Low-Power Flyback Converter Eliminates Optocoupler

Isolated outputs are required in a variety of DC/DC converter applications, not just telecom and datacom applications that require 48V isolation. Isolation is essential for noise-sensitive devices that require ground isolation from noisy input voltages such as automotive batteries, intermediate buses, and industrial inputs. Displays, programmable logic controllers, GPS systems, and some medical monitoring equipment can all be negatively affected by noisy bus voltages.

Flyback converters are widely used in isolated DC/DC applications, but they are not necessarily the designer's first choice. Power supply designers are reluctant to choose flyback converters because they have to meet lower power isolation requirements, not because flyback converters are easier to design. Flyback converters require a lot of time to design the transformer, which is further complicated by the limited selection of off-the-shelf transformers and the possibility of custom transformers. In addition, flyback converters have stability issues (due to the well-known right half plane zero in the control loop), which is further complicated by the propagation delay, aging and gain variation of the optocoupler. The LT3574 isolated monolithic flyback converter introduced by Linear Technology solves many of these flyback converter design challenges.

First, the LT3574 eliminates the need for an optocoupler, external MOSFET, secondary-side reference voltage, or an additional third winding on the power transformer, while maintaining primary-side and secondary-side isolation with only one component that must cross the isolation barrier. The LT3574 has an internal 0.65A, 60V NPN power switch, can deliver up to 3W of output power from an input voltage range of 3V to 40V, and uses a primary-side sensing circuit that detects the output voltage through the primary-side flyback switch node waveform. When the switch is off, the output diode provides 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 sum of the input voltage and the reflected output voltage, and the LT3574 can reconstruct the switch node voltage. This output voltage feedback method produces a total regulation error better than ±5% over the entire line voltage input range, the entire temperature range, and the load range from 2% to 100%. Figure 1 shows a schematic diagram of a flyback converter using the LT3574.

Figure 1: Flyback converter with primary-side output voltage sensing

The LT3574 further simplifies system design, reduces converter size and improves load regulation by using boundary mode operation. The LT3574 flyback converter turns on the internal switch immediately when the secondary current drops to zero, and turns off when the switch current reaches the predefined current limit. Therefore, the device is always in transition between continuous conduction mode (CCM) and discontinuous conduction mode (DCM) when operating, which is often called 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 connected to the RFB and RREF pins set the output voltage. The LT3574 is available in an MSOP-16 package.

Primary-Side Output Voltage Sensing
Output voltage sensing of isolated converters typically requires an optocoupler and a secondary-side reference voltage. Optocouplers send the output voltage feedback signal over an optical link while maintaining the isolation barrier. However, the transfer ratio of optocouplers varies with temperature and aging, which reduces accuracy. Optocouplers also introduce propagation delays, resulting in slower transient responses, which can be nonlinear from device to device, which can also cause a design to exhibit different characteristics in different circuit implementations. Flyback designs that use an additional transformer winding for voltage feedback instead of an optocoupler can also be used to close the feedback loop. However, this additional transformer winding can increase the size and cost of the transformer.

The LT3574 senses the output voltage at the primary side of the transformer, eliminating the need for an optocoupler or additional transformer windings. As shown in Figure 2, the output voltage can be accurately measured at the primary side switch node waveform when the power transistor is off, where N is the turns ratio of the transformer, VIN is the input voltage, and VC is the maximum clamping voltage.

Figure 2: Typical switch node waveform

Boundary Mode Operation Reduces Converter Size and Improves Regulation
Boundary mode control uses a variable frequency current mode switching circuit. When the internal power switch turns on, the transformer current increases until it reaches the preset current limit set point. The voltage on the SW pin rises to: output voltage divided by secondary to primary transformer turns ratio + input voltage. When the secondary current through the diode drops to zero, the SW pin voltage drops below VIN. The internal DCM comparator detects this event and turns the switch on again, repeating the cycle.

At the end of each cycle, boundary mode operation returns the secondary current to zero, causing a parasitic resistance voltage drop but no load regulation error. In addition, the primary flyback switch is always turned on at zero current, and there is no reverse recovery loss in the output diode. This reduction in power dissipation enables the flyback converter to operate at a relatively high switching frequency, which in turn reduces the size of the transformer (compared to lower frequency alternative designs). Figure 3 shows the SW voltage and current and the current in the output diode.

Figure 3: Flyback converter waveforms in boundary mode operation

Since the reflected output voltage is always sampled at the diode current zero crossing point, load regulation is greatly improved in boundary mode operation. The LT3574 typically provides ±3% load regulation.

Transformer Selection and Design Considerations
Transformer specification and design is perhaps the most critical part of successfully applying the LT3574. Linear Technology has worked with leading magnetics manufacturers to produce pre-designed flyback transformers, a full list of which is given 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 are typically able to withstand a breakdown voltage of 1500VAC from primary to secondary for 1 minute. Higher breakdown voltage transformers and custom transformers are also available.

Table 1: Off-the-shelf transformers for the LT3574

Linear Technology provides free simulation software LTspice, which can be downloaded from www.linear.com.cn. The LT3574 can be modeled using any of the transformers listed in Table 1, and the resulting model produces very realistic simulations that help ease the burden of designing these converters. Circuit simulations include information about how the circuit starts up, how the circuit reacts to load steps at different input voltages, and shows how common-mode currents flow under changing conditions. It is easy to change the design and see the impact of changes on circuit performance.

Transformer Turns Ratio
The user has relative freedom in selecting the transformer turns ratio to suit any given application by setting the output voltage with the RFB/RREF resistor ratio. In general, the transformer turns ratio is chosen to maximize the available output power. For low output voltages (3.3V or 5V), a turns ratio of N:1 can be used with multiple primary windings relative to the secondary winding to maximize the transformer's current gain and output power. However, the voltage at the SW pin is equal to the maximum input supply voltage + the output voltage times the turns ratio. This voltage needs to be kept below the ABS MAX rating of the SW pin to prevent breakdown of the internal power switch. These conditions combine to place an upper limit on the turns ratio (N) for a given application, which needs to satisfy the following inequality:

Where VF is the output diode voltage drop and VOUT is the output voltage.

For larger N:1 values, a physically larger transformer is required to supply the additional current and provide sufficient inductance to ensure that the off time is long enough to accurately measure the output voltage.

For lower output power values, a 1:1 or 1:N transformer can be selected to achieve the absolute minimum transformer size. Using a transformer with a turns ratio of 1:N will minimize the size of the transformer and the magnetizing inductance, but will also limit the available output power. A higher 1:N turns ratio enables very high output voltages to be provided without exceeding the breakdown voltage of the internal power switch.

Transformer Leakage Inductance
The leakage inductance of the transformer, either primary or secondary, causes a voltage spike on the primary side after the power switch is turned off. This spike is more significant at higher load currents, where more stored energy must be released. Leakage inductance can be minimized by tightly coupling the transformer windings and can be measured by reading the inductance on one transformer winding with the other winding shorted.

The simple RCD (resistor, capacitor and diode) clamp circuit shown below in Figure 4 prevents leakage inductance spikes from exceeding the breakdown voltage of the power circuit. This circuit is included in all LT3574 application circuits, and Schottky diodes are often the best choice for snubbers due to their fast turn-on time.

Figure 4: RCD clamp circuit

Demonstration Circuit
A demonstration circuit board featuring the LT3574 is shown in Figure 5. The circuit accepts an input voltage ranging from 10V to 30V and produces an isolated 5V output at currents up to 0.5A.

Figure 5: Photo of LT3574 application circuit (size: 31mm x 15mm x 6.5mm)

Conclusion
The LT3574-based circuit greatly simplifies the design of an isolated flyback DC/DC converter by eliminating the need for an optocoupler, external MOSFET, secondary-side reference voltage, and an additional third winding on the power transformer. The device maintains primary-to-secondary isolation with only one component across the isolation barrier. The LT3574 operates from a 3V to 40V input voltage range and can deliver up to 3W of continuous output power, making it ideal for a wide range of applications. Off-the-shelf transformers can be used, eliminating the need for custom transformers. Isolated converters are needed in many applications, not just telecom applications. Isolation from the bus voltage is a must for noise-sensitive applications such as GPS systems, displays, programmable logic controllers, and medical monitoring equipment.