Forward Converter DC DC SMPS Explained with Practical Circuit Included

Forward Converters DC-DC Switching Mode Power Supplies (SMPS) Explained

A forward converter is a switching power supply circuit that transfers the energy from the primary to the secondary while the switching element is “on,” which is the opposite of a flyback converter.

Forward and fly back converters are the two commonly used topologies used to either increase or decrease D.C voltages, or convert a single voltage to multiple D.C output voltages.

Components of a Typical Forward DC-DC Converter:

Transformer which is either a step-up or step-down with a single or multiple secondary windings. The type used depends on the available input voltage and desired output voltage. It also provides isolation of the load from the input voltage.
Transistor such as a MOSFET which acts as the switching device
Diodes
Capacitors
Inductor

Energy is passed directly through the transformer during the transistor’s conduction phase. The output voltage is determined by the input voltage, the transformer turns ratio and the duty cycle.

The two commonly used topologies are the single switch, and two switch forward converters.

Single-switch forward converter

Single Switch Forward Converter DC DC SMPS basic Schematic Circuit Diagram electrobuff.blogspot.com

The circuitry on the secondary (i.e. right) side is almost identical to a buck converter, and forward converters store and deliver energy in much the same way. The forward-mode controller opens and closes the switch with the appropriate duty cycle to achieve the required output voltage. The switching element is commonly a MOSFET, or IGBT but is occasionally a bipolar transistor and sometimes GaN or SiC. Various combinations of turns ratios and duty cycles can be used to achieve the required output voltage according to this equation:

Vout = Vin × Nsec / Npri × D

Vout = output voltage

Vin = input voltage

Nsec / Npri = N2 / N1 = transformer secondary to primary turns ratio

D = duty cycle = ton / (ton + toff)

When the switch is closed (MOSFET on-state), current flows through both the primary (leftmost) and secondary windings. In this state, energy is stored in the inductor and the capacitor, and no current flows through the tertiary (middle) winding due to the tertiary diode being reserve biased. It is important to note that, unlike a flyback, forward converters do not store any significant amount of energy in the transformer, and are therefore free from the corresponding limitations of doing so.

When the switch is opened, current is prevented from flowing in the primary and secondary coils. Power is supplied to the load by the inductor while the rightmost diode provides a return path for the current. However, the transformer coils still act as coupled inductors, so the tertiary winding is added in order to reset the flux stored in the transformer core, inducing a counter-clockwise flow of current that returns energy to the input voltage supply.

As the inductor continues to deliver its stored energy to the load, the inductor current will begin to drop. This current drop off continues until the next on-state, when the current rises to its previous value. This undesirable phenomenon is called output ripple, and is usually reduced to an acceptable level by placing a suitably large “smoothing” capacitor in parallel with the output. Output ripple can also be reduced by designing the converter to operate at higher switching frequencies, shortening the period of time spent in off-state.

The lower rms secondary current in the forward-mode design compared to a similar flyback design can mean lower losses in the secondary winding, even if the flyback design has a higher number of turns. This lower secondary loss is one reason forward-modes typically provide higher efficiency than similar flyback designs. Another reason is the flyback output capacitor must also handle higher current than the forward-mode design, adding capacitor losses to the equation.

Advantages of single switch forward converter

Simple construction and operation
Low input capacitor ripple current
Lower current on the secondary diodes

Disadvantages of single switch forward converter

Requires a high transistor rating (twice the input voltage)
Requirement for an active snubbers circuits for resetting the transformer core
higher conduction losses
Bigger transformer

Two-Switch Forward Mode DC DC Converter

Two Switch Forward Mode DC DC Converter SMPS typical Schematic Circuit Diagram electrobuff.blogspot.com

A two-switch forward-mode converter (also called an asymmetrical half-bridge forward converter) has two FET switches which are sometimes integrated into a single controller IC.

Two Switch Forward Converter DC DC SMPS Operations Schematic diagram electrobuff.blogspot.com

Energy is transferred from the primary to the secondary of the transformer when the two transistors are simultaneously turned on. When the transistors are off, the transformer magnetizing current flows back to the source, through diodes D1 and D2 which are now forward biased. The diodes conducts until all the magnetizing energy in the primary along with the energy stored in the leakage inductances is returned to the input supply.

To ensure a transformer reset during the OFF time, a duty cycle of less that 50% is used to give a longer OFF time than the ON time. In this operation, the primary winding of the transformer acts as the reset winding.

Advantages of two switch forward Converters

Does not require a snubber circuit (reset winding)
Less voltage stress for the MOSFET (same as input voltage)
Simple construction and operation over a wide range of input and output voltages
Ability to provide multiple isolated outputs
Low system power losses and noise

Disadvantages of two switch forward conveters

Limited frequency of operation
Slightly expensive since it uses more components
larger components (transformer and inductor)

The single switch converter is used for power outputs of up to about 100W. The two switch converter is preferred for most applications due to its reliability and efficiency and widely used in the ATX power supply units with outputs of between 150 and 750 W.

Applications of Forward DC DC Converters:

DC-DC power supplies
Portable electronics
Telecom
LED lighting
Power over Ethernet (PoE)
AC-DC power supplies

Off-the-shelf forward-mode converters are available for many applications where low cost, small size, and high efficiency are required. They are typically used in DC-DC controllers in the telecommunications (telecom) voltage range of 36 – 72 Vdc, sometimes at extended voltages ranging from 9 – 36 Vdc.

Forward-mode converters are commonly used for output current up to about 15 Amps and output power up to approximately 300 Watts. Coilcraft offers standard, off-the-shelf forward-mode wirewound transformers with power capabilities ranging from a few Watts to up to 60 Watts. When higher current and power is required, Coilcraft also offers standard planar transformers for forward-mode, push-pull, or half-bridge / full-bridge topologies in power ratings from 30 Watts to 800 Watts.