laptop computer and shaving machine, without having to pay much attention on the exact voltage and frequency levels of the utility supply.
- In contrast some of the other power supplies have a selector switch and the user is required to adjust the switch position to match the utility voltage.
- In cases user forgets to keep the selector switch at correct position, the equipment attached may get damaged.
Some common types of SMPS circuits:
- There are several different topologies for the switched mode power supply circuits.
- Some popular ones are: fly-back, forward, push-pull, C’uk, Sepic, half bridge and H-bridge circuits.
- A particular topology may be more suitable than others on the basis of one or more performance criterions like cost, efficiency, overall weight and size, output power, output regulation, voltage ripple etc.
- All the topologies listed above are capable of providing isolated voltages by incorporating a high frequency transformer in the circuit.
- There are many commercially available power supply controller ICs that can readily be used to control the duty ratio of the SMPS switches so that the final output is well regulated.
- Most of these ICs are capable of driving MOSFET type of switches. They also provide features like under voltage lock-out, output over-current protection etc.