Flyback converters have been used widely in isolated dc/dc applications for many years, but are not necessarily a designer's first choice. A flyback converter is selected out of necessity for lower power isolation requirements, not because they are easier to design with.
The flyback converter has stability issues due to the right half plane zero in the control loop, further complicated by the propagation delay, aging and gain variation of an optocoupler. A flyback converter also requires time to be devoted to designing the transformer.
However, Linear Technology's LT3748 isolated flyback controller solves many of these design difficulties.
The LT3748 eliminates the need for an optocoupler, secondary side reference voltage and extra third winding off the power transformer, while maintaining isolation between the primary and secondary side with only one part having to cross the isolation barrier. The device employs a primary side sensing scheme capable of detecting the output voltage through the flyback primary side switching node waveform.
During the switch off period, the output diode delivers the current to the output and the output voltage is reflected to the primary side of the flyback transformer. The magnitude of the switch node voltage is the summation of the input and reflected output voltages, which the LT3748 can reconstruct. This feedback technique results in better than ±5% total regulation over the full line voltage input and temperature range and over loads ranging from 2% to 100% (see fig 1).
The LT3748 accepts a 5V to 100V input, which can be applied directly without the need for a series dropping resistor. It can operate a high input voltage reliably due to the high voltage onboard low drop out regulator. In addition, an onboard gate driver powers an external NPN power switch, allowing it to deliver up to 50W, dependent upon external component selection, input voltage range and output voltage.
The use of boundary mode operation further simplifies system design and reduces overall converter size and footprint. The LT3748 turns on its internal switch immediately after the secondary side current reduces to zero and turns off when the switch current reaches the predefined current limit. Thus it always operates at the transition of continuous conduction mode and discontinuous conduction mode (DCM), commonly referred to as boundary mode or critical conduction mode. Other features include programmable soft start, adjustable current limit, undervoltage lockout and temperature compensation. The transformer turns ratio and two external resistors tied to the Rfb and Rref pins set the output voltage.
Output voltage sensing in an isolated converter normally requires an optocoupler and secondary side reference voltage. The output voltage feedback signal is transmitted through the optical link, maintaining the isolation barrier. However, an optocoupler's transfer ratio changes with temperature and aging, degrading its accuracy. Optocouplers also introduce a propagation delay, resulting in a slower transient response that can be nonlinear from unit to unit and which can cause a design to display different characteristics from circuit to circuit. While a flyback design with an extra transformer winding for voltage feedback can be used to close the feedback loop, this extra winding can increase the transformer's size and cost.
The LT3748 eliminates the need for an optocoupler or extra transformer winding by sensing the output voltage on the primary side of the transformer. The output voltage is accurately measured at the primary side switching node waveform during the off time of the power transistor (see fig 2), where N is the turns ratio of the transformer, Vin is the input voltage and Vc is the maximum clamped voltage.
Boundary mode control is a variable frequency current mode switching scheme. When the internal power switch turns on; the transformer current increases until its preset current limit set point is reached. The voltage on the SW pin rises to the output voltage divided by the secondary to primary transformer turns ratio plus the input voltage. When the secondary current through the diode falls to zero, the SW pin voltage falls to less than Vin. The internal DCM comparator detects this and turns the switch back on, thus repeating the cycle.
Boundary mode returns the secondary current to zero at the end of every cycle, so the parasitic resistive voltage drop does not cause load regulation errors. The primary flyback switch is always turned on at zero current and the output diode has no reverse recovery loss. This reduction in power loss allows the flyback converter to operate at a relatively high switching frequency, in turn reducing transformer size. Figure 3 shows the SW voltage and current, along with the output diode current.
Load regulation is much improved in boundary mode operation because the reflected output voltage always samples at the diode current zero crossing.
Transformer specification and design is probably the most critical part of applying the LT3748 successfully. In addition to the usual list of caveats – such as the high frequency isolated power supply transformer design having a low leakage inductance and close coupling – the transformer turns ratio must be tightly controlled to ensure a consistent output voltage. A tolerance of ±5% in turns ratio from transformer to transformer could result in a variation of more than ±5% in output voltage. Fortunately, most magnetic component manufacturers are capable of guaranteeing a turns ratio tolerance of ±1% or better.
Linear has worked with leading magnetic component manufacturers to produce predesigned flyback transformers for use with the LT3748. These transformers typically withstand a 1500V ac breakdown voltage for one minute from the primary to the secondary.
LTspice simulation software can be used to model the LT3748 with any of thsee transformers, producing realistic simulations to help ease the design. The simulation circuit shows how the circuit starts up, how it reaction to load steps for different input voltages and how the common mode current flows under varying conditions.
The transformer leakage inductance on either the primary or secondary side causes a voltage spike to appear at the primary after the power switch turns off. This spike is increasingly prominent at higher load currents, where more stored energy must be dissipated.
The leakage inductance can be minimised by close coupling of the transformer windings and is measured by reading the inductance on a transformer winding with the other windings shorted out.
A simple RCD clamp circuit (see fig 4) prevents the leakage inductance spike from exceeding the breakdown voltage on the power device and Schottky diodes are typically the best choice for the snubber due to their fast turn on time.
Although the design of an isolated flyback converter is not a simple task, there is now an alternative to using modules or a complex discrete implementation. An LT3748 based circuit simplifies the design of an isolated flyback converter by eliminating the need for an optocoupler, secondary side reference voltage and extra third winding off the power transformer and is suitable for a range of industrial applications.
Bruce Haug is a product marketing engineer in Linear Technology's Power Products division.
