Isolated outputs are required for a broad range of DC/DC converter applications and not just the telecom- and datacom-mandated 48V isolation requirements. Isolation can be necessary for noise-sensitive devices needing ground separation from a noisy input voltage, such as a car battery, intermediate bus, and industrial inputs. Displays, programmable logic controllers, GPS systems and some medical monitoring devices can all be negatively affected by a noisy bus voltage.
Flyback converters are widely used in isolated DC/DC applications, but they are not necessarily a designer's first choice. Power-supply designers grudgingly select a flyback topology out of necessity for lower-power isolated requirements, not because they are easier to design. A flyback converter requires a significant amount of time devoted to the design of the transformer, a task further complicated by the normally limited selection of off-the-shelf transformers and the possible necessity for a custom transformer.
Furthermore, the flyback converter has stability issues due to the well-known right-half-plane zero in the control loo, which is further complicated by the propagation delay, aging, and gain variation of an optocoupler. Linear Technology's recently released LT3574 isolated monolithic flyback converter solves many of these flyback-design difficulties.
First of all, the LT3574 eliminates the need for an optocoupler, external MOSFET, secondary-side reference voltage and extra third winding off the power transformer, all while maintaining isolation between the primary and secondary-side, with only one part having to cross the isolation barrier. It has an on-chip 0.65A, 60V NPN power switch, can deliver up to 3W of output power from an input voltage ranging from 3V to 40V, and employs a primary-side sensing scheme that is 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 voltage and reflected output voltage, which the LT3574 is able to reconstruct. This output voltage feedback technique results in better than ±5% total regulation over the full line-voltage input, temperature range, and load range of 2% to 100%. Figure 1 shows a flyback converter schematic using the LT3574.
Figure 1: Flyback converter with primary-side output-voltage sensing
(Click on image to enlarge)
The LT3574's use of boundary-mode operation further simplifies system design, reduces converter size and improves load regulation. A LT3574 flyback converter turns on its internal switch immediately after the secondary side current reduces to zero and turns off when the switch current reaches the pre-defined current limit.
Thus it always operates at the transition of continuous conduction mode (CCM) and discontinuous 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 2 external resistors tied to the RFB and RREF pins set the output voltage.
Primary-side output-voltage sensing
Output-voltage sensing for an isolated converter normally requires an optocoupler and secondary-side reference voltage. An optocoupler transmits the output-voltage feedback signal through the optical link while maintaining the isolation barrier. However, an optocoupler 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, which can also cause a design to display different characteristics from circuit to circuit. A flyback design employing an extra transformer winding for voltage feedback can also be used to close the feedback loop instead of an optocoupler. However, this extra transformer winding can increase the transformer's size and cost.
The LT3574 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 as shown in Figure 2, where N is the turns ratio of the transformer, VIN is the input voltage, and VC is the maximum clamped voltage.
Figure 2: Typical switch-node waveform
(Click on image to enlarge)
Boundary-mode operation reduces converter size, improves regulation
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 below VIN. The internal DCM comparator detects this event 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. Furthermore, 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, which in turn reduces the transformer size when compared to lower frequency alternative designs. Figure 3 shows the SW voltage and current along with the current in the output diode.
Figure 3: Flyback-converter waveforms in boundary mode
(Click on image to enlarge)
The load regulation is much improved in boundary-mode operation because the reflected output voltage always samples at the diode current zero-crossing. The LT3574 typically provides ±3% load regulation.
Transformer selection and design considerations
The transformer specification and design is perhaps the most critical part of successfully applying the LT3574. Linear Technology has worked with leading magnetic-component manufactures to produce pre-designed flyback transformers and a complete list is shown in the LT3574 data sheet. Table 1 shows an abbreviated list of recommended off-the-shelf transformers from Wurth Electronik, Pulse Engineering, and BH Electronics. These transformers typically withstand a 1500VAC breakdown voltage for one minute from primary to secondary side. Higher breakdown voltage and custom transformers can also be used.
Table 1: Off-the-shelf transformers for the LT3574
(Click on table to enlarge)
Linear Technology offers free simulation software called LTspice that can be downloaded from www.linear.com. The LT3574 can be modeled using any of the transformers listed in Table 1 which produce very realistic simulations to help ease the design of such converters. The simulation circuit includes information on how the circuit starts up, and its reaction to load steps for different input voltages, and it shows how the common-mode current flows under varying conditions. It is easy to make design changes and see the impact this has to its circuit performance.
Transformer turns ratio
By using a RFB/RREF resistor ratio to set the output voltage, the user has relative freedom in selecting a transformer turns ratio to suit any given application. Typically, the transformers turns ratio is selected to maximize the available output power. For low output voltages (3.3V or 5V), an N:1 turns ratio can be used with multiple primary windings relative to the secondary, to maximize the transformer's current gain and output power.
However, the SW pin sees a voltage that is equal to the maximum input-supply voltage plus the output voltage multiplied by the turns ratio. This voltage needs to remain below the ABS MAX rating of the SW pin to prevent breakdown of the internal power switch. Together these conditions place an upper limit on the turns ratio (N) for a given application and needs to satisfy the following equation:
where VF is the output-diode voltage drop and VOUT is the output voltage.
For larger N:1 values, a transformer with a larger physical size is needed to deliver additional current and provide a large-enough inductance to ensure that the off-time is long enough to accurately measure the output voltage.
For lower output-power levels, a 1:1 or 1:N transformer can be chosen for the absolute smallest transformer size. A 1:N transformer will minimize the transformer size and magnetizing inductance, but will also limit the available output power. A higher 1:N turns ratio makes it possible to have very high output voltages without exceeding the breakdown voltage of the internal power switch.
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 minimized 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 (resistor, capacitor, and diode) clamp circuit, Figure 4, prevents the leakage inductance spike from exceeding the breakdown voltage on the power device. This circuit is included in all the LT3574 applications circuits and Schottky diodes are typically the best choice to be used in the snubber, due to their fast turn-on time.
Figure 4: RCD clamp circuit
(Click on image to enlarge)
A demonstration board using the LT3574 is shown in Figure 5. This circuit accepts an input voltage ranging from 10V to 30V and produces an isolated 5V output at up to 0.5A.
Figure 5: LT3574 application circuit photo (Size: 31×15×6.5mm)
(Click on image to enlarge)
A LT3574-based circuit significantly simplifies the design of an isolated-flyback DC/DC converter by eliminating the need for an optocoupler, external MOSFET, secondary-side reference voltage and extra third winding off the power transformer. It maintains primary to secondary isolation with only one part crossing the isolation barrier. The LT3574 operates from a 3V to 40V input-voltage range and has the ability to deliver up to 3 watts of continuous output power, making it suitable for a wide range of applications. Readily available off-the-shelf transformers eliminate the need for a custom transformer. Isolated converters are required for a broad range of applications and not just for telecom-mandated isolation requirements.
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About the author
received his BSEE from San Jose State University in 1980. He joined Linear Technology Corp. as a product marketing engineer in April 2006. Bruce's past experience includes stints at Cherokee International, Digital Power and Ford Aerospace. He is an avid sports participant.