Understanding SMPS : Introduction, Types
If we see as a black box with two input terminals and two output terminals, the SMPS is identical to Linear power supply. The linear regulator regulates a continuous flow of current from the input to load in order to maintain a constant load voltage. The SMPS regulates the current flow by chopping up the input voltage and controlling the average current by controlling duty cycle.
The pulse width modulated SMPS are classified into two types based on the basic principle of operation. They are
(1) Forward Mode Switching Regulators (or) Forward Mode SMPS
(2) Fly-back Mode Switching Regulators (or) Flyback Mode SMPS
- Forward Mode SMPS form a large family of switching power supply topologies.
- They can identified by an L-C filter just after the power switch or after the output rectifier on the secondary of a transformer.
- A form of the forward-mode regulator is buck regulator.
- In this category, the power switch is placed directly between the input voltage and the Inductor.
- In between the power switch and the filter section(Inductor) there may be a transformer for stepping up or down the input voltage as in transformer-isolated forward regulators.
- When the switch is turned-on the load current passes from the input source, through the inductor to the load, and back again through the return lines to the input source. At this moment the diode is reverse biased.
- When the switch is turned-off the inductor still expects current to flow through it. The former current path through the input source is open-circuited at this moment. So the free wheeling diode starts to conduct and maintain a closed current loop through the load.
- When the switch turns on again, the voltage stored in the inductor reverse biases the freewheeling diode.
- In summary, the forward current is always flowing through the inductor and thus these supplies named as forward mode switching regulators.
- The amount of energy being delivered to the load is determined by duty cycle of the switch.
- Duty cycle is defined below.
D = Ton/(Ton+Toff)
- For forward mode switching regulators the value of duty cycle is in between 5 - 95%
Flyback Mode Switching Regulators [SMPS]:
- This type of SMPS use the same four basic components(L, D, Switch, C) as that of forward mode switching regulators, but the they are rearranged.
- Here the Inductor is placed directly between the input voltage source and the power switch.
- When the switch is turned On, current is being drawn through the inductor. It causes energy to be stored in the inductor.
- When the switch is turned-off, the current cannot change the direction instantaneously and it tries to flow in the same direction as before. Thus the inductor voltage reverses (or flies back).
- Thus the diode turns on and the energy from the inductor stores in the capacitor.
- Since the inductor voltage flies back above the input voltage , the voltage that appears on the output capacitor is higher than the input voltage.
- The only storage for the load is the output filter capacitor. It makes the output ripple voltage of flyback converters worse than their forward-mode regulators.
- Due to the restriction of the time required to empty the inductor’s flux into the output capacitor, the duty cycle is limited to 0-50% ( remember that the forward mode switching regulator duty cycle is in between 5 - 95%)
Based on the presence of transformer in the circuit, the SMPS is classified as follows:
(1) Non-transformer-Isolated Switching Power Supply Topologies
(2) Transformer-Isolated Switching Power Supply Topologies
Non-transformer-Isolated Switching Power Supply Topology [SMPS]:
- The non-transformer-isolated type of smps are easy to understand.
- They are used when some external component provides the DC isolation or protection in place of the switching supply.
- These external components are usually 50-60Hz transformers or isolated bulk power supplies.
- Typically they are used in local board-level voltage regulation.
- In these topologies, only the semiconductors provide the DC isolation from the input to the output. Failure of the switch leads to catastrophic failure.
- Some of the non-transformer-isolated topologies are:
- Buck regulator
- Boost regulator
- Buck-boost regulator
Transformer-Isolated Switching Power Supply Topology [SMPS]:
- Power Supplies that are intended to run directly from the AC source (offline power supplies) require a transformer to isolate the load side from AC lines.
- Transformers can also be used in power supplies where isolation is required for other reasons such as medical equipment use.
- Some of the transformer-isolated topologies are
- Flyback regulator
- Push-pull regulator
- Half-Bridge regulator
- Full-Bridge regulator